Photosynthetic production of hydrogen peroxide by Ulva rigida C. Ag. (Chlorophyta)

Abstract

Production of hydrogen peroxide has been found in Ulva rigida (Chlorophyta). The formation of H₂O₂ was light dependent with a production of 1.2 μmol·g FW⁻¹·h⁻¹ in sea water (pH 8.2) at an irradiance of 700 μmol 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 μmol·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 (7501) 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·l⁻¹) reached the highest value (3 μM) 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 pH 8.2 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.