Effect of UV radiation on the expulsion of Symbiodinium from the coral Pocillopora damicornis

- The UVR exposure reduced the content of chl a, and carotenoids in the coral.
- Solar radiation modulated the diurnal release pattern of Symbiodinium from coral.
- Release peak time was advanced for an hour when exposed to UVR.

The variation in density of the symbiotic dinoflagellate Symbiodinum in coral is a basic indicator of coral bleaching, i.e. loss of the symbiotic algae or their photosynthetic pigments. However, in the field corals constantly release their symbiotic algae to surrounding water. To explore the underlying mechanism, the rate of expulsion of zooxanthellae from the coral Pocillopora damicornis was studied over a three-day period under ultraviolet radiation (UVR, 280-400 nm) stress. The results showed that the algal expulsion rate appeared 10-20% higher under exposure to UV-A (320-395 nm) or UV-B (295-320 nm), though the differences were not statistically significant. When corals were exposed to UV-A and UV-B radiation, the maximum expulsion of zooxanthellae occurred at noon (10:00-13:00), and this timing was 1 h earlier than in the control without UVR. UVR stress led to obvious decreases in the concentrations of chl a and carotenoids in the coral nubbins after a three-day exposure. Therefore, our results suggested that although the UVR effect on algal expulsion rate was a chronic stress and was not significant within a time frame of only three days, the reduction in chl a and carotenoids may potentially enhance the possibility of coral bleaching over a longer period.