Photoinhibition of photosynthesis on a coral reef

Photoinhibition of macroalgae in the epilithic algal community (EAC) of coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. F jFm (variable to maximum fluorescence darkened samples) of shallow macroalgae declined by 50% on fine summer and winte~ days, reco.vering i~ late afternoon. Within a species, thalh from low-hght habItats were more photoinhibited (2 h at 140011mol mS-I) than those from high-light habitats. The sensitivity of Lobophora variegata (Phaeophyta) and Chlorodesmis fasti. giata (Chlorophyta) increased with depth (1 versus 20 m). However, shallow Halimeda tUlia (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas. photoinhibition and recovery were depth· and species· specific. Shallow Lobophora a~d Chlo~odesmis ~ai~t~i?ed a greater degree of QA oxidatIon durmg photomhlbltIon. In deep thaIli, reduced effective quantum yield of open photosystem II centres reflected lower proportions and excitation capture efficiencies of open centres. In Lobophora, zeaxanthin formation accompanied non-ph?tochemical fluorescence quenching (NPQ), but m Chlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may compensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity. Key-words: Chlorodesmis fastigiata; Halimeda tuna; Padina fJY~lIlosfiora; Lobofihora variegata; Laurencia illt~'iC(~t~; chlorophyll fluorescence: coral reef: macroalgae: photomhlbltion; photosynthesis,