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

Abstract

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 (C28 -C50 ) 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 ETFred 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.