Oxygen isotope fractionation during anaerobic ammonium oxidation by the marine representative Candidatus Scalindua sp.

Abstract

Analysing the nitrogen (¹⁵ε) and oxygen (¹⁸ε) isotope effects of anaerobic ammonium oxidation (anammox) is essential for accurately assessing its potential contribution to fixed-N losses in the ocean, yet the ¹⁸ε of anammox remains unexplored. Here, we determined the previously unexplored ¹⁸ε of anammox using a highly enriched culture of the marine anammox species “Ca. Scalindua sp”. Because Scalindua significantly accelerated oxygen isotope exchange between NO₂⁻ and H₂O, we introduced a new rate constant for anammox-mediated oxygen isotope exchange (k[eq], [AMX] = 8.44 ~ 13.56 × 10⁻² h⁻¹), which is substantially faster than abiotic oxygen isotope exchange (k[eq], [abio] = 1.13 × 10⁻² h⁻¹), into a numerical model to estimate the ¹⁸ε during anammox. Based on our experimental results, we successfully determined the ¹⁸ε associated with: (1) conversion of NO₂⁻ to N₂ (¹⁸ε[NO]₂₋ → [N]₂ = 10.6 ~ 16.1‰), (2) NO₂⁻ oxidation to NO₃⁻ (18ε[NO]₂₋ → [NO]₃₋ = −2.9 ~ −11.0‰, inverse fractionation), (3) incorporation of oxygen from water during NO₂⁻ oxidation to NO₃⁻ (¹⁸ε[H₂O] = 16.4 ~ 19.2‰). Our study underscores the possibility that unique anammox oxygen isotope signals may be masked due to substantial anammox-mediated oxygen isotope exchange between NO₂⁻ and H₂O. Therefore, careful consideration is required when utilizing δ¹⁸O[NO]₃₋ and δ¹⁸O[NO]₂₋ as geochemical markers to assess the potential contribution of anammox to fixed-N losses in the ocean.