Species-specific responses of freshwater diatoms to solar ultraviolet radiation

We studied species-specific growth responses of freshwater algae to solar ultraviolet radiation (UVR) by measuring growth-irradiance curves (G-E curves) of 17 algal diatoms (isolated from a periphyton community in a lotic system) in a solar simulator under photosynthetically available radiation plus UVR and minus UVR radiation condition. Algal growth responses to UVR were highly species-specific, and the photoinhibition parameter (β) of the +UVR condition differed greatly among species. We compared the degree of UVR effect on algal growth (UVR inhibition index: difference in β between +UVR and -UVR conditions) with parameters derived from the G-E curve and with algal cell morphology. UVR inhibition indexes significantly correlated with the initial slope of light-limited portion of G-E curve (α) and with the irradiance at which the growth rate starts to saturate (E_k), but not with the cell surface to volume ratios. Low-light-adapted algae tended to be more susceptible to growth inhibition by UVR than high-light-adapted ones. Algae that formed dense colonies in the growth medium were relatively tolerant of UVR irrespective of E_k values. Although past studies have emphasized that UVR effects on algae are algal-size dependent, the growth strategies dealing with solar radiation and algal growth forms appear to be more valid in correctly interpreting species-specific responses of algae to solar UVR.