Emiliania huxleyi (Lohm.) Hay et Miller is an important component of the ph
ytoplankton in open ocean waters. The sensitivity of this cosmopolitan alga
to natural levels of UVB radiation has never been tested. Since DNA is bel
ieved to be a major target of natural UVB radiation (UVBR: 280-315 nm) in l
iving cells, experiments with E. huxleyi were performed using growth rate r
eduction and DNA damage as indicators of UVBR stress. Specific growth rate,
cell volume, pigment content, and CPD (cyclobutane pyrimidine dimer) forma
tion (a measure for DNA damage) were followed during and after prolonged ex
posure of a series of cultures to a range of UVBR levels. E. huxleyi was fo
und to be very sensitive to UVBR: at a daily weighted UVBR dose of only 400
J.m(-2).d(-1) (BEDDNA300nm), growth was halted. At this UVBR level, both c
ell volume and contents of the major photosynthetic and photoprotective pig
pigments had increased. The UVBR vulnerability of huxleyi cannot be explai
ned by a high potential for cyclobutane thymine dimer formation (the most a
bundant CPD type) due to a high T content of nuclear DNA: the CG content of
this E huxleyi strain is high (68%) compared with other species. The high
UVBR sensitivity may be related to the stage of the cell cycle during UVBR
exposure, in combination with low repair capacity, It is concluded that E.
huxleyi may experience UVBR stress through the formation of cyclobutane pyr
imidine dimers, with subsequent low repair capacity and thereby arrest of t
he cell cycle.