Effect of copper on algal-host interactions in the symbiotic coral Plesiastrea versipora

Abstract Blue and green morphs of the scleractinian coral Plesiastrea versipora were exposed to increasing amounts of copper for periods of 12 (0–5.04 μM) to 36 d (two cycles of 0–7.55 μM). Algae were not expelled and there was no decrease in photosynthesis or chlorophyll a . Chlorophyll c decreased in only one out of four experiments. Total carotenoids increased in all the experiments. Two host signalling compounds that regulate algal carbon metabolism, host release factor (HRF) which stimulates algal photosynthate release, and photosynthesis inhibiting factor (PIF) which partially inhibits photosynthetic carbon fixation, were examined. PIF activity was slightly higher after 16 and 36 d of exposure to copper, but HRF activity was always similar to seawater control corals. Algae isolated from corals previously exposed to copper showed a variable response. In one experiment (12 d exposure), algae were not inhibited by PIF and stimulation by HRF was lower than in algae isolated from seawater control corals. In another experiment (36 d exposure), algae still responded to HRF but were less responsive to PIF. In parallel with the length of exposure to copper, oxidative damage was evident in coral tissues, as a decrease in the levels of tryptophan (up to 34%) and coral fluorophores (16–88%). This study shows that changes in fluorescent compounds can give an early indication of copper-induced damage in this coral before damage is outwardly visible. This is the first study to show that copper may alter algal responses to two host signalling compounds that regulate symbiotic algae.