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

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 (keq, AMX = 8.44 ~ 13.56 × 10-2 h-1), which is substantially faster than abiotic oxygen isotope exchange (keq, 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 (18εNO2- → N2 = 10.6 ~ 16.1‰), (2) NO2- oxidation to NO3- (18εNO2- → NO3- = -2.9 ~ -11.0‰, inverse fractionation), (3) incorporation of oxygen from water during NO2- oxidation to NO3- (18εH2O = 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 δ18ONO3- and δ18ONO2- as geochemical markers to assess the potential contribution of anammox to fixed-N losses in the ocean.