Abstract

Aerobic NH 4 + 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 (~20°S) during 2000, 2003, and 2004. This process was examined by comparing NH 4 + 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 NH 4 + oxidation in dark carbon fixation and NO 2 - rates of change were also explored. Thirteen samples were retrieved from the oxycline (252 to =5 µM O 2; 15 to ~65 m depth) and three from the oxygen minimum core (=5 µM O 2; 100-200 m depth). Aerobic NH 4 + oxidation rates were mainly detected in the upper part (15-30 m depth) of the oxycline, with rates ranging from 0.16 to 0.79 µM d -1, but not towards the oxycline base (40-65 m depth). In the oxygen minimum core, aerobic NH 4 + oxidation was in the upper range and higher than in the upper part of the oxycline (0.70 and 1.0 µM d -1). Carbon fixation rates through aerobic NH 4 + oxidation ranged from 0.18 to 0.43 µg C L -1 d -1 and contributed between 33% and 57% of the total dark carbon fixation, mainly towards the oxycline base and, in a single experiment, in the upper part of the oxycline. NO 2 - consumption was high (up to 10 µM d -1) towards the oxycline base and OMZ core, but was significantly reduced in experiments amended with allylthiourea, indicating that aerobic NH 4 + oxidation could contribute between 8% and 76% of NO 2 - production, which in turn could be available for denitrifiers. Overall, these results support the important role of aerobic NH 4 + oxidizers in the nitrogen and carbon cycling in the OMZ and at its upper boundary. © 2008.