The photoreduction of H(2)O(2) by Synechococcus sp. PCC 7942 and UTEX 625.

It has been claimed that the sole H(2)O(2)-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 H(2)O(2) (2.5 microM) to illuminated cells caused photochemical quenching (qP) of chlorophyll fluorescence that was relieved as the H(2)O(2) was consumed. The qP was maximal at about 50 microM H(2)O(2) with a Michaelis constant of about 7 microM. The H(2)O(2)-dependent qP strongly indicates that photoreduction can be involved in H(2)O(2) decomposition. Catalase-peroxidase activity was found to be almost completely inhibited by 10 microM NH(2)OH with no inhibition of the H(2)O(2)-dependent qP, which actually increased, presumably due to the light-dependent reaction now being the only route for H(2)O(2)-decomposition. When (18)O-labeled H(2)O(2) was presented to cells in the light there was an evolution of (16)O(2), indicative of H(2)(16)O oxidation by PS 2 and formation of photoreductant. In the dark (18)O(2) was evolved from added H(2)(18)O(2) as expected for decomposition by the catalase-peroxidase. This evolution was completely blocked by NH(2)OH, whereas the light-dependent evolution of (16)O(2) during H(2)(18)O(2) decomposition was unaffected.