Anaerobic hydrocarbon biodegradation by alkylotrophic methanogens in deep oil reservoirs

Abstract

Methanogenic biodegradation of crude oil is a common process in subsurface biodegraded oil reservoirs. This process was previously assigned to syntrophy of hydrocarbon-degrading bacteria, and methanogenic archaea. Recent studies showed that archaea of the Candidatus Methanoliparum named as alkylotrophic methanogens coupled hydrocarbon degradation and methane production in a single archaeon. To assess geochemical role of Ca. Methanoliparum, we analyzed the chemical, and microbial composition, and metabolites of 209 samples from 15 subsurface oil reservoirs across China. Gas chromatography–mass spectrometry analysis revealed that 92% of the tested samples were substantially degraded. Molecular analysis showed that 85% of the tested samples contained Ca. Methanoliparum, and 52% of the tested samples harbored multiple alkyl-coenzyme M derivatives, the intercellular metabolites of alkylotrophic archaea. According to metagenomic, and metatranscriptomic analysis, Ca. Methanoliparum dominates hydrocarbon degradation in biodegraded samples from the Changqing (CQ), Jiangsu (JS), and Shengli (SL) oilfields, and it is persistently present as shown in a 15-year long sampling effort at the SL oilfield. Together, these findings demonstrate that Ca. Methanoliparum is a widely distributed oil degrader in reservoirs of China, suggesting that alkylotrophic methanogenesis by archaea plays a key role in the alteration of oil reservoirs, thereby expanding our understanding of biogeochemical process in deep biosphere.