Limnol. Oceanogr., 44(7), 1999, 1667–1682

Colonies of Stylophora pistillata maintained for four years in indoor aquaria in the near absence of ultraviolet radiation (UVR) contained only small amounts (,5 nmol mg21 protein) of 10 identified mycosporine-like amino acids (MAAs, which act as UV sunscreens), the largest number reported in any organism. The concentrations of most MAAs increased linearly or exponentially when colonies were exposed to ultraviolet-A (UVA) and ultraviolet-B (UVB) for 8 h d21 in the presence of photosynthetically active radiation (PAR). Total MAA concentration reached 174 nmol mg21 protein after 30 d, with palythine and mycosporine-2 glycine constituting more than half of the final total. UVB specifically stimulated MAA accumulation: after 15 d, MAA levels in colonies exposed to PAR alone and to PAR and UVA did not differ (7 and 5 nmol MAA mg21 protein, respectively), while those in colonies exposed to PAR and UVA 1 UVB were significantly higher (28 nmol mg21 protein). Glyphosate, an inhibitor of the shikimate pathway, eliminated or reduced the UV-induced accumulation of most MAAs during 7 d of exposure, providing the first experimental evidence of their synthesis via this pathway in a coral symbiosis. Densities of zooxanthellae in colonies of S. pistillata, Acropora sp., and Seriatopora hystrix exposed to UVR for 15 d were only one-third of those in control colonies unexposed to UVR. This net decrease in the number of zooxanthellae in the corals (bleaching) occurred despite UV-stimulated increases in algal cytokinesis and in the host cell-specific density of zooxanthellae in hospite, increases that apparently destabilized the symbiosis and caused expulsion of the zooxanthellae. The separate and interacting effects of solar ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) on coral reef ecosystems are by now well documented 1 Present address: Observatoire Océanologique, UPRESA 7076 CNRS-UPMC, B.P. 28, F-06234 Villefranche-sur-Mer Cedex, France.