INHIBITION OF BACTERIAL ACTIVITIES BY SOLAR-RADIATION IN NEARSHORE WATERS AND THE GULF-OF-MEXICO

The effects of ultraviolet (UV) radiation on thymidine and leucine inc orporation were examined in surface waters from the Gulf of Mexico and Santa Rosa Sound, a mesotrophic estuary in northwest Florida, USA. Wh ole and 0.8 mu m filtered surface waters were incubated with H-3-thymi dine and C-14-leucine in UV transparent containers under natural solar radiation. Solar radiation was either not filtered (samples exposed t o UV-B, UV-A, and photosynthetically active radiation, PAR), filtered through Mylar 500D (samples exposed to UV-A and PAR), or filtered thro ugh Acrylite OP3 (samples exposed only to PAR). In Santa Rosa Sound, t hymidine incorporation was inhibited an average of 44% relative to dar k controls when exposed to unfiltered solar radiation. PAR contributed 23% to the total thymidine inhibition, while UV-A and UV-B contribute d 37% and 39%, respectively, to total inhibition. Leucine incorporatio n in Santa Rosa Sound was inhibited 29% by full solar radiation. The m ajority of the total leucine inhibition was due to UV-B (83%), while P AR only treatments showed leucine incorporation rates 10% higher than dark controls. For the Gulf of Mexico experiments, full solar radiatio n inhibited thymidine incorporation approximately twice as much as leu cine incorporation. However, there were no consistent patterns in diff erences due to different wavelengths. Both thymidine and leucine incor poration were inhibited to a greater extent in <0.8 mu m filtered wate r samples than in whole water samples, suggesting that the presence of primary producers may mediate the detrimental effects of solar radiat ion on bacterioplankton. Surface water was also incubated in situ with thymidine at fixed depths in UV transparent and darkened containers a t 3 locations in the Gulf of Mexico. Total inhibition was 60 to 70% at the surface and was evident to 15 m. Comparison with radiometric data and DNA dosimeters indicated that UV-B exerted the greatest effect in the upper 5 m while below that the inhibition was most Likely due to longer wavelengths. Our results suggest that both UV and visible solar radiation can negatively affect bacterial metabolism and failure to t ake into account the effects of light may result in the overestimation of bacterioplankton production in surface waters.