Effects of sludge inoculum and organic feedstock on active microbial communities and methane yield during anaerobic digestion

Ethanol-mediated Anaerobic Digestion: Functional Bacteria and Metabolic Pathways

Ethanol-mediated Anaerobic digestion (Ethanol-AD) is a biological process that converts organic waste into biogas, predominantly composed of methane (CH₄), hydrogen (H₂), and carbon dioxide (CO₂), through the breakdown of complex organic materials while ethanol is an intermediate metabolite. Ethanol improves the digestion of complex organic waste by serving as an electron precursor for interspecies electron transfer, leading to enhanced biogas production. It further serves as a substrate for acetogens or syntrophic bacteria, while mean its oxidation leads to acetate formation, which methanogens can then consume to generate methane. Methanogenesis, the final and crucial step in the anaerobic digestion in which methanogens produce methane through various metabolic routes, most notably via the hydrogenotrophic and syntrophic pathways. In hydrogenotrophic methanogenesis, methanogens consume hydrogen as an electron precursor and carbon dioxide as an electron acceptor, leading to methane generation. Alternatively, syntrophic methanogenesis, which is increasingly recognized for its efficiency, is dominated by DIET between syntrophic partners, bypassing the need for hydrogen as a mediator. This mode of electron transfer enhances the metabolic cooperation between microbes, facilitating a more efficient methanogenesis process. As research continues to explore the mechanisms underlying DIET and the role of (semi) conductive materials, there is potential for optimizing AD systems for renewable energy production by advancing the methanogenesis process, and enhancing biogas quality. The novelty of this review lies in its dual exploration of direct and indirect interspecies electron transfer (DIET and IIET) within ethanol-mediated anaerobic digestion. While DIET in ethanol-driven systems has been previously discussed, this review is the first to comprehensively examine the interplay between both direct and indirect electron transfer mechanisms, offering new insights into optimizing microbial interactions and improving methane production efficiency.

Metataxonomic characterization of the microbial present in the anaerobic digestion of turkey litter waste with the addition of two inocula: allochthonous and commercial

Turkey litter waste is lignocellulosic waste that can be sustainably used as an energy source through anaerobic digestion (AD). The 16S ribosomal RNA technique helps to unravel microbial diversity and predominant metabolic pathways. The assays were performed in 600-mL-glass bottles with 400 mL volume, for 60 days at 37 °C. The study evaluated the physicochemical parameters, the composition of the microbiota, and the functional inference in AD of different concentrations of turkey litter (T) using two inocula: granular inoculum (S) and commercial inoculum (B). The highest accumulated methane production (633 mL CH4·L-1) was observed in the test containing 25.5 g VS·L-1 of turkey litter with the addition of the two inocula (T3BS). In tests without inoculum (T3) and with commercial inoculum (T3B), there was an accumulation of acids and consequent inhibition of methane production 239 mL CH4·L-1 and 389 mL CH4·L-1, respectively. Bacteroidota, Firmicutes, and Actinobacteria were the main phyla identified. The presence of archaea Methanobacterium, Methanocorpusculum, and Methanolinea highlighted the hydrogenotrophic metabolic pathway in T3BS. Functional prediction showed enzymes involved in three metabolic pathways in turkey litter biodigestion: acetotrophic, hydrogenotrophic, and methylotrophic methanogenesis. The predominant hydrogenotrophic pathway can be observed by analyzing the microbiota, archaea involved in this specific pathway, genes involved, and relative acid consumption for T3S and T3BS samples with higher methane production. Molecular tools help to understand the main groups of microorganisms and metabolic pathways involved in turkey litter AD, such as the use of different inocula, allowing the development of strategies for the sustainable disposal of turkey litter.

Metataxonomic Studies to Evaluate the Beneficial Effect of Enzymatic Pretreatment on the Anaerobic Digestion of Waste Generated in Turkey Farming

Turkey litter waste is lignocellulosic and keratinous, requiring prior enzymatic treatment to facilitate fiber hydrolysis and utilization by microorganisms in anaerobic digestion (AD) process. The understanding of the performance of microorganisms in AD can be facilitated through molecular biology and bioinformatics tools. This study aimed to determine the taxonomic profile and functional prediction of microbial communities in the AD of turkey litter waste subjected to enzymatic pretreatment and correlate it with operational parameters. The tests involved the use of turkey litter (T) at 25 g L-1 of volatile solids, a granular inoculum (S) (10% m/v), and the addition of cellulase (C), and pectinase (P) enzymes at four concentrations. The use of enzymes increased methane production by 19% (turkey litter, inoculum, and cellulase-TSC4) and 15% (turkey litter, inoculum, and enzymatic pectinase-TSP4) compared to the control (turkey litter and inoculum-TS), being more effective in TSC4 (667.52 mLCH4), where there was consumption of acetic, butyric, and propionic acids. The pectinase assay (TSP4) showed a methane production of 648 mLCH4 and there was the accumulation of metabolites. Cellulolytic microorganisms Bacteroides, Ruminofilibacter, Lachnospiraceae, Ruminococcaceae, and Methanosaeta were favored in TSC4. In TSP4, the predominant genus was Macellibacteroides and Methanosarcina, and genes involved in methylotrophic methanogenesis were also found (mtaB, mtmB, and mtbB). Enzymes involved in hydrogenotrophic methanogenesis were identified in both assays (TSC4 and TSP4). Molecular tools helped to understand the metabolic routes involved in AD with enzymatic treatment, allowing the elaboration of strategies to improve the sustainable degradation of turkey litter waste.

A Review of Basic Bioinformatic Techniques for Microbial Community Analysis in an Anaerobic Digester

Biogas production involves various types of intricate microbial populations in an anaerobic digester (AD). To understand the anaerobic digestion system better, a broad-based study must be conducted on the microbial population. Deep understanding of the complete metagenomics including microbial structure, functional gene form, similarity/differences, and relationships between metabolic pathways and product formation, could aid in optimization and enhancement of AD processes. With advancements in technologies for metagenomic sequencing, for example, next generation sequencing and high-throughput sequencing, have revolutionized the study of microbial dynamics in anaerobic digestion. This review includes a brief introduction to the basic process of metagenomics research and includes a detailed summary of the various bioinformatics approaches, viz., total investigation of data obtained from microbial communities using bioinformatics methods to expose metagenomics characterization. This includes (1) methods of DNA isolation and sequencing, (2) investigation of anaerobic microbial communities using bioinformatics techniques, (3) application of the analysis of anaerobic microbial community and biogas production, and (4) restriction and prediction of bioinformatics analysis on microbial metagenomics. The review has been concluded, giving a summarized insight into bioinformatic tools and also promoting the future prospects of integrating humungous data with artificial intelligence and neural network software.

Evaluating the biomethane potential from the anaerobic co-digestion of palm oil mill effluent, food waste, and sewage sludge in Malaysia

The ever-increasing organic waste generation in Malaysia is a significant contributor to greenhouse gas (GHG) emissions. However, organic wastes can be utilized to produce biogas by anaerobic digestion, which is a promising option for both energy and material recovery from organic wastes with high moisture content. Therefore, this study was formulated to investigate the feasibility of anaerobic co-digestion of three types of organic wastes generated in significantly huge quantities in Malaysia, namely palm oil mill effluent (POME), food waste (FW), and sewage sludge (SWS). The biomethane potential (BMP) test was used to evaluate the biomethane potential from these organic wastes under mesophilic conditions to establish a stable and balanced microbial community, which may lack in mono-digestion, to improve biogas production. Comparative performance was made at different food to microorganism (F/M) ratios to investigate methane production in three groups of assays, namely A, B, and C. In groups A and B, the effect of F/M ratio variation on methane production was investigated, while in group C, the effect of varying the co-substrate mixture on methane yield was examined. The findings showed that the highest methane yields achieved for mono-digestion of POME and SWS in group A were 164.44 mL-CH₄/g-COD_added and 65.34 mL-CH₄/g-COD_added, respectively, at an F/M ratio of 0.8 and 197.90 mL-CH₄/g-COD_added for FW in group B at an F/M ratio of 0.5. In addition, the highest methane yield achieved from the anaerobic co-digestion was at 151.47 mL-CH₄/g-COD_added from the co-digestion of the POME and SWS (50:50) at an F/M ratio of 1.7 in group A. Both AD and AcoD were tested to fit into two kinetic models: the modified Gompertz and the transfer function models. The results showed that the modified Gompertz model had a better fit and was more adjusted to the experimental results for both AD and AcoD. The importance of this research lies in the economics of anaerobically co-digesting these abundance feedstocks and the variations in their characteristics which were found to increase their methane yield and process efficiency in anaerobic co-digestion.

Recycling anaerobic digestate enhances the co-digestion potential of agro-industrial residues: influence of different digestates as sources of microbial inoculum

Anaerobic codigestion (AcD) of agroindustrial residues was investigated. Granular sludge from bench-scale bioreactors digesting different manure were acclimated and recycled as microbial seed sludge to demonstrate inoculum-type influence on digestion performance. The biomethane potential (BMP) assay was operated for 30 days at 40 ± 2 °C in batch-type laboratory-scale reactors (100 mL). In inoculum amended reactors, codigestion showed significant, yet distinctive, biomethanation than monodigestion with a 5-fold increase (p < 0.05) in average biogas (248.3 ± 5.30 mL gVS-1) and CH4 yield (207.5 ± 4.15 mL gVS-1). The pH, soluble chemical oxygen demand (sCOD) and volatile fatty acids (VFAs) concentrations were within limits for stable AcD process with elevated total solids (TS) and volatile solids (VS) removal efficiencies. This study reinforces advancements in the recycling of digestate in biodigesters and suggests the appropriate selection of inoculum, preferably cow manure, to essentially boost methane production from these wastes.

Enhancement of Biogas Production from Macroalgae Ulva latuca via Ozonation Pretreatment

One of the dominant species of green algae growing along the Mediterranean coast of Egypt is Ulva lactuca. Pretreatment can have a major effect on biogas production because hydrolysis of the algae cell wall structure is a rate-limiting stage in the anaerobic digestion (AD) process. The use of ozone, a new pretreatment, to boost biogas production from the green algae Ulva lactuca was investigated in this study. Ozonation at various dosages was used in contrast to untreated biomass, and the effect on the performance of subsequent mesophilic AD using two separate inoculums (cow manure and activated sludge) was examined. The findings indicated that, in different studies, ozonation pretreatment showed a substantial increase in biogas yield relative to untreated algae. With an ozone dose of 249 mg O3 g–1 VS algal for Ulva lactuca, the highest biogas output (498.75 mL/g VS) was achieved using cow manure inoculum. The evaluation of FTIR, TGA, SEM, and XRD revealed the impact of O3 on the structure of the algal cell wall and integrity breakage, which was thus established as the main contributor to improving the biogas production.

Biopolymers modulate microbial communities in municipal organic waste digestion

The development of biopolymers has raised issues about their recalcitrance in the environment. Their disposal is mainly carried out with the organic fraction of municipal solid waste (OFMSW) through thermophilic anaerobic digestion and aerobic composting, bioprocesses aimed at turning organic matter into biogas and compost. However, the effects of biopolymers on OFMSW treatment, on the final compost and on the microbial communities involved are partly unexplored. In this study, the OFMSW treatment was reproduced on a laboratory-scale respecting real plant conditions and testing the impacts of mixing polylactic acid (PLA) and starch-based bioplastic (SBB) separately. The dynamics of bacterial, archaeal and fungal communities during the process was screened by high-throughput sequencing (HTS) of phylogenetic amplicons. Starch-based bioplastic showed a minor and heterogeneous microbial diversity between the anaerobic and aerobic phases. Contrariwise, PLA treatment resulted in wider and more diverse bacterial and fungal communities for the compost and the aerobic biofilm. Since the biodiversity in compost may play a crucial role in its stability and safety, the modulation of environmental microbial communities induced by higher concentrations of PLA in OFMSW treatment can pose relevant issues.

Impact of Starch from Cassava Peel on Biogas Produced through the Anaerobic Digestion Process

Cassava is a form of food that is rich in starch abundant in many countries. Several bio-products can be extracted from its starch and used as an alternative for oil-based products. This study primarily aims to investigate the influence of the starch isolated from cassava peel on the quantity and quality of the biogas produced via anaerobic digestion. Beating pre-treatment was applied for the first time to isolate the starch and mechanically pre-treat the substrate. The influence of temperature, volatile solid and sludge quantity investigations were analysed with the aid of Design of Experiments (DOE). An optimisation process was applied in calculating the energy balance at the optimal results and this was needed in evaluating the impact of the starch on the biogas produced. The study revealed that the influence of the starch on the biogas quality is quite low and, as such, negligible. The largest biogas volume as obtained was 3830 cc at 37 °C, 4.2 g-VS and 50% sludge quantity, while at the same time the maximum CH4 g−1-VS was 850 cc g−1-VS at 37 °C, 1.1 g-VS and 50% sludge quantity. The optimal results show the energy gain could be achieved based on the set criteria.

Anaerobic cultivation of waste activated sludge to inoculate solid state anaerobic co-digestion of agricultural wastes: Effects of different cultivated periods

This study investigated effects of waste activated sludge (WAS) after anaerobic cultivation as inoculum on solid-state anaerobic digestion (SSAD) of agricultural wastes. WAS was anaerobically cultivated for 0, 20 and 50 days and then used as inoculum for co-digestion at substrate/inoculum (S/I) ratios of 2 and 4, respectively. Results showed that treatments inoculated with cultivated WAS exhibited better buffering capability. The highest cumulative methane production (218 L/kg VS) was achieved when inoculating WAS after 50 days of cultivation at the S/I ratio of 2. Fresh WAS without any anaerobic cultivation as inoculum led to digestion instability with significant acidification and limited biogas production, particularly at the S/I ratio of 4. Microbial analysis deciphered that Methanosarcina was the dominant archaea in all treatments and its relative abundance increased with the extension of WAS cultivation time. Hence, WAS after prolonged anaerobic cultivation could be a favorable inoculum for SSAD of agricultural wastes.

A Methanogenic Consortium Was Active and Exhibited Long-Term Survival in an Extremely Acidified Thermophilic Bioreactor

Acid crisis characterized by acid accumulation and/or low pH is a common reason for the failure of anaerobic digestion (AD), which is usually applied for wastewater and waste treatment. Acid-tolerant methanogens are rarely reported to be active in the artificial anaerobic digester. In this study, we observed that the thermophilic methanogenesis by a consortium in the form of flocs and not granules could still be recovered during long-term operation at acetate concentration of up to 104 mM and pH 5.5 by adjusting the pH gradually or directly to pH 5.5 or 5.0. The acclimation process involving the gradual decrease in pH could enhance the resistance of the consortium against extreme acidification. The stable isotopic signature analysis of biogas revealed that Methanosarcina, which produced methane through acetoclastic methanogenesis (AM) pathway, was the predominant methane producer when the pH was decreased gradually to 5.0. Meanwhile, the abundance of Coprothermobacter increased with a decrease in pH. Contrastingly, when directly subjected to an environment of pH 5.5 and 104 mM acetate (15.84-mM free acetic acid) after a 42-day lag phase, Methanothermobacter was the predominant methanogen. Methanothermobacter initiated methane production through the hydrogenotrophic pathway and formed syntrophic relationship/consortium with the potential acetate-oxidizing bacteria, Thermacetogenium and Coprothermobacter. Comparative metagenomic and metatranscriptomic analysis on this self-adapted and acid-tolerant consortium revealed that the genes, such as GroEL, DnaK, CheY, and flagellum-related genes (FlaA, FlgE, and FliC) from Anaerobaculum, Thermacetogenium, and Coprothermobacter were highly overexpressed in response to system acidification. Microbial self-adaptation patterns (community structure adjustment, methanogenesis pathway shift, and transcriptional regulation) of thermophilic methanogenic consortium to gradual and sudden acidification were evaluated by integrated stable isotopic signature and comparative meta-omic approaches. The study elucidated the acid-resistant mechanism of thermophilic methanogenic consortium and deepened our knowledge of the function, interaction, and microbial characteristics of Methanosarcina, Methanothermobacter, and Coprothermobacter under extreme acidic environment.

Volatile fatty acids production via mixed culture fermentation: Revealing the link between pH, inoculum type and bacterial composition

The aim of the study was to investigate the effects of operational parameters, inoculum type and bacterial community on mixed culture fermentation to produce one dominant acid type in the mixture of volatile fatty acids (VFA). The study was performed using three different inocula (large&small granular and slurry) with glucose under various initial pH. The VFA production efficiency reached to 0,97 (gCOD/gSCOD) by granular sludge. VFA composition was changed by initial pH: in neutral conditions, acetic acid; in acidic conditions, acetic and butyric acids, in alkali conditions butyric acid were dominated, respectively. The VFA production was positively affected by the high relative abundance of Firmicutes. On the contrary, a negative correlation was seen between VFA production and the relative abundance of Chloroflexi. The results revealed the physical sludge structure of inoculum was the key factor for production efficiency, whereas, pH was the most important parameter to affect VFA composition.

Linking nano-ZnO contamination to microbial community profiling in sanitary landfill simulations

Nanomaterials (NMs) commercially used for various activities mostly end up in landfills. Reduced biogas productions reported in landfill reactors create a need for more comprehensive research on these greatly-diverse microbial pools. In order to evaluate the impact of one of the most widely-used NMs, namely nano-zinc oxide (nano-ZnO), simulated bioreactor and conventional landfills were operated using real municipal solid waste (MSW) for 300 days with addition nano-ZnO. Leachate samples were taken at different phases and analyzed by 16S rRNA gene amplicon sequencing. The bacterial communities were distinctly characterized by Cloacamonaceae (phylum WWE1), Rhodocyclaceae (phylum Proteobacteria), Porphyromonadaceae (phylum Bacteroidetes), and Synergistaceae (phylum Synergistetes). The bacterial community in the bioreactors shifted at the end of the operation and was dominated by Rhodocyclaceae. There was not a major change in the bacterial community in the conventional reactors. The methanogenic archaeal diversity highly differed between the bioreactors and conventional reactors. The dominance of Methanomicrobiaceae was observed in the bioreactors during the peak methane-production period; however, their prominence shifted to WSA2 in the nano-ZnO-added bioreactor and to Methanocorpusculaceae in the control bioreactor towards the end. Methanocorpusculaceae was the most abundant family in both conventional control and nano-ZnO-containing reactors.

Bioaugmentation strategy for enhancing anaerobic digestion of high C/N ratio feedstock with methanogenic enrichment culture

To investigate whether bioaugmentation could improve the digestion performance of high C/N ratio feedstock without co-digestion with nitrogen-rich substrate, different forms of enriched methanogenic culture were introduced to the continuous feed digesters. The performance efficiency of bioaugmentation on digestion improvement was compared. The effect of bioaugmentation on microbial community composition was revealed as well. Results demonstrated that routine bioaugmentation with liquid culture (containing the microbes and the medium remains) showed the best performance, with the organic loading rate (OLR), methane percentage, volumetric methane production (VMP) and volatile solid methane production (VSMP) higher at 1.0 g L^-1 d^-1, 24%, 0.22 L L^-1 d^-1 and 0.23 L g^-1 VS d^-1 respectively, compared to the non-bioaugmentation control. Whole genome pyrosequencing analysis suggested that consecutive microbial consortium addition could reconstruct the methanogens community by increasing the populations of acetoclastic methanogens Methanothrix, which could accelerate the degradation of acetate and methane production.

Adaptation of Methanogenic Inocula to Anaerobic Digestion of Maize Silage

A well-balanced microbial consortium is crucial for efficient biogas production. In turn, one of a major factor that influence on the structure of anaerobic digestion (AD) consortium is a source of microorganisms which are used as an inoculum. This study evaluated the influence of inoculum sources (with various origin) on adaptation of a biogas community and the efficiency of the biomethanization of maize silage. As initial inocula for AD of maize silage the samples from: (i) an agricultural biogas plant (ABP) which utilizes maize silage as a main substrate, (ii) cattle slurry (CS), which contain elevated levels of lignocelluloses materials, and (iii) raw sewage sludge (RSS) with low content of plant origin materials were used. The adaptation of methanogenic consortia was monitored during a series of passages, and the functionality of the adapted consortia was verified through start-up operation of AD in two-stage reactors. During the first stages of the adaptation phase, methanogenic consortia occurred very slowly, and only after several passages did the microbial community adapts to allow production of biogas with high methane content. The ABP consortium revealed highest biogas production in the adaptation and in the start-up process. The biodiversity dynamics monitored during adaptation and start-up process showed that community profile changed in a similar direction in three studied consortia. Native communities were very distinct to each other, while at the end of the Phase II of the start-up process microbial diversity profile was similar in all consortia. All adopted bacterial communities were dominated by representatives of Porphyromonadaceae, Rikenellaceae, Ruminococcaceae, and Synergistaceae. A shift from low acetate-preferring acetoclastic Methanosaetaceae (ABP and RSS) and/or hydrogenotrophic Archaea, e.g., Methanomicrobiaceae (CS) prevailing in the inoculum samples to larger populations of high acetate-preferring acetoclastic Methanosarcinaceae was observed by the end of the experiment. As a result, three independent, functional communities that syntrophically produced methane from acetate (primarily) and H₂/CO₂, methanol and methylamines were adapted. This study provides new insights into the specific process by which different inocula sampled from typical methanogenic environments that are commonly used to initiate industrial installations gradually adapted to allow biogas production from maize silage.

Application of next-generation sequencing methods for microbial monitoring of anaerobic digestion of lignocellulosic biomass

The anaerobic digestion of lignocellulosic wastes is considered an efficient method for managing the world's energy shortages and resolving contemporary environmental problems. However, the recalcitrance of lignocellulosic biomass represents a barrier to maximizing biogas production. The purpose of this review is to examine the extent to which sequencing methods can be employed to monitor such biofuel conversion processes. From a microbial perspective, we present a detailed insight into anaerobic digesters that utilize lignocellulosic biomass and discuss some benefits and disadvantages associated with the microbial sequencing techniques that are typically applied. We further evaluate the extent to which a hybrid approach incorporating a variation of existing methods can be utilized to develop a more in-depth understanding of microbial communities. It is hoped that this deeper knowledge will enhance the reliability and extent of research findings with the end objective of improving the stability of anaerobic digesters that manage lignocellulosic biomass.

Effect of humic acid on anaerobic digestion of cellulose and xylan in completely stirred tank reactors: inhibitory effect, mitigation of the inhibition and the dynamics of the microbial communities

Inhibition effect of humic acid (HA) on anaerobic digestion of cellulose and xylan and the mitigation potential of the inhibition were evaluated in controlled fed batch reactors at 30 °C and a hydraulic retention time (HRT) of 20 days. Reactor performances were evaluated by biogas production and metabolite measurements for 220 days. Microbial population dynamics of the reactors were monitored with next-generation 16S rRNA gene sequencing at nine different sampling times. Our results showed that increasing levels of HA inhibited the hydrolysis efficiency of the digestion by 40% and concomitantly reduced the methane yield. Addition of hydrolytic enzymes helped to reverse the negative effects of HA, whereas calcium addition did not reverse HA inhibition. Microbiological analyses showed that the relative abundance of hydrolytic/fermentative bacterial groups such as Clostridiales, Bacteroidales and Anaerolineales was significantly lowered by the presence of HA. HA also affected the archaeal populations. Mostly hydrogenotrophic methanogens were negatively affected by HA. The relative abundance of Methanobacteriaceae, Methanomicrobiales-WCHA208 and Unassigned Thermoplasmata WCHA1-57 were negatively affected by the presence of HA, whereas Methanosaetacea was not affected.

New insights into the key microbial phylotypes of anaerobic sludge digesters under different operational conditions

Analyses on bacterial, archaeal communities at family level and methane-production metabolism were conducted in thirteen full-scale and pilot-scale anaerobic sludge digesters. These digesters were operated at different conditions regarding solids concentration, sludge retention time, organic loading rate and feedstock composition, seeking to optimize digester capacity. Correlations between process parameters and identified microbial phylotypes were evaluated based on relative abundance of these phylotypes determined by Quantitative PCR and 16S rDNA amplicon sequencing. Results showed that, Total Solids concentration (TS), among the evaluated operational parameters, demonstrated the most positive correlation with chemical parameters (including NH3 and VFAs) and significant impact on the abundance of key microbial phylotypes regardless of other factors. Digesters were grouped into 'Higher-TS' with higher stress (TS > 44 g/L, NH3 > 90 mg/L, VFAs > 300 mg/L) and 'Lower-TS' under easier status (TS ≤ 44 g/L, NH3 < 120 mg/L, VFAs < 525 mg/L) in this study. We identified the key microbial phylotypes, i.e. the most abundant and discriminating populations, in 'Higher-TS' digesters with high biogas production rate, which were the class Clostridia, the family Methanosarcinaceae and the order Methanobacteriales. Thermoanaerobacteraceae and Syntrophomonadaceae were identified as key families of Clostridia. Methane was produced both from acetoclastic and hydrogenotrophic methanogenesis. By contrast, in 'Higher-TS' digesters with low biogas production rate, the classes Alpha-, Beta- and Gamma-proteobacteria were detected in higher percentages, of which Rhodobacteraceae, Comamonadaceae and Xanthomonadaceae were the most abundant families respectively, and Methanomicrobiales was the prevailing methanogen order. Consistently, hydrogenotrophic pathway was predominant for methanogenesis, indicating existence of syntrophic acetate oxidation in such 'high-stress', low biogas production rate digesters. These microbial phylotypes were therefore considered to be associated to 'Higher-TS' operation. In 'Lower-TS' digesters, the abundance of the class Delta-proteobacteria, the families Anaerolineaceae, Rikenellaceae, Candidatus Cloacamonas and Methanosaetaceae was obviously higher compared with those in 'Higher-TS' digesters, which were thus considered to be marker phylotypes of easy status. The influence of TS and NH3 on the microbiome should be considered when a 'TS-increasing' strategy is applied to increase digester capacity.

Metagenomic Reconstruction of Key Anaerobic Digestion Pathways in Municipal Sludge and Industrial Wastewater Biogas-Producing Systems

Anaerobic digestion (AD) is a microbial process widely used to treat organic wastes. While the microbes involved in digestion of municipal sludge are increasingly well characterized, the taxonomic and functional compositions of AD digesters treating industrial wastewater have been understudied. This study examined metagenomes from a biogas-producing digester treating municipal sludge in Shek Wu Hui (SWH), Hong Kong and an industrial wastewater digester in Guangzhou (GZ), China, and compared their taxonomic composition and reconstructed biochemical pathways. Genes encoding carbohydrate metabolism and protein metabolism functions were overrepresented in GZ, while genes encoding functions related to fatty acids, lipids and isoprenoids were overrepresented in SWH, reflecting the plants' feedstocks. Mapping of genera to functions in each community indicated that both digesters had a high level of functional redundancy, and a more even distribution of genera in GZ suggested that it was more functionally stable. While fermentation in both samples was dominated by Clostridia, SWH had an overrepresentation of Proteobacteria, including syntrophic acetogens, reflecting its more complex substrate. Considering the growing importance of biogas as an alternative fuel source, a detailed mechanistic understanding of AD is important and this report will be a basis for further study of industrial wastewater AD.

Long-Term Enrichment on Cellulose or Xylan Causes Functional and Taxonomic Convergence of Microbial Communities from Anaerobic Digesters

Cellulose and xylan are two major components of lignocellulosic biomass, which represents a potentially important energy source, as it is abundant and can be converted to methane by microbial action. However, it is recalcitrant to hydrolysis, and the establishment of a complete anaerobic digestion system requires a specific repertoire of microbial functions. In this study, we maintained 2-year enrichment cultures of anaerobic digestion sludge amended with cellulose or xylan to investigate whether a cellulose- or xylan-digesting microbial system could be assembled from sludge previously used to treat neither of them. While efficient methane-producing communities developed under mesophilic (35°C) incubation, they did not under thermophilic (55°C) conditions. Illumina amplicon sequencing results of the archaeal and bacterial 16S rRNA genes revealed that the mature cultures were much lower in richness than the inocula and were dominated by single archaeal (genus Methanobacterium) and bacterial (order Clostridiales) groups, although at finer taxonomic levels the bacteria were differentiated by substrates. Methanogenesis was primarily via the hydrogenotrophic pathway under all conditions, although the identity and growth requirements of syntrophic acetate-oxidizing bacteria were unclear. Incubation conditions (substrate and temperature) had a much greater effect than inoculum source in shaping the mature microbial community, although analysis based on unweighted UniFrac distance found that the inoculum still determined the pool from which microbes could be enriched. Overall, this study confirmed that anaerobic digestion sludge treating nonlignocellulosic material is a potential source of microbial cellulose- and xylan-digesting functions given appropriate enrichment conditions.