Anaerobic Degradation of Paraffins by Thermophilic Actinobacteria under Methanogenic Conditions

Microbial anaerobic alkane degradation
is a key process in subsurface
oil reservoirs and anoxic environments contaminated with petroleum,
with a major impact on global carbon cycling. However, the thermophiles
capable of water-insoluble paraffins (>C17) degradation
under methanogenic conditions has remained understudied. Here, we
established thermophilic (55 °C) n-paraffins-degrading
(C21–C30) cultures from an oil reservoir.
After over 900 days of incubation, the even-numbered n-paraffins were biodegraded to methane. The bacterial communities
are dominated by a novel class-level lineage of actinobacteria, ‘Candidatus Syntraliphaticia’. These ‘Ca. Syntraliphaticia’-like metagenome-assembled genomes
(MAGs) encode a complete alkylsuccinate synthases (ASS) gene operon,
as well as hydrogenases and formate dehydrogenase, and several enzymes
potentially involved in alkyl-CoA oxidation and the Wood-Ljungdahl
pathway. Metatranscriptomic analysis suggests that n-paraffins are activated via fumarate addition reaction, and oxidized
into carbon dioxide, hydrogen/formate and acetate by ‘Ca. Syntraliphaticia’, that could be further converted
to methane by the abundant hydrogenotrophic and acetoclastic methanogens.
We also found a divergent methyl-CoM reductase-like complex (MCR)
and a canonical MCR in two MAGs representing ‘Ca. Methanosuratus’ (within candidate phylum Verstraetearchaeota),
indicating the capability of methane and short-chain alkane metabolism
in the oil reservoir. Ultimately, this result offers new insights
into the degradability and the mechanisms of n-paraffins
under methanogenic conditions at high temperatures.