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

Abstract

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 μmol photons m^-2 s^-1 where the excretion rate was 330 μmol (kg Chl a)^-1s^-1. This corresponded to a concentration of 4 μMH₂O₂ in the experimental medium after 30 min (density of seaweed: 9·2 kg fresh weight m^-3). At 1200 μmol photons m^-2 s^-1, the excretion rate decreased to 190 μmol (kg Chl a)^-1 s^-1. At 600 μmol photons m^-2 s^-1, the excretion caused a concentration of 1·8 μM H₂O₂ in the experimental seawater; after a further 2 h the concentration decreased to a steady level of c. 0·8 μM. 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.

Key words :

• ascorbate peroxidase
• catalase
• hydrogen peroxide
• Mehler reaction
• photosynthesis
• Ulva rigida