Methanogenic paraffin degradation proceeds via alkane addition to fumarate by ‘Smithella’ spp. mediated by a syntrophic coupling with hydrogenotrophic methanogens

Abstract

Summary Anaerobic microbial biodegradation of recalcitrant, water‐insoluble substrates, such as paraffins, presents unique metabolic challenges. To elucidate this process, a methanogenic consortium capable of mineralizing long‐chain n ‐paraffins (C 28 ‐C 50 ) was enriched from San Diego Bay sediment. Analysis of 16S rRNA genes indicated the dominance of Syntrophobacterales (43%) and Methanomicrobiales (26%). Metagenomic sequencing allowed draft genome assembly of dominant uncultivated community members belonging to the bacterial genus Smithella and the archaeal genera Methanoculleus and Methanosaeta . Five contigs encoding homologs of the catalytic subunit of alkylsuccinate synthase ( assA ) were detected. Additionally, mRNA transcripts for these genes, including a homolog binned within the ‘ Smithella’ sp. SDB genome scaffold, were detected via RT‐PCR, implying that paraffins are activated via ‘fumarate addition’. Metabolic reconstruction and comparison with genome scaffolds of uncultivated n ‐alkane degrading ‘ Smithella’ spp. are consistent with the hypothesis that syntrophically growing ‘ Smithella’ spp. may achieve reverse electron transfer by coupling the reoxidation of ETF red to a membrane‐bound FeS oxidoreductase functioning as an ETF:menaquinone oxidoreductase. Subsequent electron transfer could proceed via a periplasmic formate dehydrogenase and/or hydrogenase, allowing energetic coupling to hydrogenotrophic methanogens such as Methanoculleus . Ultimately, these data provide fundamental insight into the energy conservation mechanisms that dictate interspecies interactions salient to methanogenic alkane mineralization.

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