UVBR-induced DNA damage in natural marine picoplankton assemblages in the tropical Atlantic Ocean

UVBR (ultraviolet-B radiation: 280 to 315 nm)-induced DNA damage, measured as cyclobutane pyrimidine dimers (CPDs), was determined in size fractions o f natural populations of bacterio- and phytoplankton collected in marine tr opical waters. Mean biologically effective UVBR doses in the wind-mixed lay er were calculated from DNA dosimeter data. Phytoplankton species compositi on in these waters was monitored using flow cytometry and pigment analyses. In terms of (divinyl-)chlorophyll a concentrations, prochlorophytes and cy anobacteria comprised the largest fraction of the phytoplankton, except in a eutrophic bay at Curacao an island located in the southern Caribbean. In terms of cell numbers and amount of DNA, small prochlorophytes and marine b acteria dominated. Small but detectable levels of UVBR-induced DNA damage w ere found at all locations. In general, more DNA damage was found in the sm all size fraction (0.2 to 1 mu m) than in the larger size fraction (1 to 10 mu m). The greatest amount of damage was found in the small size fraction collected in the central Atlantic Ocean (20 CPDs/10(6) nucleotides), despit e the fact that UVBR doses were much higher at other locations. The calcula ted mean biologically effective UVBR doses in the wind-mixed layer were 2 t o 17 times lower as compared with incident UVBR doses. CPD levels determine d in cultures of the cyanobacterium Synechococcus sp, subjected to UVBR dos es similar to those in the wind-mixed layer corresponded with CPD levels me asured in the 1 to 10 mu m fraction in the field. Our results indicate that UVBR vulnerability is size dependent. Furthermore, the low CPD levels obse rved in these field communities may be explained by the low mean biological ly effective doses received by the cells as a result of wind-induced mixing .