Reactive Oxygen Species Regulate Iron Uptake in Scenedesmus Incrassatulus

Iron-sufficient cells of the green alga Scenedesmus incrassatulus Bohl, R-83 released in the medium strong and weak Fe3+-complexing agents in response to freshly added Fe3+. Applied in the dark, photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) inhibited ferric reduction from ferric-EDTA or ferricyanide more than the chelator release. Fedeficient cells and DCMU-treated Fe-sufficient cells took up Fe nonpreferentially both from strong and from weak complexes at high rates. On the contrary, intact Fe-sufficient cells took up Fe only from strong chelates at a lower rate. In vitro, when added into fresh lipid extracts, strong Fe chelates catalyzed the generation of 5 and 8 times, respectively, less malonaldehyde compared to the content catalyzed by weak Fe complexes or inorganic Fe3+. 100 ìM DCMU prevented a Fe3+-induced release of chelators and an inorganic Fe uptake, but upregulated the uptake of exogenously added organic Fe. The hydroxyl radical (.OH) scavengers dimethylsulfoxide (DMSO), mannitol, formate, adenine and thiourea (but not urea) counteracted the effect of DCMU on ferric reduction and chelator release. DMSO did not affect the DCMU-induced stimulation of uptake of added organic Fe. Low concentrations of catalase added upregulated the Fe uptake from intact Fe-sufficient cells. Results show that Fe-sufficient cells possess a recognition system, which allows a selective uptake of Fe from non-toxic complexes. This recognition system does not operate in iron-limited and in DCMUstressed Fe-sufficient cells. The DCMU-induced inhibition of ferric reduction and chelator release is caused by reactive oxygen species such as .OH generated in the outer cell membrane. .OH is not involved in the Fe deficiencyand in DCMU-induced upregulation of Fe uptake. H2O2 is involved