Aerobic ammonium oxidation in the oxycline and oxygen minimum zone of the eastern tropical South Pacific off northern Chile (∼20°S)

Aerobic NH4+ oxidation rates were measured along the strong oxygen gradient associated with the oxygen minimum zone (OMZ) of the eastern tropical South Pacific off northern Chile (similar to 20 degrees S) during 2000, 2003, and 2004. This process was examined by comparing NH4+ rates of change during dark incubations, with and without the addition of allylthiourea, a classical inhibitor of the ammonia monooxygenase enzyme of ammonium-oxidizing bacteria. The contribution of aerobic NH4+ oxidation in dark carbon fixation and NO2- rates of change were also explored. Thirteen samples were retrieved from the oxycline (252 to <= 5 mu M O-2; 15 to similar to 65 m depth) and three from the oxygen minimum core (<= 5 mu M O-2; 100-200 m depth). Aerobic NH4+ oxidation rates were mainly detected in the upper part (15-30 m depth) of the oxydine, with rates ranging from 0.16 to 0.79 mu M d(-1), but not towards the oxycline base (40-65 m depth). In the oxygen minimum core, aerobic NH4+ oxidation was in the upper range and higher than in the upper part of the oxycline (0.70 and 1.0 mu M d(-1)). Carbon fixation rates through aerobic NH4+ oxidation ranged from 0.18 to 0.43 mu g C L-1 d(-1) and contributed between 33% and 57% of the total dark carbon fixation, mainly towards the oxydine base and, in a single experiment, in the upper part of the oxycline. NO consumption was high (up to 10 mu M d(-1)) towards the oxycline base and OMZ core, but was significantly reduced in experiments amended with allylthiourea, indicating that aerobic NH4+ oxidation could contribute between 8% and 76% of NO2- production, which in turn could be available for denitrifiers. Overall, these results support the important role of aerobic NH4+ oxidizers in the nitrogen and carbon cycling in the OMZ and at its upper boundary. (C) 2008 Published by Elsevier Ltd.