Importance of Anammox to the Nitrogen Removal in Deep Cascadian Basin Sediments

Anammox is a recently discovered N2 producing pathway where NH4+ is oxidized by NO2- to form N2. Before anammox was discovered in a waste water treatment plant in the mid 1990-ties, denitrification was considered the only removal mechanism for bio-available N in the ocean. In marine sediments, the importance of anammox is suggested to increase with water depth. At the deepest site reported (700 m), anammox account for 67-79 percent of total N2 production. If these estimates are consistent for sediments at depths >700 m, anammox would be responsible for at least 2/3 of the total N2 production in deep sediments which suggest this pathway to be highly significant in global N cycling. We investigated the role of anammox in deep sea sediments of the Cascadian basin using microbiological and biogeochemical techniques. Anammox and denitrification rates were measured at depths 2700-3200 m at eight sites in August 2006. Sediments were collected with a Multicorer and 15N isotope pairing was used together with analyses of solute pore water distribution of inorganic N and O2 to estimate activity rates and distribution of anammox and denitrification in the sediment. Pore water profiles of NO2-, NO3-, NH4+ and O2 at all sites show a removal of ammonium in the suboxic zone that strongly suggest an anaerobic ammonium oxidation by nitrite or nitrate. Minimum ammonium removal rates were estimated to vary between 1.7-31 umol NH4+ m-2 day-1 among the 8 stations. The thickness of the ammonium removal zone suggesting anammox activity increased at sites further off the coast (from 1-11 cm) and correlated to the NO2-+NO3- distribution in the sediment. Preliminary results strongly suggest an extensive anammox activity in these sediments responsible for at least 20-60 percent of total benthic N2 production in the Cascadian basin.