Knittel, Katrin

In the seabed, gaseous alkanes are oxidized by syntrophic microbial consortia that thereby reduce fluxes of these compounds into the water column. Because of the immense quantities of seabed alkane fluxes, these consortia are key catalysts of the global carbon cycle. Due to their obligate syntrophic lifestyle, the physiology of alkane-degrading archaea remains poorly understood. We have now cultivated a thermophilic, relatively fast-growing ethane oxidizer in partnership with a sulfate-reducing bacterium known to aid in methane oxidation and have retrieved the first complete genome of a short-chain alkane-degrading archaeon. This will greatly enhance the understanding of nonmethane alkane activation by noncanonical methyl-coenzyme M reductase enzymes and provide insights into additional metabolic steps and the mechanisms underlying syntrophic partnerships. Ultimately, this knowledge could lead to the biotechnological development of alkanogenic microorganisms to support the carbon neutrality of industrial processes.

ABSTRACTCold seeps and hydrothermal vents deliver large amounts of methane and other gaseous alkanes into marine surface sediments. Consortia of archaea and partner bacteria thrive on the oxidation of these alkanes and its coupling to sulfate reduction. The inherently slow growth of the involved organisms and the lack of pure cultures have impeded the understanding of the molecular mechanisms of archaeal alkane degradation. Here, using hydrothermal sediments of the Guaymas Basin (Gulf of California) and ethane as substrate we cultured microbial consortia of a novel anaerobic ethane oxidizerCandidatusEthanoperedens thermophilum (GoM-Arc1 clade) and its partner bacteriumCandidatusDesulfofervidus auxilii previously known from methane-oxidizing consortia. The sulfate reduction activity of the culture doubled within one week, indicating a much faster growth than in any other alkane-oxidizing archaea described before. The dominance of a single archaeal phylotype in this culture allowed retrieving a closed genome ofCa. Ethanoperedens, a sister genus of the recently reported ethane oxidizerCandidatusArgoarchaeum. The metagenome-assembled genome ofCa. Ethanoperedens encoded for a complete methanogenesis pathway including a methyl-coenzyme M reductase (MCR) that is highly divergent from those of methanogens and methanotrophs. Combined substrate and metabolite analysis showed ethane as sole growth substrate and production of ethyl-coenzyme M as activation product. Stable isotope probing showed that the enzymatic mechanisms of ethane oxidation inCa. Ethanoperedens is fully reversible, thus its enzymatic machinery has potential for the biotechnological development of microbial ethane production from carbon dioxide.IMPORTANCEIn the seabed gaseous alkanes are oxidized by syntrophic microbial consortia that thereby reduce fluxes of these compounds into the water column. Because of the immense quantities of seabed alkane fluxes, these consortia are key catalysts of the global carbon cycle. Due to their obligate syntrophic lifestyle, the physiology of alkane-degrading archaea remains poorly understood. We have now cultivated a thermophilic, relatively fast-growing ethane oxidizer in partnership with a sulfate-reducing bacterium known to aid in methane oxidation, and have retrieved the first complete genome of a short-chain alkane-degrading archaeon. This will greatly enhance the understanding of non-methane alkane activation by non-canonical methyl-coenzyme M reductase enzymes, and provide insights into additional metabolic steps and the mechanisms underlying syntrophic partnerships. Ultimately, this knowledge could lead to the biotechnological development of alkanogenic microorganisms to support the carbon neutrality of industrial processes.EtymologyEthanoperedens. ethano, (new Latin): pertaining to ethane;peredens(Latin): consuming, devouring;thermophilum. (Greek): heat-loving. The name implies an organism capable of ethane oxidation at elevated temperatures.LocalityEnriched from hydrothermally heated, hydrocarbon-rich marine sediment of the Guaymas Basin at 2000 m water depth, Gulf of California, Mexico.DiagnosisAnaerobic, ethane-oxidizing archaeon, mostly coccoid, about 0.7 μm in diameter, forms large irregular cluster in large dual-species consortia with the sulfate-reducing partner bacterium ‘CandidatusDesulfofervidus auxilii’.

Oil-rich sediments from the Gulf of Mexico were found to contain diverse alkane-degrading groups of archaea. The symbiotic, consortium-forming “ Candidatus Argoarchaeum” and “ Candidatus Syntrophoarchaeum” are likely responsible for the degradation of ethane and short-chain alkanes, with the help of sulfate-reducing bacteria. “ Ca. Methanoliparia” occurs as single cells associated with oil droplets. These archaea encode two phylogenetically different methyl-coenzyme M reductases that may allow this organism to thrive as a methanogen on a substrate of long-chain alkanes. Based on a library survey, we show that “ Ca. Methanoliparia ” is frequently detected in oil reservoirs and may be a key agent in the transformation of long-chain alkanes to methane. Our findings provide evidence for the important and diverse roles of archaea in alkane-rich marine habitats and support the notion of a significant functional versatility of the methyl coenzyme M reductase.

Summary The anaerobic oxidation of methane (AOM) is mediated by consortia of anaerobic methane‐oxidizing archaea (ANME) and their specific partner bacteria. In thermophilic AOM consortia enriched from Guaymas Basin, members of the ANME‐1 clade are associated with bacteria of the HotSeep‐1 cluster, which likely perform direct electron exchange via nanowires. The partner bacterium was enriched with hydrogen as sole electron donor and sulfate as electron acceptor. Based on phylogenetic, genomic and metabolic characteristics we propose to name this chemolithoautotrophic sulfate reducer Candidatus Desulfofervidus auxilii. Ca . D. auxilii grows on hydrogen at temperatures between 50°C and 70°C with an activity optimum at 60°C and doubling time of 4–6 days. Its genome draft encodes for canonical sulfate reduction, periplasmic and soluble hydrogenases and autotrophic carbon fixation via the reductive tricarboxylic acid cycle. The presence of genes for pili formation and cytochromes, and their similarity to genes of Geobacter spp., indicate a potential for syntrophic growth via direct interspecies electron transfer when the organism grows in consortia with ANME. This first ANME‐free enrichment of an AOM partner bacterium and its characterization opens the perspective for a deeper understanding of syntrophy in anaerobic methane oxidation.