Nitrogen isotope effects induced by anammox bacteria

Abstract

Nitrogen (N) isotope ratios ( 15 N/ 14 N) provide integrative constraints on the N inventory of the modern ocean. Anaerobic ammonium oxidation (anammox), which converts ammonium and nitrite to dinitrogen gas (N 2 ) and nitrate, is an important fixed N sink in marine ecosystems. We studied the so far unknown N isotope effects of anammox in batch culture experiments. Anammox preferentially removes 14 N from the ammonium pool with an isotope effect of +23.5‰ to +29.1‰, depending on factors controlling reversibility. The N isotope effects during the conversion of nitrite to N 2 and nitrate are (i) inverse kinetic N isotope fractionation associated with the oxidation of nitrite to nitrate (−31.1 ± 3.9‰), (ii) normal kinetic N isotope fractionation during the reduction of nitrite to N 2 (+16.0 ± 4.5‰), and (iii) an equilibrium N isotope effect between nitrate and nitrite (−60.5 ± 1.0‰), induced when anammox is exposed to environmental stress, leading to the superposition of N isotope exchange effects upon kinetic N isotope fractionation. Our findings indicate that anammox may be responsible for the unresolved large N isotope offsets between nitrate and nitrite in oceanic oxygen minimum zones. Irrespective of the extent of N isotope exchange between nitrate and nitrite, N removed from the combined nitrite and nitrate (NO x ) pool is depleted in 15 N relative to NO x . This net N isotope effect by anammox is superimposed on the N isotope fractionation by the cooccurring reduction of nitrate to nitrite in suboxic waters, possibly enhancing the overall N isotope effect for N loss from oxygen minimum zones.T he nitrogen (N) isotope effect associated with N loss pathways allows the isotopic tracing of N transformations in ocean waters and provides critical constraints on the global marine N budget (1-5). Culture studies have shown that heterotrophic denitrification exhibits a large N isotope effect (e)* of up to +30‰ (6-9). For decades, heterotrophic denitrification was the only known N loss pathway in the ocean (10). Consequently, strong enrichment of 15 N in residual nitrite and nitrate (NO x ) from oxygen-deficient waters, as for example in the Eastern Tropical North Pacific and the Arabian Sea, was fully attributed to water column denitrification with an N isotope effect around +25‰ (2, 11). Recent studies have highlighted the significance of anaerobic ammonium oxidation (anammox) for regional N fluxes (12, 13), with possible ramifications regarding the global N balance (14, 15). However, N isotope effects associated with the anammox metabolism were unknown and therefore their potential impacts on the distribution of oceanic N isotopes could not be addressed. This lack of knowledge severely hampers the N-isotope-based assessment of the relative importance of watercolumn N loss compared with sedimentary N loss, the most poorly constrained flux in the marine combined nitrogen budget (16). ResultsWe used the only available highly enriched (>98%) culture of anammox single cells (Kuenenia stuttgartiensis) (17, 18) to investi...

The evolution of Earth's biogeochemical nitrogen cycle

“…Ammonification can be abiotic and is just included for completeness. Metal catalysts and isotopic fractionations are taken from the literature (Brunner et al, 2013;Buick, 2007a;Casciotti, 2009;Godfrey and Glass, 2011;Hoch et al, 1992;McCready et al, 1983;Nishizawa et al, 2014;Pennock et al, 1996;Waser et al, 1998;Zerkle et al, 2008;Zhang et al, 2014 Tian et al (2011) and Zahnle (1986), 10. Schoonen & Xu (2001), 11.…”

“…See Table 1 and text for references. Adapted from Stüeken (2013) with additional information from McCready (1983), Zhang et al (2014) and Brunner et al (2013). (f) Phanerozoic, 0.5 to modern.…”

“…However, the ingredients for anammox -ammonium and nitrite -may have existed in appreciable abundances in parts of the ocean since the late Archean, especially at times when the ocean was stratified (Sections 2.1, 2.4, 4.3). Unfortunately, the nitrogen isotopic fractionations imparted by anammox bacteria are too similar to those of canonical denitrifiers to distinguishing between the two metabolisms in the geologic record (Brunner et al, 2013). …”

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

“…In their experiments incubating cultures of anammox bacteria at 31°C, Brunner et al [2013] empirically derived a value for the NO 3 À -NO 2 À equilibrium isotope effect of À60.5 ± 1‰. This value is very close to the theoretical value of À59.4‰ derived at this temperature from the vibrational frequencies of Begun and Fletcher [1960] Figure 5, blue shading) are in much better agreement with the observed values than are the predictions resulting from NH 4 + and NO 2 À oxidation and assimilation.…”

Nitrogen cycling in shallow low-oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes

“…Additional information on N-cycling processes can be obtained from the isotopic composition of NO (Casciotti, 2009;Brunner et al, 2013 .…”