Pressure and temperature effects on deep‐sea hydrocarbon‐degrading microbial communities in subarctic sediments

The Hatton–Rockall Basin (North‐East Atlantic) is an area with potential for deep‐sea (2,900 m) hydrocarbon exploration. Following the Deepwater Horizon oil spill, many investigations into the responses of sediment microbial communities to oil pollution have been undertaken. However, hydrostatic pressure is a parameter that is often omitted due to the technical difficulties associated with conducting experiments at high pressure (>10 MPa). In this study, sediments from 2,900 m in the Hatton–Rockall Basin, following a one‐week decompression period in a temperature‐controlled room at 5°C, were incubated in factorial combinations of 0.1 and 30 MPa, 5 and 20°C, and contamination with a hydrocarbon mixture or uncontaminated controls to evaluate the effect of these environmental variables on the bacterial community composition. Our results revealed varying effects of pressure, temperature, and oil contamination on the composition of the bacterial community within the sediment. Temperature was the strongest determinant of differences in the bacterial community structure between samples followed by pressure. Oil contamination did not exert a strong change in the sediment bacterial community structure when pressure and temperature conditions were held at in situ levels (30 MPa and 5°C). The γ‐proteobacteria Pseudomonas and Colwellia, and several Bacteroidetes dominated communities at 30 MPa. In contrast, hydrocarbon degraders such as Halomonas, Alcanivorax, and Marinobacter decreased in relative abundance at the same pressure. This study highlights the importance of considering hydrostatic pressure in ex situ investigations into hydrocarbon‐degrading deepwater microbial communities.