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The impact of UV-B radiation (290-315 nm) on bacterial activity and abundance in coastal water was studied in mesocosm experiments in May 1994 and May 1995 at Kristineberg Marine Research Station, Sweden. Mesocosms (6 m3) containing natural pelagic communities were exposed either to ambient irradiation (AMB), ambient irradiation with enhanced UV-B (+UV) (0.7 W m-2 4h every day around noon), or ambient irradiation screened for UV-B (-UV). Bacterial activity in the mesocosms was measured by means of thymidine incorporation in short-term tests during incubations at ambient irradiation, at ambient irradiation with enhanced UV-B, and at ambient irradiation screened for UV-B. In +UV mesocosms, bacterial activity was significantly stimulated when incubated at ambient radiation. The stimulating effect was suggested to be due to an increase in carbon or nutrient supply through a photodegradation of recalcitrant dissolved organic material (DOM). Low attenuation coefficients for UV-B and PAR (400-700 nm) in the +UV mesocosms supported this hypothesis. The bacterial activity in +UV mesocosms, however, was inhibited when incubations were made at enhanced UV-B irradiation, implying that the bacteria had become more sensitive to UV-B radiation. The increased sensitivity to UV-B exposure in bacterial assemblages that already had been exposed and stressed by UV-B radiation is suggested to be due to an overburdening of the energy- consuming DNA repair mechanism. The data suggest that increased UV-B radiation, which might occur with ozone depletion, may both stimulate and suppress bacterial activity in coastal waters, implying that the net outcome of enhanced UV-B radiation could be an unchanged bacterial activity. |
