UV DAMAGE TO PLANT LIFE IN A PHOTOBIOLOGICALLY DYNAMIC ENVIRONMENT - THE CASE OF MARINE-PHYTOPLANKTON

The effect of UV-B radiation on growth of marine phytoplankton was inv estigated in relation to DNA damage induced by a range of biologically effective doses (BEDs). Emiliania huxleyi (Prymnesiophyceae) was chos en as a model organism of the ocean's phytoplankton because of its imp ortance in global biogeochemical cycling Of carbon and sulphur, elemen ts that influence the world's climate as components of the trace gases carbon dioxide (CO2) and dimethylsulfide (DMS). A marine diatom, Cycl otella, was studied for its capacity to repair the DNA damage, quantif ied as thymine dimers by the application of a monoclonal antibody agai nst these photoproducts. DNA repair was shown to be complete after jus t a few hours of exposure to visible light; the repair rate increased with PAR intensity. E. huxleyi appeared to be most sensitive to UV-B r adiation: growth was already affected above a dose of 100 J m(-2) d(-1 ) (biologically effective radiation, weighted with Setlow's DNA action spectrum), probably through effects on the cell cycle related to dama ge to nuclear DNA: mean specific growth rates were inversely correlate d with thymine dimer contents in cells. Near the ocean's surface UV-B radiation conditions that induce the changes observed by us in culture s can be expected during the growing season of phytoplankton, not only in the tropics but also at higher latitudes. Nevertheles, blooms of s pecies such as E. huxleyi are often excessive in the field. It is sugg ested that exposure duration of cells near the surface of the ocean ca n be shorter than our artificial mixing reaches depths greater than th e layer where most UV-B is attenuated, negative effects on cells throu gh W-A-induced inhibition of photosynthesis may prevail over DNA damag e, the action spectrum of which has been shown to be limited to the UV -B part of the spectrum. Moreover, the radiation wavelengths that indu ce DNA damage repair (UV-A and visible) are attenuated vertically much less than UV-B. The photobiological situation in the upper ocean is m uch more complicated than on land, and effects of UV radiation on plan kton biota can only be modelled realistically here when both the spect rally differential attenuation in the UV and visual part of the spectr um and the rate of vertical mixing are taken into account. Action spec tra of both damage and repair of DNA and of photosynthesis inhibition of representative microalgal species are the second conditio sine qua non if we want to predict the effect of stratospheric ozone depletion on marine phytoplankton performance.