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

Because Gymnodinium breve Davis exhibits positive phototaxis and often accumulates at or near the air-water interface, the diurnal, in situ physiological responses of this red-tide toxic dinoflagellate were investigated in both cultured and field populations. Cultures were incubated outdoors in temperature-controlled waterbaths, and allowed to acclimate to attenuated (i.e. without UV) natural irradiance. Five-liter aliquots of these photoacclimated cultures were placed in cubitainers fitted with either a quartz window or a quartz window covered with acetate [providing exposure to photosynthetically 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 protein contents over three sinusoidal photoperiods in weather varying from overcast to partly cloudy to extremely sunny. In addition, surface, field populations were assessed for diurnal variations in in vivo fluorescence and in vitro pigmentation over two sinusoidal photoperiods in extremely sunny weather. The maximum quantum yield for stable charge separation at photosystem II (measured by variable fluorescence/maximum fluorescence) exhibited depressions that were roughly symmetrical about solar noon on the overcast and partly cloudy days, but exhibited a pronounced hysteresis on the sunny days 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 accumulation of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Generally, only minor adjustments occurred in the cellular chlorophyll a and fucoxanthin contents. No differences occurred between cultures exposed to PAR-only or PAR + UV treatments in the epoxidation state of the xanthophyll cycle pigments or in the maximum quantum yield for stable charge separation at photosystem II. Differences in oxygen production rates and other biochemical parameters between cultures exposed to PAR-only or PAR + UV treatments were not directly attributable to UV, indicating that G. breve possesses an inherent UV resistance and a robust photosynthetic capability.