Detection of Non-Photochemical Superoxide in Coastal and Open Ocean Seawater: Particulate Versus Dissolved Sources

Superoxide radical (O₂^-) could have a significant effect on marine metal redox chemistry, but little data exists on its marine concentrations. In this study, we measured superoxide steady-state concentrations in both filtered and unfiltered samples collected near the California coast and at Station ALOHA. Particle-generated superoxide, defined as the difference between unfiltered and filtered concentrations, ranged from undetectable to 0.019 nM at Station ALOHA and from undetectable to 0.052 nM in samples from the southern California Current. We also show that a transient superoxide signal is generated during filtering, an artifact that may have affected previously reported concentrations of particle-generated superoxide in the ocean. High concentrations of superoxide (range) were measured in filtered samples from ALOHA station and the California Current, raising concerns about possible sources of background signals.

Further study of background signals revealed that some superoxide production occurs even in artificial seawater and very aged filtered seawater samples, and that a small additional background signal is generated as the sample travels from the container to the flow cell where it is mixed with reagent for CL analysis. However, filtered seawater samples collected from the Scripps Pier had significantly higher superoxide production rates than those measured in artificial seawater, and production rates in unfiltered samples were no higher than those in filtered samples. Therefore, production by dissolved sources was the dominant non-photochemical source of superoxide in these samples. Production rates decreased in the presence of DTPA, suggesting involvement of metal ions in superoxide production. Laboratory experiments with natural organic matter (NOM) indicate that superoxide formation occurs during oxidation of reduced moieties of NOM by oxygen.