SRA

Hydrocarbon reservoirs are major sites of methane production and carbon turnover, processes with significant impacts on energy resources and global biogeochemical cycles. We applied a cultivation-independent genomic approach to define microbial community membership and predict roles for specific organisms in biogeochemical transformations in North Slope, Alaska oil fields. Samples were collected from six locations between 1,128 m (24-27°C) and 2,743 m (80 - 83°C) below the surface. Sample complexity decreased with increasing temperature. Bacteria from candidate phyla (CP) were highly represented in the lower temperature zones and to some extent the intermediate temperature zones. Most microorganisms in the intermediate and higher temperature samples were related to previously studied methanogenic and non-methanogenic archaea and thermophilic bacteria. We reconstructed a near-complete genome for an organism sampled from the lower temperature reservoir SB1 for an OD1 population that is present at an abundance level close to that of the dominant Methanosaeta harundinacea population. Consistent with prior findings for members of this lineage, the OD1 genome is small and metabolic predictions support an obligate anaerobic, fermentation-based lifestyle. At moderate abundance in samples were members of other CP bacteria, including TA06, OP11, OP9, Marinimicrobia (SAR406) and OP1. For OP1 we reconstructed the first near-complete genome. Based on the inventory of glycosyl hydrolase genes and the absence of respiratory capacity, we infer that the predominant roles for the OP1 bacterium are fermentation of mannan, cellulose or cell wall carbon polymers and hydrogen cycling. Overall, our results elucidate roles of uncultivated organisms in hydrocarbon reservoir biological processes.