Oxygen consumption associated with ferric reductase activity and iron uptake by iron-limited cells of Chlorella kessleri (Chlorophyceae)

Plasma membrane ferric reductase activity was enhanced 5-fold under iron limitation in the unicellular green alga Chlorella kessleri Fott et Novakova Furthermore, ferric reductase activity in iron-limited cells was approximately 50% higher in the light than in the dark. In contrast, iron uptake rates of iron-limited cells were unaffected by light versus dark treatments, Rates of iron uptake were much lower than rates of ferric reduction, averaging approximately 2% of the dark ferric reduction rate. Ferric reduction was associated with an increased rate of O-2 consumption in both light and dark, the increase in the light being approximately 1.5 times as large as in the dark. The increased rate of O-2 consumption could be decreased by half by the addition of catalase, indicating that H2O2 is the product of the O-2 consumption and that the increased O-2 consumption is non respiratory, The stimulation of O-2 consumption was almost completely abolished by the addition of bathophenanthroline disulfonate, a strong chelator of Fe2+. Anaerobic conditions or the presence of exogenous superoxide dismutase affected neither ferric reduction nor iron uptake. We suggest that the O-2 consumption associated with ferric reductase activity resulted from superoxide formation from the aerobic oxidation of Fe2+, which is the product of ferric reductase activity. At saturating concentrations of Fe3+ chelates, ferric reductase activity is much greater than the iron uptake rate, leading to rapid oxidation of Fe2+ and superoxide generation. Therefore, O-2 consumption is not an integral part of the iron assimilation process.