The reef coral Pocillopora damicornis and the symbiotic dinoflagellate algae commonly known as zooxanthellae (Symbiodinium microacriaticum = Gymnodinium microadriaticum) were grown in full spectrum solar radiation and in filtered sunlight lacking solar ultraviolet
(UV). Skeletal growth rate of the coral was higher in the absence of UV. Corals grown in the treatment with UV contained a greater concentration of the “S-320” UV-absorbing substances. This material apparently protected the algae and the host from UV. The zooxanthellae grown in
vivo did not appear to be damaged by full spectrum solar irradiance. Photosynthetic pigment concentration and number of algal cells per unit area were similar in both treatments. In contrast, unshielded in vitro cultures of zooxanthellae and various other unicellar algae were severely
impaired. Formation of a symbiosis apparently has allowed the plant and animal partners to derive a net benefit from intense solar radiation in spite of UV. Such symbioses appear to have a photobiological advantage over many nonsymbiotic competitors in high-UV tropical marine environments. The
impact of UV was much greater on zooxanthellae isolated from the “shade-loving” anemone Aiptasia than it was on zooxanthellae isolated from the “sun-loving” scyphozoan Cassiopea. Genetic differences in the algae as well as the host seem to be involved
in photoadaptation of symbioses to different regimes of solar irradiance. Both the 320–400 nm portion of the spectrum (UV-A) and the 280–320 nm band (UV-B) produced significant photoinhibition in these cultures.
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