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

Abstract

Production of hydrogen peroxide has been found in Ulva rigida (Chlorophyta). The formation of Hz02 was light dependent with a production of l.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 dependen!: 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 H202 was totally inhibited by 3-(3,4-d ichlo ro phenyl}- I, l­dimethylurea (DCMU). The ability oí U. rigida growing in tanks (750 1) under a natural light regime to excre te H202 was checked and found to be seven times higher at 08.00 hours than other times of the day. The H202 con­centration in the cultivation tank (density : 2 g FW·I-1 ) reached the highest value (3 µM) at 11.00 hours. Photo­syn thesis was not influenced by H202 formation. The Hz02 is suggested to come from the Mehler reaction (pseudocyclic photophosphorylation). With an oxygen evolution of 120 mmol·g FW-' ·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 H202 prod uction. The H202 production is sufficiently high to be of physio­logical and ecological signiócance, and is suggested to be a part of the defence against epi- and endophytes