Preliminary evidence for tissue retraction as a factor in photoprotection of corals incapable of xanthophyll cycling

In a 6-h experiment where xanthophyll inter-conversion was blocked by the inhibitor dithiothreitol (DTT), corals in full sunlight showed a significant increase in oxidative damage and cnidarian antioxidant enzyme concentrations compared with controls. By comparison, antioxidant enzyme concentrations did not increase within the symbiotic algae. In addition, the normal reduction in steady state chlorophyll fluorescence (F_t) and maximum fluorescence (F_m′), in response to increased irradiance, was initially suppressed in the DTT-treated corals, but after 6 h, both parameters had reduced to levels similar to controls with a functional xanthophyll cycle. Extreme host retraction was observed in corals treated with DDT in full sunlight compared with untreated corals in full sunlight and DTT-treated corals in dim irradiance. These results suggest that the coral host is capable of using behavioural (tissue retraction) and biochemical defences (antioxidant enzymes) to protect the symbiotic algae under high natural irradiance when the xanthophyll cycle is absent. While these defences could not prevent oxidative damage, nonetheless, algal numbers and algal chlorophyll levels were not affected.