Extracellular production of superoxide by marine diatoms: Contrasting effects on iron redox chemistry and bioavailability

We report the extracellular production of superoxide in cultures of the marine diatoms Thalassiosira weissflogii and Thalassiosira pseudonana. In EDTA-buffered media with similar to 45 pmol L-1 iron (Fe)(III)', over half the Fe(III) reduction is mediated by extracellular 0, production. Surprisingly, even though we saw a consistent inhibition of Fe reduction by the addition of superoxide dismutase (SOD) and enhancement of Fe reduction due to superoxide production using the xanthine-xanthine oxidase system (X-XO), we observed no effect of SOD or X-XO on Fe uptake in these cultures. We also observed no effect of SOD on the uptake of Fe from either ferrihydrite, an Fe-labeled porphyrin, or as regenerated by metazoan grazers. Our data reveal that Fe(II)' is formed by O-2 via the reduction of Fe(III)' rather than by the reduction and dissociation of FeEDTA complexes in EDTA-buffered media. This bulk-phase reduction of Fe(III)' by O-2(-) is a "futile" reductive step, as the Fe(II)' formed is rapidly reoxidized to Fe(III)' in pH 8 oxygenated seawater.