Photophysiological responses of the toxic red-tide dinoflagellate Gymnodinium breve (Dinophyceae) under natural sunlight

Because Gymnodinium breve Davis exhibits positive phototaxis and often accu mulates at or near the air-water interface, the diurnal, in situ physiologi cal responses of this red-tide toxic dinoflagellate were investigated in bo th cultured and field populations. Cultures were incubated outdoors in temp erature-controlled waterbaths, and allowed to acclimate to attenuated (i.e. without UV) natural irradiance. Five-liter aliquots of these photoacclimat ed cultures were placed in cubitainers fitted with either a quartz window o r a quartz window covered with acetate [providing exposure to photosyntheti cally available radiation (PAR) + UV or PAR-only, respectively], incubated in Sarasota Bay, Florida (USA), and then assessed for diurnal variations in in vivo fluorescence and in vitro pigmentation, lipid, carbohydrate and pr otein contents over three sinusoidal photoperiods in weather varying from o vercast to partly cloudy to extremely sunny. In addition, surface, field po pulations were assessed for diurnal variations in in vivo fluorescence and in vitro pigmentation over two sinusoidal photoperiods in extremely sunny w eather. The maximum quantum yield for stable charge separation at photosyst em II (measured by variable fluorescence/maximum fluorescence) exhibited de pressions that were roughly symmetrical about solar noon on the overcast an d partly cloudy days, but exhibited a pronounced hysteresis on the sunny da ys for both the cultured and field populations. Induction and relaxation of the xanthophyll cycle over the course of the photoperiod during the partly cloudy and sunny days resulted in stoichiometrically inverse cellular accu mulation of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Generally, only minor adjustments occurred in the cellular chlorophyll a a nd fucoxanthin contents. No differences occurred between cultures exposed t o PAR-only or PAR + UV treatments in the epoxidation state of the xanthophy ll cycle pigments or in the maximum quantum yield for stable charge separat ion at photosystem II. Differences in oxygen production rates and other bio chemical parameters between cultures exposed to PAR-only or PAR + UV treatm ents were not directly attributable to UV, indicating that G. breve possess es an inherent UV resistance and a robust photosynthetic capability.