The imprint of anammox on the stable isotope compositions of nitrogen-bearing molecules

Stable isotope measurements of nitrogen and oxygen in nitrogen-containing molecules provide important constraints on the sources, sinks and pools of these molecules in the environment. Anammox is one of two known biological processes for converting fixed nitrogen to N₂, and through its consumption of ammonium and nitrite and production of nitrate, it impacts the supply of a wide variety of fixed N molecules. Nevertheless, the isotope fractionations associated with the various anammox-associated redox reactions remain poorly constrained. We have measured the isotope effects of anammox in microbial communities enriched for the purpose of nitrogen removal from wastewater by anammox. In this system, we can replicate the ecological complexity exhibited in environmental settings, while also performing controlled experiments. We find that under a variety of conditions, the nitrogen isotope effect for the anaerobic oxidation of ammonium in this system (NH₄⁺ to N₂) is between 19‰ and 32‰, that for the reduction of nitrite (NO₂^– to N₂) is between 7‰ and 18‰, and that for the production of nitrate (NO₂^– to NO₃^–) is between -16‰ and -43‰. We propose that these ranges reflect both (1) a mixture of signals from different anammox-performing species and (2) variation of the isotope effect associated with the anammox process within a given microbial community under different conditions. We seek to understand further what factors control this variability to better interpret stable isotope measurements of N-bearing molecules in environmental settings.