In situ experiments were conducted at various depths in the water column to determine the impact of solar UV radiation (280 to 400 nm) upon photosynthesis and DNA of natural phytoplankton assemblages from mid-latitudes of Patagonia (Bahia Bustamante, Chubut, Argentina; 45 degreesS, 66.5 degreesW). The effects of UV radiation were significant at the surface; however, the impact decreased rapidly with depth: at 3 m there was no measurable DNA damage accumulation, whereas at 6 m photosynthetic inhibition was almost zero. UV-A radiation (315 to 400 nm) was mostly responsible for photosynthetic inhibition, while UV-B radiation (280 to 315 nm) had a lesser effect on this process. However, UV-B radiation was very effective in damaging the DNA through the formation of cyclobutane pyrimidine dimers (CPDs) in surface waters. The high initial CPD level found in the natural phytoplankton assemblage decreased when samples were incubated at 3 or 6 m, indicating that at these depths repair, dilution or disappearance of damage occurred. Phytoplankton assemblages were dominated by cells less than 2 mum in effective diameter; this cell size category seems to be more resistant to photosynthetic inhibition, but vulnerable to CPD accumulation, as compared with larger eukaryotic phytoplankters (i.e., Phaeodactylum sp.).
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