Patterns of DNA damage and photoinhibition in temperate South-Atlantic picophytoplankton exposed to solar ultraviolet radiation

Natural marine phytoplankton assemblages from Bahia Bustamante (Chubut, Arg entina, 45 degreesS, 66.5 degreesW), mainly consisting of cells in the pico plankton size range (0.2-2 mum), were exposed to various UVBR (280-315 nm) and UVAR (315-400 mn) regimes in order to follow wavelength-dependent patte rns of cyclobutane pyrimidine dimer (CPD) induction and repair. Simultaneou sly, UVR induced photosynthetic inhibition was studied in radiocarbon incor poration experiments. Biological weighting functions (BWFs) for photoinhibi tion and for CPD induction, the latter measured in bare calf thymus DNA, di ffered in the UVAR region: carbon incorporation was reduced markedly due to UVAR, whereas no measurable UVAR effect was found on CPD formation. In con trast, BWFs for inhibition of photosynthesis and CPD accumulation were fair ly similar in the UVBR region, especially above 300 um. Incubation of phyto plankton under full solar radiation caused rapid CPD accumulation over the day, giving maximum damage levels exceeding 500 CPD.MB-1 at the end of the afternoon. A clear daily pattern of CPD accumulation was found, in keeping with the DNA effective dose measured by a DNA dosimeter. In contrast, UVBR induced photosynthetic inhibition was not dose related and remained nearly constant during the day. Screening of U-VBR or UVR did not cause significan t CPD removal, indicating that photoreactivation either by PAR or UVAR was of minor importance in these organisms. Fligh CPD levels were found in situ early in the morning, which remained unaffected notwithstanding treatments favoring photorepair. These results imply that a proportion of cells had b een killed by UVBR exposure prior to the treatments. Our data suggest that the limited potential for photoreactivation in picophytoplankton assemblage s from the southern Atlantic Ocean causes high CPD accumulation as a result of UVBR exposure. (C) 2001 Elsevier Science BY All rights reserved.