PHOTOSYSTEM-II QUANTUM YIELDS AND XANTHOPHYLL-CYCLE PIGMENTS OF THE MACROALGA SARGASSUM NATANS (PHAEOPHYCEAE) - RESPONSES UNDER NATURAL SUNLIGHT

Our understanding of the physiological mechanisms that allow marine ph otoautotrophs to thrive in a high light environment is limited. The pe lagic phaeophyte, Sargassum natans (L.) Gaillon, exists at the air-sea interface and often is exposed to high irradiances. During a cruise i n the Gulf of Mexico, aggregates of S. natans were collected and maint ained in a shipboard incubator under natural sunlight. In vivo fluores cence and pigmentation dynamics were assessed over two daily cycles to characterize the photophysiological responses of this taxon to varyin g irradiance (i.e. overcast and sunny conditions). The relative propor tion of the photosynthetic carotenoid, violaxanthin, to the photoprote ctive carotenoid, zeaxanthin, decreased during daylight hours. This mi rrored the dynamics in the maximum quantum yield for stable charge sep aration at photosystem II (F-V/F-M [variable fluorescence / maximum fl uorescence]), which decreased (relative to predawn levels) by 50%-60% during periods of sustained bright light and recovered to predawn valu es 3 h after sunset. The ratio of de-epoxidized to epoxidized componen ts of the xanthophyll-cycle pigment pool (violaxanthin, zeaxanthin) wa s associated with energy dissipation activity within the pigment bed. The operational quantum yield for photosystem II activity (phi(IIe)) w as substantially lower then F-V/F-M due to both a decreased probabilit y that absorbed photons reached open reaction centers and to the induc tion of nonphotochemical fluorescence quenching (which was rapidly rev ersible). Bright light also affected the rate of electron flow from th e reaction center chlorophyll through to the secondary electron accept or, quinone B (Q(B)); specifically, single turnover decay curves indic ated that the proportion of Q(B) bound to the D1-D2 complex in photosy stem II decreased during the protracted periods of bright light. Kauts ky curves suggested that the relative proportion of inactive light-har vesting complexes also increased during periods of bright light. Taken together, these findings suggest that S. natans can tolerate high irr adiances by down-regulating its quantum yield during the day, decreasi ng its functional absorption coefficient through the uncoupling of lig ht-harvesting complexes, and decreasing the efficiency with which abso rbed light is utilized. These cellular responses appear to be driven b y the absolute flux of light and not by an endogenous rhythm, which is phased to a particular time of day.