Dynamics of non-photochemical superoxide production and decay in the Great Barrier Reef lagoon

Superoxide (O2 2 ) and hydrogen peroxide (H2 O2) concentrations ranging from 87 to 1120 pmol L21 and 5 to 107 nmol L21, 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 2 ranging from 1 to 16 pmol L21 s 21 were also determined and calculated to be similar in magnitude to the likely abiotic photochemical O2 2 production rates in GBR surface waters. Manipulative experiments using 0.22-mm 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 2 decay rate constants were very low at O2 2 concentrations , 1 nmol L21 (values in filtered samples ranged from 0.7 to 4.3 3 1022 s 21) but increased with increasing O2 2 concentration toward a value of , 0.2 s21 at O2 2 concentrations . 10 nmol L21. 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 2 at low O2 2 concentrations, thereby inhibiting catalyzed O2 2 disproportionation. This notion was supported by the observation that addition of superoxide dismutase dramatically increased rates of H2 O2 production in samples. We suggest that, under these conditions, O2 2 can maintain a biologically useful reducing microenvironment around cells without resulting in significant accumulation of potentially harmful H2 O2.