Production, scavenging and toxicity of hydrogen peroxide in the green seaweed Ulva rigida

Some aspects of hydrogen peroxide (H2O2) production, scavenging and toxicity in Ulva rigida (Chlorophyta) have been investigated. The seaweed produced H2O2 in light and excretion increased exponentially with increasing light up to 800 mu mol photons m(-2) s(-1) where the excretion rate was 330 mu mol (kg Chl a)(-1) s(-1). This corresponded to a concentration of 4 mu M H2O2 in the experimental medium after 30 min (density 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 concentration 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 the 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 H2O2 caused an 80% reduction in photosynthesis while no effect was seen at 0.1 mM. Addition of 3 mM H2O2 caused decreased variable fluorescence and decreased light-dependent oxygen evolution; these effects persisted even after the H2O2 was washed away. These observations suggest that the endogenous production by U. rigida has no direct auto-toxic effect.