PHOTOSYNTHETIC RESPONSE OF THE GIANT-KELP MACROCYSTIS-PYRIFERA (PHAEOPHYCEAE) TO ULTRAVIOLET-RADIATION

Solar ultraviolet radiation (UVA + UVB) impairs photosynthesis in mari ne algae. Canopy blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh are exposed to high levels of solar UV in the field. To determ ine the effects of UV radiation on photosynthesis in the giant kelp an d to identify sites of UV damage, O-2 evolution, reaction center organ ization, light harvesting, and energy transfer efficiency were measure d in canopy blades that had been exposed to elevated levels of UV in t he laboratory. UV treatment reduced both the light-saturated rate and the light-limited rate of photosynthesis by 50% but produced no signif icant change in the rate of dark respiration. A significant impairment of photosystem II (PSII) reaction center function was observed, sugge sting the PSII is a major site of damage in chromophytes. Reduced quan tum efficiency of photosynthesis and loss of energy transfer from ligh t-harvesting pigments (fucoxanthin, chlorophyll a, and chlorophyll c) to PSII indicate that the major light-harvesting complex of M. pyrifer a, the fucoxanthin-chlorophyll protein complex (FCPC), was another sit e of UV damage. These measures provide the first evidence of a direct effect of UV radiation on specific sites in the photosynthetic apparat us of chromophytes and indicate that in situ fluorescence excitation a nalysis may be a simple means to detect UV stress in algae.