THE EFFECTS OF EXCESS IRRADIANCE ON PHOTOSYNTHESIS IN THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM

The response of Phaeodactylum tricornutum to excess light was remarkab ly similar to that observed in higher plants and green algae and was c haracterized by complex changes in minimal fluorescence yields of full y dark-adapted samples and declines in maximum variable fluorescence l evels and oxygen evolution rates. In our study the parallel decreases in the effective rate constant for photosystem II (PSII) photochemistr y, the variable fluorescence yield of a dark-adapted sample, and light -limited O-2 evolution rates after short (0-10 min) exposures to photo inhibitory conditions could not be attributed to damage or down-regula tion of PSII reaction centers. Instead, these changes were consistent with the presence of nonphotochemical quenching of PSII excitation ene rgy in the antennae. This quenching was analogous to that component of nonphotochemical quenching studied in higher plants that is associate d with photoinhibition of photosynthesis and/or processes protecting a gainst photoinhibition in that it did not relax readily in the dark an d persisted in the absence of a bulk transthylakoid proton gradient. T he quenching was most likely associated with photoprotective processes in the PSII antenna that reduced the extent of photoinhibitory damage , particularly after longer exposures. Our results suggest that a larg e population of damaged, slowly recovering PSII centers did not form i n Phaedactylum even after 60 min of exposure to excess actinic light.