Temperature response of denitrification and anaerobic ammonium oxidation rates and microbial community structure in Arctic fjord sediments

The temperature dependency of denitrification and anaerobic ammonium oxidation (anammox) rates from Arctic fjord sediments was investigated in a temperature gradient block incubator for temperatures ranging from -1 to 40°C. Community structure in intact sediments and slurry incubations was determined using Illumina SSU rRNA gene sequencing. The optimal temperature (Topt ) for denitrification was 25-27°C, whereas anammox rates were optimal at 12-17°C. Both denitrification and anammox exhibited temperature responses consistent with a psychrophilic community, but anammox bacteria may be more specialized for psychrophilic activity. Long-term (1-2 months) warming experiments indicated that temperature increases of 5-10°C above in situ had little effect on the microbial community structure or the temperature response of denitrification and anammox. Increases of 25°C shifted denitrification temperature responses to mesophilic with concurrent community shifts, and anammox activity was eliminated above 25°C. Additions of low molecular weight organic substrates (acetate and lactate) caused increases in denitrification rates, corroborating the hypothesis that the supply of organic substrates is a more dominant control of respiration rates than low temperature. These results suggest that climate-related changes in sinking particulate flux will likely alter rates of N removal more rapidly than warming.