Abiotic and biotic transformation of petroleum hydrocarbons coupled with redox cycling of structural iron in clay mineral

Geothermal systems are hot spots for interaction among minerals, microorganisms, and hydrocarbons. Coupled hydrocarbon transformation and redox cycling of iron in minerals is important to ecosystem functions but remains poorly understood. This work studied abiotic transformation of petroleum hydrocarbons by reactive oxygen species produced upon oxygenation of a reduced clay mineral (nontronite NAu-2). Subsequently, the impact of such abiotic petroleum-clay interactions on coupled reduction of structural Fe(III) in clay mineral and petroleum transformation was studied under anaerobic condition. In the abiotic phase, hydrocarbons in a crude oil sample were oxidized by hydroxyl radicals (OH) generated upon oxygenation of reduced NAu-2, forming partially oxygenated compounds with hydroxyl, carbonyl, and carboxyl groups. In the subsequent bio-reduction experiments, these more bioavailable compounds significantly enhanced Fe(III) bio-reduction by a thermophilic microbial community enriched from a terrestrial hot spring. In particular, Sphingomonas and Phyllobacterium were enriched, both of which possessed genes for anaerobic hydrocarbon activation and Fe(III) reduction. Notably, the thermophilic community possessed more genes for breaking down C, H, and O-containing compounds. In contrast, when the same thermophilic community was exposed to the original (unoxidized) petroleum hydrocarbons, they possessed more genes for degrading C and H compounds. These findings enhance our understanding of the important role of minerals in regulating hydrocarbon transformation and in shaping subsurface microbial community.