Denitrification and anammox activity in Arctic marine sediments

We measured rates of N2 production through anaerobic NH 4 + oxidation with NO 2 −(anammox) and denitrification in permanently cold (from −1.7°C to 4°C) sediments off the east and west coasts of Greenland. The investigated sites (36– to 100–m water depth) covered sediments in which carbon contents ranged from 0.3 to 3.2 dry weight %, O 2 uptake rates ranged from 3.4 to 8.3 mmol m −2 d −1 , O 2 penetration depths ranged from 0.25 to 1.70 cm, and bottom‐water NO 3 −concentrations ranged from 0.3 to 15.3 µmol L −1 . Total N 2 production was 34–344 µmol N m −2 d −1 , of which anammox accounted for 1–92 µmol N m −2 d −1 (1–35% of total) and denitrification for 33–265 µmol N m −2 d −1 . At one of the high‐Arctic sites, anammox activity had an optimum temperature ( T opt ) of 12°C, while that of bacterial denitrification was 24°C. According to the classical temperature scheme for metabolic growth, the anammox response was psychrophilic, while denitrification was psychrotrophic. Although T opt was considerably higher than in situ temperatures, rates of denitrification and anammox were still high at −1.3°C, reaching 17% and 40%, respectively, of those found at T opt . The activation energies, E a , of anammox and denitrification were 51.0 and 60.6 kJ mol −1 , respectively, and the corresponding Q 10 values were 2.2 and 2.4. Rates of anammox were linearly correlated with bottom‐water NO 3 −concentrations ( r 2 = 0.96, p < 0.0001, n = 11) at the investigated sites. We suggest that the slow‐growing anammox bacteria are favored in sediments with high and stable NO 3 −conditions. This may be a general pattern in deeper waters at other latitudes as well.