Concerted Metabolic Shifts Give New Insights Into the Syntrophic Mechanism Between Propionate-Fermenting Pelotomaculum thermopropionicum and Hydrogenotrophic Methanocella conradii

Cometabolism of ferrihydrite reduction and methyl-dismutating methanogenesis by Methanosarcina mazei

Recent discoveries have shown that some Methanosarcina species can reduce Fe(III), reshaping our understanding of Methanosarcina ecophysiology. However, the specific minerals reduced, the products formed, and the underlying metabolic mechanisms remain elusive. Here, we report on the cometabolic process of Fe(III) reduction and methylotrophic methanogenesis in Methanosarcina mazei zm-15. Biogeochemical and mineralogical analyses were conducted to investigate Fe(III) reduction from three mineral preparations-ferrihydrite, goethite, and hematite. The results revealed that 38% of the 6 mM Fe(III) in ferrihydrite was reduced within 4 days, and this percentage increased to 75% with the addition of 100 µM anthraquinone-2,6-disulfonate (AQDS). Active Fe(III) reduction occurred immediately and preceded rapid methanogenesis. The addition of ferrihydrite and AQDS together significantly enhanced the maximal CH₄ production rate. However, Fe(III) reduction did not occur in goethite or hematite, even with the addition of 100 µM AQDS. Vivianite was identified as the major product from ferrihydrite reduction. Transcriptomic analysis revealed that gene expression related to the oxidation branch of the methyl-dismutating pathway and the membrane-associated electron transport chain (ETC) was significantly upregulated, whereas the expressions of genes associated with the reduction branch of the methyl-dismutating pathway were downregulated. In conclusion, M. mazei zm-15 demonstrates a strong ability to reduce poorly crystalline ferrihydrite, but not highly crystalline goethite and hematite. During the cometabolism of Fe(III) reduction and CH₄ production from methanol, the methyl-oxidation and membrane ETC pathways are enhanced, while the methyl-reduction pathway is downregulated. The mechanism of electron relay from cells to ferrihydrite, however, remains unclear and warrants further investigation.IMPORTANCEThe recent discovery that certain Methanosarcina species can grow by reducing Fe(III) challenges the traditional understanding of methanogens. However, the underlying metabolic mechanisms remain largely unexplored. Using a combination of biogeochemical, mineralogical, and microbiological approaches, we investigated the ability of Methanosarcina mazei zm-15. It exhibited a strong capacity to reduce poorly crystalline ferrihydrite but not highly crystalline goethite and hematite. The formation of vivianite from ferrihydrite reduction is likely due to the high rate of Fe(III) reduction and the presence of excess phosphorus in incubations. During the cometabolism of Fe(III) reduction and CH4 production from methanol, the methyl-oxidation and membrane electron transport pathways are upregulated, while the methyl-reduction pathway is downregulated. Our research uncovers a differential regulation of metabolic pathways during the cometabolism of Fe(III) reduction and CH4 production from methanol. The findings shed new light on the adaptive strategies employed by M. mazei in environments with the presence of Fe(III) and suggestthat Methanosarcina can play a significant role in methane production and iron cycling in natural environments.

Syntrophic entanglements for propionate and acetate oxidation under thermophilic and high-ammonia conditions

Propionate is a key intermediate in anaerobic digestion processes and often accumulates in association with perturbations, such as elevated levels of ammonia. Under such conditions, syntrophic ammonia-tolerant microorganisms play a key role in propionate degradation. Despite their importance, little is known about these syntrophic microorganisms and their cross-species interactions. Here, we present metagenomes and metatranscriptomic data for novel thermophilic and ammonia-tolerant syntrophic bacteria and the partner methanogens enriched in propionate-fed reactors. A metagenome for a novel bacterium for which we propose the provisional name 'Candidatus Thermosyntrophopropionicum ammoniitolerans' was recovered, together with mapping of its highly expressed methylmalonyl-CoA pathway for syntrophic propionate degradation. Acetate was degraded by a novel thermophilic syntrophic acetate-oxidising candidate bacterium. Electron removal associated with syntrophic propionate and acetate oxidation was mediated by the hydrogen/formate-utilising methanogens Methanoculleus sp. and Methanothermobacter sp., with the latter observed to be critical for efficient propionate degradation. Similar dependence on Methanothermobacter was not seen for acetate degradation. Expression-based analyses indicated use of both H2 and formate for electron transfer, including cross-species reciprocation with sulphuric compounds and microbial nanotube-mediated interspecies interactions. Batch cultivation demonstrated degradation rates of up to 0.16 g propionate L-1 day-1 at hydrogen partial pressure 4-30 Pa and available energy was around -20 mol-1 propionate. These observations outline the multiple syntrophic interactions required for propionate oxidation and represent a first step in increasing knowledge of acid accumulation in high-ammonia biogas production systems.

Syntrophic Propionate Oxidation: One of the Rate-Limiting Steps of Organic Matter Decomposition in Anoxic Environments

Syntrophic propionate oxidation is one of the rate-limiting steps during anaerobic decomposition of organic matter in anoxic environments. Syntrophic propionate-oxidizing bacteria (SPOB) are members of the "rare biosphere" living at the edge of the thermodynamic limit in most natural habitats. Hitherto, only 10 bacterial species capable of syntrophic propionate oxidization have been identified. SPOB employ different metabolisms for propionate oxidation (e.g., methylmalonyl-CoA pathway and C6 dismutation pathway) and show diverse life strategies (e.g., obligately and facultatively syntrophic lifestyle). The flavin-based electron bifurcation/confurcation (FBEB/C) systems have been proposed to help solve the thermodynamic dilemma during the formation of the low-potential products H2 and formate. Molecular ecological approaches, such as DNA stable isotope probing (DNA-SIP) and metagenomics, have been used to detect SPOB in natural environments. Furthermore, the biogeographical pattern of SPOB has been recently described in paddy soils. A comprehensive understanding of SPOB is essential for better predicting and managing organic matter decomposition and carbon cycling in anoxic environments. In this review, we described the critical role of syntrophic propionate oxidation in anaerobic decomposition of organic matter, phylogenetic and metabolic diversity, life strategies and ecophysiology, composition of syntrophic partners, and pattern of biogeographic distribution of SPOB in natural environments. We ended up with a few perspectives for future research.

Histone variants in archaea - An undiscovered country

Exchanging core histones in the nucleosome for paralogous variants can have important functional ramifications. Many of these variants, and their physiological roles, have been characterized in exquisite detail in model eukaryotes, including humans. In comparison, our knowledge of histone biology in archaea remains rudimentary. This is true in particular for our knowledge of histone variants. Many archaea encode several histone genes that differ in sequence, but do these paralogs make distinct, adaptive contributions to genome organization and regulation in a manner comparable to eukaryotes? Below, we review what we know about histone variants in archaea at the level of structure, regulation, and evolution. In all areas, our knowledge pales when compared to the wealth of insight that has been gathered for eukaryotes. Recent findings, however, provide tantalizing glimpses into a rich and largely undiscovered country that is at times familiar and eukaryote-like and at times strange and uniquely archaeal. We sketch a preliminary roadmap for further exploration of this country; an undertaking that may ultimately shed light not only on chromatin biology in archaea but also on the origin of histone-based chromatin in eukaryotes.

Effects of ferroferric oxide on propionate methanogenesis in sequencing batch reactors: Microbial community structure and metagenomic analysis

This study investigated the effects of ferroferric oxide (Fe₃O₄) on propionate methanogenesis in anaerobic sequencing batch reactor (ASBR). Compared to ASBR_C (without Fe₃O₄ addition), the addition of 10 g/L Fe₃O₄ (ASBR_Fe) decreased the maximum methane production rate by 69.6 % when propionate was used as the sole substrate. The addition of Fe₃O₄ reduced the contents of humic substances, riboflavin and nicotinamide adenine dinucleotide in extracellular polymeric substances. Therefore, Fe₃O₄ inhibited interspecies electron transfer of microorganisms through electronic mediators. Microbial community analysis revealed that Fe₃O₄ addition increased the relative abundance of acetate oxidizing bacterium (Mesotoga), but decreased the abundance of hydrogenotrophic methanogen (Methanobacterium). Further metagenomics analysis indicated that Fe₃O₄ increased the abundance of acetate oxidation genes and decreased that of hydrogenotrophic methanogenesis, quorum sensing and V/A-type ATPase genes. Thus, Fe₃O₄ reduced propionate methanogenesis during anaerobic digestion. The overall results indicate that Fe₃O₄ addition inhibits methanogenesis for treatment of propionate-contaminated wastewater in ASBR.

Effect of magnetite addition on transcriptional profiles of syntrophic Bacteria and Archaea during anaerobic digestion of propionate in wastewater sludge

Anaerobic digestion (AD) is an important technology for the effective conversion of waste and wastewater to methane. Here, syntrophic bacteria transfer molecular hydrogen (H₂ ), formate, or directly supply electrons (direct interspecies electron transfer, DIET) to the methanogens. Evidence is accumulating that the methanation of short-chain fatty acids can be enhanced by the addition of conductive material to the anaerobic digester, which has often been attributed to the stimulation of DIET. Since little is known about the transcriptional response of a complex AD microbial community to the addition of conductive material, we added magnetite to propionate-fed laboratory-scale reactors that were inoculated with wastewater sludge. Compared to the control reactors, the magnetite-amended reactors showed improved methanation of propionate. A genome-centric metatranscriptomics approach identified the active SCFA-oxidizing bacteria that affiliated with Firmicutes, Desulfobacterota and Cloacimonadota. The transcriptional profiles revealed that the syntrophic bacteria transferred acetate, H₂ and formate to acetoclastic and hydrogenotrophic methanogens, whereas transcription of potential determinants for DIET such as conductive pili and outer-membrane cytochromes did not significantly change with magnetite addition. Overall, changes in the transcriptional profiles of syntrophic Bacteria and Archaea in propionate-fed lab-scale reactors amended with magnetite refute a major role of DIET in the studied system.

Syntrophic propionate-oxidizing bacteria in methanogenic systems

The mutual nutritional cooperation underpinning syntrophic propionate degradation provides a scant amount of energy for the microorganisms involved, so propionate degradation often acts as a bottleneck in methanogenic systems. Understanding the ecology, physiology and metabolic capacities of syntrophic propionate-oxidizing bacteria (SPOB) is of interest in both engineered and natural ecosystems, as it offers prospects to guide further development of technologies for biogas production and biomass-derived chemicals, and is important in forecasting contributions by biogenic methane emissions to climate change. SPOB are distributed across different phyla. They can exhibit broad metabolic capabilities in addition to syntrophy (e.g. fermentative, sulfidogenic and acetogenic metabolism) and demonstrate variations in interplay with cooperating partners, indicating nuances in their syntrophic lifestyle. In this review, we discuss distinctions in gene repertoire and organization for the methylmalonyl-CoA pathway, hydrogenases and formate dehydrogenases, and emerging facets of (formate/hydrogen/direct) electron transfer mechanisms. We also use information from cultivations, thermodynamic calculations and omic analyses as the basis for identifying environmental conditions governing propionate oxidation in various ecosystems. Overall, this review improves basic and applied understanding of SPOB and highlights knowledge gaps, hopefully encouraging future research and engineering on propionate metabolism in biotechnological processes.

Integrated Metagenomic and Metaproteomic Analyses Unravel Ammonia Toxicity to Active Methanogens and Syntrophs, Enzyme Synthesis, and Key Enzymes in Anaerobic Digestion

During anaerobic digestion, the active microbiome synthesizes enzymes by transcription and translation, and then enzymes catalyze multistep bioconversions of substrates before methane being produced. However, little information is available on how ammonia affects truly active microbes containing the expressed enzymes, enzyme synthesis, and key enzymes. In this study, an integrated metagenomic and metaproteomic investigation showed that ammonia suppressed not only the obligate acetotrophic methanogens but also the syntrophic propionate and butyrate oxidation taxa and their assistant bacteria (genus Desulfovibrio), which declined the biotransformations of propionate and butyrate → acetate → methane. Although the total population of the hydrolyzing and acidifying bacteria was not affected by ammonia, the bacteria with ammonia resistance increased. Our study also revealed that ammonia restrained the enzyme synthesis process by inhibiting the RNA polymerase (subunits A' and D) during transcription and the ribosome (large (L3, L12, L13, L22, and L25) and small (S3, S3Ae, and S7) ribosomal subunits) and aminoacyl-tRNA synthesis (aspartate-tRNA synthetase) in translation. Further investigation suggested that methylmalonyl-CoA mutase, acetyl-CoA C-acetyltransferase, and CH₃-CoM reductase, which regulate propionate and butyrate oxidation and acetoclastic methanation, were significantly downregulated by ammonia. This study provides intrinsic insights into the fundamental mechanisms of how ammonia inhibits anaerobic digestion.

Growth Coordination Between Butyrate-Oxidizing Syntrophs and Hydrogenotrophic Methanogens

Syntrophy is a thermodynamically required mutualistic cooperation between fatty acid-oxidizing bacteria and methanogens that plays the important role in organic decomposition and methanogenesis in anoxic environments. In this study, three experiments were conducted to evaluate the cell-to-cell interaction in a thermophilic coculture consisting of Syntrophothermus lipocalidus and Methanocella conradii and a mesophilic coculture consisting of Syntrophomonas wolfei and Methanococcus maripaludis. First, syntrophs and methanogens were inoculated at different initial cell ratios to evaluate the growth synchronization. The quantitative PCR analysis revealed that the organism with a lower relative abundance at the beginning always grew faster, and the cell ratio converged over time to relative constant values in both the thermophilic and mesophilic cocultures. Next, intermittent ultrasound and constant shaking treatments were used to evaluate the influence of physical disturbance on microbial aggregation in the mesophilic coculture. The fluorescence in situ hybridization and scanning electron microscopy revealed that the tendency of syntrophic aggregation was not affected by the physical disturbances, although the activity was slightly depressed. Syntrophomonas dominated in the initial microbial aggregates, which, however, did not grow until Methanococcus was attached and increased to a significant extent, indicating the local growth synchronization during the formation and maturation of syntrophic aggregates. Last, microfluidic experiments revealed that whether or not Syntrophomonas or Methanococcus was loaded first, the second organism preferred moving to the place where the first organism was located, suggesting the cell-to-cell attraction between Syntrophomonas and Methanococcus. Collectively, our study demonstrated the growth synchronization and cell-to-cell attraction between the butyrate-oxidizing bacteria and methanogens for optimizing the syntrophic cooperation.

Enrichment and description of novel bacteria performing syntrophic propionate oxidation at high ammonia level

Inefficient syntrophic propionate degradation causes severe operating disturbances and reduces biogas productivity in many high-ammonia anaerobic digesters, but propionate-degrading microorganisms in these systems remain unknown. Here, we identified candidate ammonia-tolerant syntrophic propionate-oxidising bacteria using propionate enrichment at high ammonia levels (0.7-0.8 g NH₃ L^-1 ) in continuously-fed reactors. We reconstructed 30 high-quality metagenome-assembled genomes (MAGs) from the propionate-fed reactors, which revealed two novel species from the families Peptococcaceae and Desulfobulbaceae as syntrophic propionate-oxidising candidates. Both MAGs possess genomic potential for the propionate oxidation and electron transfer required for syntrophic energy conservation and, similar to ammonia-tolerant acetate degrading syntrophs, both MAGs contain genes predicted to link to ammonia and pH tolerance. Based on relative abundance, a Peptococcaceae sp. appeared to be the main propionate degrader and has been given the provisional name "Candidatus Syntrophopropionicum ammoniitolerans". This bacterium was also found in high-ammonia biogas digesters, using quantitative PCR. Acetate was degraded by syntrophic acetate-oxidising bacteria and the hydrogenotrophic methanogenic community consisted of Methanoculleus bourgensis and a yet to be characterised Methanoculleus sp. This work provides knowledge of cooperating syntrophic species in high-ammonia systems and reveals that ammonia-tolerant syntrophic propionate-degrading populations share common features, but diverge genomically and taxonomically from known species.

Response of Microbial Community to Induced Failure of Anaerobic Digesters Through Overloading With Propionic Acid Followed by Process Recovery

In order to effectively use microbial-based strategies to manage anaerobic digesters, it is necessary to distinguish between community shifts that are part of the natural dynamic of the system and shifts caused by environmental or operational disturbances. The objective of this research study was to evaluate the significance of changes in the microbial community of anaerobic digesters during failure in correlation to operational parameters such as an organic acid overload. Five continuously stirred 0.5 L reactors were set-up as semi-continuously-fed, mesophilic dairy manure digesters with a 30-day hydraulic retention time. After a 120-day stabilization period, two digesters were kept as controls, while the organic loading rates in the triplicate set were increased step-wise to ultimately provide a shock-load leading to failure using propionic acid spikes. Acidosis resulting in near cessation of biogas and termination of methane production occurred between 4 and 7 weeks, after which all the digesters continued to be fed only dairy manure. The shock loading of propionic acid led to an accumulation of mainly acetate and propionate, with low levels of iso-butyrate, butyrate, iso-valerate, and valerate. High-throughput Illumina sequencing of the V4 region of the bacterial and archaeal 16S rRNA gene in digester samples showed a significant change in the microbial community composition during propionic acid overload, followed by a return to the original composition with regular feedstock. Bacterial genera whose relative abundance decreased during the inhibition stage included Sedimentibacter, Syntrophomonas, TSCOR003.O20, and Marinilabiaceae, while the relative abundance of Lachnospiraceae, Ruminococcus, Mogibacteriaceae, Pyramidobacter, and Bacteroides increased. The relative abundance of dominant methanogens, Methanosarcina and Methanobacterium, although initially resistant, were decreased (from 91.71 to 12.14% and from 2.98 to 0.73%, respectively) during inhibition, while Methanobrevibacter and Methanosphaera that were prominent in the manure feedstock increased from 17.36 to 79.45% and from 0.14 to 1.12%, respectively. Shifts in bacterial and archaeal compositions, back to their pre-shock steady state after failure, highlight the digester's microbial resilience and recovery potential.

Insights into Ammonia Adaptation and Methanogenic Precursor Oxidation by Genome-Centric Analysis

Ammonia released from the degradation of protein and/or urea usually leads to suboptimal anaerobic digestion (AD) when N-rich organic waste is used. However, the insights behind the differential ammonia tolerance of anaerobic microbiomes remain an enigma. In this study, the cultivation in synthetic medium with different carbon sources (acetate, methanol, formate, and H2/CO2) shaped a common initial inoculum into four unique ammonia-tolerant syntrophic populations. Specifically, various levels of ammonia tolerance were observed: consortia fed with methanol and H2/CO2 could grow at ammonia levels up to 7.25 g NH+-N/L, whereas the other two groups (formate and acetate) only thrived at 5.25 and 4.25 g NH+-N/L, respectively. Metabolic reconstruction highlighted that this divergent microbiome might be achieved by complementary metabolisms to maximize biomethane recovery from carbon sources, thus indicating the importance of the syntrophic community in the AD of N-rich substrates. Besides, sodium/proton antiporter operon, osmoprotectant/K+ regulator, and osmoprotectant synthesis operon may function as the main drivers of adaptation to the ammonia stress. Moreover, energy from the substrate-level phosphorylation and multiple energy-converting hydrogenases (e.g., Ech and Eha) could aid methanogens to balance the energy request for anabolic activities and contribute to thriving when exposed to high ammonia levels.

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.

Evolution of microbial community during dry storage and recovery of aerobic granular sludge

Aerobic granular sludge (AGS) was imbedded in agar and stored at 4 °C for 30 days, and then the stored granules were recovered in a sequencing batch reactor fed real wastewater within 11 days. Variations in microbial community compositions were investigated during dry storage and recovery of AGS, aiming to elucidate the mechanism of granular stability loss and recovery. The storage and recovery of AGS involved microbial community evolution. The dominant bacterial genera of the mature AGS were Zoogloea (relative abundance of 22.39%), Thauera (16.03%) and Clostridium_sensu_stricto (11.17%), and those of the stored granules were Acidovorax (26.79%), Macellibacteroides (12.83%) and Pseudoxanthomonas (5.69%), respectively. However, the dominant genera were Streptococcus (43.64%), Clostridium_sensu_stricto (12.3.6%) and Lactococcus (11.47%) in the recovered AGS. Methanogens were always the dominant archaeal species in mature AGS (93.01%), stored granules (99.99%) and the recovered AGS (94.84%). Facultative anaerobes and anaerobes proliferated and dominated in the stored granules, and their metabolic activities gradually led to granular structure destruction and property deterioration. However, the stored granules served as carriers for the microbes originated from the real septic tank wastewater during recovery. They proliferated rapidly and secreted a large number of extracellular polymeric substances which helped to recover the granular structure in 11 days.

Stimulation of Smithella-dominating propionate oxidation in a sediment enrichment by magnetite and carbon nanotubes

Recent studies have shown that application of conductive materials including magnetite and carbon nanotubes (CNTs) can promote the methanogenic decomposition of short-chain fatty acids and even more complex organic matter in anaerobic digesters and natural habitats. The linkage to microbial identity and the mechanisms, however, remain poorly understood. Here, we evaluate the effects of nanoscale magnetite (nanoFe₃ O₄ ) and multiwalled CNTs on the syntrophic oxidation of propionate in an enrichment obtained from lake sediment. The microbial populations were composed mainly of Smithella, Syntrophomonas, Methanosaeta, Methanosarcina and Methanoregula. In addition to acetate, butyrate was transiently accumulated indicating that propionate was oxidized by Smithella via the dismutation pathway and part of the leaked butyrate was oxidized by Syntrophomonas. Propionate oxidation and CH₄ production were significantly accelerated in the presence of nanoFe₃ O₄ and CNTs. While propionate oxidation was suppressed upon H₂ application and suspended completely upon formate application in the control, this suppressive effect was substantially compromised in the presence of nanoFe₃ O₄ and CNTs. The tests on hydrogenotrophic methanogenesis of a pure culture methanogen and of the enrichment culture without propionate showed negative effect by both materials. The positive effect of nanoFe₃ O₄ disappeared when it was insulated by surface-coating with silica. Observations made with fluorescence in situ hybridization and scanning electron microscope indicated the extensive formation of microbial cell-conductive material mixture aggregates. Our results suggest that direct interspecies electron transfer is likely activated by the conductive materials and operates in concert with H₂ /formate-dependent electron transfer for syntrophic propionate oxidation in the sediment enrichment.

Novel energy conservation strategies and behaviour of Pelotomaculum schinkii driving syntrophic propionate catabolism

Under methanogenic conditions, short-chain fatty acids are common byproducts from degradation of organic compounds and conversion of these acids is an important component of the global carbon cycle. Due to the thermodynamic difficulty of propionate degradation, this process requires syntrophic interaction between a bacterium and partner methanogen; however, the metabolic strategies and behaviour involved are not fully understood. In this study, the first genome analysis of obligately syntrophic propionate degraders (Pelotomaculum schinkii HH and P. propionicicum MGP) and comparison with other syntrophic propionate degrader genomes elucidated novel components of energy metabolism behind Pelotomaculum propionate oxidation. Combined with transcriptomic examination of P. schinkii behaviour in co-culture with Methanospirillum hungatei, we found that formate may be the preferred electron carrier for P. schinkii syntrophy. Propionate-derived menaquinol may be primarily re-oxidized to formate, and energy was conserved during formate generation through newly proposed proton-pumping formate extrusion. P. schinkii did not overexpress conventional energy metabolism associated with a model syntrophic propionate degrader Syntrophobacter fumaroxidans MPOB (i.e., CoA transferase, Fix and Rnf). We also found that P. schinkii and the partner methanogen may also interact through flagellar contact and amino acid and fructose exchange. These findings provide new understanding of syntrophic energy acquisition and interactions.