Singlet Oxygen and Other Reactive Oxygen Species are Involved in Regulation of Release of Iron-Binding Chelators from Scenedesmus cells

Freshly-added iron only slightly affected the growth of iron-sufficient cells of the green alga Scenedesmus incrassatulus Bohl, strain R-83, but induced accumulation of malondialdehyde (MDA) in cells and excretion of MDA in the medium. These effects were stronger in response to Fe2+ as compared to Fe3+, but Fe3+ induced the release of more iron-binding chelators from these cells than Fe2+. Fe3+ added either in dark or in light induced release of equal concentrations of iron-complexing agents, part of which formed strong chelates with iron in the medium. Exogenously added hydrogen peroxide inhibited iron-induced release of chelators but the effect was removed by addition of the hydroxyl radical scavenger dimethylsulfoxide (DMSO). Malondialdehyde also inhibited the release of chelators. Release of chelators was induced in the absence of iron salts by photoexcited chlorophyll (Chl). The Chl-induced release was efficiently inhibited by singlet oxygen scavengers such as dimethylfuran, β-carotene, sodium azide and vitamin B6, and stimulated in D2O or DMSO. Exogenously added catalase inhibited the release more than added superoxide dismutase. The Fe3-induced release of chelators was also inhibited by scavengers of singlet oxygen, but was not affected by sodium azide and by ethanol. Hence both H2O2 and singlet oxygen were involved in induction of chelator release in the absence of iron in light. The induction of chelator release by iron in dark involved H2O2, but not singlet oxygen.