Tepidanaerobacter acetatoxydans sp. nov., an anaerobic, syntrophic acetate-oxidizing bacterium isolated from two ammonium-enriched mesophilic methanogenic processes

Impact of additives on syntrophic propionate and acetate enrichments under high-ammonia conditions

High ammonia concentrations in anaerobic degradation systems cause volatile fatty acid accumulation and reduced methane yield, which often derive from restricted activity of syntrophic acid-oxidising bacteria and hydrogenotrophic methanogens. Inclusion of additives that facilitate the electron transfer or increase cell proximity of syntrophic species by flocculation can be a suitable strategy to counteract these problems, but its actual impact on syntrophic interactions has yet to be determined. In this study, microbial cultivation and molecular and microscopic analysis were performed to evaluate the impact of conductive (graphene, iron oxide) and non-conductive (zeolite) additives on the degradation rate of acetate and propionate to methane by highly enriched ammonia-tolerant syntrophic cultures derived from a biogas process. All additives had a low impact on the lag phase but resulted in a higher rate of acetate (except graphene) and propionate degradation. The syntrophic bacteria 'Candidatus Syntrophopropionicum ammoniitolerans', Syntrophaceticus schinkii and a novel hydrogenotrophic methanogen were found in higher relative abundance and higher gene copy numbers in flocculating communities than in planktonic communities in the cultures, indicating benefits to syntrophs of living in close proximity to their cooperating partner. Microscopy and element analysis showed precipitation of phosphates and biofilm formation in all batches except on the graphene batches, possibly enhancing the rate of acetate and propionate degradation. Overall, the concordance of responses observed in both acetate- and propionate-fed cultures highlight the suitability of the addition of iron oxide or zeolites to enhance acid conversion to methane in high-ammonia biogas processes. KEY POINTS: • All additives promoted acetate (except graphene) and propionate degradation. • A preference for floc formation by ammonia-tolerant syntrophs was revealed. • Microbes colonised the surfaces of iron oxide and zeolite, but not graphene.

Enhancing cellulose acetate biodegradability in cigarette filters: an in-depth analysis of thermal alkaline pretreatment, microbial dynamics, and breakdown pathway prediction

BACKGROUND

The demand for bioplastics has increased exponentially as they have emerged as alternatives to petrochemical plastics. However, there is a substantial lack of knowledge regarding bioplastic degradation. This study developed a novel pretreatment method to improve the accessibility of a bioplastic substrate for biodegradation. In this study, cellulose acetate, a bioplastic found in the world's most littered waste, e.g. cigarette filters, was selected as a potential substrate. Before anaerobic digestion, three thermal alkaline pretreatments: TA 30 °C, TA 90 °C, and TA 121 °C, were used to evaluate their effects on the chemical alterations of cellulose acetate.

RESULT

The ester groups in cellulose acetate were significantly reduced by the TA 30 °C pretreatment, as seen by a decrease in C = O stretching vibrations and shortening of C - O stretches (1,270 ∼ 1,210 cm- 1), indicating effective removal of acetyl groups. This pretreatment significantly enhanced cellulose acetate biodegradability to a maximum of 91%, surpassing the previously reported cellulose acetate degradation. Methane production increased to 695.0 ± 4 mL/g of volatile solid after TA 30 °C pretreatment, indicating enhanced cellulose acetate accessibility to microorganisms, which resulted in superior biogas production compared to the control (306.0 ± 10 mL/g of volatile solid). Diverse microbes in the anaerobic digestion system included hydrolytic (AB240379_g, Acetomicrobium, FN436103_g, etc.), fermentative, and volatile fatty acids degrading bacteria (JF417922_g, AB274492_g, Coprothermobacter, etc.), with Methanobacterium and Methanothermobacter being the sole hydrogenotrophic methanogens in the anaerobic digestion system. Additionally, an attempt to predict the pathway for the effective degradation of cellulose acetate from the microbial community in different pretreatment conditions.

CONCLUSIONS

To the best of our knowledge, this is the first study to estimate the maximum cellulose acetate degradation rate, with a simple and cost-effective pretreatment procedure. This approach holds promise for mitigating the environmental impact of cellulose acetate of cigarette filters and presents a sustainable and economically viable waste management strategy.

Genome-centric metagenomics and methanogenic pathway analysis for acclimated anaerobic digestion of chicken manure with high ammonia stressed under thermophilic condition

Thermophilic anaerobic digestion (AD) of animal manure offers various environmental benefits but the process requires a microbial community acclimatized to high ammonia. In current study, a lab-scale continuous stirred tank reactor (CSTR) fed with chicken manure was operated under thermophilic condition for 450 days in total. Results showed that the volumetric methane production decreased from 445 to 328 and sharply declined to 153 mL L-1·d-1 with feeding total solid (TS) step increased from 5% to 7.5% and 10%, respectively. While, after a long-term stop feeding for 80 days, highly disturbed reactor was able to recover methane generation to 739 mL L-1·d-1 at feeding TS of 10%. Isotope analysis indicted acetate converted to methane through the syntrophic acetate oxidation and hydrogenotrophic methanogenesis (SAO-HM) pathway increased from 33% to 63% as the concentration of ammonium increased from 2493 to 6258 mg L-1. Significant different in the genome expression of the SAO bacterial from 0.09% to 1.23%, combining with main hydrogenotrophic partners (Methanoculleus spp. and Methanothermobacter spp.) contented of 2.1% and 99.9% during inhibitory and recovery stages, respectively. The highly expressed KEGG pathway in level 3 (enzyme genes) for the Recovery sludge combining with the extraordinary high abundance of genera Halocella sp. suggested that Halocella sp. might be a highly efficient hydrolytic and acidogenic microorganism and enhance the process of SAO during carbon metabolic flow to methane. This report will be a basis for further study of AD studies on high nitrogen content of poultry manure.

Metabolism of novel potential syntrophic acetate-oxidizing bacteria in thermophilic methanogenic chemostats

Acetate is a major intermediate in the anaerobic digestion of organic waste to produce CH4. In methanogenic systems, acetate degradation is carried out by either acetoclastic methanogenesis or syntrophic degradation by acetate oxidizers and hydrogenotrophic methanogens. Due to challenges in the isolation of syntrophic acetate-oxidizing bacteria (SAOB), the diversity and metabolism of SAOB and the mechanisms of their interactions with methanogenic partners are not fully characterized. In this study, the in situ activity and metabolic characteristics of potential SAOB and their interactions with methanogens were elucidated through metagenomics and metatranscriptomics. In addition to the reported SAOB classified in the genera Tepidanaerobacter, Desulfotomaculum, and Thermodesulfovibrio, we identified a number of potential SAOB that are affiliated with Clostridia, Thermoanaerobacteraceae, Anaerolineae, and Gemmatimonadetes. The potential SAOB possessing the glycine-mediated acetate oxidation pathway dominates SAOB communities. Moreover, formate appeared to be the main product of the acetate degradation by the most active potential SAOB. We identified the methanogen partner of these potential SAOB in the acetate-fed chemostat as Methanosarcina thermophila. The dominated potential SAOB in each chemostat had similar metabolic characteristics, even though they were in different fatty-acid-fed chemostats. These novel syntrophic lineages are prevalent and may play critical roles in thermophilic methanogenic reactors. This study expands our understanding of the phylogenetic diversity and in situ biological functions of uncultured syntrophic acetate degraders and presents novel insights into how they interact with methanogens.IMPORTANCECombining reactor operation with omics provides insights into novel uncultured syntrophic acetate degraders and how they perform in thermophilic anaerobic digesters. This improves our understanding of syntrophic acetate degradation and contributes to the background knowledge necessary to better control and optimize anaerobic digestion processes.

Diverse electron carriers drive syntrophic interactions in an enriched anaerobic acetate-oxidizing consortium

In many anoxic environments, syntrophic acetate oxidation (SAO) is a key pathway mediating the conversion of acetate into methane through obligate cross-feeding interactions between SAO bacteria (SAOB) and methanogenic archaea. The SAO pathway is particularly important in engineered environments such as anaerobic digestion (AD) systems operating at thermophilic temperatures and/or with high ammonia. Despite the widespread importance of SAOB to the stability of the AD process, little is known about their in situ physiologies due to typically low biomass yields and resistance to isolation. Here, we performed a long-term (300-day) continuous enrichment of a thermophilic (55 °C) SAO community from a municipal AD system using acetate as the sole carbon source. Over 80% of the enriched bioreactor metagenome belonged to a three-member consortium, including an acetate-oxidizing bacterium affiliated with DTU068 encoding for carbon dioxide, hydrogen, and formate production, along with two methanogenic archaea affiliated with Methanothermobacter_A. Stable isotope probing was coupled with metaproteogenomics to quantify carbon flux into each community member during acetate conversion and inform metabolic reconstruction and genome-scale modeling. This effort revealed that the two Methanothermobacter_A species differed in their preferred electron donors, with one possessing the ability to grow on formate and the other only consuming hydrogen. A thermodynamic analysis suggested that the presence of the formate-consuming methanogen broadened the environmental conditions where ATP production from SAO was favorable. Collectively, these results highlight how flexibility in electron partitioning during SAO likely governs community structure and fitness through thermodynamic-driven mutualism, shedding valuable insights into the metabolic underpinnings of this key functional group within methanogenic ecosystems.

Volatile fatty acids bio-production using extracellular polymeric substances disengaged from sludge for carbon source recycling

Excessive waste-activated sludge (WAS) and insufficient carbon source (CS) for biological nitrogen removal (BNR) often coexist in municipal sewage treatment. Although the production of volatile fatty acids (VFAs) from WAS has been recognized as a promising solution, the development is limited by low VFAs production efficiency and dewatering deterioration of sludge. This study extracted the extracellular polymeric substances (EPS) from sludge by low-temperature thermal-hydrolysis (LTH) and high-speed hydro-cyclone (HSHC) pretreatment and recovered it for high-quality VFAs bio-production in thermophilic fermentation. Microbial mechanism analysis disclosed that interspecific interaction networks composed of functional flora, which accumulate VFAs by bio-converting EPS primarily and supplemented by EPS synthesis, guaranteed the efficient bio-production of VFAs. This process scheme shows promise in providing alternative denitrification CSs and avoiding deterioration of sludge dewaterability.

Does carbon cloth really improve thermophilic anaerobic digestion performance on a larger scale? focusing on statistical analysis and microbial community dynamics

Currently, the phenomenon of direct interspecies electron transfer (DIET) is of great interest in the technology of anaerobic digestion (AD) due to potential performance benefits. However, the conditions for the occurrence of DIET and its limits on improving AD under conditions close to real have not been studied enough. This research is concentrated on the effect of conductive carbon cloth (R3), in comparison with a dielectric fiberglass cloth (R2) and control (R1), on the AD performance in large (90 L) thermophilic reactors, fed with a mixture of simulated organic fraction of municipal solid waste and sewage sludge. While organic loading rate (OLR) was gradually increased from 2.4 to 8.66 kg VS/(m³ day), a statistically significant (p < 0.05) difference in biogas production was observed between R1 and both R2 and R3. However, at a maximum OLR of 12.12 kg VS/(m³ day) in R3, an increase in biogas production (p < 0.05) was observed both compared to R1 (by 8.97%) and R2 (by 4.24%). The content of volatile fatty acids in R3 as a whole was the lowest, especially at the maximum OLR. Biofilm on carbon cloth was rich in syntrophic microorganisms of the genera Tepidanaerobacter, as well as Defluviitoga, capable of DIET in mixed cultures with Methanothrix, which was the most abundant methanogen in biofilm. Suspended Bifidobacterium, Fervidobacterium and Anaerobaculum were negatively affected, while Defluviitoga, Methanothermobacter and Methanosarcina, on the contrary, were positively affected by the increase in OLR and showed, respectively, a negative and positive correlation (p < 0.05) with the main AD performance parameters.

Reprogramming the metabolism of an acetogenic bacterium to homoformatogenesis

Methyl groups are abundant in anoxic environments and their utilization as carbon and energy sources by microorganisms involves oxidation of the methyl groups to CO₂, followed by transfer of the electrons to an acceptor. In acetogenic bacteria, the electron acceptor is CO₂ that is reduced to enzyme bound carbon monoxide, the precursor of the carboxyl group in acetate. Here, we describe the generation of a mutant of the acetogen Acetobacterium woodii in which the last step in methyl group oxidation, formate oxidation to CO₂ catalyzed by the HDCR enzyme, has been genetically deleted. The mutant grew on glycine betaine as methyl group donor, and in contrast to the wild type, formed formate alongside acetate, in a 1:2 ratio, demonstrating that methyl group oxidation stopped at the level of formate and reduced electron carriers were reoxidized by CO₂ reduction to acetate. In the presence of the alternative electron acceptor caffeate, CO₂ was no longer reduced to acetate, formate was the only product and all the carbon went to formate. Apparently, acetogenesis was not required to sustain formatogenic growth. This is the first demonstration of a genetic reprogramming of an acetogen into a formatogen that grows by homoformatogenesis from methyl groups. Formate production from methyl groups is not only of biotechnological interest but also for the mechanism of electron transfer in syntrophic interactions in anoxic environments.

Effects of phenyl acids on different degradation phases during thermophilic anaerobic digestion

Aromatic compounds like phenyl acids (PA) can accumulate during anaerobic digestion (AD) of organic wastes due to an increased entry of lignocellulose, secondary plant metabolites or proteins, and thermodynamic challenges in degrading the benzene ring. The effects of aromatic compounds can be various – from being highly toxic to be stimulating for methanogenesis – depending on many parameters like inoculum or molecular characteristics of the aromatic compound. To contribute to a better understanding of the consequences of PA exposure during AD, the aim was to evaluate the effects of 10 mM PA on microbial communities degrading different, degradation phase–specific substrates in thermophilic batch reactors within 28  days: Microcrystalline cellulose (MCC, promoting hydrolytic to methanogenic microorganisms), butyrate or propionate (promoting syntrophic volatile fatty acid (VFA) oxidisers to methanogens), or acetate (promoting syntrophic acetate oxidisers to methanogens). Methane production, VFA concentrations and pH were evaluated, and microbial communities and extracellular polymeric substances (EPS) were assessed. The toxicity of PA depended on the type of substrate which in turn determined the (i) microbial diversity and composition and (ii) EPS quantity and quality. Compared with the respective controls, methane production in MCC reactors was less impaired by PA than in butyrate, propionate and acetate reactors which showed reductions in methane production of up to 93%. In contrast to the controls, acetate concentrations were high in all PA reactors at the end of incubation thus acetate was a bottle-neck intermediate in those reactors. Considerable differences in EPS quantity and quality could be found among substrates but not among PA variants of each substrate. Methanosarcina spp. was the dominant methanogen in VFA reactors without PA exposure and was inhibited when PA were present. VFA oxidisers and Methanothermobacter spp. were abundant in VFA assays with PA exposure as well as in all MCC reactors. As MCC assays showed higher methane yields, a higher microbial diversity and a higher EPS quantity and quality than VFA reactors when exposed to PA, we conclude that EPS in MCC reactors might have been beneficial for absorbing/neutralising phenyl acids and keeping (more susceptible) microorganisms shielded in granules or biofilms.

Proteiniphilum and Methanothrix harundinacea became dominant acetate utilizers in a methanogenic reactor operated under strong ammonia stress

Microorganisms in anaerobic digestion (AD) are easily affected by ammonia, especially acetoclastic methanogens. Thus, in ammonia-suppressed AD systems, acetate degradation is reported to be carried out mainly by the cooperation of syntrophic acetate oxidizers and hydrogenotrophic methanogens. Previous studies have revealed ammonia inhibition on microbial flora by AD performance, but the effect mechanism of ammonia on microbial metabolism remains poorly understood. In this study, we constructed a mesophilic chemostat fed with acetate as the sole carbon source, gradually increased the total ammonia nitrogen (TAN) concentration from 1 g L^-1 to 6 g L^-1, and employed the 16S rRNA gene, metagenomics, and metatranscriptomics analysis to characterize the microbial community structure and metabolic behavior. The results showed that even at the TAN of 6 g L^-1 (pH 7.5), the methanogenesis kept normal, the biogas production was approximately 92% of that at TAN of 1 g L^-1 and the acetate degradation ratio reached 99%, suggesting the strong TAN tolerance of the microbial community enriched. 16S rRNA gene analysis suggested that the microbial community structure changed along with the TAN concentration. Methanothrix predominated in methanogens all the time, in which the dominant species was gradually replaced from M. soehngenii to M. harundinacea with the increased TAN. Dominant bacterial species also changed and Proteiniphilum showed a significant positive correlation with increased TAN. Meta-omics analysis showed that the absolute dominant microorganisms at TAN of 6 g L^-1 were M. harundinacea and Proteiniphilum, both of which highly expressed genes for anti-oxidative stress. M. harundinacea and the second dominant methanogen Methanosarcina highly expressed both acetate cleavage and CO₂ reduction pathways, suggesting the possibility that these two pathways contributed to methanogenesis together. Proteiniphilum and some other species in Firmicutes and Synergistetes were likely acetate oxidizers in the community as they highly expressed genes for syntrophic acetate oxidization, H₂ generation, and electron transfer. These results suggested that Proteiniphilum as well as M. harundinacea have strong ammonia tolerance and played critical roles in acetate degradation under ammonia-suppressed conditions. The achievements of the study would contribute to the regulation and management of the AD process.

Syntrophic Acetate-Oxidizing Microbial Consortia Enriched from Full-Scale Mesophilic Food Waste Anaerobic Digesters Showing High Biodiversity and Functional Redundancy

Syntrophic acetate oxidation (SAO) coupled with hydrogenotrophic methanogenesis (HM) plays a vital role in the anaerobic digestion of protein-rich feedstocks such as food wastes. However, current knowledge of the biodiversity and genetic potential of the involved microbial participants, especially syntrophic acetate-oxidizing bacteria (SAOB), is limited due to the low abundance of these microorganisms and challenges in their isolation. The intent of this study was to enrich and identify potential SAOB. Therefore, we conducted continuous acetate feeding under high ammonia concentrations using two separate inoculum consortia of microorganisms that originated from full-scale mesophilic food waste digesters, which lasted for more than 200 days. Using 16S rRNA gene amplicon and metagenomic analyses, we observed a convergence of the experimental microbial communities during the enrichment regarding taxonomic composition and metabolic functional composition. Stable carbon isotope analyses of biogas indicated that SAO-HM was the dominant methanogenic pathway during the enrichment process. The hydrogenotrophic methanogen Methanoculleus dominated the archaeal community. The enriched SAO community featured high biodiversity and metabolic functional redundancy. By analyzing the metagenome-assembled genomes, the known SAOB Syntrophaceticus schinkii and six uncultured populations were identified to have the genetic potential to perform SAO through the conventional reversed Wood-Ljungdahl pathway, while another six bacteria were found to encode the reversed Wood-Ljungdahl pathway combined with a glycine cleavage system as novel SAOB candidates. These results showed that the food waste anaerobic digesters harbor diverse SAOB and highlighted the importance of the glycine cleavage system for acetate oxidation. IMPORTANCE Syntrophic acetate oxidation to CO2 and H2, together with hydrogenotrophic methanogenesis, contributes to much of the carbon flux in the anaerobic digestion of organic wastes, especially at high ammonia concentrations. A deep understanding of the biodiversity, metabolic genetic potential, and ecology of the SAO community can help to improve biomethane production from wastes for clean energy production. Here, we enriched the SAO-HM functional guild obtained from full-scale food waste anaerobic digesters and recorded dynamic changes in community taxonomic composition and functional profiles. By reconstructing the metabolic pathways, diverse known and novel bacterial members were found to have SAO potential via the reversed Wood-Ljungdahl (WL) pathway alone, or via the reversed WL pathway with a glycine cleavage system (WLP-GCS), and those catalyzing WLP-GCS showed higher microbial abundance. This study revealed the biodiversity and metabolic functional redundancy of SAOB in full-scale anaerobic digester systems and provided inspiration for further genome-centric studies.

Genome-centric metagenomics analysis revealed the metabolic function of abundant microbial communities in thermal hydrolysis-assisted thermophilic anaerobic digesters under propionate stress

The ecological roles of microbial communities and how they interact with each other in thermal hydrolysis process (THP) assisted thermophilic anaerobic digestion (THP-AD) reactors remain largely unknown, especially under propionate stress. Two thermophilic THP-AD reactors had methane yield of 240-248 mL/g VS_added, but accumulated approximately 2000 mg/L propionate. Genome-centric metagenomics analysis showed that 68 metagenome-assembled genomes (MAGs) were recovered, 32 MAGs of which were substantially enriched. Firmicutes spp. dominated the enriched microbial community, including hydrolytic/fermentative bacteria and syntrophs. Methanogenic activities were mainly mediated by Methanosarcina sp. and Methanothermobacter spp. In addition to hydrogenotrophic methanogens, Thermodesulfovibrio sp. could also be a vital H₂ scavenger, contributing to maintaining low H₂ partial pressure in the bioreactors. The remarkable accumulation of propionate could be likely attributed to the weak syntrophic propionate-oxidizing activity or its absence. These findings advanced our knowledge about the mutualistic symbiosis of carbon metabolism in thermophilic THP-AD reactors.

Analysis of the anaerobic digestion metagenome under environmental stresses stimulating prophage induction

BACKGROUND

The viral community has the potential to influence the structure of the microbiome and thus the yield of the anaerobic digestion process. However, the virome composition in anaerobic digestion is still under-investigated. A viral induction experiment was conducted on separate batches undergoing a series of DNA-damaging stresses, in order to coerce temperate viruses to enter the lytic cycle.

RESULTS

The sequencing of the metagenome revealed a viral community almost entirely composed of tailed bacteriophages of the order Caudovirales. Following a binning procedure 1,092 viral and 120 prokaryotic genomes were reconstructed, 64 of which included an integrated prophage in their sequence. Clustering of coverage profiles revealed the presence of species, both viral and microbial, sharing similar reactions to shocks. A group of viral genomes, which increase under organic overload and decrease under basic pH, uniquely encode the yopX gene, which is involved in the induction of temperate prophages. Moreover, the in-silico functional analysis revealed an enrichment of sialidases in viral genomes. These genes are associated with tail proteins and, as such, are hypothesised to be involved in the interaction with the host. Archaea registered the most pronounced changes in relation to shocks and featured behaviours not shared with other species. Subsequently, data from 123 different samples of the global anaerobic digestion database was used to determine coverage profiles of host and viral genomes on a broader scale.

CONCLUSIONS

Viruses are key components in anaerobic digestion environments, shaping the microbial guilds which drive the methanogenesis process. In turn, environmental conditions are pivotal in shaping the viral community and the rate of induction of temperate viruses. This study provides an initial insight into the complexity of the anaerobic digestion virome and its relation with the microbial community and the diverse environmental parameters. Video Abstract.

Bioaugmentation with syntrophic volatile fatty acids-oxidizing consortia to alleviate the ammonia inhibition in continuously anaerobic digestion of municipal sludge

Ammonia inhibition easily affects the performance of anaerobic digestion (AD) for municipal sludge and the oxidization of volatile fatty acids (VFAs) is the rate-limiting step of this process. Bioaugmentation is considered to be an effective method to alleviate ammonia inhibition of AD, but most study used the hydrogenotrophic methanogens as the bioaugmentation culture. In this study, bioaugmentation of mesophilic AD (MAD) and thermophilic AD (TAD) under ammonia inhibition with syntrophic acetate and propionate oxidizing consortia was investigated. The results showed that the bioaugmented reactors recovered earlier than control reactors with 20 (MAD) and 8 (TAD) days, respectively. The high-throughput 16S rRNA gene sequencing indicated that the relative abundance of carbohydrates fermenter (Lentimicrobium), syntrophic VFAs-oxidizing bacteria (Rikenellaceae_DMER64, Smithella and Syntrophobacter) and acetoclastic and hydrogenotrophic methanogens (Methanosaeta, Methanolinea and Methanospirillum) increased in MAD after bioaugmentation. However, part of the bioaugmentation culture could not adapt to the high free ammonia (FAN) concentration in MAD and the effect was weakened. In TAD, proteolytic bacteria (Keratinibaculum and Tepidimicrobium), syntrophic VFAs-oxidizing bacteria (Syntrophomonas) and hydrogenotrophic methanogen (Methanosarcina) were strengthened. The effect of bioaugmentation in TAD was durable even at higher organic loading rate (OLR), due to its positive influence on microbial community. These results suggested that the different bioaugmentation mechanism occurred in MAD and TAD, which are derived from the synergetic effects of ammonia tolerance and microbial interactions. Our study revealed the VFAs-oxidizing consortia as bioaugmented culture could be the potential strategy to alleviate the ammonia stress of AD.

Similar Methanogenic Shift but Divergent Syntrophic Partners in Anaerobic Digesters Exposed to Direct versus Successive Ammonium Additions

During anaerobic digestion (AD) of protein-rich wastewater, ammonium (NH4+) is released by amino acid degradation. High NH4+ concentrations disturb the AD microbiome balance, leading to process impairments. The sensitivity of the AD microbiome to NH4+ and the inhibition threshold depend on multiple parameters, especially the previous microbial acclimation to ammonium stress. However, little is known about the effect of different NH4+ acclimation strategies on the differential expression of key active microbial taxa. Here, we applied NH4+ inputs of increasing intensity (from 1.7 to 15.2 g N-NH4+ liters-1) in batch assays fed with synthetic wastewater, according to two different strategies: (i) direct independent inputs at a unique target concentration and (ii) successive inputs in a stepwise manner. In both strategies, along the NH4+ gradient, the active methanogens shifted from acetoclastic Methanosaeta to Methanosarcina and eventually hydrogenotrophic Methanoculleus. Despite shorter latency times, the successive input modality led to lower methane production rate, lower soluble chemical oxygen demand (sCOD) removal efficiency, and lower half maximal inhibitory concentration, together with higher volatile fatty acid (VFA) accumulation, compared to the independent input modality. These differential performances were associated with a drastically distinct succession pattern of the active bacterial partners in both experiments. In particular, the direct exposure modality was characterized by a progressive enrichment of VFA producers (mainly Tepidimicrobium) and syntrophic VFA oxidizers (mainly Syntrophaceticus) with increasing NH4+ concentration, while the successive exposure modality was characterized by a more dynamic succession of VFA producers (mainly Clostridium, Sporanaerobacter, Terrisporobacter) and syntrophic VFA oxidizers (mainly Tepidanaerobacter, Syntrophomonas). These results bring relevant insights for improved process management through inoculum adaptation, bioaugmentation, or community-driven optimization. IMPORTANCE Anaerobic digestion (AD) is an attractive biotechnological process for wastewater bioremediation and bioenergy production in the form of methane-rich biogas. However, AD can be inhibited by ammonium generated by protein-rich effluent, commonly found in agro-industrial activities. Insights in the microbial community composition and identification of AD key players are crucial for anticipating process impairments in response to ammonium stress. They can also help in defining an optimal microbiome adapted to high ammonium levels. Here, we compared two strategies for acclimation of AD microbiome to increasing ammonium concentration to better understand the effect of this stress on the methanogens and their bacterial partners. Our results suggest that long-term cumulative exposure to ammonia disrupted the AD microbiome more strongly than direct (independent) ammonium additions. We identified bioindicators with different NH4+ tolerance capacity among VFA producers and syntrophic VFA oxidizers.

Combined microbial transcript and metabolic analysis reveals the different roles of hydrochar and biochar in promoting anaerobic digestion of waste activated sludge

Hydrothermal pretreatment of waste activated sludge (WAS) could eliminate the rate limiting step of anaerobic digestion (AD) -hydrolysis. However, the high organic loading rate may cause acid accumulation, thus leading to an unstable system. This study compared the effect of different hydrochar (HC2-260°C and HC3-320°C) and biochar (BC5-500°C and BC7-700°C) on AD of hydrothermal pretreated WAS (HPS). Results demonstrated that hydrochar was superior to biochar in the methane yield and production rate, especially HC2. HC2 had the highest surface oxygen-containing functional groups that could facilitate direct interspecies electron transfer (DIET). The enhanced methane yield was related with the increased protein utilization, and hydrochar and biochar enriched different microbes related to protein degradation. Metabolomic analysis showed the significantly changed metabolites induced by hydrochar and biochar were involved in fatty acids and amino acids-related metabolism, indicating the rapid conversion of intermediated products, which was consistent with the microbial community structure results. Hydrochar and biochar also induced upregulation of metabolites related to microbial metabolic activity and extracellular electron transfer. Although biochar induced the same metabolic changes, the alterations of these metabolites were weaker than those of hydrochar. The results of this study offered new insights into the molecular mechanisms of enhanced AD of HPS by hydrochar and biochar.

Gradual development of ammonia-induced syntrophic acetate-oxidizing activities under mesophilic and thermophilic conditions quantitatively tracked using multiple isotopic approaches

Insights into microbiota adaptation to increased ammonia stress, and identification of indicator microorganisms can help to optimize the operation of anaerobic digesters. To identify microbial indicators and investigate their metabolic contribution to acetoclastic methanogenesis (AM), syntrophic acetate oxidation (SAO) or hydrogenotrophic methanogenesis (HM), 40 anaerobic batch reactors fed with acetate of 110 mmol/L were set up at NH₄⁺-N concentrations of 0.14 g/L, 5.00 g/L or 7.00 g/L, inoculated with thermophilic or mesophilic microbiota with or without pre-exposure to ammonia stress. Four stable carbon isotope probing approaches were applied in parallel, with [1,2-¹³C]-CH₃COOH, [2-¹³C]-CH₃COOH, [¹³C]NaHCO₃ or non-labeled CH₃COOH used individually. The last three approaches were used to quantify the methanogenic pathways by tracking labeled ¹³C or natural ¹³C signatures in the resulting CH₄ and CO₂, and consistently detected the dynamic transition of dominant pathways from AM to SAO-HM under ammonia stress. Results of quantitative PCR and fluorescence in-situ hybridization illustrated the procedure, acetotrophic methanogens being outcompeted by acetate-oxidizing syntrophs. The first and last isotope-labeling approaches were designed to probe the active acetate-mineralizing microbes with DNA-SIP. Known acetate-oxidizing bacteria like Syntrophaceticus and Tepidanaerobacter, as well as novel members of Pseudomonas, Bacillus and Symbiobacteraceae were detected, with Methanoculleus as the predominant H₂/CO₂-utilizing partner. Using NanoSIMS, some bacterial cells were observed to be fixing CO₂ from [¹³C]NaHCO₃. In this study, Methanosaeta was only active with ammonia < 200 mg-N/L; the syntrophs catalyzing SAO-HM started to compete with AM-conducting Methanosarcina at intermediate concentrations of ammonia, i.e. 200-500 mg-N/L, and outcompeted the acetotrophic methanogens with ammonia > 500 mg-N/L. Under ammonia stress, diverse known and novel microbial taxa were involved in acetate mineralization, comparable with those identified in previous studies.

Microbiological Surveillance of Biogas Plants: Targeting Acetogenic Community

Acetogens play a very important role in anaerobic digestion and are essential in ensuring process stability. Despite this, targeted studies of the acetogenic community in biogas processes remain limited. Some efforts have been made to identify and understand this community, but the lack of a reliable molecular analysis strategy makes the detection of acetogenic bacteria tedious. Recent studies suggest that screening of bacterial genetic material for formyltetrahydrofolate synthetase (FTHFS), a key marker enzyme in the Wood-Ljungdahl pathway, can give a strong indication of the presence of putative acetogens in biogas environments. In this study, we applied an acetogen-targeted analyses strategy developed previously by our research group for microbiological surveillance of commercial biogas plants. The surveillance comprised high-throughput sequencing of FTHFS gene amplicons and unsupervised data analysis with the AcetoScan pipeline. The results showed differences in the acetogenic community structure related to feed substrate and operating parameters. They also indicated that our surveillance method can be helpful in the detection of community changes before observed changes in physico-chemical profiles, and that frequent high-throughput surveillance can assist in management towards stable process operation, thus improving the economic viability of biogas plants. To our knowledge, this is the first study to apply a high-throughput microbiological surveillance approach to visualise the potential acetogenic population in commercial biogas digesters.

The fate of anaerobic syntrophy in anaerobic digestion facing propionate and acetate accumulation

How the acetate and propionate accumulation impact anaerobic syntrophy during methane formation is not well understood. To investigate such effect, continuous acetate (35 g/L), propionate (11.25 g/L) and bicarbonate (30 g/L) supplementation were used during mesophilic anaerobic digestion. The high throughput sequencing (16S rRNA and mcrA), Real-Time quantitative PCR, and stable carbon isotope fingerprinting were applied to investigate the structure and activity of microbial community members. The results demonstrated that the abundance of syntrophic acetate oxidizing bacteria exhibited a gradual decrease coupled with heavier stable carbon isotopic signature of methane (δ ¹³CH₄) in the three reagents impacted reactors. The increased acetate and propionate concentrations exerted negative influence on biogas production but the relatively stable hydrogenotrophic methanogens together with syntrophic acetate/propionate oxidizing bacteria kept the stable methane formation facing acetate and propionate accumulation. The functional genes copy number of the hydrogenotrophic Methanocellaceae and Methanomicrobiaceae correlated significantly with δ ¹³CH₄ (R² > 0.74), but only the abundance of Methanocellaceae fitted well with δ ¹³CH₄ (p < 0.05). The δ ¹³CH₄ signatures can predict methanogenesis, as it directly reflects the main methanogenic pathway; yet, further investigation of isotope fractionation in acetate/propionate coupled with δ ¹³CH₄ is needed. Collectively, these results provide deep insight into anaerobic syntrophy and reveal changes of synergistic relationships, both of which may contribute to the stability of biogas reactors.

Sugarcane vinasse extreme thermophilic digestion: a glimpse on biogas free management

The high temperature in which sugarcane vinasse (SV) is generated (~ 90 °C) and the positive effect of higher temperatures in biochemical reactions have motivated the evaluation of SV anaerobic digestion (AD) under extreme temperature conditions. Two-stage (acidogenic/methanogenic) and single-stage (methanogenic) AD of SV were evaluated under 70 °C in structured-bed reactors. The extreme temperature was beneficial to the acidogenic step of the two-stage AD process. The methane production, however, was hindered at 70 °C. The VMP of the single and two-stage reactors accounted, respectively, for only 13% and 7% of the production rate reported in sugarcane vinasse AD at 55 °C. At 70 °C, the main genera responsible for methane production was Methanothermobacter and the acetoclastic methanogenesis did not occur, resulting in acetic acid build up (15,800 mg L^-1). These results brought a new perspective for sugarcane vinasse management, with acetic acid production alternatively to methanization. In this perspective, two-stage process would be composed of acidogenic and acetogenic reactors, and beyond acetate, hydrogen and other soluble compounds could be recovered in a complete biorefinery process.

Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis

Acetate is a pivotal intermediate product during anaerobic decomposition of organic matter. Its generation and consumption network is quite complex, which almost covers the most steps in anaerobic digestion (AD) process. Besides acidogenesis, acetogenesis and methanogenesis, syntrophic acetate oxidation (SAO) replaced acetoclastic methanogenesis to release the inhibition of AD at some special conditions, and the importance of considering homoacetogenesis had also been proved when analysing anaerobic fermentations. Syntrophic acetate-oxidizing bacteria (SAOB), with function of SAO, can survive under high temperature and ammonia/ volatile fatty acids (VFAs) concentrations, while, homoacetogens, performed homoacetogenesis, are more active under acidic, alkaline and low temperature (10°C-20°C) conditions, This review summarized the roles of SAO and homoacetogenesis in AD process, which contains the biochemical reactions, metabolism pathways, physiological characteristics and energy conservation of functional bacteria. The specific roles of these two processes in the subprocess of AD (i.e., acidogenesis, acetogenesis and methanogenesis) were also analyzed in detail. A two phases anaerobic digester is proposed for protein-rich waste(water) treatment by enhancing the functions of homoacetogens and SAOB compared to the traditional two-phases anaerobic digesters, in which the first phase is fermentation phase including acidogens and homoacetogens for acetate production, and second phase is a mixed culture coupling syntrophic fatty acids bacteria, SAOB and hydrogenotrophic methanogens for methane production. This review provides a new insight into the network on production and consumption of acetate in AD process.

Sludge digestibility and functionally active microorganisms in methanogenic sludge digesters revealed by E. coli-fed digestion and microbial source tracking

Methanogenic sludge digestion plays a pivotal role in attenuating and hygienizing the massively-produced waste activated sludge (WAS), which is predominantly composed of microbial cells and extracellular polymeric substances (EPS). The efficient sludge digestion requires a variety of functionally active microorganisms working together closely to convert sludge organic matter into biogas. Nonetheless, the digestion efficiency (or digestibility quantified as carbon removal efficiency) of major sludge constituents (i.e., microbial cells and EPS) and associated functionally active microorganisms in sludge digesters remain elusive. In this study, we identified the digestibility of sludge microbial cells and the associated functionally active microorganisms by using Escherichia coli (E. coli)-fed digestion and microbial source tracking. The average carbon removals in four digesters fed with fresh WAS (WAS-AD), thermal pretreated WAS (Thermal-WAS-AD), E. coli cells (E.coli-AD) and thermal pretreated E. coli cells (Thermal-E.coli-AD) were 30.6 ± 3.4%, 45.8 ± 2.9%, 69.0 ± 3.4% and 68.9 ± 4.6%, respectively. Compared to WAS-AD and Thermal-WAS-AD, the significantly higher carbon removals in E. coli-AD and Thermal-E. coli-AD suggested the remarkably higher digestibility of microbial cells than EPS, and releasing organic matter from EPS might be a rate-limiting step in sludge digestion. Functionally active microorganisms for microbial cell digestion predominantly included fermenters (e.g., Petrimonas and Lentimicrobium), syntrophic acetogens (e.g., Synergistaceae) and methanogens (e.g., Methanosaeta and Methanosarcina). Microbial source tracking estimation showed that the microbial cell-digesting populations accounted for 35.6 ± 9.1% and 70.3 ± 10.1% of total microbial communities in the WAS-AD and Thermal-WAS-AD, respectively. Accordingly, the functionally active microorganisms for digestion of both microbial cells and EPS accounted for 64.5 ± 12.1% and 97.3 ± 2.0% of total digestion sludge microbiome in WAS-AD and Thermal-WAS-AD, respectively. By contrast, feeding WAS-derived microorganisms accounted for 23.2 ± 4.4% and 2.3 ± 1.2% of total microbial communities in the WAS-AD and Thermal-WAS-AD, respectively.

Lignin intermediates lead to phenyl acid formation and microbial community shifts in meso- and thermophilic batch reactors

BACKGROUND

Lignin intermediates resulting from lignocellulose degradation have been suspected to hinder anaerobic mineralisation of organic materials to biogas. Phenyl acids like phenylacetate (PAA) are early detectable intermediates during anaerobic digestion (AD) of aromatic compounds. Studying the phenyl acid formation dynamics and concomitant microbial community shifts can help to understand the microbial interdependencies during AD of aromatic compounds and may be beneficial to counteract disturbances.

RESULTS

The length of the aliphatic side chain and chemical structure of the benzene side group(s) had an influence on the methanogenic system. PAA, phenylpropionate (PPA), and phenylbutyrate (PBA) accumulations showed that the respective lignin intermediate was degraded but that there were metabolic restrictions as the phenyl acids were not effectively processed. Metagenomic analyses confirmed that mesophilic genera like Fastidiosipila or Syntrophomonas and thermophilic genera like Lactobacillus, Bacillus, Geobacillus, and Tissierella are associated with phenyl acid formation. Acetoclastic methanogenesis was prevalent in mesophilic samples at low and medium overload conditions, whereas Methanoculleus spp. dominated at high overload conditions when methane production was restricted. In medium carbon load reactors under thermophilic conditions, syntrophic acetate oxidation (SAO)-induced hydrogenotrophic methanogenesis was the most important process despite the fact that acetoclastic methanogenesis would thermodynamically be more favourable. As acetoclastic methanogens were restricted at medium and high overload conditions, syntrophic acetate oxidising bacteria and their hydrogenotrophic partners could step in for acetate consumption.

CONCLUSIONS

PAA, PPA, and PBA were early indicators for upcoming process failures. Acetoclastic methanogens were one of the first microorganisms to be impaired by aromatic compounds, and shifts to syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis occurred in thermophilic reactors. Previously assumed associations of specific meso- and thermophilic genera with anaerobic phenyl acid formation could be confirmed.

Microbial community and molecular ecological network in the EGSB reactor treating antibiotic wastewater: Response to environmental factors

In this study, one lab-scale EGSB reactor (1.47 L volume) was designed to treat the antibiotic wastewater under different environmental factors, including the addition of cephalexin (CFX), Temperature (T) and Hydraulic Retention Time (HRT). The microbial community structure in EGSB reactor was analyzed with high-throughput sequencing technology to investigate their response to environmental factors changes, and then the random-matrix-theory (RMT)-based network analysis was used to investigate the microbial community's molecular ecological network in EGSB systems treating antibiotics wastewater. Moreover, the explanatory value of each environmental factor on the change of microbial community structure was obtained through the result of redundancy analysis (RDA). The results showed that the addition of cephalexin (CFX), decline of T and decline of HRT (8 h) would decrease the removal efficiency of COD decreasing. And the removal efficiency of CFX would not be affected by decline of T and HRT, except the producing and degrading process of CFX by-products was changed obviously. The result of RDA analysis suggested the environmental factors mainly affected bacterial and fungal microbial community structure but not archaeal ones. The result of high-throughput sequencing showed the relative abundance (RA) of Firmicutes had been obviously affected by T and HRT, which might be main reason leading to the decrease of COD removal efficiency. In addition, molecular ecological network analysis showed the growth of Bacteroidetes occupied the niche of functional microorganism and led to the unstable operation of EGSB when T declined. What's more, the molecular ecological network analysis revealed that Exophiala which belonged to fungi Ascomycota phylum was the hub genus to degrade complex refractory organic pollutants, and Aceticlastic methanogens Methanosaeta was the core functional archaea genus.

Ammonia-induced inhibition of manure-based continuous biomethanation process under different organic loading rates and associated microbial community dynamics

Three Continuously Stirred Tank Reactors (CTSRs) were operating at steady state conditions with Organic Loading Rates (OLR) of 2.09, 3.024 and 4.0 g VS L^-1 d^-1. Glucose was used as the sole factor for increasing the OLR, linking the increase of the OLR with the C/N ratio increase. The reactors were stressed by increasing the ammonia concentration to 5 g L^-1 from 1.862 g L^-1. The results showed elevating inhibition of the anaerobic process by increasing the C/N ratio just by increasing the OLR, under the high ammonia concentration. A different response of the bacterial and archaeal community under ammonia stressed conditions was also observed. Under the high ammonia concentration, hydrogen-depended methylotrophic was the dominant methanogenesis route at OLR of 2.09 g VS L^-1d^-1, while the hydrogenotrophic route was the dominant at the high OLR of 4 g VS L^-1d^-1, which coincided with high acetate and propionate concentrations.

Symbiotic chitin degradation by a novel anaerobic thermophilic bacterium Hydrogenispora sp. UUS1-1 and the bacterium Tepidanaerobacter sp. GT38

Chitin is the second most abundant organic compound in nature. Although mesophilic bacteria degrade insoluble chitin, there is a paucity of data describing degradation of insoluble chitin by anaerobic thermophilic bacteria. In this report, we screened cow manure compost for new chitin degradation systems, and identified a chitinolytic bacterial community (CBC) that showed high chitin degradation activity under thermophilic conditions, i.e., 1% (w/v) chitin powder degraded completely within 7 days at 60 °C. Metagenomic analysis revealed that the CBC was dominated by two bacterial genera from Hydrogenispora, an uncultured taxonomic group, and Tepidanaerobacter. Hydrogenispora were abundant in the early-to-mid stages of culturing with chitin, whereas the population of Tepidanaerobacter increased during the later stages of culturing. Strains UUS1-1 and GT38, which were isolated as pure cultures using the roll-tube method with colloidal chitin, N-acetyl-d-glucosamine, and glucose as carbon sources, were found to be closely related to H. ethanolica and T. acetatoxydans, respectively. Strain UUS1-1 readily degraded chitin and is the first anaerobic thermophilic chitinolytic bacterium reported, whereas strain GT38 showed no chitinolytic activity. Based on phylogenetic analysis, UUS1-1 and GT38 should be classified as novel genera and species. Zymogram analysis revealed that UUS1-1 produces at least two chitinases with molecular weights of 150 and 40 kDa. A coculture of UUS1-1 and GT38 degraded crystalline chitin faster with lower accumulation of lactate compared with UUS1-1 alone, indicating that the strains maintained a symbiotic association through assimilation of organic acids in chitin degradation and that strain GT38 consumed end-products to reduce end-product inhibition and enhance the degradation of crystalline chitin.

Koleobacter methoxysyntrophicus gen. nov., sp. nov., a novel anaerobic bacterium isolated from deep subsurface oil field and proposal of Koleobacteraceae fam. nov. and Koleobacterales ord. nov. within the class Clostridia of the phylum Firmicutes

An anaerobic thermophilic, rod-shaped bacterium possessing a unique non-lipid sheathed-like structure enveloping a single-membraned cell, designated strain NRmbB1^T was isolated from at the deep subsurface oil field located in Yamagata Prefecture, Japan. Growth occurred with 40-60°C (optimum, 55°C), 0-2% (2%), NaCl and pH 6.0-8.5 (8.0). Fermentative growth with various sugars was observed. Glucose-grown cells generated acetate, hydrogen, pyruvate and lactate as the main end products. Syntrophic growth occurred with glucose, pyruvate and 3,4,5-trimethoxybenzoate in the presence of an H₂-scavenging partner, and growth on 3,4,5-trimethoxybenzoate was only observed under syntrophic condition. The predominant cellular fatty acids were C_16:0, iso-C_16:0, anteiso-C_15:0, and iso-C_14:0. Respiratory quinone was not detected. The genomic G+C content was 40.8mol%. Based on 16S rRNA gene phylogeny, strain NRmbB1^T belongs to a distinct order-level clade in the class Clostridia of the phylum Firmicutes, sharing low similarity with other isolated organisms (i.e., 87.5% for top hit Moorella thermoacetica DSM 2955^T). In total, chemotaxonomic, phylogenetic and genomic characterization revealed that strain NRmbB1^T (=KCTC 25035^T, =JCM 39120^T) represents a novel species of a new genus. In addition, we also propose the associated family and order as Koleobacteraceae fam. nov and Koleobacterales ord. nov., respectively.

Genome recovery and metatranscriptomic confirmation of functional acetate-oxidizing bacteria from enriched anaerobic biogas digesters

In many cases, it is difficult to isolate the key microbial organisms from their communities present in natural environments. Metagenomic methods can recover near-complete genomes of the dominant microbial organisms in communities, and metatrancriptomic data could further reveal important genes and pathways related to their functions. In this study, three draft genomes of Clostridium ultunense-like bacteria were recovered based on metagenomic analyses, which is an essential syntrophic acetate-oxidizing bacteria (SAOB) member for maintaining high methane production in high-ammonium biogas digesters but difficult to isolate from its syntrophic partners. Firstly, syntrophic acetate-oxidizing bacteria in a microbial community series were enriched from a biogas digester by adding sodium acetate in the medium. Global analyses of C. ultunense suggested that it would combine the pyruvate-serine-glycine pathway and part of the Wood-Ljungdahl pathway for syntrophic acetate oxidization. Moreover, metatranscriptomic analyses showed that all of the genes of the proposed syntrophic acetate-oxidizing pathway present in the genome were actively transcribed in the microbiota. The functional bacterial enrichment and refined assembly method identify rare microbial genome in complex natural microbiota, which help to recover the syntrophic acetate-oxidizing pathway in C. ultunense strains in this study.

Novel Syntrophic Isovalerate-Degrading Bacteria and Their Energetic Cooperation with Methanogens in Methanogenic Chemostats

Isovalerate is an important intermediate in anaerobic degradation of proteins/amino acids. Little is known about how this compound is degraded due to challenges in cultivation and characterization of isovalerate-degrading bacteria, which are thought to symbiotically depend on methanogenic archaea. In this study, we successfully enriched novel syntrophic isovalerate degraders (uncultivated Clostridiales and Syntrophaceae members) through operation of mesophilic and thermophilic isovalerate-fed anaerobic reactors. Metagenomics- and metatranscriptomics-based metabolic reconstruction of novel putative syntrophic isovalerate metabolizers uncovered the catabolic pathway and byproducts (i.e., acetate, H₂, and formate) of isovalerate degradation, mechanisms for electron transduction from isovalerate degradation to H₂ and formate generation (via electron transfer flavoprotein; ETF), and biosynthetic metabolism. The identified organisms tended to prefer formate-based interspecies electron transfer with methanogenic partners. The byproduct acetate was further converted to CH₄ and CO₂ by either Methanothrix (mesophilic) and Methanosarcina (thermophilic), which employed different approaches for acetate degradation. This study presents insights into novel mesophilic and thermophilic isovalerate degraders and their interactions with methanogens.

Syntrophic acetate oxidation replaces acetoclastic methanogenesis during thermophilic digestion of biowaste

BACKGROUND

Anaerobic digestion (AD) is a globally important technology for effective waste and wastewater management. In AD, microorganisms interact in a complex food web for the production of biogas. Here, acetoclastic methanogens and syntrophic acetate-oxidizing bacteria (SAOB) compete for acetate, a major intermediate in the mineralization of organic matter. Although evidence is emerging that syntrophic acetate oxidation is an important pathway for methane production, knowledge about the SAOB is still very limited.

RESULTS

A metabolic reconstruction of metagenome-assembled genomes (MAGs) from a thermophilic solid state biowaste digester covered the basic functions of the biogas microbial community. Firmicutes was the most abundant phylum in the metagenome (53%) harboring species that take place in various functions ranging from the hydrolysis of polymers to syntrophic acetate oxidation. The Wood-Ljungdahl pathway for syntrophic acetate oxidation and corresponding genes for energy conservation were identified in a Dethiobacteraceae MAG that is phylogenetically related to known SAOB. 16S rRNA gene amplicon sequencing and enrichment cultivation consistently identified the uncultured Dethiobacteraceae together with Syntrophaceticus, Tepidanaerobacter, and unclassified Clostridia as members of a potential acetate-oxidizing core community in nine full-scare digesters, whereas acetoclastic methanogens were barely detected.

CONCLUSIONS

Results presented here provide new insights into a remarkable anaerobic digestion ecosystem where acetate catabolism is mainly realized by Bacteria. Metagenomics and enrichment cultivation revealed a core community of diverse and novel uncultured acetate-oxidizing bacteria and point to a particular niche for them in dry fermentation of biowaste. Their genomic repertoire suggests metabolic plasticity besides the potential for syntrophic acetate oxidation. Video Abstract.

Microbial Consortiums of Hydrogenotrophic Methanogenic Mixed Cultures in Lab-Scale Ex-Situ Biogas Upgrading Systems under Different Conditions of Temperature, pH and CO

In this study, hydrogenotrophic methanogenic mixed cultures taken from 13 lab-scale ex-situ biogas upgrading systems under different temperature (20–70 °C), pH (6.0–8.5), and CO (0–10%, v/v) variables were systematically investigated. High-throughput 16S rRNA gene sequencing was used to identify the microbial consortia, and statistical analyses were conducted to reveal the microbial diversity, the core functional microbes, and their correlative relationships with tested variables. Overall, bacterial community was more complex than the archaea community in all mixed cultures. Hydrogenotrophic methanogens Methanothermobacter, Methanobacterium, and Methanomassiliicoccus, and putative syntrophic acetate-oxidizing bacterium Coprothermobacter and Caldanaerobacter were found to predominate, but the core functional microbes varied under different conditions. Multivariable sensitivity analysis indicated that temperature (p < 0.01) was the crucial variable to determine the microbial consortium structures in hydrogenotrophic methanogenic mixed cultures. pH (0.01 < p < 0.05) significantly interfered with the relative abundance of dominant archaea. Although CO did not affect community (p > 0.1), some potential CO-utilizing syntrophic metabolisms might be enhanced. Understanding of microbial consortia in the hydrogenotrophic methanogenic mixed cultures related to environmental variables was a great advance to reveal the microbial ecology in microbial biogas upgrading process.

Dynamic shifts within volatile fatty acid-degrading microbial communities indicate process imbalance in anaerobic digesters

Buildup of volatile fatty acids (VFAs) in anaerobic digesters (ADs) often results in acidification and process failure. Understanding the dynamics of microbial communities involved in VFA degradation under stable and overload conditions may help optimize anaerobic digestion processes. In this study, five triplicate mesophilic completely mixed AD sets were operated at different organic loading rates (OLRs; 1-6 g chemical oxygen demand [COD] L_R^-1day^-1), and changes in the composition and abundance of VFA-degrading microbial communities were monitored using amplicon sequencing and taxon-specific quantitative PCRs, respectively. AD sets operated at OLRs of 1-4 g COD L_R^-1day^-1 were functionally stable throughout the operational period (120 days) whereas process instability (characterized by VFA buildup, pH decline, and decreased methane production rate) occurred in digesters operated at ≥ 5 g COD L_R^-1day^-1. Though microbial taxa involved in propionate (Syntrophobacter and Pelotomaculum) and butyrate (Syntrophomonas) degradation were detected across all ADs, their abundance decreased with increasing OLR. The overload conditions also inhibited the proliferation of the acetoclastic methanogen, Methanosaeta, and caused a microbial community shift to acetate oxidizers (Tepidanaerobacter acetatoxydans) and hydrogenotrophic methanogens (Methanoculleus). This study's results highlight the importance of operating ADs with conditions that promote the maintenance of microbial communities involved in VFA degradation.

pH and Phosphate Induced Shifts in Carbon Flow and Microbial Community during Thermophilic Anaerobic Digestion

pH is a central environmental factor influencing CH4 production from organic substrates, as every member of the complex microbial community has specific pH requirements. Here, we show how varying pH conditions (5.0-8.5, phosphate buffered) and the application of a phosphate buffer per se induce shifts in the microbial community composition and the carbon flow during nine weeks of thermophilic batch digestion. Beside monitoring the methane production as well as volatile fatty acid concentrations, amplicon sequencing of the 16S rRNA gene was conducted. The presence of 100 mM phosphate resulted in reduced CH4 production during the initial phase of the incubation, which was characterized by a shift in the dominant methanogenic genera from a mixed Methanosarcina and Methanoculleus to a pure Methanoculleus system. In buffered samples, acetate strongly accumulated in the beginning of the batch digestion and subsequently served as a substrate for methanogens. Methanogenesis was permanently inhibited at pH values ≤5.5, with the maximum CH4 production occurring at pH 7.5. Adaptations of the microbial community to the pH variations included shifts in the archaeal and bacterial composition, as less competitive organisms with a broad pH range were able to occupy metabolic niches at unfavorable pH conditions.

Salt tolerance of Cressa cretica and its rhizosphere microbiota

The dwarf shrub Cressa cretica is a thermocosmopolitan halophilic species. Different mechanisms confer salt stress tolerance such as tissue and osmotic tolerance and ion exclusion, as well as the associated microbiota. The aims were (i) to investigate the best conditions for C. cretica seed germination and to examine the tolerance of germinated seeds and seedlings to different salt concentrations and (ii) to characterize the rhizosphere and bulk soil microbiota. Germination and growth experiments were conducted to address plant salt tolerance, and with Illumina sequencing the microbiota of rhizosphere and bulk soil was investigated. While high salt concentrations (600, 800, and 1000 mM NaCl) inhibited C.cretica seed germination, recovery of ungerminated seeds under non-saline conditions was high, indicating osmotic rather than toxic effects of high salt concentrations. The microbiota found in rhizosphere and bulk soil showed high similarity with that found in previous studies on halophyte-associated microbiota, among the Planctomyces, Halomonas and Jeotgalibacillus. Concluding, salt stress responses on the plant level were shown, as has the involvement of associated halotolerant bacteria. Still, the potential role for mitigating saline stress by the bacterial associates of C. cretica, most of them microbial dark matter, will have to be investigated, as will the contributions of archaea and fungi.

Microbial community dynamics in mesophilic and thermophilic batch reactors under methanogenic, phenyl acid-forming conditions

BACKGROUND

Proteinaceous wastes exhibit high theoretical methane yields and their residues are considered valuable fertilisers. The routine anaerobic degradation of proteins often raises problems like high aromatic compound concentrations caused by the entry of aromatic amino acids into the system. A profound investigation of the consequences of aromatic compound exposure on various microorganisms, which cascade-like and interdependently degrade complex molecules to biogas, is still pending.

RESULTS

In mesophilic samples, methane was predominantly produced via acetoclastic methanogenesis. The highest positive correlation was observed between phenylacetate (PAA) and Psychrobacter spp. and between phenylpropionate (PPA) and Haloimpatiens spp. Moreover, Syntrophus spp. negatively correlated with PAA (Spearman's rank correlations coefficient (rs) = - 0.46, p < 0.05) and PPA concentrations (rs = - 0.44, p < 0.05) and was also associated with anaerobic benzene ring cleavage. In thermophilic samples, acetate was predominantly oxidised by Tepidanaerobacter spp. or Syntrophaceticus spp. in syntrophic association with a hydrogenotrophic methanogen. The genera Sedimentibacter and Syntrophaceticus correlated positively with both PAA and PPA concentrations. Moreover, Sedimentibacter spp., Tepidanaerobacter spp., Acetomicrobium spp., and Sporanaerobacter spp. were significant LEfSe (linear discriminant analysis effect size) biomarkers for high meso- as well as thermophilic phenyl acid concentrations. Direct negative effects of phenyl acids on methanogenic properties could not be proven.

CONCLUSIONS

Anaerobic phenyl acid formation is not restricted to specific microbial taxa, but rather done by various meso- and thermophilic bacteria. The cleavage of the highly inert benzene ring is possible in methanogenic batch reactors-at least in mesophilic fermentation processes. The results indicated that phenyl acids rather affect microorganisms engaged in preceding degradation steps than the ones involved in methanogenesis.

Different inhibitory mechanisms of chlortetracycline and enrofloxacin on mesophilic anaerobic degradation of propionate

In anaerobic digestion, propionate is a key intermediate whose degradation is thermodynamically challenging and accumulation is detrimental to the process. Many wastewater streams contain antibiotics due to its globally increasing use, and these compounds can inhibit methane production. However, the effect of antibiotics on propionate degradation in anaerobic digestion remains unclear. In this study, the influence of two antibiotics (chlortetracycline [CTC] and enrofloxacin [EFX]) on biogas production and mesophilic propionate-degrading microbial community was investigated. CTC strongly repressed propionate oxidation, acetate utilization, and methane production, while EFX only inhibited propionate oxidation and methane production to a lesser extent. Microbial community analyses showed that syntrophic propionate-oxidizing bacteria (SPOB) Syntrophobacter had strong tolerance to both CTC and EFX. CTC inhibition mainly acted on the activity of acetate-oxidizing bacteria (Mesotoga, Geovibrio, Tepidanaerobacter, unclassified Bacteroidetes, and unclassified Clostridia) and acetoclastic methanogen, while EFX inhibition applied to the SPOB Smithella and acetoclastic methanogen. Network analysis further indicated that more complicated correlation among bacterial genera occurred in CTC treatments. These results suggested that CTC and EFX inhibited propionate degradation via different mechanisms, which was the result of joint action by antibiotics and microbial interactions.

Enhanced hydrolysis-acidification of high-solids and low-organic-content sludge by biological thermal-alkaline synergism

In this study, a biological thermal-alkaline synergistic system was successfully established to enhance the hydrolysis-acidification efficiency of high-solids and low-organic-content sludge (HS-LOC-S). The results indicated that the highest hydrolysis rate was obtained at pH of 12 (52.62%) leading to the highest production of soluble chemical oxygen demand (SCOD) and soluble protein (SP). The highest acidification rate was observed at pH of 10 (32.15%), leading to the highest production of volatile fatty acids (VFAs). At pH of 10, average sludge size reduced by 24.60%, and the proportion of biodegradable dissolved organic matter (DOM) produced by synergistic system increased by 15.82%, when compared with those of raw sludge. Moreover, results of 16S rRNA clearly validated that the relative abundance of hydrolytic and acidogenic microbes (e.g. Tepidimicrobium, Coprothermobacter) abundantly enriched at pH of 10 (49.88%) was greatly higher than others, which was the main reason for its maximum VFAs accumulation.

Energy-Conserving Enzyme Systems Active During Syntrophic Acetate Oxidation in the Thermophilic Bacterium Thermacetogenium phaeum

The thermophilic acetogen Thermacetogenium phaeum uses the Wood-Ljungdahl pathway (WLP) in both directions, either for the production of acetate from various compounds or for the oxidation of acetate in syntrophic cooperation with methanogens. In this study, energy-conserving enzyme systems in T. phaeum were investigated in both metabolic directions. A gene cluster containing a membrane-bound periplasmically oriented formate dehydrogenase directly adjacent to putative menaquinone synthesis genes was identified in the genome. The protein products of these genes were identified by total proteome analysis, and menaquinone MK-7 had been found earlier as the dominant quinone in the membrane. Enzyme assays with membrane preparations and anthraquinone-2,6-disulfonate as electron acceptor verified the presence of a quinone-dependent formate dehydrogenase. A quinone-dependent methylene-THF reductase is active in the soluble fraction and in the membrane fraction. From these results we conclude a reversed electron transport system from methyl-THF oxidation to CO₂ reduction yielding formate as reduced product which is transferred to the methanogenic partner. The redox potential difference between methyl-THF (E_o' = -200 mV) and formate (E_o' = -432 mV) does not allow electron transfer through syntrophic formate removal alone. We postulate that part of the ATP conserved by substrate-level phosphorylation has to be invested into the generation of a transmembrane proton gradient by ATPase. This proton gradient could drive the endergonic oxidation of methyl-THF in an enzyme reaction similar to the membrane-bound reversed electron transport system previously observed in the syntrophically butyrate-oxidizing bacterium Syntrophomonas wolfei. To balance the overall ATP budget in acetate oxidation, we postulate that acetate is activated through an ATP-independent path via aldehyde:ferredoxin oxidoreductase (AOR) and subsequent oxidation of acetaldehyde to acetyl-CoA.

Air-side ammonia stripping coupled to anaerobic digestion indirectly impacts anaerobic microbiome

Air-side stripping without a prior solid-liquid phase separation step is a feasible and promising process to control ammonia concentration in thermophilic digesters. During the process, part of the anaerobic biomass is exposed to high temperature, high pH and aerobic conditions. However, there are no studies assessing the effects of those harsh conditions on the microbial communities of thermophilic digesters. To fill this knowledge gap, the microbiomes of two thermophilic digesters (55°C), fed with a mixture of pig manure and nitrogen-rich co-substrates, were investigated under different organic loading rates (OLR: 1.1-5.2 g COD l-1  day-1 ), ammonia concentrations (0.2-1.5 g free ammonia nitrogen l-1 ) and stripping frequencies (3-5 times per week). The bacterial communities were dominated by Firmicutes and Bacteroidetes phyla, while the predominant methanogens were Methanosarcina sp archaea. Increasing co-substrate fraction, OLR and free ammonia nitrogen (FAN) favoured the presence of genera Ruminiclostridium, Clostridium and Tepidimicrobium and of hydrogenotrophic methanogens, mainly Methanoculleus archaea. The data indicated that the use of air-side stripping did not adversely affect thermophilic microbial communities, but indirectly modulated them by controlling FAN concentrations in the digester. These results demonstrate the viability at microbial community level of air side-stream stripping process as an adequate technology for the ammonia control during anaerobic co-digestion of nitrogen-rich substrates.

Methanogenesis stimulation and inhibition for the production of different target electrobiofuels in microbial electrolysis cells through an on-demand control strategy using the coenzyme M and 2-bromoethanesulfonate

Electron allocation through the suppression or the stimulation of methanogenesis is critical for microbial electrolysis cells (MECs) to produce the desired target product (e.g., CH₄ or H₂). In this study, selective methanogenesis control using the coenzyme M (CoM) and 2-bromoethanesulfonate (2-BES) was investigated in a two-chambered MEC to evaluate the effect of CoM and 2-BES on the production of different electrobiofuels, net energy conversion efficiency and microbial community structure. Because the CoM is a crucial methyl-group carrier in the final process of methanogenesis, it was postulated that CoM would stimulate methanogenic activity at the anode, while a structural analog of the CoM (i.e., 2-BES) was expected to improve cathodic H₂ yield using electrons conserved because of methanogen inhibition (electron equivalence: 8 mol e^- = 1 mol CH₄ = 4 mol H₂). CoM injection in MECs significantly enhanced their CH₄ production rate, purity, and yield by 4.5-fold, 14.5%, and 76.1%, respectively, compared to the control. Moreover, microbial community analysis indicated that Methanosaeta, the major acetoclastic methanogen, continued to dominate the microbial community but steadily decreased in relative abundance after the CoM injection. On the other hand, drastic increases in hydrogenotrophic methanogens, such as Methanoculleus and Methanolinea, were observed along with potential syntrophic acetate-oxidizing bacteria. In contrast, CH₄ production in the 2-BES injected trials was significantly inhibited by 79.5%, resulting in a corresponding increase of H₂ production by 145.5% compared to the control. Unlike the CoM, the microbial community did not noticeably change when 2-BES was injected, although the population size gradually decreased over time. Also, a single injection of CoM and 2-BES, even at low concentrations (500 μM), enabled the desired allocation of electrons as characterized by a high sensitivity, fast response, and negligible interference. In terms of energy conversion efficiency, methanogenesis stimulation approach resulted in higher net energy production than inhibition approach, whereas the remained electrons were not fully converted to hydrogen in case of the inhibition trial, thus producing less energy.

Elucidating Syntrophic Butyrate-Degrading Populations in Anaerobic Digesters Using Stable-Isotope-Informed Genome-Resolved Metagenomics

Linking the genomic content of uncultivated microbes to their metabolic functions remains a critical challenge in microbial ecology. Resolving this challenge has implications for improving our management of key microbial interactions in biotechnologies such as anaerobic digestion, which relies on slow-growing syntrophic and methanogenic communities to produce renewable methane from organic waste. In this study, we combined DNA stable-isotope probing (SIP) with genome-centric metagenomics to recover the genomes of populations enriched in 13C after growing on [13C]butyrate. Differential abundance analysis of recovered genomic bins across the SIP metagenomes identified two metagenome-assembled genomes (MAGs) that were significantly enriched in heavy [13C]DNA. Phylogenomic analysis assigned one MAG to the genus Syntrophomonas and the other MAG to the genus Methanothrix. Metabolic reconstruction of the annotated genomes showed that the Syntrophomonas genome encoded all the enzymes for beta-oxidizing butyrate, as well as several mechanisms for interspecies electron transfer via electron transfer flavoproteins, hydrogenases, and formate dehydrogenases. The Syntrophomonas genome shared low average nucleotide identity (<95%) with any cultured representative species, indicating that it is a novel species that plays a significant role in syntrophic butyrate degradation within anaerobic digesters. The Methanothrix genome contained the complete pathway for acetoclastic methanogenesis, indicating that it was enriched in 13C from syntrophic acetate transfer. This study demonstrates the potential of stable-isotope-informed genome-resolved metagenomics to identify in situ interspecies metabolic cooperation within syntrophic consortia important to anaerobic waste treatment as well as global carbon cycling.IMPORTANCE Predicting the metabolic potential and ecophysiology of mixed microbial communities remains a major challenge, especially for slow-growing anaerobes that are difficult to isolate. Unraveling the in situ metabolic activities of uncultured species may enable a more descriptive framework to model substrate transformations by microbiomes, which has broad implications for advancing the fields of biotechnology, global biogeochemistry, and human health. Here, we investigated the in situ function of mixed microbiomes by combining stable-isotope probing with metagenomics to identify the genomes of active syntrophic populations converting butyrate, a C4 fatty acid, into methane within anaerobic digesters. This approach thus moves beyond the mere presence of metabolic genes to resolve "who is doing what" by obtaining confirmatory assimilation of the labeled substrate into the DNA signature. Our findings provide a framework to further link the genomic identities of uncultured microbes with their ecological function within microbiomes driving many important biotechnological and global processes.

The Wood-Ljungdahl pathway as a key component of metabolic versatility in candidate phylum Bipolaricaulota (Acetothermia, OP1)

The Wood-Ljungdahl (WL) pathway is an important component of the metabolic machinery in multiple anaerobic prokaryotes, including numerous yet-uncultured bacterial phyla. The pathway can operate in the reductive and oxidative directions, enabling a wide range of metabolic processes. Here, we present a detailed analysis of 14 newly acquired, previously analysed, and publicly available genomic assemblies belonging to the candidate phylum Bipolaricaulota (candidate division OP1, and candidatus Acetothermia), where the occurrence of WL pathway appears to be universal. In silico analysis of predicted metabolic capabilities indicates that the pathway enables homoacetogenic fermentation of sugars and amino acids in all three Bipolaricaulota orders (RBG-16-55-9, UBA7950 and Bipolaricaulales). In addition, members of RBG-16-55-9 appear to possess the additional capacity for syntrophic acetate oxidation using the WL pathway; as well as for respiratory growth using oxygen or nitrate. Anabolically, all UBA7950, and the majority of the Bipolaricaulales genomes possess the capacity for autotrophic growth using the WL pathway. Our results highlight the WL-enabled metabolic versatility in the Bipolaricaulota, emphasize the need for examining the WL pathway in context of the overall metabolic circuitry in uncultured taxa, and demonstrate the value of comparative genomic analysis for providing a detailed overview of metabolic potential in a target microbial lineage and its potential functional niche in an ecosystem.

Impact of total carbon/sulfate on methane production and sulfate removal from co-digestion of sulfate-containing wastewater and corn stalk

During the process of preparing furfural by straw depolymerization with dilute sulfuric acid, large amounts of high temperature sulfate-rich organic wastewater were produced. It cannot be treated directly by anaerobic digestion and converted to bioenergy due to high concentrations of sulfate. In this study, anaerobic co-digestion of sulfate containing wastewater and corn stalk was performed at thermophilic conditions to investigate the influences of total carbon (TC)/sulfate (6, 16, 35 and 110) on methane production and sulfate removal. The results showed that the highest methane production of 260.14 mL g^-1 volatile solid (VS) was achieved at TC/sulfate of 35, which was significantly higher than 12.53 mL g^-1 VS obtained at TC/sulfate of 6. Moreover, the results of sulfate balance analysis showed a maximum sulfate removal of 93.43% was achieved at TC/sulfate of 16, and sulfate concentration in biogas slurry was less than 0.1 g/L regardless of TC/sulfate after 28 days of co-digestion. The microbial community was analyzed using 16S rDNA sequencing technology, the results showed that methane was mainly produced by Methanoculleus and Methanosarcina, and sulfate was removed via Desulfotomaculum, and the relative abundance of methanogenic archaea (MA) and sulfate reducing bacteria (SRB) were significantly correlated with methane production and sulfate removal. It can concluded that higher methane production and sulfate removal can be obtained by anaerobic co-digestion of sulfate containing wastewater and corn stalk at properly TC/sulfate.

Mitigation of ammonia inhibition through bioaugmentation with different microorganisms during anaerobic digestion: Selection of strains and reactor performance evaluation

The effect of bioaugmentation with different microorganisms on anaerobic digestion to mitigate the ammonia inhibition problem was investigated. Seven pure strains of microorganisms (including obligate aceticlastic methanogen, facultative aceticlastic methanogen, hydrogenotrophic methanogen, syntrophic acetate oxidizing bacteria (SAOB) were selected and thirteen bioaugmentation approaches were tested. Bioaugmentation with hydrogenotrophic methanogen Methanobrevibacter smithii (MBS) and SAOB Syntrophaceticu schinkii together was the optimal choice, methane production (MP) was 71.1% higher than that in Blank, the activity of hydrogenotrophic methanogenesis was greatly heightened according to specific methanogenic activity analysis. Bioaugmentation with facultative aceticlastic methanogen Methanosarcina barkeri (MSB) alone without SAOB addition was also proven efficient (MP was 59.7% higher than that in Blank), both aceticlastic and hydrogenotrophic methanogenesis were enhanced. Further evaluation with carbon isotope fractionations analysis indicated that balancing the activities of the aceticlastic and hydrogenotrophic methanogenic pathways is of great importance. 16s rRNA gene sequencing results showed that Methanobacterium spp. and Methanosaeta spp. were the dominant archaea in all 14 reactors. Nevertheless, bioaugmentation with Methanosaeta spp. did not result in a positive effect on MP. On the other hand, Methanobrevibacter spp. and Methanosarcina spp. were non-dominant archaea (even after bioaugmentation with MBS or MSB, the relative abundances were still poor (<2%)), but displayed pivotal roles in determining the overall microbial consortium and, in turn, improved the overall performance.

Growth Characteristics and Thermodynamics of Syntrophic Acetate Oxidizers

Syntrophic acetate oxidation (SAO) plays a pivotal role in biogas production processes when aceticlastic methanogens are inhibited. Despite the importance of SAO, the metabolic interactions and syntrophic growth of the organisms involved are still poorly understood. Therefore, we studied growth parameters and interactions within constructed defined cocultures comprising the methanogen Methanoculleus bourgensis and one, or several, of the syntrophic acetate oxidizers Syntrophaceticus schinkii, [ Clostridium] ultunense, and Tepidanaerobacter acetatoxydans and a novel, uncharacterized bacterium. Cultivation experiments in a design-of-experiment approach revealed positive effects on methane production rate of increased ammonium levels (up to 0.2 M), temperature (up to 45 °C), and acetate concentrations (0.15-0.30 M). Molecular analyses and thermodynamic calculations demonstrated close interlinkages between the microorganisms, with available energies of -10 kJ/mol for acetate oxidation and -20 kJ/mol for hydrogenotrophic methanogenesis. The estimated generation time varied between 3 and 20 days for all syntrophic microorganisms involved, and the acetate minimum threshold level was 0.40-0.45 mM. The rate of methanogenesis depended on the SAO bacteria present in the culture. These data are beneficial for interpretation of SAO prevalence and competiveness against aceticlastic methanogens in anaerobic environments.

Meta-omics based analyses of microbiome involved in biomethanation of rice straw in a thermophilic anaerobic bioreactor under optimized conditions

Biomethanation of rice straw was performed at 55 °C without thermochemical pretreatment using cattle dung supplemented with Methanothermobacter thermautotrophicus strains. Methane yield of 323 ml g^-1 VS obtained under optimized conditions such as particle size (1 mm), carbon to nitrogen ratio (15:1), substrate to inoculum ratio (1:1), organic loading rate (7.5% w/v) and hydraulic retention time (20 days), was one of the highest ever reported from rice straw. Metagenome analysis revealed several putative novel taxa among resident microbes. The genomes of Clostridium, Hungateiclostridium, Alkaliphilus, Anaerocolumna, Olsenella, Paenibacillus, Pseudoclostridium, Tepidanaerobacter and Turicibacter were recovered as metagenome assisted genomes. Clostridium spp. and M. thermautotrophicus were the dominant hydrolytic and methanogenic microbes, respectively. Syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis was found to be the major pathway for methane production. Efficient thermophilic biomethanation of rice straw without thermochemical pretreatment using cattle dung supplemented with M. thermautotrophicus is reported for the first time.