Chemistry of Superoxide Radical in Seawater:  Reactions with Organic Cu Complexes

Organically complexed Cu can be a significant sink of photoproduced superoxide (O2-) in seawater. Using pulse radiolysis, we examined the rate constant of catalytic dismutation (kcat) of O2- in the presence of Cu complexed by strong (L1) and weaker (L2) organic ligands present in coastal seawater samples and produced by cultures of Synechococcus sp. In the coastal samples examined, CuL2 complexes were almost as reactive towards O2- as inorganically complexed Cu species (kcat = 2.9−8.1 × 108 M-1 s-1). Furthermore, kcat was invariant with added Cu (5−60 nM), implying that the added Cu formed a single type of Cu complex or a mixture of complexes with very similar O2- reactivities. Significant differences between kcat values of estuarine and coastal samples were observed, suggesting an effect of organic ligands from terrestrial sources. CuL1 complexes produced by Synechococcus sp. were found to be less reactive (kcat < 1 × 108 M-1 s-1). The high reactivity of CuL2 complexes decreases estimates of steady-state concentrations of O2- in sunlit marine waters by at least a factor of 10.