PHOTOINHIBITION OF PHOTOSYNTHESIS ON A CORAL-REEF

Photoinhibition of macroalgae in the epilithic algal community (EAC) o f coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. F-v/F-m (variable to maximum fluorescence, darkened samples) of shallow macroalgae declined by 50% on fine summer and winter days, recovering in late afternoon. Within a species, thalli from low-light habitats were more photoinhibi ted (2h at 1400 mu mol m(-2) s(-1)) than those from high-light habitat s. The sensitivity of Lobophora variegata (Phaeophyta) and Chlorodesmi s fastigiata (Chlorophyta) increased with depth (1 versus 20 m). Howev er, shallow Halimeda tuna (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas. Photoinhibi tion and recovery were depth- and species-specific. Shallow Lobophora and Chlorodesmis maintained a greater degree of Q(A) oxidation during photoinhibition. In deep thalli, reduced effective quantum yield of op en photosystem II centres reflected lower proportions and excitation c apture efficiencies of open centres. In Lobophora, zeaxanthin formatio n accompanied non-photochemical fluorescence quenching (NPQ), but in C hlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may c ompensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity.