Responses of two species of Caribbean shallow-water branching corals to changes in ultraviolet radiation

Clear coral reefs waters are highly transparent to ultraviolet radiation (UVR) and therefore, reef organisms should be adapted to tolerate present levels of UVR. However, UVR can severely damage coral tissues and overall physiology. The effects of changes in UVR on the growth, fecundity, and photosynthetic and photoprotective pigments production of the Caribbean shallow-water branching corals Acropora cervicornis (Lamarck 1816) and Porites furcata (Lamarck 1816) were studied by either reducing or enhancing normal UVR doses in two separate experiments. First. UVR was artificially depleted with HyzodRTM acrylic panels and SaranRTM meshes. Second, different colonies were exposed to enhanced UVR by either transplanting colonies of A. cervicornis from deep to shallow areas, or exposing colonies of P. furcata to 10% increased UVR in aquariums located under UV fluorescent bulbs. Growth rates were measured using the Alizarin Red staining method. Fecundity was estimated after histological analysis. Pigments were quantified through High Performance Liquid Chromatography (HPLC) analysis. A positive correlation was found between growth and photosynthetic pigments concentration, and reduced UVR, while the concentration of UV-absorbing compounds (mycosporine-like amino acids or MAA's) was negatively correlated with reduced UVR. Severe bleaching experienced by A. cervicornis colonies transplanted from deep to shallow areas resulted in significantly decreased growth rates and photosynthetic pigments concentration compared to controls, although no significant changes were observed in zooxanthellae densities. This suggests that a specific targeted effect of UVR on the photosynthetic capacity of the zooxanthellae caused the bleaching. Bleached colonies survived by significantly increasing the UVR protection with increased MAA's concentrations and a possible relocation of resources. Similarly, colonies of P. furcata exposed to artificially enhanced UVR significantly reduced their growth rates and photosynthetic pigments concentrations compared to controls exposed to normal UVR. A significant increase in MAA's was also found in colonies of P. furcata under enhanced UVR, while no differences were observed in fecundity compared to controls. While several physical factors may influence reef corals physiology, these results suggest that shallow-water corals could be significantly affected by increases in UVR resulting from the thinning of the Earth's ozone layer.