Denitrification exceeds anammox as a nitrogen loss pathway in the Arabian Sea oxygen minimum zone

The oxygen minimum zones (OMZs) of the world oceans are sites of enhanced anaerobic nitrogen transformations, such that the largest OMZ, the Arabian Sea, is responsible for 30–50% of the oceanic water column fixed N loss. Measurements of excess N 2 and nitrate deficits have been interpreted to indic...

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Published in:	Deep-sea research. Part I, Oceanographic research papers Vol. 57; no. 3; pp. 384 - 393
Main Authors:	Bulow, Silvia E., Rich, Jeremy J., Naik, Hema S., Pratihary, Anil K., Ward, Bess B.
Format:	Journal Article
Language:	English
Published:	Kidlington Elsevier Ltd 01-03-2010 Elsevier Pergamon Press Inc
Subjects:	Ammonia Anammox Animal and plant ecology Animal, plant and microbial ecology Arabian Sea Biological and medical sciences Boundaries Carbon Deep sea Denitrification Earth, ocean, space Exact sciences and technology External geophysics Fundamental and applied biological sciences. Psychology Marine Nitrates Nitrites Nitrogen Oceanography Oceans Oxidation Oxygen Oxygen minimum zone Physical and chemical properties of sea water Physics of the oceans Sea water ecosystems Stations Synecology Tracers Transformations Indian Ocean Arabian Sea ISW, Arabian Sea Denitrification Anammox Arabian Sea Oxygen minimum zone Microbial activity ammonia(compound) Nitrogen 15 bacteria Radiolabelling oxidation nitrogen cycle biogeochemistry denitrification Ship observation prokaryotes marine environment Anaerobe Nitrogen fixation Observation data Hypoxia
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Summary:	The oxygen minimum zones (OMZs) of the world oceans are sites of enhanced anaerobic nitrogen transformations, such that the largest OMZ, the Arabian Sea, is responsible for 30–50% of the oceanic water column fixed N loss. Measurements of excess N 2 and nitrate deficits have been interpreted to indicate a large net loss of fixed nitrogen through N 2 production in waters with very low oxygen concentrations. Rates of N 2 production both by denitrification and anaerobic ammonium oxidation (anammox) were investigated by measuring 15N–N 2 production from 15 NH 4 + , 15 NO 2 − , and 46N 2O in incubations at 3–4 depths at each of the three stations in the central region of the Arabian Sea OMZ. In addition to conventional measurements of anammox and denitrification rates from incubations in 12 ml exetainers purged with He, we also investigated the effect of ammonium and organic carbon additions on N 2 production rate. Denitrification was detected at every depth at every station; the maximum rate (25.3±9.1 nmol l −1 d −1 from the 15NO 2 tracer and 21.2±5.6 nmol l −1 d −1 from the 46N 2O tracer) occurred at the depth of the secondary nitrite maximum in the core of the OMZ. The 46N 2O tracer was the most direct method to measure the denitrification rate. Rates of denitrification correlated strongly with water column nitrite concentration. Addition of organic carbon or ammonium did not consistently stimulate denitrification rates. Distribution of anammox was patchy; low rates of anammox production of 29N 2 in 15 NH 4 + incubations were detected at only four of eleven depths. The highest rate of anammox production (4.23±0.35 nmol l −1 d −1) occurred near the upper boundary of the OMZ at one station. Overall, denitrification dominated N 2 production at this time in the Arabian Sea OMZ.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	0967-0637 1879-0119
DOI:	10.1016/j.dsr.2009.10.014