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

Abstract

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.