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Inhibitory Effect of Solar Radiation on Thymidine and Leucine incorporation by Freshwater and Marine Bacterioplankton In press: Applied Environmental Microbiology.(1997) Ruben Sommaruga,1* Ingrid Obernosterer,2 Gerhard J. Herndl,2 and Roland Psenner1 Institute of Zoology and Limnology, University of Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria,1 and Institute of Zoology, Dept. of Marine Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria2 *Corresponding author: Ruben.Sommaruga@uibk.ac.at ABSTRACT We studied the effect of solar radiation on the incorporation of [3H]-Thymidine (TdR) and [14C]-Leucine (Leu) by bacterioplankton in a high mountain lake and the northern Adriatic Sea. After short-term exposure (3-4 h) of natural bacterial assemblages to sunlight just beneath the surface, the incorporation rate of TdR and Leu was reduced at both sites by up to ~ 70% compared to the dark control. Within the solar UV radiation (290-400 nm) the inhibition was exclusively caused by UV-A (320-400 nm) radiation. However, PAR (400-700 nm) contributed almost equally to this effect. Experiments made in the high mountain lake showed that at 2.5 m depth the inhibition was caused almost exclusively by UV-A radiation. At 8.5 m depth, where chlorophyll a concentrations were higher than in the upper water column, the incorporation rates of TdR were higher in those samples exposed to full sunlight or to UV-A plus PAR than in the dark control. In laboratory experiments with artificial UV light, the incorporation of TdR and Leu by mixed bacterial lake cultures was also mainly inhibited by UV-A. In contrast, in the presence of the green alga Chlamydomonas geitleri at a chlorophyll a concentration of 2.5 µg l-1, inhibition by UV radiation was significantly reduced. These results suggest that there may be complex interactions among UV radiation, heterotrophic bacteria, and phytoplankton and their release of extracellular organic carbon. Our findings indicate that the wavelengths which caused the strongest inhibition of TdR and Leu incorporation by bacterioplankton in the water column were in the UV-A range. However, it may be premature to extrapolate this effect to estimates of bacterial production before more precise information is obtained on how solar radiation affects the transport of TdR and Leu into the cell.