Evidence for the involvement of two heterodisulfide reductases in the energy-conserving system ofMethanomassiliicoccus luminyensis

Diversity and function of soluble heterodisulfide reductases in methane-metabolizing archaea

Soluble heterodisulfide reductase subunit A (HdrA) is an ancient protein central to energy metabolism, facilitating the recycling of intermediates in methane metabolism and performing flavin-based electron bifurcation for energy conservation. In this study, we investigated the functional diversity and evolutionary dynamics of HdrA in methane-metabolizing archaea. An analysis of 1,152 HdrA sequences from 624 genomes revealed that HdrA diversified through internal domain modifications, resulting in 28 distinct classes and 4 major types (types I, Ia, II, and III). Functional genes in HdrA gene clusters revealed variations in mid-potential electron donors, including NADH, F420H2, H2, and formate. Two major types of HdrA have not previously been studied in detail. Type II HdrA resulted from a fusion of two different classes of type I HdrA. Particularly, a consistent gene cluster containing type II HdrA, molybdopterin oxidoreductase, and F420 dehydrogenase was identified in anaerobic methane-oxidizing archaea and methanogens. Protein sequence and structural predictions suggested that the molybdopterin oxidoreductase protein had lost its catalytic function, and F420H2 served as the mid-potential electron donor or acceptor for the Hdr protein complex. This gene cluster may expand to include additional type I HdrA and HdrD, potentially supporting two electron bifurcation events to lower electron potential for ferredoxin reduction. Type III HdrA, with an inserted GltD domain compared to type I HdrA, appears to have altered the electron transfer route and may use NADH as its mid-potential electron donor or acceptor. The remarkable functional flexibility of HdrA likely helps methane-metabolizing archaea adapt to diverse anaerobic environments.IMPORTANCEAll methanogenic archaea use heterodisulfide of coenzymes M and B as the terminal electron acceptor. In anaerobic methane- and alkane-oxidizing archaea, the reverse reaction occurs. The cycling of heterodisulfide is vital to the energy conservation of these anaerobic microorganisms. Soluble heterodisulfide reductase is an ancient protein fulfilling this function via flavin-based electron bifurcation or confurcation. Despite being present in the vast majority of methane- and alkane-metabolizing archaea, the diversity and evolution of this key protein have not been investigated. This study reveals substantial domain variation and structural changes in the key bifurcating subunit HdrA in methane- and alkane-metabolizing archaea. The resulting flexibility of HdrA enables the protein complex to vary its interacting subunits and electron carriers based on the organisms' primary metabolism. Our findings shed light on how methane- and alkane-metabolizing archaea thrive in various anaerobic environments, contributing to our broader understanding of carbon cycling and energy conservation.

Innovations in the electron transport chain fuel archaeal methane metabolism

Methanogenic archaea (or methanogens) produce methane as a by-product of energy metabolism. Strategies for energy conservation differ across methanogens. Some lineages use an electron transport chain (ETC) with an endogenously produced heterodisulfide as an electron acceptor. Of late, culture-independent -omics techniques and genome editing tools have provided new insights into the evolution and function of bioenergetic complexes in methanogen ETCs, which will be the primary focus of this review. We will also discuss how the ETC enhances metabolic flexibility in methanogens and can even permit anaerobic respiration decoupled from methanogenesis. Finally, we expand on how innovations in the ETC might have enabled anaerobic methane oxidation in a closely related group of microorganisms called anaerobic methanotrophic archaea (ANME).

Effect of D-limonene on volatile fatty acids production from anaerobic fermentation of waste activated sludge under pH regulation: performance and mechanisms

D-limonene extracted from citrus peels possesses an inhibitory effect on methanogenic archaea. This study is aimed to bridge the research gap on the influence of D-limonene on volatile fatty acids (VFA) production from waste activated sludge (WAS) and to address the low VFA yield in standalone anaerobic fermentation of WAS. When the initial pH was not controlled, 1.00 g/g TSS D-limonene resulted in a VFA accumulation of 1175.45 ± 101.36 mg/L (174.45 ± 8.13 mgCOD/gVS). When the initial pH was controlled at 10 and the D-limonene concentration was 0.50 g/g TSS, the VFA accumulation reached 2707.44 ± 183.65 mg/L (445.51 ± 17.10 mgCOD/gVS). The pH-regulated D-limonene treatment enhanced solubilization and acidification, slightly inhibited hydrolysis, and significantly suppressed methanogenesis. D-limonene under alkaline conditions can increase the relative abundance of Clostridium_sensu_stricto, significantly enhancing acidification. Moreover, it markedly inhibited methanogenesis by particularly reducing the relative abundance of Methanothrix that was responsible for acetate consumption, thus favoring the accumulation of VFA. The research reveals the potential mechanism of pH regulation and D-limonene on anaerobic fermentation acid production, providing a theoretical basis for improving the acid production performance of the anaerobic fermentation of WAS.

Effect of incubation temperature on identification of key odorants of sewage sludge using headspace GC analysis

Currently, headspace gas chromatography-mass spectrometry is a widely used method to identify the key odorants of sludge. However, the effect of incubation temperature on the generation and emission of key odorants from sludge was still uncertain. Thus, in this paper, headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS) and headspace gas chromatography-coupled ion mobility spectrometry (HS-GC-IMS) were carried out to analyze the volatiles emitted from the sludge incubated at different temperatures (30 °C, 50 °C, 60 °C, and 80 °C). The results indicated that the total volatile concentration of the sludge increased with temperatures, which affected the identified proportion of sludge key odorants to a certain extent. Differently from the aqueous solutions, the variation of volatile emission from the sludge was inconsistent with temperature changes, suggesting a multifactorial influence of incubation temperature on the identification of sludge odorants. The microbial community structure and adenosine triphosphate (ATP) metabolic activity of the sludge samples were analyzed at the initial state, 30 °C, and 80 °C. Although no significant effect of incubation temperature on the microbial community structure of the sludge, the incubation at 80 °C led to a noticeable decrease in microbial ATP metabolic activity, accompanied by a significant change in the proportion of odor-related microorganisms with low relative abundances. Changes in the composition and activity of these communities jointly contributed to the differences in odor emission from sludge at different temperatures. In summary, the incubation temperature affects the production and emission of volatiles from sludge through physicochemical and biochemical mechanisms, by which the microbial metabolism playing a crucial role. Therefore, when analyzing the key odorants of sludge, these factors should be considered.

Cultivation and visualization of a methanogen of the phylum Thermoproteota

Methane is the second most abundant climate-active gas, and understanding its sources and sinks is an important endeavour in microbiology, biogeochemistry, and climate sciences1,2. For decades, it was thought that methanogenesis, the ability to conserve energy coupled to methane production, was taxonomically restricted to a metabolically specialized group of archaea, the Euryarchaeota1. The discovery of marker genes for anaerobic alkane cycling in metagenome-assembled genomes obtained from diverse habitats has led to the hypothesis that archaeal lineages outside the Euryarchaeota are also involved in methanogenesis3-6. Here we cultured Candidatus Methanosuratincola verstraetei strain LCB70, a member of the archaeal class Methanomethylicia (formerly Verstraetearchaeota) within the phylum Thermoproteota, from a terrestrial hot spring. Growth experiments combined with activity assays, stable isotope tracing, and genomic and transcriptomic analyses demonstrated that this thermophilic archaeon grows by means of methyl-reducing hydrogenotrophic methanogenesis. Cryo-electron tomography revealed that Ca. M. verstraetei are coccoid cells with archaella and chemoreceptor arrays, and that they can form intercellular bridges connecting two to three cells with continuous cytoplasm and S-layer. The wide environmental distribution of Ca. M. verstraetei suggests that they might play important and hitherto overlooked roles in carbon cycling within diverse anoxic habitats.

Metabolic profiling of petroleum-degrading microbial communities incubated under high-pressure conditions

While pressure is a significant characteristic of petroleum reservoirs, it is often overlooked in laboratory studies. To clarify the composition and metabolic properties of microbial communities under high-pressure conditions, we established methanogenic and sulfate-reducing enrichment cultures under high-pressure conditions using production water from the Jilin Oilfield in China. We utilized a metagenomics approach to analyze the microbial community after a 90-day incubation period. Under methanogenic conditions, Firmicutes, Deferribacteres, Ignavibacteriae, Thermotogae, and Nitrospirae, in association with the hydrogenotrophic methanogen Archaeoglobaceae and acetoclastic Methanosaeta, were highly represented. Genomes for Ca. Odinarchaeota and the hydrogen-dependent methylotrophic Ca. Methanosuratus were also recovered from the methanogenic culture. The sulfate-reducing community was dominated by Firmicutes, Thermotogae, Nitrospirae, Archaeoglobus, and several candidate taxa including Ca. Bipolaricaulota, Ca. Aminicenantes, and Candidate division WOR-3. These candidate taxa were key pantothenate producers for other community members. The study expands present knowledge of the metabolic roles of petroleum-degrading microbial communities under high-pressure conditions. Our results also indicate that microbial community interactions were shaped by syntrophic metabolism and the exchange of amino acids and cofactors among members. Furthermore, incubation under in situ pressure conditions has the potential to reveal the roles of microbial dark matter.

Methanomethylophilus alvi gen. nov., sp. nov., a Novel Hydrogenotrophic Methyl-Reducing Methanogenic Archaea of the Order Methanomassiliicoccales Isolated from the Human Gut and Proposal of the Novel Family Methanomethylophilaceae fam. nov

The methanogenic strain Mx-05T was isolated from the human fecal microbiome. A phylogenetic analysis based on the 16S rRNA gene and protein marker genes indicated that the strain is affiliated with the order Methanomassiliicoccales. It shares 86.9% 16S rRNA gene sequence identity with Methanomassiliicoccus luminyensis, the only member of this order previously isolated. The cells of Mx-05T were non-motile cocci, with a diameter range of 0.4-0.7 μm. They grew anaerobically and reduced methanol, monomethylamine, dimethylamine, and trimethylamine into methane, using H2 as an electron donor. H2/CO2, formate, ethanol, and acetate were not used as energy sources. The growth of Mx-05T required an unknown medium factor(s) provided by Eggerthella lenta and present in rumen fluid. Mx-05T grew between 30 °C and 40 °C (optimum 37 °C), over a pH range of 6.9-8.3 (optimum pH 7.5), and between 0.02 and 0.34 mol.L-1 NaCl (optimum 0.12 mol.L-1 NaCl). The genome is 1.67 Mbp with a G+C content of 55.5 mol%. Genome sequence annotation confirmed the absence of the methyl branch of the H4MPT Wood-Ljungdahl pathway, as described for other Methanomassiliicoccales members. Based on an average nucleotide identity analysis, we propose strain Mx-05T as being a novel representative of the order Methanomassiliicoccales, within the novel family Methanomethylophilaceae, for which the name Methanomethylophilus alvi gen. nov, sp. nov. is proposed. The type strain is Mx-05T (JCM 31474T).

Insights into pyrrolysine function from structures of a trimethylamine methyltransferase and its corrinoid protein complex

The 22nd genetically encoded amino acid, pyrrolysine, plays a unique role in the key step in the growth of methanogens on mono-, di-, and tri-methylamines by activating the methyl group of these substrates for transfer to a corrinoid cofactor. Previous crystal structures of the Methanosarcina barkeri monomethylamine methyltransferase elucidated the structure of pyrrolysine and provide insight into its role in monomethylamine activation. Herein, we report the second structure of a pyrrolysine-containing protein, the M. barkeri trimethylamine methyltransferase MttB, and its structure bound to sulfite, a substrate analog of trimethylamine. We also report the structure of MttB in complex with its cognate corrinoid protein MttC, which specifically receives the methyl group from the pyrrolysine-activated trimethylamine substrate during methanogenesis. Together these structures provide key insights into the role of pyrrolysine in methyl group transfer from trimethylamine to the corrinoid cofactor in MttC.

Expanding the phylogenetic distribution of cytochrome b-containing methanogenic archaea sheds light on the evolution of methanogenesis

Methane produced by methanogenic archaea has an important influence on Earth's changing climate. Methanogenic archaea are phylogenetically diverse and widespread in anoxic environments. These microorganisms can be divided into two subgroups based on whether or not they use b-type cytochromes for energy conservation. Methanogens with b-type cytochromes have a wider substrate range and higher growth yields than those without them. To date, methanogens with b-type cytochromes were found exclusively in the phylum "Ca. Halobacteriota" (formerly part of the phylum Euryarchaeota). Here, we present the discovery of metagenome-assembled genomes harboring methyl-coenzyme M reductase genes reconstructed from mesophilic anoxic sediments, together with the previously reported thermophilic "Ca. Methylarchaeum tengchongensis", representing a novel archaeal order, designated the "Ca. Methylarchaeales", of the phylum Thermoproteota (formerly the TACK superphylum). These microorganisms contain genes required for methyl-reducing methanogenesis and the Wood-Ljundahl pathway. Importantly, the genus "Ca. Methanotowutia" of the "Ca. Methylarchaeales" encode a cytochrome b-containing heterodisulfide reductase (HdrDE) and methanophenazine-reducing hydrogenase complex that have similar gene arrangements to those found in methanogenic Methanosarcinales. Our results indicate that members of the "Ca. Methylarchaeales" are methanogens with cytochromes and can conserve energy via membrane-bound electron transport chains. Phylogenetic and amalgamated likelihood estimation analyses indicate that methanogens with cytochrome b-containing electron transfer complexes likely evolved before diversification of Thermoproteota or "Ca. Halobacteriota" in the early Archean Eon. Surveys of public sequence databases suggest that members of the lineage are globally distributed in anoxic sediments and may be important players in the methane cycle.

Metagenomic insights into the environmental adaptation and metabolism of Candidatus Haloplasmatales, one archaeal order thriving in saline lakes

The KTK 4A-related Thermoplasmata thrives in the sediment of saline lakes; however, systematic research on its taxonomy, environmental adaptation and metabolism is lacking. Here, we detected this abundant lineage in the sediment of five artificially separated ponds (salinity 7.0%-33.0%) within a Chinese soda-saline lake using culture-independent metagenomics and archaeal 16S rRNA gene amplicons. The phylogenies based on the 16S rRNA gene, and 122 archaeal ubiquitous single-copy proteins and genome-level identity analyses among the metagenome-assembled genomes demonstrate this lineage forming a novel order, Candidatus Haloplasmatales, comprising four genera affiliated with the identical family. Isoelectric point profiles of predicted proteomes suggest that most members adopt the energetically favourable 'salt-in' strategy. Functional prediction indicates the lithoheterotrophic nature with the versatile metabolic potentials for carbohydrate and organic acids as well as carbon monoxide and hydrogen utilization. Additionally, hydrogenase genes hdrABC-mvhADG are linked with incomplete reductive citrate cycle genes in the genomes, suggesting their functional connection. Comparison with the coupling of HdrABC-MvhADG and methanogenesis pathway provides new insights into the compatibility of laterally acquired methanogenesis with energy metabolism in the related order Methanomassiliicoccales. Globally, our research sheds light on the taxonomy, environmental adaptative mechanisms, metabolic potentials and evolutional significance of Ca. Haloplasmatales.

Bioaugmentation with a propionate-degrading methanogenic culture to improve methane production from chicken manure

Volatile fatty acid (VFA) accumulation caused by high ammonia concentrations is often encountered during the anaerobic digestion (AD) of ammonia-rich substrates. In this study, propionate-degrading methanogenic cultures were introduced to augment the semi-continuous AD of chicken manure under high ammonia levels. Introduction of a methanogenic culture enhanced the methane yield in the bioaugmented digester by 17-26% when the organic loading rate (OLR) was 2-4 g L^-1d^-1 compared to that in the control. When the OLR was further increased from 4.0 L^-1d^-1 to 5.0 g L^-1d^-1, and bioaugmentation ceased, methane yield improved by 15-18% under a high total ammonia nitrogen level of 5.0-8.4 g NH₄⁺-N/L. Moreover, bioaugmentation reconstructed the methanogenic community in the digester, promoting the dominance of hydrogenotrophic Methanobacterium and slightly increasing the abundance of aceticlastic Methanothrix and the syntrophic propionate-oxidizing bacteria Syntrophobacter, which were the key contributors to the improved AD under high ammonia concentrations.

Diversion of food waste into the sulfate-laden sewer: Interaction and electron flow of sulfidogenesis and methanogenesis

Diverting food waste (FW) into the sulfate-laden sewer may pose a significant influence on the production of methane and sulfide in sewers. Identifying microbial electron utilization is essential to understanding the interaction of sulfidogenesis and methanogenesis in depth. Here, we reported sulfide and methane production from the sewer bioreactors receiving sulfate-laden wastewater (160 mg S/L), with and without FW addition. Long-term monitoring showed that the addition of FW (1 g/L) could boost both sulfide (by 39%) and methane (by 44%) production. As for the electrons used for sulfidogenesis and methanogenesis, about 98% flowed to sulfidogenesis. Cryosection-fluorescence in situ hybridization showed that high sulfate content suppressed the accumulation of methanogens in biofilm outer layer, whereas methanogens in the inner layer were enriched with FW addition. Moreover, the FW addition fostered the diversity of the fermentative bacteria and changed the type of methanogens in biofilms, and up-regulated the key enzymes expressions for sulfidogenesis and methanogenesis. A model-based investigation suggests that increased FW-to-sewage ratios would exert a significant impact on methane production than on sulfide production. The microbial electron flows were highly dependent on sulfate concentration and FW-to-sewage ratios. The findings of this study suggest that sulfate and substrate levels play a key role in microbial electron utilization for sulfide and methane production, and diverting FW into the sulfate-laden sewer may exert negative impacts on sewer management and the environment.

Process of energy conservation in the extremely haloalkaliphilic methyl-reducing methanogen Methanonatronarchaeum thermophilum

The recently isolated methanogen Methanonatronarchaeum thermophilum is an extremely haloalkaliphilic and moderately thermophilic archaeon and belongs to the novel class Methanonatronarchaeia in the phylum Halobacteriota. The knowledge about the physiology and biochemistry of members of the class Methanonatronarchaeia is still limited. It is known that M. thermophilum performs hydrogen or formate-dependent methyl-reducing methanogenesis. Here, we show that the organism was able to grow on all tested C₁ -methylated substrates (methanol, trimethylamine, dimethylamine, monomethylamine) in combination with formate or molecular hydrogen. A temporary accumulation of intermediates (dimethylamine or/and monomethylamine) in the medium occurred during the consumption of trimethylamine or dimethylamine. The energy conservation of M. thermophilum was dependent on a respiratory chain consisting of a hydrogenase (VhoGAC), a formate dehydrogenase (FdhGHI), and a heterodisulfide reductase (HdrDE) that were well adapted to the harsh physicochemical conditions in the natural habitat. The experiments revealed the presence of two variants of energy-conserving oxidoreductase systems in the membrane. These included the H₂ : heterodisulfide oxidoreductase system, which has already been described in Methanosarcina species, as well as the novel formate: heterodisulfide oxidoreductase system. The latter electron transport chain, which was experimentally proven for the first time, distinguishes the organism from all other known methanogenic archaea and represents a unique feature of the class Methanonatronarchaeia. Experiments with 2-hydroxyphenazine and the inhibitor diphenyleneiodonium chloride indicated that a methanophenazine-like cofactor might function as an electron carrier between the hydrogenase/ formate dehydrogenase and the heterodisulfide reductase.

The hydrogen threshold of obligately methyl-reducing methanogens

Methanogenesis is the final step in the anaerobic degradation of organic matter. The most important substrates of methanogens are hydrogen plus carbon dioxide and acetate, but also the use of methanol, methylated amines, and aromatic methoxy groups appears to be more widespread than originally thought. Except for most members of the family Methanosarcinaceae, all methylotrophic methanogens require external hydrogen as reductant and therefore compete with hydrogenotrophic methanogens for this common substrate. Since methanogenesis from carbon dioxide consumes four molecules of hydrogen per molecule of methane, whereas methanogenesis from methanol requires only one, methyl-reducing methanogens should have an energetic advantage over hydrogenotrophic methanogens at low hydrogen partial pressures. However, experimental data on their hydrogen threshold is scarce and suffers from relatively high detection limits. Here, we show that the methyl-reducing methanogens Methanosphaera stadtmanae (Methanobacteriales), Methanimicrococcus blatticola (Methanosarcinales), and Methanomassiliicoccus luminyensis (Methanomassiliicoccales) consume hydrogen to partial pressures < 0.1 Pa, which is almost one order of magnitude lower than the thresholds for M. stadtmanae and M. blatticola reported in the only previous study on this topic. We conclude that methylotrophic methanogens should outcompete hydrogenotrophic methanogens for hydrogen and that their activity is limited by the availability of methyl groups.

Comparative population genomic analyses of transporters within the Asgard archaeal superphylum

Upon discovery of the first archaeal species in the 1970s, life has been subdivided into three domains: Eukarya, Archaea, and Bacteria. However, the organization of the three-domain tree of life has been challenged following the discovery of archaeal lineages such as the TACK and Asgard superphyla. The Asgard Superphylum has emerged as the closest archaeal ancestor to eukaryotes, potentially improving our understanding of the evolution of life forms. We characterized the transportomes and their substrates within four metagenome-assembled genomes (MAGs), that is, Odin-, Thor-, Heimdall- and Loki-archaeota as well as the fully sequenced genome of Candidatus Prometheoarchaeum syntrophicum strain MK-D1 that belongs to the Loki phylum. Using the Transporter Classification Database (TCDB) as reference, candidate transporters encoded within the proteomes were identified based on sequence similarity, alignment coverage, compatibility of hydropathy profiles, TMS topologies and shared domains. Identified transport systems were compared within the Asgard superphylum as well as within dissimilar eukaryotic, archaeal and bacterial organisms. From these analyses, we infer that Asgard organisms rely mostly on the transport of substrates driven by the proton motive force (pmf), the proton electrochemical gradient which then can be used for ATP production and to drive the activities of secondary carriers. The results indicate that Asgard archaea depend heavily on the uptake of organic molecules such as lipid precursors, amino acids and their derivatives, and sugars and their derivatives. Overall, the majority of the transporters identified are more similar to prokaryotic transporters than eukaryotic systems although several instances of the reverse were documented. Taken together, the results support the previous suggestions that the Asgard superphylum includes organisms that are largely mixotrophic and anaerobic but more clearly define their metabolic potential while providing evidence regarding their relatedness to eukaryotes.

Metabolic Potential for Reductive Acetogenesis and a Novel Energy-Converting [NiFe] Hydrogenase in Bathyarchaeia From Termite Guts - A Genome-Centric Analysis

Symbiotic digestion of lignocellulose in the hindgut of higher termites is mediated by a diverse assemblage of bacteria and archaea. During a large-scale metagenomic study, we reconstructed 15 metagenome-assembled genomes of Bathyarchaeia that represent two distinct lineages in subgroup 6 (formerly MCG-6) unique to termite guts. One lineage (TB2; Candidatus Termitimicrobium) encodes all enzymes required for reductive acetogenesis from CO₂ via an archaeal variant of the Wood–Ljungdahl pathway, involving tetrahydromethanopterin as C₁ carrier and an (ADP-forming) acetyl-CoA synthase. This includes a novel 11-subunit hydrogenase, which possesses the genomic architecture of the respiratory Fpo-complex of other archaea but whose catalytic subunit is phylogenetically related to and shares the conserved [NiFe] cofactor-binding motif with [NiFe] hydrogenases of subgroup 4 g. We propose that this novel Fpo-like hydrogenase provides part of the reduced ferredoxin required for CO₂ reduction and is driven by the electrochemical membrane potential generated from the ATP conserved by substrate-level phosphorylation; the other part may require the oxidation of organic electron donors, which would make members of TB2 mixotrophic acetogens. Members of the other lineage (TB1; Candidatus Termiticorpusculum) are definitely organotrophic because they consistently lack hydrogenases and/or methylene-tetrahydromethanopterin reductase, a key enzyme of the archaeal Wood–Ljungdahl pathway. Both lineages have the genomic capacity to reduce ferredoxin by oxidizing amino acids and might conduct methylotrophic acetogenesis using unidentified methylated compound(s). Our results indicate that Bathyarchaeia of subgroup 6 contribute to acetate formation in the guts of higher termites and substantiate the genomic evidence for reductive acetogenesis from organic substrates, possibly including methylated compounds, in other uncultured representatives of the phylum.

Functional diversity of prokaryotic HdrA(BC) modules: Role in flavin-based electron bifurcation processes and beyond

In methanogenic archaea, the archetypical complex of heterodisulfide reductase (HdrABC) and hydrogenase (MvhAGD) couples the endergonic reduction of CO₂ by H₂ to the exergonic reduction of the CoB-S-S-CoM heterodisulfide by H₂ via flavin-based electron bifurcation. Presently known enzymes containing HdrA(BC)-like components play key roles in methanogenesis, acetogenesis, respiratory sulfate reduction, lithotrophic reduced sulfur compound oxidation, aromatic compound degradation, fermentations, and probably many further processes. This functional diversity is achieved by a modular architecture of HdrA(BC) enzymes, where a big variety of electron input/output modules may be connected either directly or via adaptor modules to the HdrA(BC) components. Many, but not all HdrA(BC) complexes are proposed to catalyse a flavin-based electron bifurcation/confurcation. Despite the availability of HdrA(BC) crystal structures, fundamental questions of electron transfer and energy coupling processes remain. Here, we address the common properties and functional diversity of HdrA(BC) core modules integrated into electron-transfer machineries of outstanding complexity.

New Insights into the Ecology and Physiology of Methanomassiliicoccales from Terrestrial and Aquatic Environments

Members of the archaeal order Methanomassiliicoccales are methanogens mainly associated with animal digestive tracts. However, environmental members remain poorly characterized as no representatives not associated with a host have been cultivated so far. In this study, metabarcoding screening combined with quantitative PCR analyses on a collection of diverse non-host-associated environmental samples revealed that Methanomassiliicoccales were very scarce in most terrestrial and aquatic ecosystems. Relative abundance of Methanomassiliicoccales and substrates/products of methanogenesis were monitored during incubation of environmental slurries. A sediment slurry enriched in Methanomassiliicoccales was obtained from a freshwater sample. It allowed the reconstruction of a high-quality metagenome-assembled genome (MAG) corresponding to a new candidate species, for which we propose the name of Candidatus 'Methanomassiliicoccus armoricus MXMAG1'. Comparison of the annotated genome of MXMAG1 with the published genomes and MAGs from Methanomassiliicoccales belonging to the 2 known clades ('free-living'/non-host-associated environmental clade and 'host-associated'/digestive clade) allowed us to explore the putative physiological traits of Candidatus 'M. armoricus MXMAG1'. As expected, Ca. 'Methanomassiliicoccus armoricus MXMAG1' had the genetic potential to produce methane by reduction of methyl compounds and dihydrogen oxidation. This MAG encodes for several putative physiological and stress response adaptations, including biosynthesis of trehalose (osmotic and temperature regulations), agmatine production (pH regulation), and arsenic detoxication, by reduction and excretion of arsenite, a mechanism that was only present in the 'free-living' clade. An analysis of co-occurrence networks carried out on environmental samples and slurries also showed that Methanomassiliicoccales detected in terrestrial and aquatic ecosystems were strongly associated with acetate and dihydrogen producing bacteria commonly found in digestive habitats and which have been reported to form syntrophic relationships with methanogens.

Evidence for non-methanogenic metabolisms in globally distributed archaeal clades basal to the Methanomassiliicoccales

Recent discoveries of mcr and mcr-like genes in genomes from diverse archaeal lineages suggest that methane metabolism is an ancient pathway with a complicated evolutionary history. One conventional view is that methanogenesis is an ancestral metabolism of the class Thermoplasmata. Through comparative genomic analysis of 12 Thermoplasmata metagenome-assembled genomes (MAGs) basal to the Methanomassiliicoccales, we show that these microorganisms do not encode the genes required for methanogenesis. Further analysis of 770 Ca. Thermoplasmatota genomes/MAGs found no evidence of mcrA homologues outside of the Methanomassiliicoccales. Together, these results suggest that methanogenesis was laterally acquired by an ancestor of the Methanomassiliicoccales. The 12 analysed MAGs include representatives from four orders basal to the Methanomassiliicoccales, including a high-quality MAG that likely represents a new order, Ca. Lunaplasma lacustris ord. nov. sp. nov. These MAGs are predicted to use diverse energy conservation pathways, including heterotrophy, sulfur and hydrogen metabolism, denitrification, and fermentation. Two lineages are widespread among anoxic, sedimentary environments, whereas Ca. Lunaplasma lacustris has thus far only been detected in alpine caves and subarctic lake sediments. These findings advance our understanding of the metabolic potential, ecology, and global distribution of the Thermoplasmata and provide insight into the evolutionary history of methanogenesis within the Ca. Thermoplasmatota.

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.

Several ways one goal-methanogenesis from unconventional substrates

Abstract

Methane is the second most important greenhouse gas on earth. It is produced by methanogenic archaea, which play an important role in the global carbon cycle. Three main methanogenesis pathways are known: in the hydrogenotrophic pathway H₂ and carbon dioxide are used for methane production, whereas in the methylotrophic pathway small methylated carbon compounds like methanol and methylated amines are used. In the aceticlastic pathway, acetate is disproportionated to methane and carbon dioxide. However, next to these conventional substrates, further methanogenic substrates and pathways have been discovered. Several phylogenetically distinct methanogenic lineages (Methanosphaera, Methanimicrococcus, Methanomassiliicoccus, Methanonatronarchaeum) have evolved hydrogen-dependent methylotrophic methanogenesis without the ability to perform either hydrogenotrophic or methylotrophic methanogenesis. Genome analysis of the deep branching Methanonatronarchaeum revealed an interesting membrane-bound hydrogenase complex affiliated with the hardly described class 4 g of multisubunit hydrogenases possibly providing reducing equivalents for anabolism. Furthermore, methylated sulfur compounds such as methanethiol, dimethyl sulfide, and methylmercaptopropionate were described to be converted into adapted methylotrophic methanogenesis pathways of Methanosarcinales strains. Moreover, recently it has been shown that the methanogen Methermicoccus shengliensis can use methoxylated aromatic compounds in methanogenesis. Also, tertiary amines like choline (N,N,N-trimethylethanolamine) or betaine (N,N,N-trimethylglycine) have been described as substrates for methane production in Methanococcoides and Methanolobus strains. This review article will provide in-depth information on genome-guided metabolic reconstructions, physiology, and biochemistry of these unusual methanogenesis pathways.

Key points

• Newly discovered methanogenic substrates and pathways are reviewed for the first time.

• The review provides an in-depth analysis of unusual methanogenesis pathways.

• The hydrogenase complex of the deep branching Methanonatronarchaeum is analyzed.

Ground food waste discharge to sewer enhances methane gas emission: A lab-scale investigation

Emission of sulfide and methane from sewerage system has been a major concern for a long time. Sewers are now facing emerging challenges, such as receiving food waste (FW) to relieve the burdens on solid waste treatment. However, the knowledge of the direct impact of FW addition on sulfide and methane production in and emission from sewers is still lacking. In this study, two lab-scale sewer reactors, one without and one with FW addition, were continuously operated to investigate the production of sulfide and methane and microbial communities arising from FW discharge to freshwater sewerage system. The 190-day long-term monitoring and the batch tests on days 69 and 124 suggest that the FW addition has little impact on sulfide production possibly due to the limited sulfate concentration (40 mg S/L) but enhanced methane production by up to 60%. Moreover, cryosection-fluorescence in situ hybridization (FISH) revealed that the FW addition significantly stimulated the accumulation of methanogenic archaea (MA) in sewer biofilms and altered the spatial distributions of sulfate-reducing bacteria (SRB) and MA. Moreover, the relative abundance of MA in biofilms with FW addition was higher than that without FW addition, whereas the relative abundance of SRB was similar. Metabolic pathway analysis for sulfidogenesis and methanogenesis indicates that sufficient substrates derived from the FW addition were biodegraded during fermentation to produce acetate and hydrogen, and consequently facilitate methanogenesis. These findings shed light on the impacts of changes in wastewater compositions (e.g., FW addition) on sulfide and methane production in the freshwater sewerage system for improved policy-making on sewer management.

Archaea, specific genetic traits, and development of improved bacterial live biotherapeutic products: another face of next-generation probiotics

Trimethylamine (TMA) and its oxide TMAO are important biomolecules involved in disease-associated processes in humans (e.g., trimethylaminuria and cardiovascular diseases). TMAO in plasma (pTMAO) stems from intestinal TMA, which is formed from various components of the diet in a complex interplay between diet, gut microbiota, and the human host. Most approaches to prevent the occurrence of such deleterious molecules focus on actions to interfere with gut microbiota metabolism to limit the synthesis of TMA. Some human gut archaea however use TMA as terminal electron acceptor for producing methane, thus indicating that intestinal TMA does not accumulate in some human subjects. Therefore, a rational alternative approach is to eliminate neo-synthesized intestinal TMA. This can be achieved through bioremediation of TMA by these peculiar methanogenic archaea, either by stimulating or providing them, leading to a novel kind of next-generation probiotics referred to as archaebiotics. Finally, specific components which are involved in this archaeal metabolism could also be used as intestinal TMA sequesters, facilitating TMA excretion along with stool. Referring to a standard pharmacological approach, these TMA traps could be synthesized ex vivo and then delivered into the human gut. Another approach is the engineering of known probiotic strain in order to metabolize TMA, i.e., live engineered biotherapeutic products. These alternatives would require, however, to take into account the necessity of synthesizing the 22nd amino acid pyrrolysine, i.e., some specificities of the genetics of TMA-consuming archaea. Here, we present an overview of these different strategies and recent advances in the field that will sustain such biotechnological developments. KEY POINTS: • Some autochthonous human archaea can use TMA for their essential metabolism, a methyl-dependent hydrogenotrophic methanogenesis. • They could therefore be used as next-generation probiotics for preventing some human diseases, especially cardiovascular diseases and trimethylaminuria. • Their genetic capacities can also be used to design live recombinant biotherapeutic products. • Encoding of the 22nd amino acid pyrrolysine is necessary for such alternative developments.

Effect of a side-stream voltage supplied by sludge recirculation to an anaerobic digestion reactor

This study showed that side stream voltage supplied by sludge recirculation from an auxiliary bio-electrochemical anaerobic digestion (ABEAD) reactor appears to have a similar effect as main stream voltage supply to an anaerobic digestion (AD) reactor. The increased sludge recirculation rate enhanced the operation stability at a high OLR. H₂-producing bacterial community was improved in bio-electrochemical anaerobic digestion (BEAD) and ABEAD reactors and was increased with increase in sludge recirculation rate. Despite the dominance of hydrogenotrophic methanogens in all reactors, high operational performances of BEAD and ABEAD reactors supports the results of H₂-producing bacteria increase in those reactors. The ABEAD reactors having 1/7 of the capacity of the main AD reactor showed possibility of integration of BEAD technology into new and existing facilities economically. The findings of this study would provide useful information for approaching the commercialization of BEAD and suggest direction of further research for practical applications.

Methylotrophic methanogens everywhere - physiology and ecology of novel players in global methane cycling

Research on methanogenic Archaea has experienced a revival, with many novel lineages of methanogens recently being found through cultivation and suggested via metagenomics approaches, respectively. Most of these lineages comprise Archaea (potentially) capable of methanogenesis from methylated compounds, a pathway that had previously received comparably little attention. In this review, we provide an overview of these new lineages with a focus on the Methanomassiliicoccales. These lack the Wood-Ljungdahl pathway and employ a hydrogen-dependent methylotrophic methanogenesis pathway fundamentally different from traditional methylotrophic methanogens. Several archaeal candidate lineages identified through metagenomics, such as the Ca. Verstraetearchaeota and Ca. Methanofastidiosa, encode genes for a methylotrophic methanogenesis pathway similar to the Methanomassiliicoccales. Thus, the latter are emerging as a model system for physiological, biochemical and ecological studies of hydrogen-dependent methylotrophic methanogens. Methanomassiliicoccales occur in a large variety of anoxic habitats including wetlands and animal intestinal tracts, i.e. in the major natural and anthropogenic sources of methane emissions, respectively. Especially in ruminant animals, they likely are among the major methane producers. Taken together, (hydrogen-dependent) methylotrophic methanogens are much more diverse and widespread than previously thought. Considering the role of methane as potent greenhouse gas, resolving the methanogenic nature of a broad range of putative novel methylotrophic methanogens and assessing their role in methane emitting environments are pressing issues for future research on methanogens.

Immediate Effects of Ammonia Shock on Transcription and Composition of a Biogas Reactor Microbiome

The biotechnological process of biogas production from organic material is carried out by a diverse microbial community under anaerobic conditions. However, the complex and sensitive microbial network present in anaerobic degradation of organic material can be disturbed by increased ammonia concentration introduced into the system by protein-rich substrates and imbalanced feeding. Here, we report on a simulated increase of ammonia concentration in a fed batch lab-scale biogas reactor experiment. Two treatment conditions were used simulating total ammonia nitrogen concentrations of 4.9 and 8.0 g/L with four replicate reactors. Each reactor was monitored concerning methane generation and microbial composition using 16S rRNA gene amplicon sequencing, while the transcriptional activity of the overall process was investigated by metatranscriptomic analysis. This allowed investigating the response of the microbial community in terms of species composition and transcriptional activity to a rapid upshift to high ammonia conditions. Clostridia and Methanomicrobiales dominated the microbial community throughout the entire experiment under both experimental conditions, while Methanosarcinales were only present in minor abundance. Transcription analysis demonstrated clostridial dominance with respect to genes encoding for enzymes of the hydrolysis step (cellulase, EC 3.2.1.4) as well as dominance of key genes for enzymes of the methanogenic pathway (methyl-CoM reductase, EC 2.8.4.1; heterodisulfide reductase, EC 1.8.98.1). Upon ammonia shock, the selected marker genes showed significant changes in transcriptional activity. Cellulose hydrolysis as well as methanogenesis were significantly reduced at high ammonia concentrations as indicated by reduced transcription levels of the corresponding genes. Based on these experiments we concluded that, apart from the methanogenic archaea, hydrolytic cellulose-degrading microorganisms are negatively affected by high ammonia concentrations. Further, Acholeplasma and Erysipelotrichia showed lower abundance under increased ammonia concentrations and thus might serve as indicator species for an earlier detection in order to counteract against ammonia crises.

Metabolic potential of microbial communities from ferruginous sediments

Ferruginous (Fe-rich, SO₄ -poor) conditions are generally restricted to freshwater sediments on Earth today, but were likely widespread during the Archean and Proterozoic Eons. Lake Towuti, Indonesia, is a large ferruginous lake that likely hosts geochemical processes analogous to those that operated in the ferruginous Archean ocean. The metabolic potential of microbial communities and related biogeochemical cycling under such conditions remain largely unknown. We combined geochemical measurements (pore water chemistry, sulfate reduction rates) with metagenomics to link metabolic potential with geochemical processes in the upper 50 cm of sediment. Microbial diversity and quantities of genes for dissimilatory sulfate reduction (dsrAB) and methanogenesis (mcrA) decrease with increasing depth, as do rates of potential sulfate reduction. The presence of taxa affiliated with known iron- and sulfate-reducers implies potential use of ferric iron and sulfate as electron acceptors. Pore-water concentrations of acetate imply active production through fermentation. Fermentation likely provides substrates for respiration with iron and sulfate as electron donors and for methanogens that were detected throughout the core. The presence of ANME-1 16S and mcrA genes suggests potential for anaerobic methane oxidation. Overall our data suggest that microbial community metabolism in anoxic ferruginous sediments support coupled Fe, S and C biogeochemical cycling.

Physiology and central carbon metabolism of the gut bacterium Prevotella copri

The human gut microbiota is a crucial factor for the host's physiology with respect to health and disease. Metagenomic shotgun sequencing of microbial gut communities revealed that Prevotella copri is one of the most important players in the gastrointestinal tract of many individuals. Because of the importance of this bacterium we analyzed the growth behavior and the central metabolic pathways of P. copri. Bioinformatic data, transcriptome profiling and enzyme activity measurements indicated that the major pathways are based on glycolysis and succinate production from fumarate. In addition, pyruvate can be degraded to acetate and formate. Electron transport phosphorylation depends on fumarate respiration with NADH and reduced ferredoxin as electron donors. In contrast to Bacteroides vulgatus, P. copri showed a more pronounced dependency on the addition of CO₂ or bicarbonate for biomass formation, which is a remarkable difference between P. copri and Bacteroides spp. with important implication in the context of gut microbial competition. The analysis of substrate consumption and product concentrations from many P. copri cultures with different optical densities allowed a prediction of the carbon and electron flow in the central metabolism and a detailed calculation of growth yields as well as carbon and redox balances.

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.

Methanol metabolism and archaeal community changes in a bioelectrochemical anaerobic digestion sequencing batch reactor with copper-coated graphite cathode

In this study, the metabolism of methanol and changes in an archaeal community were examined in a bioelectrochemical anaerobic digestion sequencing batch reactor with a copper-coated graphite cathode (BEAD-SBR_Cu). Copper-coated graphite cathode produced methanol from food waste. The BEAD-SBR_Cu showed higher methanol removal and methane production than those of the anaerobic digestion (AD)-SBR. The methane production and pH of the BEAD-SBR_Cu were stable even under a high organic loading rate (OLR). The hydrogenotrophic methanogens increased from 32.2 to 60.0%, and the hydrogen-dependent methylotrophic methanogens increased from 19.5 to 37.7% in the bulk of BEAD-SBR_Cu at high OLR. Where methanol was directly injected as a single substrate into the BEAD-SBR_Cu, the main metabolism of methane production was hydrogenotrophic methanogenesis using carbon dioxide and hydrogen released by the oxidation of methanol on the anode through bioelectrochemical reactions.

Electron Bifurcation and Confurcation in Methanogenesis and Reverse Methanogenesis

Reduction of the disulfide of coenzyme M and coenzyme B (CoMS–SCoB) by heterodisulfide reductases (HdrED and HdrABC) is the final step in all methanogenic pathways. Flavin-based electron bifurcation (FBEB) by soluble HdrABC homologs play additional roles in driving essential endergonic reactions at the expense of the exergonic reduction of CoMS–SCoM. In the first step of the CO₂ reduction pathway, HdrABC complexed with hydrogenase or formate dehydrogenase generates reduced ferredoxin (Fdx^2-) for the endergonic reduction of CO₂ coupled to the exergonic reduction of CoMS–SCoB dependent on FBEB of electrons from H₂ or formate. Roles for HdrABC:hydrogenase complexes are also proposed for pathways wherein the methyl group of methanol is reduced to methane with electrons from H₂. The HdrABC complexes catalyze FBEB-dependent oxidation of H₂ for the endergonic reduction of Fdx driven by the exergonic reduction of CoMS–SCoB. The Fdx^2- supplies electrons for reduction of the methyl group to methane. In H₂^- independent pathways, three-fourths of the methyl groups are oxidized producing Fdx^2- and reduced coenzyme F₄₂₀ (F₄₂₀H₂). The F₄₂₀H₂ donates electrons for reduction of the remaining methyl groups to methane requiring transfer of electrons from Fdx^2- to F₄₂₀. HdrA1B1C1 is proposed to catalyze FBEB-dependent oxidation of Fdx^2- for the endergonic reduction of F₄₂₀ driven by the exergonic reduction of CoMS–SCoB. In H₂^- independent acetotrophic pathways, the methyl group of acetate is reduced to methane with electrons derived from oxidation of the carbonyl group mediated by Fdx. Electron transport involves a membrane-bound complex (Rnf) that oxidizes Fdx^2- and generates a Na⁺ gradient driving ATP synthesis. It is postulated that F₄₂₀ is reduced by Rnf requiring HdrA2B2C2 catalyzing FBEB-dependent oxidation of F₄₂₀H₂ for the endergonic reduction of Fdx driven by the exergonic reduction of CoMS–SCoB. The Fdx^2- is recycled by Rnf and HdrA2B2C2 thereby conserving energy. The HdrA2B2C2 is also proposed to play a role in Fe(III)-dependent reverse methanogenesis. A flavin-based electron confurcating (FBEC) HdrABC complex is proposed for nitrate-dependent reverse methanogenesis in which the oxidation of CoM-SH/CoB-SH and Fdx^2- is coupled to reduction of F₄₂₀. The F₄₂₀H₂ donates electrons to a membrane complex that generates a proton gradient driving ATP synthesis.

Revival of Archaeal Methane Microbiology

The methane concentration in the Earth's atmosphere is rising, and, as methane is a potent greenhouse gas, it contributes considerably to climate change. It is produced by methanogenic archaea that thrive in anoxic habitats and can be oxidized by methane-oxidizing bacteria or archaea. In this Perspective, recent innovations and discoveries in archaeal methane microbiology are discussed and a future outlook on how novel methane-metabolizing archaea might be cultivated is provided.

Acclimation of acid-tolerant methanogenic propionate-utilizing culture and microbial community dissecting

The acid-tolerant methanogenic propionate degradation culture was acclimated in a propionate-fed semi-continuous bioreactor by daily adjusting the digestate pH. The performance of propionate fermentation, the respond of microbial community structure to the acidic environment, and the microbial network for propionate degradation in the acid-tolerant culture was investigated. The results demonstrated that after long term of acclimation to low pH, the digester could produce methane from propionate at pH 4.8-5.5 with 0.3-0.4 L g^-1 propionic acid (HPr) d^-1 of the volatile solids (VS) methane production. The predominant methanogens shifted from acetoclastic methanogens (∼87%) to hydrogenotrophic methanogens (∼67%) in the bioreactor with the dropping pH, indicating that hydrogenotrophic methanogens were more acid-tolerant than acetoclastic methanogens. Smithella (∼11%), Syntrophobacter (∼7%) and Pelotomaculum (∼3%) were the main propionate oxidizers in the acid-tolerant propionate-utilizing culture. Methanothrix dominant acetoclastic methanogens, while Methanolinea and Methanospirillum were the major H₂ scavengers to support Syntrophobacter and Pelotomaculum syntrophic propionate degradation.

Vertical distribution of microbial community and metabolic pathway in a methanogenic propionate degradation bioreactor

The methanogenic propionate degradation consortia were enriched in a propionate-fed semi-continuous bioreactor. The microbial community shift with depth, the microbial network and its correlation with metabolic pathway were also investigated. The results demonstrated that the maximum organic loading rate (OLR) of the reactor was 2.5g propionic acid (HPr) L^-1d^-1 with approximately 1.20LL^-1d^-1 of volumetric methane production (VMP). The organisms in the enrichment were spanning 36 bacterial phyla and 7 archaeal orders. Syntrophobacter, the main Hpr oxidizer in the digester, dominated bacteria with relative abundance changing from 63% to 37% with depth. The predominant methanogens shift from hydrogenotrophic Methanoculleus (∼60%) at the upper liquid layer to acetoclastic Methanothrix (∼51%) at the lower sediment layer in the bioreactor. These methanogens syntrophically support Syntrophobacter by degrading HPr catabolism by-products (H₂ and acetate). Other bacteria could scavenge anabolic products (carbohydrate and protein) presumably derived from detrital biomass produced by the HPr-degrading community.

Growth Characteristics of Methanomassiliicoccus luminyensis and Expression of Methyltransferase Encoding Genes

DNA sequence analysis of the human gut revealed the presence a seventh order of methanogens referred to as Methanomassiliicoccales. Methanomassiliicoccus luminyensis is the only member of this order that grows in pure culture. Here, we show that the organism has a doubling time of 1.8 d with methanol + H2 and a growth yield of 2.4 g dry weight/mol CH4. M. luminyensis also uses methylamines + H2 (monomethylamine, dimethylamine, and trimethylamine) with doubling times of 2.1-2.3 d. Similar cell yields were obtained with equimolar concentrations of methanol and methylamines with respect to their methyl group contents. The transcript levels of genes encoding proteins involved in substrate utilization indicated increased amounts of mRNA from the mtaBC2 gene cluster in methanol-grown cells. When methylamines were used as substrates, mRNA of the mtb/mtt operon and of the mtmBC1 cluster were found in high abundance. The transcript level of mtaC2 was almost identical in methanol- and methylamine-grown cells, indicating that genes for methanol utilization were constitutively expressed in high amounts. The same observation was made with resting cells where methanol always yielded the highest CH4 production rate independently from the growth substrate. Hence, M. luminyensis is adapted to habitats that provide methanol + H2 as substrates.

Genomic composition and dynamics among Methanomicrobiales predict adaptation to contrasting environments

Members of the order Methanomicrobiales are abundant, and sometimes dominant, hydrogenotrophic (H₂-CO₂ utilizing) methanoarchaea in a broad range of anoxic habitats. Despite their key roles in greenhouse gas emissions and waste conversion to methane, little is known about the physiological and genomic bases for their widespread distribution and abundance. In this study, we compared the genomes of nine diverse Methanomicrobiales strains, examined their pangenomes, reconstructed gene flow and identified genes putatively mediating their success across different habitats. Most strains slowly increased gene content whereas one, Methanocorpusculum labreanum, evidenced genome downsizing. Peat-dwelling Methanomicrobiales showed adaptations centered on improved transport of scarce inorganic nutrients and likely use H⁺ rather than Na⁺ transmembrane chemiosmotic gradients during energy conservation. In contrast, other Methanomicrobiales show the potential to concurrently use Na⁺ and H⁺ chemiosmotic gradients. Analyses also revealed that the Methanomicrobiales lack a canonical electron bifurcation system (MvhABGD) known to produce low potential electrons in other orders of hydrogenotrophic methanogens. Additional putative differences in anabolic metabolism suggest that the dynamics of interspecies electron transfer from Methanomicrobiales syntrophic partners can also differ considerably. Altogether, these findings suggest profound differences in electron trafficking in the Methanomicrobiales compared with other hydrogenotrophs, and warrant further functional evaluations.

Methanogenesis and the Wood-Ljungdahl Pathway: An Ancient, Versatile, and Fragile Association

Methanogenesis coupled to the Wood–Ljungdahl pathway is one of the most ancient metabolisms for energy generation and carbon fixation in the Archaea. Recent results are sensibly changing our view on the diversity of methane-cycling capabilities in this Domain of Life. The availability of genomic sequences from uncharted branches of the archaeal tree has highlighted the existence of novel methanogenic lineages phylogenetically distant to previously known ones, such as the Methanomassiliicoccales. At the same time, phylogenomic analyses have suggested a methanogenic ancestor for all Archaea, implying multiple independent losses of this metabolism during archaeal diversification. This prediction has been strengthened by the report of genes involved in methane cycling in members of the Bathyarchaeota (a lineage belonging to the TACK clade), representing the first indication of the presence of methanogenesis outside of the Euryarchaeota. In light of these new data, we discuss how the association between methanogenesis and the Wood–Ljungdahl pathway appears to be much more flexible than previously thought, and might provide information on the processes that led to loss of this metabolism in many archaeal lineages. The combination of environmental microbiology, experimental characterization and phylogenomics opens up exciting avenues of research to unravel the diversity and evolutionary history of fundamental metabolic pathways.