Nitrogen transformations as inferred from the activities of key enzymes in the Arabian Sea oxygen minimum zone |
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| Affiliation: | 1. State Key Laboratory of Marine Environmental Science, Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies, College of the Environment and Ecology, Xiamen University, Xiamen 361102, China;2. Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China;3. Fujian Provincial Academy of Environmental Science, Fuzhou 350003, China;1. The Centre for Environment, Fisheries and Aquaculture Science (Cefas), Pakefield Road, Lowestoft, Suffolk NR33 OHT, UK;2. Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB, Den Burg (Texel), The Netherlands;3. School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, UK;1. Department of Earth and Environmental Sciences, Tulane University, 6823 St. Charles Avenue, New Orleans, LA 70118, USA;2. College of Marine Sciences, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701, USA;3. National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, 266 Woods Hole Road, MA 02543, USA |
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| Abstract: | Vertical distributions of the potential activities of some key enzymes mediating nitrification and denitrification were investigated within the oxygen (O2) minimum zone of the Arabian Sea at a number of locations between latitudes 17°N and 21°N and longitudes 63°E and 68°E so as to get an insight into the predominant biochemical mode(s) of production and consumption of nitrous oxide (N2O). Results revealed that the dissimilatory nitrate (NO−3) reduction activity was generally very low or absent within the σθ range 26.6–26.8, which corresponds to the Persian Gulf Watermass (PGW). Depth profiles of nitrate reductase (NaR), nitrite reductase (NiR) and ammonia monooxygenase (AMO) activities were compared with those of O2, NO−3, nitrite (NO−2) and N2O, and it is concluded that nitrifier denitrification rather than heterotrophic denitrification is active within the core of PGW. The presence of multiple peaks of AMO activity coinciding with distinct maxima in the O2 profile and with a trend opposite to that of NaR activity indicates that the two processes, viz., classical and nitrifier denitrification, occur in discrete layers, probably determined by the variations in the ambient O2 concentrations at various depths surrounding the PGW core. Further, it appears that at the depths where nitrifier denitrification is active in the absence of heterotrophic denitrification, N2O builds up as its consumption may be inhibited by O2. Possible reasons for the occurrence of appreciable nitrate deficit within the core of PGW, where dissimilatory NO−3 reduction is lacking, are discussed. |
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