Multiple metabolisms constrain the anaerobic nitrite budget in the Eastern Tropical South Pacific

Abstract

The Eastern Tropical South Pacific is one of the three major oxygen deficient zones (ODZs) in the global ocean and is responsible for approximately one third of marine water column nitrogen loss. It is the best studied of the ODZs and, like the others, features a broad nitrite maximum across the low oxygen layer. How the microbial processes that produce and consume nitrite in anoxic waters interact to sustain this feature is unknown. Here we used 15 N-tracer experiments to disentangle five of the biologically mediated processes that control the nitrite pool, including a high-resolution profile of nitrogen loss rates. Nitrate reduction to nitrite likely depended on organic matter fluxes, but the organic matter did not drive detectable rates of denitrification to N 2 . However, multiple lines of evidence show that denitrification is important in shaping the biogeochemistry of this ODZ. Significant rates of anaerobic nitrite oxidation at the ODZ boundaries were also measured. Iodate was a potential oxidant that could support part of this nitrite consumption pathway. We additionally observed N 2 production from labeled cyanate and postulate that anammox bacteria have the ability to harness cyanate as another form of reduced nitrogen rather than relying solely on ammonification of complex organic matter. The balance of the five anaerobic rates measured-anammox, denitrification, nitrate reduction, nitrite oxidation, and dissimilatory nitrite reduction to ammonium-is sufficient to reproduce broadly the observed nitrite and nitrate profiles in a simple one-dimensional model but requires an additional source of reduced nitrogen to the deeper ODZ to avoid ammonium overconsumption.

Cyanate distribution and uptake above and within the Eastern Tropical South Pacific oxygen deficient zone

“…OCN − , urea, and uptake were also measured below the euphotic zone in the ODZ, and uptake rates of these compounds increased with depth and exceeded rates of anammox attributed to those same substrates at the SNM (Fig. ; Babbin et al ). Uptake rates of all three N compounds were of similar magnitude within the ODZ (0.1–0.5 nmol N L −1 h −1 ; Fig.…”

“…Error bars are ± 1 SD (concentration, 29 N 2 production, n = 3) and ± 1 average deviation (uptake, n = 2). 29 N 2 production rates and , , and oxygen profiles are reproduced from Babbin et al ().…”

“…While these indirect pathways of OCN − and urea utilization by anammox organisms are plausible, abiotic reactions are slow and a two‐step process is not likely to be linear over short time scales. The linear production of 29 N 2 over 12 h following the addition of OC 15 N − suggests that OCN − is used directly by anammox bacteria in a “cyanammox” pathway (Babbin et al ). In contrast to OCN − , there was a 36 h lag in 29 N 2 production from 15 N‐urea (Babbin et al ), suggesting that this pathway involves a slow conversion of urea to OCN − or followed by OCN − ‐supported or traditional anammox.…”

“…We compared OCN − concentrations with rates of OCN − uptake and uptake of urea, , , and nitrate ( ) in the shallow oxic layer. We also compared the distributions of these compounds with respect to their uptake rates in the ODZ and to previously published rates of 29 N 2 production (Babbin et al ) from OCN − , urea, and at this site.…”

Isotopic Constraints on Nitrogen Transformation Rates in the Deep Sedimentary Marine Biosphere

“…This reoxidation dynamic has been invoked in other zones where NO 2 − accumulation occurs, particularly in ODZs of the global ocean (Beman et al, ; Buchwald et al, ; Füssel et al, ; Martin & Casciotti, ; Peters et al, ). Indeed, reoxidation of nitrite under anoxic conditions, while still not well understood, may underpin a greater part of the natural nitrogen cycle than previously recognized (Babbin et al, ; Granger & Wankel, ; Sun et al, ). For example, recent studies have suggested that the larger than expected Δδ 15 N values may actually be representative of isotopic equilibration between nitrite and nitrate, occurring in connection with highly reversible enzymatic activity (Figure b).…”