The influence of reactive oxygen species on "‘respiration" isotope effect

The triple-oxygen isotope (17O/16O, 18O/16O) measurement of oxygen-bearing species represents one of the most robust tools to directly trace oxygen cycling in the environment. One particularly consequential application of this isotope system is the analysis of dissolved oxygen (O2) in aquatic environments to determine gross oxygen production. This approach assumes that photosynthesis, microbial respiration, and gas exchange are the main drivers of dissolved O2 isotope compositions, and that each process is described by predictable, consistent triple-oxygen isotope effects. However, there currently exists a large disagreement in the literature on the triple-oxygen isotope effect of respiration, which carries major implications for global primary productivity estimates. Recent work has additionally highlighted the ubiquitous production of extracellular reactive oxygen species (ROS) such as superoxide and hydrogen peroxide by microorganisms; this flux maybe responsible for as much as 20% of net oxygen utilization in the ocean.

To examine the influence of ROS-mediated O2 recycling on the oxygen utilization isotope effect, we measured the triple-oxygen isotope fractionations and mass laws of superoxide dismutase, catalase, and iron-mediated H2O2 degradation. We incorporate these constraints into an oxygen isotope flux model to explore the influence of ROS-mediated oxygen cycling on "respiration" isotope effects in previous studies. We find that ROS-mediated oxygen cycling can reconcile the previously reported range of triple-oxygen isotope fractionation factors and that typical marine isotope effects are broadly consistent with independent estimates of superoxide-mediated oxygen loss. These data are described further in the related publication, Sutherland et al., 2022 (doi: 10.1016/j.gca.2022.02.033).