RAPID RECOVERY OF MARINE BACTERIOPLANKTON ACTIVITY AFTER INHIBITION BY UV-RADIATION IN COASTAL WATERS

Laboratory and in situ experiments were performed in order to evaluate the role of W radiation on bacterial activity. Particular attention w as given to the determination of the role of W-A and photosynthetic ac tive radiation (PAR) and different nutrient conditions on the recovery of bacterial activity. Laboratory experiments with nearly natural rad iation intensities indicated a 20 to 40% reduction from the initial le vel of bacterial activity after W-B exposure for 2 to 4 h. Bacterial a ctivity in freshly collected seawater showed a more pronounced inhibit ion and faster recovery than bacterial activity in aged, nutrient-depl eted seawater. The results of in situ experiments with filtered water (0.8-mu m-pore-size filter) and natural surface solar radiation levels agreed with those of the laboratory experiments and revealed that W-A and PAR are important for the recovery of bacterial activity and resu lt in levels of bacterial activity that are higher than those prior to exposure to full solar radiation. Bacterioplankton exposed to full so lar radiation for 3 h and subsequently incubated at different depths w ithin the upper mixed water column showed an increase in bacterial act ivity with increased depth; the highest bacterial activity was detecte d at depths of 5.5 to 10.5 m, where the short-wavelength W-B was alrea dy largely attenuated, but enough long wavelength W-A and short PAR we re available to allow recovery, This elevated bacterial activity follo wing exposure to UV-B was attributed to the photolysis of dissolved or ganic matter (DOM) exposed to near-surface radiation and to the rapid recovery of bacteria from UV stress once they were mixed into deeper l ayers of the upper mixed water column, where they efficiently utilize the photolytically cleaved DOM. It is concluded that studies on the ro le of W on the carbon and energy flux through the upper layer of the o cean should take into account the highly dynamic radiation conditions.