PHOTOSYNTHETIC PRODUCTION OF HYDROGEN-PEROXIDE BY ULVA-RIGIDA C-AG (CHLOROPHYTA)

Production of hydrogen peroxide has been found in Ulva rigida (Chlorophyta). The formation of H2O2 was light dependent with a production of 1.2 mu mol . g FW-1. h(-1) in sea water (pH 8.2) at an irradiance of 700 mu mol photons m(-2). s(-1). The excretion was also pH dependent: in pH 6.5 the production was not detectable (< 5 nmol . g FW-1. h(-1)) but at pH 9.0 the production was 5.0 mu mol . g FW(-1)h(-1). The production of H2O2 was totally inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). The ability of U. rigida growing in tanks (750 1) under a natural light regime to excrete H2O2 was checked and found to be seven times higher at 08.00 hours than other times of the day. The H2O2 concentration in the cultivation tank (density: 2 g FW . 1(-1)) reached the highest value (3 mu M) at 11.00 hours. Photosynthesis was not influenced by H2O2 formation. The H2O2 is suggested to come from the Mehler reaction (pseudocyclic photophosphorylation). With an oxygen evolution of 120 mmol . g FW(-1)h(-1) at pH8.2 and 90 mmol . g FW-1. h(-1) at pH 9.0, 0.5% and 2.7% of the electrons were used for extracellular H2O2 production. The H2O2 production is sufficiently high to be of physiological and ecological significance, and is suggested to be a part of the defence against epi- and endophytes.