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 of natural populations of bacterio- and phytoplankton collected in marine tropical waters. Mean biologically effective UVBR doses in the wind-mixed layer were calculated from DNA dosimeter data. Phytoplankton species composition in these waters was monitored using flow cytometry and pigment analyses. In terms of (divinyl-)chlorophyll a concentrations, prochlorophytes and cyanobacteria 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 bacteria dominated. Small but detectable levels of UVBR-induced DNA damage were found at all locations. In general, more DNA damage was found in the small 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), despite the fact that UVBR doses were much higher at other locations. The calculated mean biologically effective UVBR doses in the wind-mixed layer were 2 to 17 times lower as compared with incident UVBR doses. CPD levels determined in cultures of the cyanobacterium Synechococcus sp, subjected to UVBR doses similar to those in the wind-mixed layer corresponded with CPD levels measured 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 observed in these field communities may be explained by the low mean biologically effective doses received by the cells as a result of wind-induced mixing.