H sub(2)O sub(2)-Induced Inhibition of Photosynthetic O sub(2) Evolution by Anabaena variabilis Cells

Hydrogen peroxide inhibits photosynthetic O sub(2) evolution. It has been shown that H sub(2)O sub(2) destroys the function of the oxygen-evolving complex (OEC) in some chloroplast and Photosystem (PS) II preparations causing release of manganese from the OEC. In other preparations, H sub(2)O sub(2) did not cause or caused only insignificant release of manganese. In this work, we tested the effect of H sub(2)O sub(2) on the photosynthetic electron transfer and the state of OEC manganese in a native system (intact cells of the cyanobacterium Anabaena variabilis). According to EPR spectroscopy data, H sub(2)O sub(2) caused an increase in the level of photooxidation of P700, the reaction centers of PS I, and decreased the rate of their subsequent reduction in the dark by a factor larger than four. Combined effect of H sub(2)O sub(2), CN super(-), and EDTA caused more than eight- to ninefold suppression of the dark reduction of P700 super(+). EPR spectroscopy revealed that the content of free (or loosely bound) Mn super(2+) in washed cyanobacterial cells was 20% of the total manganese pool. This content remained unchanged upon the addition of CN super(-) and increased to 25-30% after addition of H sub(2)O sub(2). The content of the total manganese decreased to 35% after the treatment of the cells with EDTA. The level of the H sub(2)O sub(2)-induced release of manganese increased after the treatment of the cells with EDTA. Incubation of cells with H sub(2)O sub(2) for 2 h had no effect on the absorption spectra of the photosynthetic pigments. More prolonged incubation with H sub(2)O sub(2) (20 h) brought about degradation of phycobilins and chlorophylla and lysis of cells. Thus, H sub(2)O sub(2) causes extraction of manganese from cyanobacterial cells, inhibits the OEC activity and photosynthetic electron transfer, and leads to the destruction of the photosynthetic apparatus. H sub(2)O sub(2) is unable to serve as a physiological electron donor in photosynthesis.