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Ultraviolet Radiation Effects on a Microscopic Green Alga and the Protective Effects of Natural Dissolved Organic Matter

Abstract— The population and photosynthetic responses of a microscopic green alga (Selenastrum capricornutum) to realistic levels of UV radiation (UVA and UVB) were assessed in natural lake waters of different dissolved organic carbon (DOC) concentration. Specific growth rates and photosynthetic competence (as reflected by Fv/Fm [measure of maximal quantum efficiency of photosystem II] and t1/2 [estimate of electrons transported to the plastoquinone pool] measured by in vivo variable chlorophyll a fluorescence) were compared between two exposure levels of UVR and two concentrations of DOC (2.5 mg C L−1 7.7 mg C L−1). Exposure periods of 6–9 days (five to nine generations) were used. Exposure to UVA primarily affected the efficiency of photosystem II, as evidenced by significant decreases of Fv/Fm but not growth rates or t1/2 Exposure to UVB, in the presence of UVA, did not cause significant additional decreases of Fv/Fm but did diminish growth rates. In the low DOC water, t1/2 was also diminished, suggesting different proximate sites of action from those for UVA. The high DOC water decreased the effective exposure to both UVA and UVB and diminished the negative impact of UV radiation on the cells, but the apparent protection was not explicable solely by the shading action of the DOC. Control values for Fv/Fm, growth rates and t1/2 were all lower in the high DOC water, suggesting a negative side effect to the apparent protective action of the DOC against UVB.