Anaerobic ammonium oxidation in deep-sea sediments off the Washington margin

The significance of anaerobic ammonium oxidation (anammox) for nitrogen removal in deep continental margin sediments was studied with 15N amendments to suboxic sediments collected from 2800-3100-m water depth at eight sites in the Cascadia Basin (eastern North Pacific Ocean). Consistent with earlier data from deep continental margin sediments, pore-water distributions of inorganic N indicated NH⁴₄ removal from suboxic zone sediments, likely due to reaction with nitrate. Anammox rates estimated from suboxic sediment incubations with ¹⁵N-labled substrates ranged between 0.065 and 1.7 nmol N mL^-1 h^-1 (wet sediment), which suggested that anammox was responsible for the observed NH⁺₄ removal. Anammox and denitrification rates derived from NH⁺₄ and NO^-₄ pore-water profiles were 32-82 μmol N m ^-2 d^-1 and 50-110 μmol N m^-2 d^-1, respectively. The average contribution of anammox to total N₂ production was 40% (¹⁵N-amended sediment incubations) to 42% (flux from pore-water inorganic N), which does not support earlier reports that suggested that the relative importance of anammox increased with water depth and thereby should dominate over denitrification at depths greater than 1000 m.