Photoinhibition by visible and ultraviolet radiation in the red macroalga Porphyra umbilicalis grown in the laboratory

Photosynthetic oxygen production and PAM fluorescence measurements were use d to follow photoinhibition in the red macroalga Porphyra umbilicalis. Expo sure to simulated solar radiation caused inhibition of the effective photos ynthetic quantum yield from which the thalli partially recovered in the sha de in subsequent hours. There were no significant differences between sampl es exposed to unfiltered radiation and those exposed to radiation from whic h increasing portions of UV radiation had been removed indicating that the thalli are well adapted to current levels of solar PAR and UV radiation. Th is notion was supported by the finding of high concentrations of UV screeni ng pigments which were even enhanced by exposure to increased UV radiation. However, when exposed to (only) UV radiation about 50% higher than that en countered by the organisms in their natural habitat, the photosynthetic yie ld decreased slowly and did not show any recovery even when the degree of i nhibition did not exceed 10%.