Anaerobic N-2 production in Arctic sea ice

We quantified anaerobic N-2 production through bacterial denitrification and anaerobic NH4+ oxidation (anammox) in first-year ice from Young Sound (74degreesN) and in an ice floe off Northeast Greenland (79degreesN). Bacterial denitrification activity (100-300 nmol N L-1 sea ice d(-1)) occurred in the lower 0.5 m of the sea ice, which had high concentrations of NO3-, NH4+, and dissolved organic carbon (DOC). Despite sea-ice algal production in the lower sea-ice layers, heterotrophic activity resulted in a net O-2 consumption of 13 mumol O-2 L-1 sea ice d(-1) in the lower 0.5-m ice layers. Together with melting of deoxygenated ice crystals, this led to anoxic conditions in the brine system favoring conditions for anaerobic NO3- reduction. Numbers of anaerobic NO3--reducing bacteria in the same ice layers were high (1.1 x 10(5) cells ml(-1) sea ice, corresponding to 1.2 x 10(6) cells ml(-1) brine). Area-integrated denitrification rates were 10-45 mumol N m(-2) sea ice d(-1), which corresponds to 7-50% of the sediment activity in the area. Although the proportion of anammox to total N2 production was up to 19% in layers of the ice floe from the Greenland Sea, the integrated rate only accounted for 0-5% of total NO3- reduction at the investigated localities.