ROLE OF ULTRAVIOLET-B RADIATION ON BACTERIOPLANKTON AND THE AVAILABILITY OF DISSOLVED ORGANIC-MATTER

Attenuation of ultraviolet (UV)-radiation into the water column is hig hly correlated with the concentration of the dissolved organic matter (DOM). Thus UV penetrates deeper into marine waters than into freshwat er systems. DOM is efficiently cleaved by solar surface radiation leve ls consuming more oxygen than bacterial metabolism. This photolyticall y cleaved DOM exhibits higher absorbance ratios (250/365 nm) than untr eated DOM. Natural bacterioplankton reach higher abundance if inoculat ed in previously solar-exposed DOM than in untreated DOM; during bacte rial growth the absorbance ratio declines steadily indicating the util ization of the photolytically cleaved DOM. On the other hand, bacterio plankton are greatly reduced in their activity if exposed to surface s olar radiation levels. Photoenzymatic repair of DNA induced by W-A rad iation, however, leads to an efficient recovery of bacterial activity once the UV-B stress is released. Turbulent mixing of the upper layers of the water column leads to a continuous alteration of the UV exposu re regime. Close to the surface, bacteria and DOM are exposed to high levels of UV-B leading to a reduction in bacterial activity and to pho tolysis of DOM. Once mixed into deeper layers where W-B is attenuated, but sufficient UV-A is remaining to allow photoenzymatic repair, the photolytically cleaved DOM is efficiently taken up by bacterioplankton leading to even higher bacterial activity than prior to the exposure. Thus, the overall effect of UV on bacterioplankton is actually an enh ancement of bacterial activity despite their lack of protective pigmen ts.