Hydrogen peroxide inhibits photosynthetic electron transport in cells of cyanobacteria

The effect of H2O2 on photosynthetic O-2 evolution and photosynthetic elect ron transfer in cells of cyanobacteria Anabaena variabilis and Anacystis ni dulans was studied. The following experiments were performed: 1) directly t esting the effect of exogenous H2O2; 2) testing the effect of intracellular H2O2 generated with the use of methyl viologen (MV); 3) testing the effect of inhibiting intracellular H2O2 decomposition by salicylic acid (SA) and 3-amino-1,2,4-triazole (AT). H2O2 inhibited photosynthetic O-2 evolution an d light-induced reduction of p-benzoquinone (BQ) + ferricyanide (FeCy) in t he Hill reaction. The I-50 value for H2O2 was similar to0.75 mM. Photosynth etic electron transfer in the cells treated with H2O2 was not maintained by H2O2, NH2OH, 1,5-diphenylcarbazide, tetraphenylboron, or butylated hydroxy toluene added as artificial electron donors for Photosystem (PS) II. The H2 O --> CO2, H2O --> MV (involving PSII and PSI) and H2O --> BQ + FeCy (chief ly dependent on PSII) electron transfer reactions were inhibited upon incub ation of the cells with MV, SA, or AT. The N,N,N',N'-tetramethyl-p-phenylen ediamine --> MV (chiefly dependent on PSI) electron transfer was inhibited by SA and AT but was resistant to MV. The results show that H2O2 inhibits p hotosynthetic electron transfer. It is unlikely that H2O2 could be a physio logical electron donor in oxygenic photosynthesis.