Influence of UV-B radiation on bacterial activity in coastal waters

The impact of UV-B radiation (290-315 nm) on bacterial activity and abundan ce in coastal water was studied in mesocosm experiments in May 1994 and May 1995 at Kristineberg Marine Research Station, Sweden. Mesocosms (6 m(3)) c ontaining natural pelagic communities were exposed either to ambient irradi ation (AMB), ambient irradiation with enhanced UV-B (+UV) (0.7 W m(-2) 4 h every day around noon), or ambient irradiation screened for UV-B (-UV). Bac terial activity in the mesocosms was measured by means of thymidine incorpo ration in short-term tests during incubations at ambient irradiation, at am bient irradiation with enhanced UV-B, and at ambient irradiation screened f or UV-B. In +UV mesocosms, bacterial activity was significantly stimulated when incubated at ambient radiation. The stimulating effect was suggested t o be due to an increase in carbon or nutrient supply through a photodegrada tion of recalcitrant dissolved organic material (DOM). Low attenuation coef ficients 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 sensit ivity to UV-B exposure in bacterial assemblages that already had been expos ed and stressed by UV-B radiation is suggested to be due to an overburdenin g of the energy-consuming DNA repair mechanism. The data suggest that incre ased UV-B radiation, which might occur with ozone depiction, may both stimu late and suppress bacterial activity in coastal waters, implying that the n et outcome of enhanced UV-B radiation could be an unchanged bacterial activ ity.