Chemistry of superoxide radical in seawater: Reactions with organic Cu complexes

Organically complexed Cu can be a significant sink of photoproduced superox ide (O-2(-)) in seawater. Using pulse radiolysis, we examined the rate cons tant of catalytic dismutation (k(cat)) of O-2(-) in the presence of Cu comp lexed by strong (L-1) and weaker (L-2) organic ligands present in coastal s eawater samples and produced by cultures of Synechococcus sp. In the coasta l samples examined, CuL2 complexes were almost as reactive towards O-2(-) a s inorganically complexed Cu species (k(cat) = 2.9-8.1 x 10(8) M-1 s(-1)). Furthermore, k(cat) was invariant with added Cu (5-60 nM), implying that th e added Cu formed a single type of Cu complex or a mixture of complexes wit h very similar O-2(-) reactivities. Significant differences between k(cat) values of estuarine and coastal samples were observed, suggesting an effect of organic ligands from terrestrial sources. CuL1 complexes produced by Sy nechococcus sp. were found to be less reactive (k(cat) < 1 x 10(8) M-1 s(-1 )). The high reactivity of CuL2 complexes decreases estimates of steady-sta te concentrations of O-2(-) in sunlit marine waters by at least a factor of 10.