First insights into bacterial communities in pre-salt oil reveal a far-from-sterile environment

Abstract

The ultra-deep pre-salt oil reservoirs have attracted attention owing to their high oil quality and unique characteristics, such as high pressure and high carbon dioxide content. However, its microbiome has been poorly explored. We conducted 16S rRNA sequencing of two pre-salt oils, three production water, and three rock samples from Brazilian oil reservoirs. Both pre-salt oil samples (respectively, Oil1 from a 6,000-meter-deep and Oil2 from 4,000-meter deep reservoirs) demonstrated higher operational taxonomic unit (OTU) richness when compared to rock and production water. Pre-salt samples showed 60.8% of exclusive genera, among which 122 with no report in the literature concerning oil degradation. Only 11.67% of the pre-salt oil genera was shared with rock and 5.44% with production water groups. Ralstonia and Vogesella dominated the pre-salt oil samples (Oil1 and Oil2) with 39.3% and 37.6%, respectively. To detect taxonomic signatures of different environments and whether the pre-salt oil resembles oil-contaminated biodiversity, we conducted a broad comparison with contaminated, non-contaminated, and oil reservoir samples worldwide. The differences in microbial communities between samples concerning community membership were measured with the Bray-Curtis dissimilarity matrix. Similarity analysis (ANOSIM) showed that microbiomes grouped according to the BioProject, presenting a distinct differentiation (R = 0.95, P ≤ 0.001). The candidate phylum FCPU426 is mainly enriched in pre-salt samples. The functional inference revealed a greater abundance of microorganisms associated with aromatic compounds degradations in pre-salt samples, possibly related to high oil quality in these reservoirs. In conclusion, pre-salt oil exhibited higher biodiversity than other oil field samples. It revealed an unexpected indigenous microbial richness on deep reservoirs not subjected to enhanced oil recovery processes, indicating a microbiota adapted to this inhospitable environment, with a high occurrence of uncultivated or poorly classified taxa, representing a gene pool whose importance for biogeochemical cycles should be further investigated.