Effects of ultraviolet (UV) radiation on marine microalgal-invertebrate symbioses .1. Response of the algal symbionts in culture and in hospite

As ultraviolet (UV) radiation has been implicated in recent reports of ''environmental stress'' in shallow water marine ecosystems, we have studied the effect of UV on two symbiotic associations; the temperate dinoflagellate Symbiodinium californium and its host Anthopleura elegantissima, and the tropical dinoflagellate S. microadriaticum and its host Cassiopeia xamachana. The algae were exposed to UV in the presence of photosynthetically active radiation (PAR) in culture and in hospite, and their responses compared to cells exposed to PAR only. Exposure to UV affected the size of neither S. californium nor S. microadriaticum, but growth rates in culture were significantly reduced under the influence of UV in the former, but not the latter. Synthesis of chlorophylls a and c(2) was depressed by UV in S. microadriaticum, but not in S. californium. UV-depressed cell motility in culture in both species. While S. microadriaticum showed no obvious morphological responses to UV in culture or in hospite, S. californium in culture developed multiple-layered cell walls after 4-wk exposure to UV, and this phenomenon disappeared after the cells were returned to culture conditions in the absence of UV. The production of multiple cell walls was not observed in hospite.