Anaerobic ammonium oxidation in the oxygen-deficient waters off northern Chile

Abstract

We investigated the pathways of N 2 production in the oxygen-deficient water column of the eastern tropical South Pacific off Iquique, Chile, at 20uS, through short anoxic incubations with 15 N-labelled nitrogen compounds. The location was characterized by steep chemical gradients, with oxygen decreasing to below detection at ,50-m depth, while nitrite reached 6 mmol L 21 and ammonium was less than 50 nmol L 21 . Ammonium was oxidized to N 2 with no lag phase during the incubations, and when only NH þ 4 was 15 N-labeled, 15 N appeared in the form of 14 N 15 N, whereas 15 N 15 N was not detected. Likewise, nitrite was reduced to N 2 at rates similar to the rates of ammonium oxidation, and when only NO 2 2 was 15 N-labeled, 15 N appeared mainly as 14 N 15 N, whereas 15 N 15 N appeared in only one incubation. These observations indicate that ammonium was oxidized and nitrite was reduced through the anammox reaction, whereas denitrification was generally not detected and, therefore, was a minor sink for nitrite. Anammox rates were highest, up to 0.7 nmol N 2 L 21 h 21 , just below the oxycline, whereas rates were undetectable, ,0.2 nmol N 2 L 21 h 21 , deeper in the oxygen-deficient zone. Instead of complete denitrification to N 2 , oxidation of organic matter during the incubations may have been coupled to reduction of nitrate to nitrite. This process was evident from strong increases in nitrite concentrations toward the end of the incubations. The results point to anammox as an active process in the major open-ocean oxygen-deficient zones, which are generally recognized as important sites of denitrification. Still, denitrification remains the simplest explanation for most of the nitrogen deficiency in these zones.

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

“…When integrated over the depth range of the OMZ, the highest N-loss due to anammox occurred over the Omani shelf at a rate of up to 4.5±0.4 mmol of N m À2 per day (Figure 4), comparable with those reported for the Namibian and Peruvian shelves (Kuypers et al, 2005;Hamersley et al, 2007). Together with previous findings of substantial anammox, but low to undetectable denitrification in other marine oxygen-deficient waters (Kuypers et al, 2005;Thamdrup et al, 2006;Hamersley et al, 2007;Jensen et al, 2008), the current study confirms the importance of anammox in global oceanic N-loss. At just about 500 km offshore, the depth-integrated N-loss was reduced by two orders of magnitude, and down to only potential rates in the central-NE basin (Figure 4).…”

“…Similar to the findings in the Peruvian and Chilean OMZ (Thamdrup et al, 2006;Hamersley et al, 2007), anammox rates determined from both 15 N-incubations were the highest in the upper OMZ over the shelf (B2. þ and 15 NO 2 À incubations, respectively).…”

“…Anammox and denitrification rates were calculated from the (Thamdrup and Dalsgaard, 2002;Thamdrup et al, 2006;Supplementary SI Text). In the case of DNRA-anammox coupling, total anammox rates were calculated in a similar manner as in nitrification-anammox coupling (Jensen et al, 2008;Supplementary SI Text).…”

“…The major oxygen minimum zones (OMZs) in the world, found in the Arabian Sea and the eastern tropical North and South Pacific, are responsible for 20-40% of global N-loss from the oceans, whereas 10-20% is thought to occur in the Arabian Sea (Naqvi, 1987;Codispoti et al, 2001;Gruber, 2004;Devol et al, 2006). Recent findings from incubation experiments with 15 Nlabeled compounds indicated substantial N-loss due to anammox (NH 4 þ þ NO 2 À -N 2 þ 2H 2 O) in the OMZs off Peru, Chile and Namibia, whereas denitrification was generally below detection (Kuypers et al, 2005;Thamdrup et al, 2006;Hamersley et al, 2007). These studies strongly question the traditional view that heterotrophic denitrification is the only significant driver of oceanic N-loss.…”

Enhanced hyporheic exchange flow around woody debris does not increase nitrate reduction in a sandy streambed

“…Anaerobic ammonium oxidation (anammox) is less widely reported in freshwater environments but has recently been shown to play a significant role in N 2 production in permeable sediments (Lansdown et al 2016;Zhou et al 2014). Anammox is performed by obligate anaerobic bacteria which use nitrite to oxidise ammonia to produce di-nitrogen gas (Van de Graaf et al 1996), the stoichiometry of which actually makes it a more efficient sink for reactive nitrogen over denitrification (Lansdown et al 2016;Thamdrup et al 2006). Finally, dissimilatory nitrate reduction to ammonia (DNRA) conserves nitrogen within the biosphere by reducing nitrate to ammonium.…”