Effects of Magnesium and Chloride Ions on Light-Induced Electron Transport in Membrane Fragments from a Blue-Green Alga

The effects of magnesium and chloride ions on photosynthetic electron transport were investigated in membrane fragments of a blue-green alga, Nostoc muscorum (Strain 7119), noted for their stability and high rates of electron transport from water or reduced dichlorophenolindophenol to NADP+. Magnesium ions were required not only for light-induced electron transport from water to NADP+ but also for protection in the dark of the integrity of the water-photooxidizing system (Photosystem II). Membrane fragments suspended in the dark in a medium lacking Mg2+ lost the capacity to photoreduce NADP+ with water on subsequent illumination. Chloride ions could substitute, but less effectively, for each of these two effects of magnesium ions. By contrast, the photoreduction of NADP+ by DCIPH2 was independent of Mg2+ (or Cl−) for the protection of the electron transport system in the dark or during the light reaction proper. Furthermore, high concentrations of MgCl2 produced a strong inhibition of NADP+ photoreduction with DCIPH2 without significantly affecting the rate of NADP+ photoreduction with water. The implications of these findings for the differential involvement of Photosystem I and Photosystem II in the photoreduction of NADP+ with different electron donors are discussed.