Methanogenesis inhibitors used in bio-electrochemical systems: A review revealing reality to decide future direction and applications

Microbial fuel cell (MFC) is a robust technology capable of treating real wastewaters by utilizing mixed anaerobic microbiota as inoculum for producing electricity from oxidation of the biodegradable matters. However, these mixed microbiota comprises of both electroactive microorganisms (EAM) and substrate/electron scavenging microorganisms such as methanogens. Hence, in order to maximize bioelectricity from MFC, different physio-chemical techniques have been applied in past investigations to suppress activity of methanogens. Interestingly, recent investigations exhibit that methanogens can produce electricity in MFC and possess the cellular machinery like cytochrome c and Type IV pili to perform extracellular electron transfer (EET) in the presence of suitable electron acceptors. Hence, in this review, in-depth analysis of versatile behaviour of methanogens in both MFC and natural anaerobic conditions with different inhibition techniques is explored. This review also discusses the future research directions based on the latest scientific evidence on role of methanogens for EET in MFC.

Assessment of microbial diversity associated with CH4 emission from sugarcane vinasse storage and transportation systems

Sugarcane bioethanol has favorable energy and greenhouse gas balance, although the production process generates several residues including vinasse, which deserves attention because of its significant methane (CH₄) emission during storage and transportation stages. Considering that CH₄ emissions are dependent on the structure and abundance of microbial communities, we hypothesized that different vinasse transportation systems would harbor different microbial community composition, resulting in distinct CH₄ patterns. To test this hypothesis, we used high-throughput 16S rRNA sequencing with real-time PCR to evaluate the composition and abundance of microorganisms in the two main systems of vinasse storage and transportation (i.e. open channels and tanks systems) in Brazil. Our results showed higher microbial diversity and CH₄ emissions in channel system, especially in the uncoated section. Significant differences in microbial community structure, diversity, and abundance between the uncoated/coated open channel and tanks indicated a clear selection at taxonomic and functional levels, especially in relation to CH₄ production. These responses included higher methanogens diversity in the uncoated section of the channel and are in agreement with the methanogen abundance determined by mcrA and mba genes copy number (1.5 × 10⁷ and 4.3 × 10¹⁰) and subsequent positive correlation with CH₄ emissions (R² = 0.8). The most representative methanogen genus across the samples was Methanobrevibacter. The results observed herein shows that the use of the coating in the bottom of channels and tanks prevent the growth and development of a methanogen-related community. We concluded that the improvements in vinasse storage and transportation systems would significantly change the microbial community and reduce CH₄ emissions, thereby making bioethanol a greener biofuel.

Simultaneous H2S mitigation and methanization enhancement of chicken manure through the introduction of the micro-aeration approach

The impact on H₂S alleviation and methane yield enhancement after submitting the anaerobic digestion of chicken manure to a finite amount of air was investigated. The largest reduction in the H₂S biogas content (58% lower) occurred when air intensity of 30 ml/g VS_in was injected into the reactors. Consequently, a maximum methane yield (335 mL-g VS_in^-1), which was 77% higher than the control, was concurrently achieved. Slight sulfate accumulation (<330 mg L^-1) was observed inside the micro-aerated digesters with higher air intensities, suggesting a suppression of sulfide inhibition. Bacterial diversity/richness was enhanced in these digesters while the relative abundance of Methanocelleus increased by 36%. The most important contributing factor to enhancement was the synergistic effect resulting from increments in the hydrolysis rate and the suppression of sulfide inhibition. The results highlighted the potential of in situ H₂S mitigation with the added benefit of methane yield enhancement.

Deciphering the effects of temperature on bio-methane generation through anaerobic digestion

Anaerobic digestion (AD) is a sustainable wastewater treatment technology which facilitates energy, nutrient, and water recovery from organic wastes. The agricultural and industrial wastes are suitable substrates for the AD, as they contain a high level of biodegradable compounds. The aim of this study was to examine the AD of three different concentrations of phenol (100, 200, and 300 mg/L) containing wastewater with and without co-substrate (acetate) at four different temperatures (25, 35, 45, and 55 °C) to produce methane (CH₄)-enriched biogas. It was observed that the chemical oxygen demand (COD) and phenol removal efficiencies of up to 76% and 72%, respectively, were achieved. The CH₄ generation was found higher in anaerobic batch reactors (ABRs) using acetate as co-substrate, with the highest yield of 189.1 μL CH₄ from 500 μL sample injected, obtained using 200 mg/L of phenol at 35 °C. The results revealed that the performance of ABR in terms of degradation efficiency, COD removal, and biogas generation was highest at 35 °C followed by 55, 45, and 25 °C indicating 35 °C to be the optimum temperature for AD of phenolic wastewater with maximum energy recovery. Scanning electron microscopy (SEM) revealed that the morphology of the anaerobic sludge depends greatly on the temperature at which the system is maintained which in turn affects the performance and degradation of toxic contaminants like phenol. It was observed that the anaerobic sludge maintained at 35 °C showed uniform channels leading to higher permeability through enhanced mass transfer to achieve higher degradation rates. However, the denser sludge as in the case of 55 °C showed lesser permeability leading to limited transfer and thus reduced treatment. Quantitative real-time PCR (qPCR) analysis revealed a more noteworthy change in the population of the microbial communities due to temperature than the presence of phenol with the methanogens being the dominating species at 35 °C. The findings suggest that the planned operation of the ABR could be a promising choice for CH₄-enriched biogas and COD removal from phenolic wastewater.

Digestive tract methanodrome: Physiological roles of human microbiota-associated methanogens

Methanogens are the archaea most commonly found in humans, in particular in the digestive tract and are an integral part of the digestive microbiota. They are present in humans from the earliest moments of life and represent the only known source of methane production to date. They are notably detected in humans by microscopy, fluorescent in situ hybridization, molecular biology including PCR-sequencing, metagenomics, matrix-assisted laser desorption ionization time-of-flight mass spectrometry and culture. Methanogens present in the human digestive tract play major roles, in particular the use of hydrogen from the fermentation products of bacteria, thus promoting digestion. They are also involved in the transformation of heavy metals and in the use of trimethylamine produced by intestinal bacteria, thus preventing major health problems, in particular cardiovascular diseases. Several pieces of evidence suggest their close physical contacts with bacteria support symbiotic metabolism. Their imbalance during dysbiosis is associated with many pathologies in humans, particularly digestive tract diseases such as Crohn's disease, ulcerative colitis, diverticulosis, inflammatory bowel disease, irritable bowel syndrome, colonic polyposis, and colorectal cancer. There is a huge deficit of knowledge and partially contradictory information concerning human methanogens, so much remains to be done to fully understand their physiological role in humans. It is necessary to develop new methods for the identification and culture of methanogens from clinical samples. This will permit to isolate new methanogens species as well as their phenotypic characterization, to explore their genome by sequencing and to study the population dynamics of methanogens by specifying in particular their exact role within the complex flora associated with the mucous microbiota of human.

The enrichment of anaerobic fungi and methanogens showed higher lignocellulose degrading and methane producing ability than that of bacteria and methanogens

In this study, rumen content was used to obtain three enrichments of anaerobic fungi and methanogens (F + M enrichment), bacteria and methanogens (B + M enrichment), and whole rumen content (WRC enrichment), to evaluate their respective ability to degrade lignocellulose and produce methane. Among the treatments, F + M enrichment elicited the strongest lignocellulose degradation and methane production ability with both rice straw and wheat straw as substrates. Quantitative real-time PCR analysis and diversity analyses of methanogens in the three enrichment treatments demonstrated that F + M had larger number of 16S rRNA gene copies of methanogens and higher relative abundance of Methanobrevibacter, the predominant methanogen found in all enrichments. Caecomyces was the main anaerobic fungal genus for co-culturing to provide substrates for methanogens in this enrichment. Importantly, the F + M enrichment was stable and could be maintained with transfers supplied every 3 days, confirming its potential utility in anaerobic digestion for lignocellulose degradation and methane production.

Effect of long term fertilization management strategies on methane emissions and rice yield

Optimum fertilization is an efficient method to maintain rice yield and reduce N-losses. It is essential though to evaluate methane emissions from paddy fields, to further understand its impact on greenhouse gas budget. Therefore, a field experiment was conducted to investigate the effect of long-term optimum fertilization on CH₄ emissions and rice yield. We collected data in the 7th and 8th year from a field experiment initiated in 2010. Four optimum fertilization strategies, reduced N-fertilizer and zero-P treatment (RNP, 200 kg N/ha), sulfur-coated urea combined with uncoated urea treatment (SCU, 200 kg N/ha), organic fertilizer combined chemical fertilizer treatment (OCN, 200 kg N/ha), organic fertilizer treatment (OF, 200 kg N/ha); and two controls, the farmers' N management (FN, 270 kg N/ha) and zero-N treatment (N0), were employed. The results showed the rice yields achieved for the optimum fertilization treatments (RNP, SCU, OCN, and OF) were similar with those for the FN. No significant differences in CH₄ emissions among all treatments. Cumulative seasonal CH₄ emissions were negatively correlated with grain yield (P < 0.05). In the RNP and SCU treatments, soil available K, mcrA gene and available P were the key variables affecting CH₄ emissions; soil available K, available P and SOC contents were the key emissions factors for OCN and OF treatments. The SCU achieved the highest rice yield and lowest CH₄ emission intensity among optimum fertilization treatments. These results suggest that long-term application of sulfur-coated urea combined with uncoated urea can maintain rice yield and reduce methane emissions from rice paddies.

Superior resolution characterisation of microbial diversity in anaerobic digesters using full-length 16S rRNA gene amplicon sequencing

In the past decade, the characterisation of the microbial community in anaerobic digestion was primarily done by using high-throughput short-read amplicon sequencing. However, the short-read approach has inherent primer bias and low phylogenetic resolution. Our previous study using Illumina MiSeq suggested that the heterogeneity of AD microbiome was operation-driven. To advance our knowledge towards the complexity of the AD microbiome, we performed full-length 16S rRNA gene amplicon sequencing using PacBio Sequel for a more accurate phylogenetic identification. To this end, purified DNA samples from 19 global anaerobic digesters were sequenced. Sixteen methanogenic archaea were identified at the species level. Among them, Methanosarcina horonobensis and Methanosarcina flavescens had significant presence under specific operating conditions. Methanothrix concilii presented in all digesters sequenced. Unexpectedly, over 90% of the Smithella detected were closely related to alkane-degrading Smithella strains D17 and M82, not Smithella propionica. Using LEfSe and network analysis, the interspecies relationship between the fermentative and syntrophic bacteria was addressed. Comparison of the short- and long-read sequencing results were performed and discussed. From sample preparation to data analysis, this work characterised the digester microbiomes in a superior resolution.

Effect of tannic acid combined with fluoride and lignosulfonic acid on anaerobic digestion in the agricultural waste management chain

Livestock waste is stored and used as soil fertilizer or directly as substrate for biogas production. Methane emissions from manure storages and ammonia inhibition of anaerobic digesters fed with manure, are well-known problems related to manure management. This study examines the effect of adding tannic acid with fluoride (TA-NaF) and lignosulfonic acid (LS) on methanogenic activity in batch reactors with ammonia inhibited maize silage digestate and in batch reactors with manure. Lignosulfonic acid counteracted urea induced ammonia inhibition of methanogenesis, whereas TA-NaF inhibited methanogenesis itself. Stable carbon isotope ratio analysis and methanogen community analysis suggested that TA-NaF affected acetoclastic methanogens the most. The combined findings suggest that TA-NaF could be used to reduce methane emissions from stored manure. Conversely, LS could be used as supplement in anaerobic digesters prone to urea induced ammonia inhibition.

Genomic and transcriptomic insights into methanogenesis potential of novel methanogens from mangrove sediments

BACKGROUND

Methanogens are crucial to global methane budget and carbon cycling. Methanogens from the phylum Euryarchaeota are currently classified into one class and seven orders, including two novel methanogen taxa, Methanofastidiosa and Methanomassiliicoccales. The relative importance of the novel methanogens to methane production in the natural environment is poorly understood.

RESULTS

Here, we used a combined metagenomic and metatranscriptomic approach to investigate the metabolic activity of methanogens in mangrove sediments in Futian Nature Reserve, Shenzhen. We obtained 13 metagenome-assembled genomes (MAGs) representing one class (Methanofastidiosa) and five orders (Methanomassiliicoccales, Methanomicrobiales, Methanobacteriales, Methanocellales, and Methanosarcinales) of methanogens, including the two novel methanogens. Comprehensive annotation indicated the presence of an H₂-dependent methylotrophic methanogenesis pathway in Methanofastidiosa and Methanomassiliicoccales. Based on the functional gene analysis, hydrogenotrophic and methylotrophic methanogenesis are the dominant pathways in mangrove sediments. MAG mapping revealed that hydrogenotrophic Methanomicrobiales were the most abundant methanogens and that methylotrophic Methanomassiliicoccales were the most active methanogens in the analyzed sediment profile, suggesting their important roles in methane production.

CONCLUSIONS

Partial or near-complete genomes of two novel methanogen taxa, Methanofastidiosa and Methanomassiliicoccales, in natural environments were recovered and analyzed here for the first time. The presented findings highlight the ecological importance of the two novel methanogens and complement knowledge of how methane is produced in mangrove ecosystem. This study implies that two novel methanogens play a vital role in carbon cycle. Video Abstract.

Response of methanogens in calcified anaerobic granular sludge: Effect of different calcium levels

Inhibition of high calcium during anaerobic wastewater treatment has been studied in recent years, focusing on calcium precipitates in anaerobic granule but neglecting the effect of functional microbes. In this study, key factors of calcification and microbial behaviors especially methanogens of calcified anaerobic granule (AnGS) were investigated in batch assays with calcium level varying from 0 to 5 g L^-1. The results showed that the COD removal efficiency and specific methane activity of calcified AnGS were restrained with calcium addition, especially high calcium (>2 g L^-1), and little tolerance of calcified AnGS to Ca²⁺ was underlined compared with non-calcified AnGS. Analysis of calcium mass flow from solution to sludge validated the formation of calcium precipitates influenced by calcium concentration, pH and HCO₃^-. Besides, death of microbes in outer layer of anaerobic granules was triggered by calcium precipitation. Most importantly, aceticlastic Methanothrix genus was the dominant methanogen, and its relative abundance was correlative negatively with cumulative decrease of bulk Ca²⁺. Hydrogenotrophic Methanobacterium was enriched at higher calcium level, and it suggested that hydrogenotrophic methanogenesis could play a role in alleviating the inhibition of high calcium.

Effect of sulfate addition on carbon flow and microbial community composition during thermophilic digestion of cellulose

Substrates with high sulfate levels pose problems for biogas production as they allow sulfate reducing bacteria to compete with syntrophic and methanogenic members of the community. In addition, the end product of sulfate reduction, hydrogen sulfide, is toxic and corrosive. Here we show how sulfate addition affects physiological processes in a thermophilic methanogenic system by analyzing the carbon flow and the microbial community with quantitative PCR and amplicon sequencing of the 16s rRNA gene. A sulfate addition of 0.5 to 3 g/L caused a decline in methane production by 73-92%, while higher sulfate concentrations had no additional inhibitory effect. Generally, sulfate addition induced a shift in the composition of the microbial community towards a higher dominance of Firmicutes and decreasing abundances of Bacteroidetes and Euryarchaeota. The abundance of methanogens (e.g., Methanoculleus and Methanosarcina) was reduced, while sulfate reducing bacteria (especially Candidatus Desulforudis and Desulfotomaculum) increased significantly in presence of sulfate. The sulfate addition had a significant impact on the carbon flow within the system, shifting the end product from methane and carbon dioxide to acetate and carbon dioxide. Interestingly, methane production quickly resumed, when sulfate was no longer present in the system. Despite the strong impact of sulfate addition on the carbon flow and the microbial community structure during thermophilic biogas production, short-term process disturbances caused by unexpected introduction of sulfate may be overcome due to the high resilience of the engaged microorganisms.

Influence of non-dechlorinating microbes on trichloroethene reduction based on vitamin B12 synthesis in anaerobic cultures

In this study, the YH consortium, an ethene-producing culture, was used to evaluate the effect of vitamin B₁₂ (VB₁₂) on trichloroethene (TCE) dechlorination by transferring the original TCE-reducing culture with or without adding exogenous VB₁₂. Ultra-high performance liquid chromatography - tandem mass spectrometry (UPLC-MS/MS) was applied to detect the concentrations of VB₁₂ and its lower ligand 5,6-dimethylbenzimidazole (DMB) in the cultures. After three successive VB₁₂ starvation cycles, the dechlorination of TCE stopped mostly at cis-dichloroethene (cDCE), and no ethene was found; methane production increased significantly, and no VB₁₂ was detected. Results suggest that the co-cultured microbes may not be able to provide enough VB₁₂ as a cofactor for the growth of Dehalococcoides in the YH culture, possibly due to the competition for corrinoids between Dehalococcoides and methanogens. The relative abundances of 16 S rRNA gene of Dehalococcoides and reductive dehalogenase genes tceA or vcrA were lower in the cultures without VB₁₂ compared with the cultures with VB₁₂. VB₁₂ limitation changed the microbial community structures of the consortia. In the absence of VB₁₂, the microbial community shifted from dominance of Chloroflexi to Proteobacteria after three consecutive VB₁₂ starvation cycles, and the dechlorinating genus Dehalococcoides declined from 42.9% to 13.5%. In addition, Geobacter, Clostridium, and Desulfovibrio were also present in the cultures without VB₁₂. Furthermore, the abundance of archaea increased under VB₁₂ limited conditions. Methanobacterium and Methanosarcina were the predominant archaea in the culture without VB₁₂.

Insights into the toxicity of troclocarban to anaerobic digestion: Sludge characteristics and methane production

Triclocarban (TCC), as the most typical antibacterial agent, is widely discovered in many ecological environment, especially in sludge. However, so far, no studies have reported the effect of TCC exposure on the properties of excess sludge. Therefore, in this study, TCC's toxicities to waste activated sludge (WAS) were analyzed by investigating the variation of physicochemical properties of sludge. It was found that TCC exposure has no effect on sludge pH, while it facilitated organic substances release from sludge, e.g. dissolved organic matter (DOM), protein and polysaccharide, which caused an increase of sludge reduction and changed the structure of functional groups and surface morphology of sludge. Moreover, we explored the effect of TCC on anaerobic digestion of WAS and found methane production was seriously inhibited by TCC. The related mechanism tests had illustrated that TCC exposure did not affect the hydrolysis process, but promoted the acidification and acetogenesis, and importantly inhibited the methanogenesis process. Methanogenic community was further evaluated and observed that the presence of TCC could vary the microbial community of methanogens with the abundance of aceticlastic methanogens increasing and hydrogenotrophic methanogens decreasing. These findings reached in this study would widen the understanding scope for TCC's toxicity to WAS.

Manure amendment increased the abundance of methanogens and methanotrophs but suppressed the type I methanotrophs in rice paddies

Methane (CH₄) emission is the consequence of CH₄ production and consumption performed by methanogens and methanotrophs, respectively. Fertilization is an important factor that regulates the behavior of methanogens and methanotrophs; however, the effect of manure and rice straw addition combined with inorganic fertilizers on these communities is not well understood. This study aimed to explore how manure and rice straw amendments together with inorganic fertilizers influenced the methanogenic and methanotrophic communities in a 31-year fertilized rice paddy. Manure amendment significantly increased the abundance of mcrA and pmoA genes by 61.2% and 63.3% compared with the unfertilized control, whereas inorganic NPK fertilization alone or rice straw addition did not affect their abundances. Manure and rice straw amendments greatly decreased the Shannon index and ACE index of the methanogenic communities, whereas inorganic NPK fertilization alone increased the ACE index of the methanotrophic communities compared with the unfertilized control. Methanosarcinaceae and Methylococcaceae dominated at the family level, representing 23.1-35.0% and 48.7-67.2% of the total reads, for the methanogenic and methanotrophic communities, respectively. Application of manure together with inorganic fertilizers suppressed the Methanocellales methanogens and the type I methanotrophs (Methylococcus and Methylobacter). Fertilization greatly altered the community structure of methanogens and methanotrophs, and manure addition had more apparent effects than rice straw. Moreover, total nitrogen, soil organic carbon, available phosphorus, and available potassium correlated significantly to the abundance, composition, and community structure of methanogens and methanotrophs. In conclusion, our study revealed that long-term manure amendment in combination with inorganic fertilizers significantly increased the abundance of methanogens and methanotrophs, but suppressed the type I methanotrophs in rice paddies.

Employing anaerobic fungi in biogas production: challenges & opportunities

Anaerobic fungi (AF, phylum Neocallimastigomycota) are best known for their ability to efficiently break down lignocellulosic biomass. Their unique combination of mechanical and enzymatic attacks on recalcitrant plant structures bears great potential for enhancement of the anaerobic digestion (AD) process. Although scientists in this field have long agreed upon the potential of AF for biotechnology, research is only recently gaining traction. This delay was largely due to difficulties in culture-dependent and culture-independent analysis of those high-maintenance organisms with their still unknown complex growth requirements. In this review, we will summarize current research efforts on bioaugmentation with AF and further point out, how the lack of basic knowledge on AF nutritional needs hampers their implementation on an industrial scale. Through this, we hope to further kindle interest into basic research on AF in order to advance their stable integration into biotechnological processes.

Generation of zero valent sulfur from dissimilatory sulfate reduction under methanogenic conditions

Dissimilatory sulfate reduction mediated by sulfate-reducing microorganisms (SRMs) has a pivotal role in the sulfur cycle, from which the generation of zero valent sulfur (ZVS) represents a novel pathway. Nonetheless, information on ZVS production from the dissimilatory sulfate reduction remains scarce. This study successfully showed the ZVS production from the dissimilatory sulfate reduction both in a bioreactor and batch experiments under the methanogenic condition. The ZVS was produced in the form of polysulfide and largely located at extracellular sites. In the bioreactor, interestingly, ZVS could be generated first from partial sulfide oxidation mediated by sulfide-oxidizing bacteria (e.g., Thiobacillus) and later from the dissimilatory sulfate reduction in SRMs when changing the reactor operation from anoxic to obligate anaerobic and black condition. In batch experiments, increasing sulfate concentration was shown to enhance ZVS production. Based on these results, together with thermodynamic calculations, a scenario was proposed for the ZVS production from dissimilatory sulfate reduction, in which SRMs might utilize sulfate-to-ZVS as an alternative pathway to sulfate-to-sulfide to increase the thermodynamic favorability and alleviate the inhibitive effects of sulfide. This study expands our understanding of the SRMs-mediated dissimilatory sulfate reduction and may have important implications in environmental bioremediation.

Potential influences of exogenous pollutants occurred in waste activated sludge on anaerobic digestion: A review

Anaerobic digestion is a promising approach for waste activated sludge (WAS) disposal. However, a wide range of exogenous pollutants (e.g. heavy metals, nanoparticles) exists in WAS and their influences on anaerobic digestion are neglected. This study investigates the correlations between exogenous pollutants and anaerobic digestion performance. The results indicate that exogenous pollutants exhibit dose-dependent influences on WAS digestion. Most of the pollutants improve the performance of anaerobic digestion by partially or wholly promoting the hydrolysis, acidification and methanogenesis processes at low dose, but exhibit negative effects at high levels due to their toxicity. Generally, methanogens are more vulnerable than those hydrolytic and acidogenic bacteria. Poly-aluminum chloride and polyacrylamide show strong inhibition on WAS digestion, which are primarily attributed to their physical enmeshments of organic matters in WAS. The synergistic effects of different mixed pollutants and the mitigating strategies for typical pollutants inhibition deserve more attention in light of WAS anaerobic digestion.

Advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge

Low acid production and acid-forming process instability are becoming the major issues to limit the popularization of anaerobic fermentation to produce volatile fatty acid. Considerable research efforts have been made to address these problems, from studying the microorganisms that are primarily responsible for or detrimental to this process, to determining their biochemical pathways and developing mathematical models that facilitate better prediction of process performance to identify the mechanism and optimization of process control. A limited understanding of the complex microbiology and biochemistry of anaerobic fermentation is the primary cause of acid production upset or failure. This review critically assesses the recent advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge from micro to macro scale, particularly relating to the microbiology, biochemistry, impact factors, and enhancement methods. Previous results suggest that further studies are necessary to substantially promote the efficiency and stability of acid production. One of the promising directions appears to be integrating the existing and growing pretreatment technologies and fermentation processes to enhance metabolic pathways of acetogens but inhibit activities of methanogens, which this study hopes to partially achieve.

Culture of salivary methanogens assisted by chemically produced hydrogen

Methanogen cultures require hydrogen produced by fermentative bacteria such as Bacteroides thetaiotaomicron (biological method). We developed an alternative method for hydrogen production using iron filings and acetic acid with the aim of cultivating methanogens more efficiently and more quickly (chemical method). We developed this new method with a reference strain of Methanobrevibacter oralis, compared the method to the biological reference method with a reference strain of Methanobrevibacter smithii and finally applied the method to 50 saliva samples. Methanogen colonies counted using ImageJ software were identified using epifluorescence optical microscopy, real-time PCR and PCR sequencing. For cultures containing pure strains of M. oralis and M. smithii, colonies appeared three days postinoculation with the chemical method versus nine days with the biological method. The average number of M. smithii colonies was significantly higher with the chemical method than with the biological method. There was no difference in the delay of observation of the first colonies in the saliva samples between the two methods. However, the average number of colonies was significantly higher with the biological method than with the chemical method at six days and nine days postinoculation (Student's test, p = 0.005 and p = 0.04, respectively). The chemical method made it possible to isolate four strains of M. oralis and three strains of M. smithii from the 50 saliva samples. Establishing the chemical method will ease the routine isolation and culture of methanogens.

Specific affinity and relative abundance of methanogens in acclimated anaerobic sludge treating low-strength wastewater

Kinetic parameters affecting effluent water quality including half saturation constant (K_s), maximum specific growth rate (μ_max), and specific affinity ([Formula: see text], defined as μ_max/K_s) were investigated using three types of anaerobic sludge (raw anaerobic digestion sludge referred to as unacclimated sludge, unacclimated sludge after endogenous decay, and sludge acclimated to low-strength wastewater in an anaerobic membrane bioreactor (AnMBR) for 360 days). Long-term acclimation to low-strength wastewater resulted in sludge with high specific affinity (1.6 × 10^-3 L/mg COD/day for acclimated sludge compared to 4.1 × 10^-4 L/mg COD/day for unacclimated sludge). The μ_max values for unacclimated sludge and acclimated sludge were 0.08 and 0.07 day^-1, respectively. The K_s values for unacclimated sludge and acclimated sludge were 194 ± 81 mg COD/L and 45 ± 13 mg COD/L, respectively. Although the K_s of unacclimated sludge after endogenous decay increased to 772 ± 74 mg COD/L, μ_max increased to 0.35 day^-1 as well, resulting in no statistically significant difference of [Formula: see text] between the two types of unacclimated sludge. Overall, [Formula: see text] is a better indicator than μ_max or K_s alone for determining effluent water quality, as effluent substrate concentration is approximately inversely proportional to the specific affinity. 16S rRNA sequencing data analysis indicated a high abundance (85.8% of total archaea) of Methanosaeta in the microbial community after long-term acclimation. High [Formula: see text] associated with the enrichment of Methanosaeta appears to ensure successful anaerobic treatment of low-strength wastewater.

Synergetic alginate conversion by a microbial consortium of hydrolytic bacteria and methanogens

Sludge, of which alginate-like biomaterial is a major organic component, is an increasing environmental problem. Thus, efficient anaerobic degradation of alginate provides a new method for sludge utilization. In this study, anaerobic alginate hydrolytic bacteria (AHB) were proposed to enrich with methanogens synergetically to reduce the inhibition of intermediate metabolites. The COD of produced methane reached 80.7 ± 1.9% (n = 4) of initial alginate COD. After considering the microbial growth (8%-18% of COD), a good COD balance indicated that alginate was fully consumed and the main final metabolites were methane and CO₂. Methanogenesis could promote alginate conversion by AHB. The enriched bacteria for alginate degradation in this study were different from that of former known AHB. The metabolic pathway of alginate degradation was revealed by metagenomics, in which oligo-alginate lyase was detected in twelve bacteria, and typical carbon metabolic pathways to convert alginate to methane were identified. More studies of bacterial isolation and biofuel production are still needed in the future.

Microbial community composition and methanogens' biodiversity during a temperature shift in a methane fermentation chamber

More information on the connection between anaerobic digestion (AD) parameters and composition of the microbial community involved in the AD process is required to gain a better understanding of how a bioreactor functions. The aim of this study was to analyse the composition of microbial communities and the dynamics of methanogens' biodiversity changes during the shift from mesophilic (38°C) to thermophilic (55°C) conditions during biogas production. The total microbial composition was examined via the metagenomic approach based on 16S rRNA gene sequencing, whereas the methanogen communities were analysed using PCR-DGGE (Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis) of mcrA. Even though the temperature is one of the crucial parameters affecting microorganisms involved in the AD process, the results presented here revealed that there were no statistically significant differences in bacterial community composition between the mesophilic and thermophilic phases of the process. The most abundant phyla were found to be Proteobacteria, Firmicutes, Actinobacteria and Bacteroidetes. However, the methanogens' community genotypic structure as examined by the PCR-DGGE method changed under thermophilic conditions. The temperature had the strongest impact on the archaeal methanogens in the fermentation chamber directly after implementing the temperature shift. A relatively higher biogas yield and average content of CH₄ in the produced biogas were observed under thermophilic conditions.

Performance of different methanogenic species for the microbial electrosynthesis of methane from carbon dioxide

Microbial electrosynthesis (MES) is a promising technology to convert CO₂ and electricity into the biofuel methane using methanogens. Until now, most investigations on electro-methanogenesis are "proof-of-principle" studies. In this paper, different strains were quantitatively compared in regard to final methane concentration, yields based on CO₂-conversion, productivities as well as Coulombic efficiencies in order to identify suitable organisms for MES. Methanococcus vannielii, Methanococcus maripaludis, Methanolacinia petrolearia, Methanobacterium congolense, and Methanoculleus submarinus were able to produce methane via MES at -700 mV vs. standard hydrogen electrode (SHE). Beside methane also biological H₂ production was detected during MES, which might be due to the involvement of hydrogenases. A direct electron transfer pathway is most likely. Obviously, M. maripaludis is the most resource efficient methane producer in microbial electrosynthesis regarding the methane productivity (8.81 ± 0.51 mmol m^-2 d^-1) and the Coulombic efficiency (58.9 ± 0.8%).

Progress towards catalyzing electro-methanogenesis in anaerobic digestion process: Fundamentals, process optimization, design and scale-up considerations

Electro-methanogenesis represents an emerging bio-methane production pathway that can be achieved through integrating microbial electrolysis cell (MEC) with conventional anaerobic digester (AD). Since 2009, a significant number of publications have reported superior methane productivity and kinetics from MEC-AD integrated systems. The overall objective of this review is to communicate the recent advances towards promoting electro-methanogenesis in the anaerobic digestion process. Firstly, the electro-methanogenesis pathways and functional roles of key microbial members are summarized. Secondly, various extrinsic process parameters, such as applied voltage/potential, pH, and temperature are discussed with emphasis on process optimization. Moreover, available methods for the inoculation and start-up of MEC-AD process are critically reviewed. Finally, system design and scale-up considerations, such as the selection of electrode materials, surface area and surface chemistry of electrode materials, and electrode spacing are summarized.