Ultraviolet-B radiation and bacterial metabolism in coastal waters

Abstract

The impact of ultraviolet-B (UV-B) radiation on bacterial density and production and on extracellular enzymatic activity was investigated the northern Adriatic Sea. Samples were incubated in quartz bottles and exposed to natural solar radiation (0.5 W m -2 ) as well as to artificial UV-B (0.4 W m -2 ) sources. Exposure to artificial UV-B sources over a period of 12 h revealed a constant decline in bacterial density to about 60% of the corresponding dark value. Total lipase and leucine-aminopeptidase activity showed a decrease to 38.8 and 21.9%, respectively, of the dark control; dissolved leucine-aminopeptidase activity was significantly more affected (15.3 % of the corresponding dark value) than dissolved lipase activity (43.2 % of the corresponding dark value). Samples exposed for 6 h to artificial UV-B or for 4 h to natural solar radiation exhibited rapid recovery during subsequent dark incubation. Following UV-B exposure (0.4 W m -2 ) bacterial density recovered rapidly from 74.6 % to 84.1%, lipase activity recovered from 64 % to 80 % and leucine-aminopeptidase activity from 53 % to 71% of the corresponding dark values during 6 h of subsequent dark incubation. Recovery of bacteria following exposure to natural solar UV-B radiation with similar intensity was even higher. In these experiments bacterial density reached similar values as in the dark control, bacterial production even exceeded the dark control production rates after 6 h of dark incubation following UV-B exposure. This difference might be attributed to photorepair induced by UV-A and to increased availability of dissolved organic matter due to UV-B mediated photolysis. UV-B radiation levels of 0.4 W m -2 as used in this study are detectable in the surface layers of the northern Adriatic Sea up to 0.5 m depth for at least 3 to 5 h d -1 during summer. Thus our results suggest that microbial life might be affected by UV-B radiation and consequently also the carbon and energy flow in aquatic systems.