Chemistry of superoxide radical in seawater: CDOM associated sink of superoxide in coastal waters

Colored dissolved organic matter (CDOM) and humic substances contain a nonm etallic redox-cycling component capable of catalyzing superoxide (O-2(-)) d ismutation. First-Order rate coefficients (k(pseudo)) measured for this O-2 (-) sink in a number of coastal and Chesapeake Bay water samples range up t o 1.4 s(-1), comparable in magnitude to catalyzed dismutation by Cu species . A significant (r(2) = 0.73) correlation is observed between k(pseudo) and the optical absorption and salinity of individual coastal water samples, s uggesting an association with non-marine-derived CDOM. The activity of this sink is not changed by acidification or boiling of samples but is removed by photooxidation, indicating that it is an organic compound, but that it i s neither enzymatic nor likely to consist of tightly bound metals. The stoi chiometry of hydrogen peroxide formation from O-2(-) decay indicates that t his sink is capable of a redox cycle catalyzing the dismutation of O-2(-) T his CDOM sink combined with the organic copper sink previously described wi ll produce a steady-state superoxide concentration in coastal waters that i s 100-1000-fold lower than that predicted from bimolecular dismutation alon e. Catalyzed O-2(-) decay was also observed in a variety of humic and fulvi c acid samples, possibly occurring through quinone functionalities. Althoug h the presence of quinone moieties in humic and fulvic acids has been demon strated, there do not appear to be good correlations between several measur es of quinone content and the O-2(-) dismutation rates of these samples.