Bacterial anomalies in seabed sediments associated with deep water hydrocarbon seepage

Deep sea hydrocarbon seep detection relies predominantly on geochemical analyses of seabed marine sediment cores to identify the presence of gas or oil. The presence of seeping hydrocarbons in these locations alters resident microbial community structure, leading to culture-based biodegradation assays as a complement to geochemical tools for seep detection. Biodiversity surveys of microbial communities can offer a similar proxy for seeping hydrocarbons, but this strategy has not been extensively investigated in deep water settings. In this study, 16S rRNA gene sequencing of bacterial communities was performed on sediment cores obtained in >2500 m water depth at 43 different locations in the NW Atlantic Ocean. Core samples from as deep as 10 metres below seafloor (mbsf) were assessed for gas composition, gas isotopes and liquid hydrocarbons. Over 650 bacterial 16S rRNA gene amplicon libraries were constructed from different sediment depths at these locations. Select sites showed strong evidence for the presence of thermogenic or biogenic hydrocarbons such that bacterial population analyses revealed significant differences between hydrocarbon seep and non-seep locations. Specific bacterial indicators were associated with different sediment depth intervals. Caldatribacteriota and Campilobacterota OTUs were observed in high relative sequence abundance in hydrocarbon seep sediments, particularly in the 20-50 cmbsf interval. Furthermore, these groups were differentially abundant between sites with thermogenic and biogenic hydrocarbons. The patterns revealed here suggest that microbial screening has the potential to play a key role in hydrocarbon seep detection and characterisation in remote deep-sea environments.