Dynamics of nonphotochemical superoxide production in the Great Barrier Reef lagoon

Abstract

Superoxide (O2–) and hydrogen peroxide (H2O2) concentrations ranging from 87 to 1120 pmol L−1 and 5 to 107 nmol L−1, respectively, were measured in samples of surface water from the Great Barrier Reef (GBR) lagoon in the absence of photochemistry. Nonphotochemical, particle‐associated net production rates of O2‐ ranging from 1 to 16 pmol L‐1 s‐1 were also determined and calculated to be similar in magnitude to the likely abiotic photochemical O2‐ production rates in GBR surface waters. Manipulative experiments using 0.22‐µm filtration and addition of biological inhibitors demonstrated that the majority of this particle‐associated production was biological and likely driven by photosynthetic organisms. Pseudo‐first‐order O2‐ decay rate constants were very low at O2‐ concentrations < 1 nmol L‐1 (values in filtered samples ranged from 0.7 to 4.3 x 10−2 s−1) but increased with increasing O2‐ concentration toward a value of ~ 0.2 s−1 at O2‐ concentrations > 10 nmol L‐1. This was thought to occur because reduced forms of metals such as iron and copper, or redox‐active organic moieties, preferentially react with O2 rather than O2‐ at low O2‐ concentrations, thereby inhibiting catalyzed O2‐ disproportionation. This notion was supported by the observation that addition of superoxide dismutase dramatically increased rates of H2O2 production in samples. We suggest that, under these conditions, O2‐ can maintain a biologically useful reducing microenvironment around cells without resulting in significant accumulation of potentially harmful H2O2.