The effect of organic carbon on fixed nitrogen loss in the eastern tropical South Pacific and Arabian Sea oxygen deficient zones

Abstract

The three major oxygen deficient zones (ODZs) of the world oceans (eastern tropical North and South Pacific (ETNP and ETSP, respectively), and Arabian Sea (AS) host the vast majority of pelagic fixed nitrogen (N) loss and up to half of total marine N loss. The input of organic matter is an important control on the absolute and relative importance of the two main pathways of N removal (denitrification and anammox). We investigated the response of N loss in the ETSP and AS ODZs to additions of organic matter in the form of glucose and naturally derived dissolved and particulate organic matter (DOM and POM, respectively). In the ETSP ODZ, the addition of glucose stimulated denitrification (1.6-fold increase after 5 d) but not anammox (14-fold decrease after 5 d). In the AS ODZ, only POM, not DOM, significantly increased rates of denitrification at the base of the oxycline (5.4-6.4-fold increase after 2 d), but not at the secondary nitrite maximum. These results suggest that denitrification was generally limited by organic matter supply at the time of this study in both the ETSP and AS ODZs, although the lability of the organic matter supplied was important. Interestingly, 15 N 2 produced in ETSP and AS incubations was not binomially distributed relative to the reactants after the influence of anammox was taken into account, suggesting an unknown production mechanism or pathway of N removal.

The influence of anoxia and substrate availability on N2O cycling by denitrification in the upper boundary of the oxygen minimum zone off northern Chile

“…These data suggest that denitrifiers in the offshore UBOMZ, stimulated by electron acceptor and donors, reduced the N 2 O to N 2 at high rates (58.71 nM d −1 ) but were not able to reduce all the N 2 O produced by denitrification, leading to an N 2 O surplus of ∼25.5% in the offshore UBOMZ. Similar results were obtained in experiments developed in 2005 in the OMZ off Peru where the addition of glucose also stimulated denitrification (1.6-fold; Chang et al 2014). Although both studies support the carbon limitation of denitrification in the eastern tropical South Pacific (at the base of the oxycline only) previously suggested by Ward et al (2008), we have to consider that the influence of nitrite and organic carbon separately was not measured in this study and requires further investigation to determine their effect on N 2 O cycling by autotrophic and heterotrophic communities in the UBOMZ.…”

“…Denitrification rates measured in this study at the UBOMZ are similar to rates previously measured (∼20-62 nM d −1 ) in the UBOMZ (Table 2) by Farías et al (2009) and in the OMZ core (200 m depth) by Castro- González and Farías (2004) and by Chang et al (2014). However, they are lower than those reported for other suboxic areas, such as the Arabian Sea (110 nM d −1 ; Naqvi et al 1993), the Indian Shelf (300 nM d −1 ; Naqvi et al 2000), and the Baltic Sea (140 nM d −1 ; Brettar and Rheinheimer 1992).…”

“…Additionally, denitrification in the OMZ depends on the following: N-oxide acceptors, such as NO − 3 and NO − 2 (Codispoti et al 1986); electron donor availability, such as dissolved organic carbon (DOC; Körner and Zumft 1989;Brettar and Rheinheimer 1992;Kalvelage et al 2013;Chang et al 2014); trace metal availability, such as Fe and Cu (; Granger and Ward 2003;Morel and Price 2003;Ward et al 2008); and O 2 levels (Kalvelage et al 2013), which determine whether the end product of denitrification is N 2 or N 2 O, given the variable sensitivity of NO − 3 , NO − 2 , and N 2 O reductases to O 2 (Zumft 1997). The aim of this study was to analyze the role of the natural denitrifying community in the production and/or consumption of N 2 O along the upper oxycline and to discern the effects of decreasing O 2 levels and electron donor/acceptor availability on this process.…”

Extreme N<sub>2</sub>O accumulation in the coastal oxygen minimum zone off Peru

“…During denitrification, the canonical reduction of nitrate to molecular nitrogen, N 2 O evolves as an intermediate product. Denitrification is stimulated by the supply of organic carbon or hydrogen sulfide (Chang et al, 2014;Dalsgaard et al, 2014;Galan et al, 2014), and active denitrification is restricted to suboxic to anoxic conditions (e.g. Firestone et al, 1980;Dalsgaard et al, 2014).…”

Intact polar lipids of Thaumarchaeota and anammox bacteria as indicators of N cycling in the eastern tropical North Pacific oxygen-deficient zone

“…Other studies have called attention to the relevance of organic matter fluxes as a control over these metabolic pathways and ultimately over the balance between the two mainly responsible for the N 2 removal from the oceans, i.e., anammox and denitrification Chang et al, 2014;Kalvelage et al, 2013;Koevef and Kähler, 2010;Ward, 2013;Ward et al, 2008Ward et al, , 2009). Specifically, variations in the C / N ratio content of the particulate organic matter (POM) entering the ODZ may play a prominent role in determining anammox and heterotrophic denitrification rates, with anammox being favored by nitrogen-rich OM .…”