PRODUCTION, SCAVENGING AND TOXICITY OF HYDROGEN-PEROXIDE IN THE GREENSEAWEED ULVA-RIGIDA

Some aspects of hydrogen peroxide (H2O2) production, scavenging and to xicity in Ulva rigida (Chlorophyta) have been investigated. The seawee d produced H2O2 in light and excretion increased exponentially with in creasing light up to 800 mu mol photons m(-2) s(-1) where the excretio n rate was 330 mu mol (kg Chl a)(-1) s(-1). This corresponded to a con centration of 4 mu M H2O2 in the experimental medium after 30 min (den sity of seaweed: 9.2 kg fresh weight m(-3)). At 1200 mu mol photons m( -2) s(-1), the excretion rate decreased to 190 mu mol (kg Chl a)(-1) s (-1). At 600 mu mol photons m(-2) s(-1), the excretion caused a concen tration of 1.8 mu M H2O2 in the experimental seawater; after a further 2 h the concentration decreased to a steady level of c. 0.8 mu M. The enzymatic degradation of H2O2 is suggested to be primarily through th e activity of ascorbate peroxidase (62 mmol H2O2 (kg Chl a)(-1) s(-1)) since no catalase activity was detected. The H2O2 levels in U. rigida are reduced by diffusion through the plasmalemma. Addition of 1 mM H2 O2 caused an 80% reduction in photosynthesis while no effect was seen at 0.1 mM. Addition of 3 mM H2O2 caused decreased variable fluorescenc e and decreased light-dependent oxygen evolution; these effects persis ted even after the H2O2 was washed away. These observations suggest th at the endogenous production by U. rigida has no direct auto-toxic eff ect.