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A combination of stable isotopes (15N) and molecular ecological approaches was used to investigate the vertical distribution and mechanisms of biological N2 production along a transect from the Omani coast to the central-northeastern (NE) Arabian Sea. The Arabian Sea harbors the thickest oxygen minimum zone (OMZ) in the world's oceans, and is considered to be a major site of oceanic nitrogen (N) loss. Short (<48 h) anoxic incubations with 15N-labeled substrates and functional gene expression analyses showed that the anammox process was highly active, whereas denitrification was hardly detectable in the OMZ over the Omani shelf at least at the time of our sampling. Anammox was coupled with dissimilatory nitrite reduction to ammonium (DNRA), resulting in the production of double-15N-labeled N2 from 15NO2−, a signal often taken as the lone evidence for denitrification in the past. Although the central-NE Arabian Sea has conventionally been regarded as the primary N-loss region, low potential N-loss rates at sporadic depths were detected at best. N-loss activities in this region likely experience high spatiotemporal variabilities as linked to the availability of organic matter. Our finding of greater N-loss associated with the more productive Omani upwelling region is consistent with results from other major OMZs. The close reliance of anammox on DNRA also highlights the need to take into account the effects of coupling N-transformations on oceanic N-loss and subsequent N-balance estimates. [ABSTRACT FROM AUTHOR]

We performed a suite of 15N incubations (15NO2−, 15NO3− and 15NH4+) with and without the organic-nitrogen (N) compound allylthiourea (ATU), in the suboxic waters of the Arabian Sea. Production of 29N2 in control (-ATU) incubations with either 15NH4++14NO2−, or their analogues, 15NO2−+14NH4+, though small, confirmed the presence of anammox. In contrast, when we added ATU, along with 15NO2− and 14NH4+, there was a much greater production of 29N2, with 92% of the 15N-label being recovered as 29N2 on average. Such stimulated production of 29N2 could not be due to anammox, as the addition of ATU, along with 15NH4++14NO2−, only produced 29N2 equivalent to that in the controls. The ratios of 29N2 to 30N2 produced also precluded stimulation of denitrification. We present this as evidence for a hitherto uncharacterised metabolism potentially capable of oxidising organic-N (e.g. NH2 groups) directly to N2 gas at the expense of NO2−. [ABSTRACT FROM AUTHOR]