Earth & Environmental Sciences

The naturally occurring nitrogen (N) isotopes, ¹⁵N and ¹⁴N, exhibit different reaction rates during many microbial N transformation processes, which results in N isotope fractionation. Such isotope effects are critical parameters for interpreting natural stable isotope abundances as proxies for biological process rates in the environment across scales. The kinetic isotope effect of ammonia oxidation (AO) to nitrite (NO₂ ^-), performed by ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), is generally ascribed to the enzyme ammonia monooxygenase (AMO), which catalyzes the first step in this process. However, the kinetic isotope effect of AMO, or ε _{A M O} , has been typically determined based on isotope kinetics during product formation (cumulative product, NO₂ ^-) alone, which may have overestimated ε _{A M O} due to possible accumulation of chemical intermediates and alternative sinks of ammonia/ammonium (NH₃/NH₄ ⁺). Here, we analyzed ¹⁵N isotope fractionation during archaeal ammonia oxidation based on both isotopic changes in residual substrate (RS, NH₄ ⁺) and cumulative product (CP, NO₂ ^-) pools in pure cultures of the soil strain Nitrososphaera viennensis EN76 and in highly enriched cultures of the marine strain Nitrosopumilus adriaticus NF5, under non-limiting substrate conditions. We obtained ε _{A M O} values of 31.9-33.1‰ for both strains based on RS (δ¹⁵NH₄ ⁺) and showed that estimates based on CP (δ¹⁵NO₂ ^-) give larger isotope fractionation factors by 6-8‰. Complementary analyses showed that, at the end of the growth period, microbial biomass was ¹⁵N-enriched (10.1‰), whereas nitrous oxide (N₂O) was highly ¹⁵N depleted (-38.1‰) relative to the initial substrate. Although we did not determine the isotope effect of NH₄ ⁺ assimilation (biomass formation) and N₂O production by AOA, our results nevertheless show that the discrepancy between ε _{A M O} estimates based on RS and CP might have derived from the incorporation of ¹⁵N-enriched residual NH₄ ⁺ after AMO reaction into microbial biomass and that N₂O production did not affect isotope fractionation estimates significantly.