4.2 Article

Ammonium cycling under a strong oxygen gradient associated with the Oxygen Minimum Zone off northern Chile (similar to 23 degrees S)

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MARINE ECOLOGY PROGRESS SERIES
卷 288, 期 -, 页码 35-43

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INTER-RESEARCH
DOI: 10.3354/meps288035

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ammonium regeneration; nitrification potential; bacterioplankton; heterotrophic nanoflagellates; Oxygen Minimum Zone; upwelling area; northern Chile

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Ammonium (NH4+) cycling rates under different dissolved oxygen (DO) conditions in the water column, at a coastal upwelling area off northern Chile (similar to 23 degrees S), were estimated. Net NH4+ regeneration (eukaryotes and prokaryotes) and NH4+ oxidation (nitrifying bacteria) rates were examined by means of selective inhibitor assays (cycloheximide and allylthiourea) under dark conditions. Whole water samples for incubations were taken in the oxycline, low-DO zone (30 m; 69 mu m DO), and in the upper boundary of the Oxygen Minimum Zone (OMZ), suboxic zone (50 m; < 5 mu m DO). Net NH4+ regeneration and consumption were also determined in size-fractionated (< 200 mu m) experiments with water samples obtained from the suboxic zone (50 m) and the base of the mixed layer, oxic zone (15 m; 104 mu M DO). Results indicate that, in the oxycline, prokaryotes were responsible for most of the NH4+ cycling, with regeneration and oxidation rates of similar to 1.3 and 0.56 mu M d(-1), respectively. This, in turn, favoured NH4+ and NO3- accumulation in this layer, characterised by strong physical-chemical gradients (temperature, salinity and DO), and accompanied by lower abundances of cyanobacteria (0.09 X 10(5) cells ml(-1)) and heterotrophic nanoflagellates (HNF; 15.6 cells ml(-1)). In the oxic and suboxic layers, eukaryotes appear to be the main contributors to net NH4+ regeneration (4.6 to 17.7 mu M d(-1)), supporting a high net NH4+ dark prokaryote consumption, including high potential NH4+ oxidation (0.95 to 1.34 mu M d(-1)) in the suboxic zone. The abundances of bacteria, cyanobacteria and HNF were higher in these layers (> 1.2 x 10(6), > 0.9 X 10(5) and > 37 cells ml(-1), respectively), indicating a potential large impact on NH4+ cycling.

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