Anaerobic oxidation of ammonium (anammox) is an important process in the marine nitrogen cycle. It has been estimated to contribute up to 50% of nitrogen loss from the ocean, and is especially prevalent within oxygen minimum zones (OMZs). Here we studied the presence and distribution of anammox in the extended OMZ of the Eastern Tropical North Pacific (ETNP) using ladderane fatty acids, specific biomarkers for anammox bacteria. The validity of ladderane fatty acids as proxies for anammox bacteria was demonstrated by their excellent correspondence with anammox 16S rRNA functional gene abundances and their expression and intact polar ladderane lipid concentrations in suspended particulate matter (SPM) from the Arabian Sea. In the ETNP, SPM was collected from various water depths at four stations along a northwest to southeast cruise transect and ladderane fatty acids were analyzed at each station. In all SPM samples where ladderane lipids were detected, C18 ladderane fatty acids were on average 5 fold more abundant than C20 ladderane fatty acids. Maximum concentrations in ladderane fatty acids (1.1 - 2.3 ng/l) were recorded at 400-600 m water depth, often corresponding to the depth of the secondary nitrite maximum. In one of the four stations, a second maximum in the ladderane fatty acid concentration was noted at a shallower depth (i.e. at 85 m), coinciding with higher nitrite concentrations at this water depth. The availability of nitrite probably limits anammox activity in the ETNP. Anammox lipids were abundant within the OMZ at all stations and concentrations were comparable to those in other OMZs, suggesting that anammox may be responsible for a significant loss of nitrogen in the OMZ of the ETNP.
