Abstract
Oil reservoirs contain microbial populations that are both autochthonously and allochthonously introduced by industrial development. These microbial populations are greatly influenced by external factors including, but not limited to, salinity and temperature. In this study, we used metagenomics to examine the microbial populations within five wells of the same hydrocarbon reservoir system in the Gulf of Mexico. These elevated salinity (149–181 ppt salinity, 4–5× salinity of seawater) reservoirs have limited taxonomic and functional microbial diversity dominated by methanogens, Halanaerobium and other Firmicutes lineages, and contained less abundant lineages such as Deltaproteobacteria. Metagenome assembled genomes (MAGs) were generated and analyzed from the various wells. Methanogen MAGs were closely related to Methanohalophilus euhalobius, a known methylotrophic methanogen from a high salinity oil environment. Based on metabolic reconstruction of genomes, the Halanaerobium perform glycine betaine fermentation, potentially produced by the methanogens. Industrial introduction of methanol to prevent methane hydrate formation to this environment is likely to be consumed by these methanogens. As such, this subsurface oil population may represent influences from industrial processes.
Highlights
Methanogens have been noted in numerous anoxic, elevated salinity environments including deep sea brines, hypersaline microbial mats and soda lakes (McGenity, 2010)
Considering the incongruent phylogeny of 16S rRNA genes, concatenated ribosomal proteins and McrA sequences, we examined the average nucleotide identity (ANI) and average amino acid identity (AAI) of the Metagenome assembled genomes (MAGs) versus close relatives (Table 3)
Since the Methanohalophilus genus is well known for the production of glycine betaine (GB), used as a compatible solute to allow salt tolerance (Guan et al, 2019), and GB can be fermented by Halanaerobium spp., producing trimethylamine, which in turn can feed Methanohalophilus spp. (Daly et al, 2016), we examined our data for genes that could indicate GB fermentation
Summary
Methanogens have been noted in numerous anoxic, elevated salinity environments including deep sea brines, hypersaline microbial mats and soda lakes (McGenity, 2010). A recent study of produced fluids from unconventional shale reservoirs, in which salinity increased during continued production, showed that methylotrophic methanogens and halotolerant bacteria are likely linked through the fermentation of glycine betaine (Daly et al, 2016). In this environment, the production of trimethylamine occurred during glycine betaine fermentation in a Halanaerobium species, which likely fueled the growth of Methanohalophilus, a halophilic and methylotrophic methanogen. The production of trimethylamine occurred during glycine betaine fermentation in a Halanaerobium species, which likely fueled the growth of Methanohalophilus, a halophilic and methylotrophic methanogen These interactions are likely aided by the salt tolerance of the organisms.
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