# Oxygen Isotopic Fractionation of O2 Consumption by Methane and Ammonia Monooxygenases

## TL;DR

This study examines how oxygen isotopes change during methane and ammonia oxidation by microbes, to better understand oxygen consumption in aquatic ecosystems.

## Contribution

The study experimentally determines oxygen isotopic fractionation values for specific microbial enzymes and evaluates their role in observed discrepancies in oxygen isotope measurements.

## Key findings

- Oxygen isotopic fractionation values for pMMO and AMO were similar to those of heterotrophic respiration.
- The sMMO enzyme showed a more negative isotopic fractionation value compared to prior reports.
- The observed discrepancy in oxygen isotope measurements may be due to substrate diffusion limitations rather than enzyme activity.

## Abstract

Understanding stable
isotopic fractionation of dissolved O2 in aquatic environments
is crucial to constrain and accurately
model the processes responsible for biological O2 consumption,
which are closely linked to the overall health of an ecosystem. This
study aimed to investigate whether O2 consumption by microbial
methane and ammonia oxidation may contribute to the observed discrepancy
in O2 isotopic fractionation (18ϵ) between
heterotrophic O2 respiration in laboratory incubations
(−18 to −24 ‰) and in situ measurements
of O2 consumption in lakes and oceans (−10 to −18
‰). To estimate the in vivo
18ϵ
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 18ϵ values for pMMO and AMO ranged from −18 ± 12
to −24 ± 5 ‰, not significantly different from 18ϵ values typical for heterotrophic respiration. The 18ϵ value determined for sMMO (−22 ± 2 ‰)
was in the same range, yet more negative than the previously reported 18ϵ value for the isolated enzyme. Our results provide
insights into the potential reaction mechanisms of pMMO and AMO and
indicate that O2 consumption by sMMO, pMMO, or AMO cannot
explain the observed discrepancy between in situ and
laboratory 18ϵ values for “community”
O2 consumption in aquatic environments. Instead, the apparent
difference may be attributed to aspects involving substrate diffusion
limitation.