Depth-dependant response to light of the reef building coral, Pocillopora verrucosa: Implication of oxidative stress

Several environmental factors have been described to trigger bleaching in cnidarian/dinoflagellate endosymbiosis. However, the molecular mechanisms underlying this process still need more investigations. Symbiosis breakdown is known to result from physiological damage to animal host cells and/or symbionts. Cellular oxidation appears to be an essential player in this damage. Indeed, oxidative stress is a direct consequence of increase in irradiance and temperature, the two main environmental factors involved in bleaching. In this study, we examined the role of irradiance in inducing dissociation and oxidative stress in cnidarians and dinoflagellates. We used the bleaching-sensitive scleractinian coral Pocillopora verrucosa in a field cross-transplantation experiment performed between 5 m and 20 m depth at Grande Glorieuse Island (Indian Ocean), a preserved area subject to minimal anthropogenic influence. Cellular damage and increase in antioxidant defense were correlated with bleaching in upward transplanted samples. Downward transplanted colonies presented no associated alterations similar to the controls. We therefore conclude that increasing light induced bleaching via a prooxidative period. Remarkably, the distribution of Symbiodinium over depth was invariant; all colonies were monomorph for clade C, suggesting that bleaching sensitivity of P. verrucosa might not be associated with clade specificity.