The risk of oil spills in cold marine environments is expected to increase in response to trans-Arctic shipping and as Arctic oil reserves get exploited. Marine hydrocarbon-degrading microbes can reduce the impact of spilled hydrocarbons, but their degradation capabilities at low temperature are yet to be uncovered.

We combined DNA amplicon sequencing and chemometrics to investigate the effect of decreasing temperature (0–15 °C) on the succession and function of hydrocarbon-degrading bacteria in seawater. The bacterial community and degradation patterns were investigated at time points when a similar amount of hydrocarbons was mineralised at the different temperatures. This allowed decomposing the effect of temperature into a main component related to the reduced microbial activity at low temperature and a secondary effect. The reduced microbial activity at low temperature delayed the microbial community succession and degradation rates. The secondary effect of temperature was most pronounced at 0 °C, where (1) degradation of the least water-soluble n-alkanes (>C12) was suppressed in contrast to a relative stronger degradation of the most water-soluble n-alkanes (<C12) and polycyclic aromatic hydrocarbons; and (2) bacterial taxa which we identified as psychrosensitive were inhibited, whereas taxa identified as psychrophilic flourished.