Abstract
Understanding stable isotopic fractionation of dissolved O(2) in aquatic environments is crucial to constrain and accurately model the processes responsible for biological O(2) consumption, which are closely linked to the overall health of an ecosystem. This study aimed to investigate whether O(2) consumption by microbial methane and ammonia oxidation may contribute to the observed discrepancy in O(2) isotopic fractionation ((18)ϵ) between heterotrophic O(2) respiration in laboratory incubations (-18 to -24 ‰) and in situ measurements of O(2) consumption in lakes and oceans (-10 to -18 ‰). To estimate the in vivo (18)ϵ values of soluble methane monooxygenase (sMMO), particulate methane monooxygenase (pMMO), and ammonia monooxygenase (AMO), which are the first enzymes required for the oxidation of methane and ammonia, experiments were performed with three methanotrophic bacteria and one comammox (complete-ammonia-oxidizing) bacterium. The resulting (18)ϵ values for pMMO and AMO ranged from -18 ± 12 to -24 ± 5 ‰, not significantly different from (18)ϵ values typical for heterotrophic respiration. The (18)ϵ value determined for sMMO (-22 ± 2 ‰) was in the same range, yet more negative than the previously reported (18)ϵ value for the isolated enzyme. Our results provide insights into the potential reaction mechanisms of pMMO and AMO and indicate that O(2) consumption by sMMO, pMMO, or AMO cannot explain the observed discrepancy between in situ and laboratory (18)ϵ values for "community" O(2) consumption in aquatic environments. Instead, the apparent difference may be attributed to aspects involving substrate diffusion limitation.