Nitrogen cycling in the secondary nitrite maximum of the eastern tropical North Pacific off Costa Rica

Abstract

Nitrite is a central intermediate in the marine nitrogen cycle and represents a critical juncture where nitrogen can be reduced to the less bioavailable N 2 gas or oxidized to nitrate and retained in a more bioavailable form. We present an analysis of rates of microbial nitrogen transformations in the oxygen deficient zone (ODZ) within the eastern tropical North Pacific Ocean (ETNP). We determined rates using a novel one-dimensional model using the distribution of nitrite and nitrate concentrations, along with their natural abundance nitrogen (N) and oxygen (O) isotope profiles. We predict rate profiles for nitrate reduction, nitrite reduction, and nitrite oxidation throughout the ODZ, as well as the contributions of anammox to nitrite reduction and nitrite oxidation. Nitrate reduction occurs at a maximum rate of 25 nM d À1 at the top of the ODZ, at the same depth as the maximum rate of nitrite reduction, 15 nM d À1 . Nitrite oxidation occurs at maximum rates of 10 nM d À1 above the secondary nitrite maximum, but also in the secondary nitrite maximum, within the ODZ. Anammox contributes to nitrite oxidation within the ODZ but cannot account for all of it. Nitrite oxidation within the ODZ that is not through anammox is also supported by microbial gene abundance profiles. Our results suggest the presence of nitrite oxidation within the ETNP ODZ, with implications for the distribution and physiology of marine nitrite-oxidizing bacteria, and for total nitrogen loss in the largest marine ODZ.

Oxidation kinetics and inverse isotope effect of marine nitrite-oxidizing isolates

“…In the Arabian Sea OMZ, Nitrospina spp., and a novel lineage of NxrA clustering between anammox and Nitrospina sequences were identified (Lüke et al 2016). Nitrospina was further found near Costa Rica in the eastern tropical North Pacific (Buchwald et al 2015), and Nitrospina and Nitrococcus make up ~9% of the microbial community in the Namibian OMZ (Füssel et al 2012). There are numerous studies addressing the ubiquitous occurrence of the genus Nitrospina, which make our results to some extent representative for the marine NOB community in at least some sampling sites.…”

“…comm.). The use of such a lower isotope effect may be appropriate because other OMZ regions (like the eastern tropical North Pacific off Costa Rica OMZ, the Namibian OMZ, the Black Sea OMZ, and the Baltic Sea; Labrenz et al 2007, Fuchsman et al 2011, Füssel et al 2012, Spieck et al 2014, Buchwald et al 2015) host predominantly Nitrospina, for which we find a low isotope effect in this study. This shows that a more accurate assessment of isotope effects and the kinetic parameters that determine these effects is urgently needed to better constrain biogeochemical models.…”

“…Generally, a wide range of isotope effects has been applied in different models: In the OMZ of the eastern tropical North Pacific off Costa Rica, an isotope effect of 30 ‰ was calculated for nitrite oxidation (Buchwald et al 2015). In another model study focusing on the eastern tropical South Pacific, measurements of δ 15 N-NO 2 − could not be reproduced using currently published isotope effects -an isotope effect of 32 ‰ (close to that determined experimentally for anammox bacteria; Brunner et al 2013) would have been required to reproduce the data (Casciotti et al 2013).…”

Enzyme‐level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean

“…In natural abundance studies, the same conclusion of NO 2 À oxidation is reached through the observation of a decoupling of NO N in suboxic waters; these observations have been attributed to forcings from the combined isotope effects of NO 3 À reduction, NO 2 À reduction and oxidation, and anammox Casciotti, 2009;Casciotti et al, 2013;Gaye et al, 2013;Buchwald et al, 2015;Martin and Casciotti, 2016]. Again, the perplexing aspect of interpretations suggesting NO 2 À oxidation in ODZ waters is the lack of appropriate oxidized species in the water column to serve as electron acceptors [Buchwald et al, 2015;Peng et al, 2015]. …”

“…A counterintuitive finding in the ocean's subsurface oxygen-deficient zones (ODZs), both from 15 N tracer studies [Lipschultz et al, 1990;Füssel et al, 2012;Beman et al, 2013;Peng et al, 2015] and natural abundance isotope observations Casciotti et al, 2013;Gaye et al, 2013;Buchwald et al, 2015], is that NO 2 À appears to undergo oxidation to NO 3 À despite the lack of appropriate oxidants in the environment. In tracer studies, this has been shown through the addition of 15 N-labeled NO 2 À to seawater samples collected from suboxic zones and the subsequent detection of 15 N-labeled NO 3 À after incubation under anoxic conditions [Füssel et al, 2012;Beman et al, 2013;Peng et al, 2015].…”