Role of nitrogen versus phosphorus availability on the effect of UV radiation on bacterioplankton and their recovery from previous UV stress

The influence of nitrogen (N) versus phosphorus (P) availability on the sensitivity of marine bacterioplankton to solar radiation and on their recovery from UV stress was assessed in laboratory experiments under high versus low substrate concentrations. Bacterioplankton were exposed to artificial solar radiation closely resembling natural radiation levels for 4 h and aliquots were subsequently exposed to different radiation ranges for 3 h. Bacterial activity was significantly reduced after exposure to solar radiation, as compared to the activity measured prior to the exposure, only in P-deplete conditions under both, high, and low substrate conditions. Exposure of the bacterioplankton to different radiation ranges following exposure to the full range of solar radiation revealed that nucleotide excision repair is probably more important than the photoenzymatic DNA repair mechanism. Recovery from previous UV stress was similar under N- and P-deplete bacterial growth at high substrate conditions. Under low substrate conditions, however, the recovery efficiency was significantly lower under P- than under N-deplete conditions. Thus, we conclude that to a large extent, P availability determines the sensitivity of bacterioplankton to UV radiation and the recovery efficiency from previous UV stress in oligotrophic surface waters.