UV-B induces DNA damage and DNA synthesis delay in the marine diatom Cyclotella sp.

The effect of UV-B on the occurrence of DNA damage and consequences for the cell cycle were studied in the marine diatom Cyclotella sp. DNA damage was quantified by immunofluorescent detection of thymine dimers in nuclear DNA of single cells using flow cytometry. A total UV-B dose (biologically effective dose BE(DNA300nm)) of 1.05 kJ m(-2) caused detectable damage in about 20% of the exposed population. In the period after the UV-B treatment, when the culture was exposed to PAR (450 mu mol m(-2)s(-1)) only, thymine dimers were removed; after 8 hours none of these photoproducts remained. Cellular DNA content measurements and quantification of the fraction of recently divided cells revealed that the DNA synthesis rate as well as the rate of cell division were reduced during and shortly after UV-B exposure. Apparently, UV-B irradiation extends the cell cycle of Cyclotella sp. in the S (DNA synthesis) phase until the dimers are removed.