The photoreduction of H2O2 by Synechococcus sp PCC 7942 and UTEX 625

It has been claimed that the sole H2O2-scavenging system in the cyanobacterium Synechococcus sp. PCC 7942 is a cytosolic catalase-peroxidase. We have measured in vivo activity of a light-dependent peroxidase in Synechococcus sp. PCC 7942 and UTEX 625. The addition of small amounts of H2O2 (2.5 mu M) to illuminated cells caused photochemical quenching (qP) of chlorophyll fluorescence that was relieved as the H2O2 was consumed. The qP was maximal at about 50 mu M H2O2 With a Michaelis constant of about 7 mu M. The H2O2-dependent qP strongly indicates that photoreduction can be involved in H2O2 decomposition. Catalase-peroxidase activity was found to be almost completely inhibited by 10 mu M NH2OH with no inhibition of the H2O2-dependent qP, which actually increased, presumably due to the light-dependent reaction now being the only route for H2O2-decomposition. When O-18-labeled H2O2 was presented to cells in the light there was an evolution of O-16(2), indicative of (H2O)-O-16 oxidation by PS 2 and formation of photoreductant. In the dark O-18(2) was evolved from added (H2O2)-O-18 as expected for decomposition by the catalase-peroxidase. This evolution was completely blocked by NH2OH, whereas the light-dependent evolution of O-16(2) during (H2O2)-O-18 decomposition was unaffected.