Long-term effects of natural sunlight under various ultraviolet radiation conditions on growth and photosynthesis of intertidal Ulva rigida (Chlorophyceae) cultivated in situ

Long-term effects of full-spectrum solar radiation, solar radiation without UVB, and solar radiation without total UV (UVA+UVB) radiation were studied in intertidal Ulva rigida C. Agardh (Chlorophyta). The experiment was carr ied out under natural conditions and at prevailing winter levels of UV radi ation. Changes in relative growth rate, photosynthesis and pigment content were studied after 7 and 20 d of cultivation under the three solar radiatio n conditions. Relative growth rate was enhanced (50 %) in the absence of UV B radiation after one week, but no difference was found after 20 d. Pigment content (chlorophyll a, chlorophyll b and carotenoids) was significantly h igher in the presence of UVB, suggesting an efficient protective-pigment me chanism. When UVB was removed, photosynthetic performance measured as oxyge n exchange and fluorescence parameters exhibited higher values than under t he other treatments after 7 d of incubation. When samples were cultivated u nder PAR alone, photosynthetic performance was always negatively affected c ompared to in the presence of UV radiation. On the whole, intertidal U. rig ida exhibited a remarkable capacity to cope with fluctuating light conditio ns.