Intensive nitrogen loss over the Omani Shelf due to anammox coupled with dissimilatory nitrite reduction to ammonium

Abstract

A combination of stable isotopes ( 15 N) and molecular ecological approaches was used to investigate the vertical distribution and mechanisms of biological N 2 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 (o48 h) anoxic incubations with 15 N-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-15 N-labeled N 2 from 15 NO 2 À , 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.

Nitrite oxidation in the Namibian oxygen minimum zone

“…at St 225) seemed to occur at very low O 2 levels (o1 mM) in the lower OMZ and the BBL, respectively ( Figure 2). As these rates were in fact determined in helium-purged water samples, in which oxygen had been reduced down to pB0.5 mM as verified in the current and previous studies with microsensors (Dalsgaard et al, 2003;Jensen et al, 2008Jensen et al, , 2011, our results showed that nitrite oxidation occurred at microaerobic or apparently anoxic conditions. The effect of O 2 on nitrite oxidation was further examined in 15 N-labeling experiments with varying O 2 concentrations (0-11 mM) in the Namibian OMZ waters.…”

“…Nitrite oxidation rates were determined as the net production of 15 , respectively. Except for the latter two, all samples were purged with helium for 15 min before incubations, to reduce O 2 down to pB0.5 mM (Dalsgaard et al, 2003;Jensen et al, 2008Jensen et al, , 2011. To examine the effect of oxygen on NO 2 À oxidation, additional experiments were conducted for two samples (St 206-100 m and St 252-105 m) with O 2 adjusted to four different controlled levels (B1-11 mM).…”

Macrofauna regulate heterotrophic bacterial carbon and nitrogen incorporation in low-oxygen sediments

“…In this region, oxygen limitation and the absence of mega-and macrofauna are important factors promoting OM preservation (for example, Woulds et al, 2007;Jeffreys et al, 2009 and references therein). In contrast, phyto C and phyto N retention in the OMZ boundary (800-1100 m) was variable, with high phyto C and phyto N concentrations in bulk OM and HAA pools at T1 800 m, compared with the relatively low values at T2 800 m and T2 1100 m. This increased variability occured concomitantly with the presence of macrofauna (Table 1) and oxygen concentrations high enough to support nitrification but still sufficiently low to sustain denitrification and anaerobic ammoniaoxidising pathways (Devol, 1978;Jensen et al, 2011). D/L-Ala ratios demonstrate that in the OMZ core, bacterial assimilation accounted for 30-90% of the phyto C and phyto N labelling of the HAAs.…”

Microbial oceanography of anoxic oxygen minimum zones

“…This observation, when coupled with known oceanographic nitrite distributions, suggests that essentially oxygen-free waters occupy a volume of at least 2.4 × 10 14 km 3 in the ETSP (3). Essentially oxygen-free waters have also been found using STOX sensors in the nitrite-rich regions of the Arabian Sea (18). It is clear, therefore, that vast volumes of nitrite-rich essentially anoxic waters (AMZs) occur in the ETSP and the Arabian Sea.…”