UV-ABSORBING SUBSTANCES IN THE TUNIC OF A COLONIAL ASCIDIAN PROTECT ITS SYMBIONT, PROCHLORON SP, FROM DAMAGE BY UV-B RADIATION

Photosynthesis by cells of Prochloron sp. freshly isolated from the as cidian host, Lissoclinum patella, collected from shallow waters in Pal au, was severely inhibited by ultraviolet (UV) irradiation. No photoin hibition, however, was observed in Prochloron cells isolated from inta ct colonies after UV irradiation, suggesting some protection by the as cidian host. It was shown that UV protection was brought about by the thick gelatinous tunic covering the whole ascidian colony. Analysis re vealed that the surface tunic of L. patella, although transparent to v isible light, contains several UV-absorbing substances, identified by highperformance liquid chromatography as mycosporine-like amino acids (MAAs). The predominant MAA identified was shinorine (lambda(max) = 33 4 nm), followed by mycosporine-glycine (lambda(max) = 310 nm) and a sm all amount of palythine (lambda(max) = 320 nm). Although isolated Proc hloron cells also contained shinorine, on a protein-weight basis it wa s less than half of that observed in the host tunic. These results sug gest that the surface tunic of a L. patella colony, which is transpare nt to visible light for photosynthesis, also contains UV-absorbing com pounds that protect its photoautotrophic symbiont, Prochloron sp., fro m damage by the intense UV-irradiation that they receive daily in shal low, tropical marine waters.