Nitrogen transformations as inferred from the activities of key enzymes in the Arabian Sea oxygen minimum zone

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 σ_θ 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.