Metagenomic investigation of a low diversity, high salinity offshore oil reservoir

Oil reservoirs can represent extreme environments for microbial life due to low water availability, high salinity, high pressure, and naturally occurring radionuclides. This study investigated the microbiome of saline formation water samples from a Gulf of Mexico oil reservoir. Metagenomic analysis and associated anaerobic enrichment cultures enabled investigations into metabolic potential for microbial persistence in this envi-ronment despite the high salinity (4.5%) and low nutrient availability. Preliminary 16S rRNA gene amplicon sequencing revealed a very low microbial diversity in this oil reservoir. Accordingly, deep shotgun sequencing resulted in 9 metagenome-assembled genomes (MAGs), including members of novel lineages QPJE01 (genus level) within the Halanaerobiaceae, and BM520 (family level) within the Bacteroidales. Genomes of the 9 organisms included respiratory pathways such as nitrate reduction (in Arhodomonas, Flexistipes, Geotoga, and Marinobacter MAGs) and thiosulfate reduction (in Arhodomonas, Flexistipes, and Geotoga MAGs). Genomic evidence for adaptation to high salinity, with-standing radioactivity, and metal acquisition was also observed in different MAGs, possibly explaining their occurrence in this extreme habitat. Other metabolic features included the potential for quorum sensing and biofilm formation, and genes for forming endospores in some cases. Understanding the microbiomes of deep biosphere environments sheds light on the capabilities of uncultivated subsurface microorganisms and their potential roles in subsurface settings, including during oil recovery operations.