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

Abstract

Our understanding of the physiological mechanisms that allow marine photoautotrophs to thrive in a high light environment is limited. The pelagic phaeophyte, Sargassum natans (L.) Gaillon, exists at the air–sea interface and often is exposed to high irradiances. During a cruise in the Gulf of Mexico, aggregates of S. natans were collected and maintained in a shipboard incubator under natural sunlight. In vivo fluorescence and pigmentation dynamics were assessed over two daily cycles to characterize the photophysiological responses of this taxon to varying irradiance (i.e. overcast and sunny conditions). The relative proportion of the photosynthetic carotenoid, violaxanthin, to the photoprotective carotenoid, zeaxanthin, decreased during daylight hours. This mirrored the dynamics in the maximum quantum yield for stable charge separation at photosystem II (FV/FM[variable fluorescence/maximum fluorescence]), which decreased (relative to predawn levels) by 50%–60% during periods of sustained bright light and recovered to predawn values 3 h after sunset. The ratio of de‐epoxidized to epoxidized components of the xanthophyll‐cycle pigment pool (violaxanthin, zeaxanthin) was associated with energy dissipation activity within the pigment bed. The operational quantum yield for photosystem II activity (φIIe) was substantially lower than FV/FM due to both a decreased probability that absorbed photons reached open reaction centers and to the induction of nonphotochemical fluorescence quenching (which was rapidly reversible). Bright light also affected the rate of electron flow from the reaction center chlorophyll through to the secondary electron acceptor, quinone B (QB); specifically, single turnover decay curves indicated that the proportion of QB bound to the D1–D2 complex in photosystem II decreased during the protracted periods of bright light. Kautsky curves suggested that the relative proportion of inactive light‐harvesting complexes also increased during periods of bright light. Taken together, these findings suggest that S. natans can tolerate high irradiances by down‐regulating its quantum yield during the day, decreasing its functional absorption coefficient through the uncoupling of light‐harvesting complexes, and decreasing the efficiency with which absorbed light is utilized. These cellular responses appear to be driven by the absolute flux of light and not by an endogenous rhythm, which is phased to a particular time of day.