Dynamics of nonphotochemical superoxide production and decay in the Great Barrier Reef lagoon

Superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)) 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 O(2)(-) 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 O(2)(-) production rates in GBR surface waters. Manipulative experiments using 0.22-mu m 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 O(2)(-) decay rate constants were very low at O(2)(-) concentrations < 1 nmol L(-1) (values in filtered samples ranged from 0.7 to 4.3 3 10(-2) s(-1)) but increased with increasing O(2)(-) concentration toward a value of similar to 0.2 s(-1) at O(2)(-) 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 O(2) rather than O(2)(-) at low O(2)(-) concentrations, thereby inhibiting catalyzed O(2)(-) disproportionation. This notion was supported by the observation that addition of superoxide dismutase dramatically increased rates of H(2)O(2) production in samples. We suggest that, under these conditions, O(2)(-) can maintain a biologically useful reducing microenvironment around cells without resulting in significant accumulation of potentially harmful H(2)O(2).