In January 1999, unialgal cultures of the diatom Thalassiosira sp., solate from natural phyto-plankton assemblages from Potter Cove, Antarctica, were exposed to solar ultraviolet radiation (UVR, 280400 nm) in order to study the long-term acclimation of this species. Ultraviolet radiation B (UVBR, 280-315 nm) inhibited the growth rate during the first and second days of exposure. No UVBR inhibition was observed on the third day. The initial content of &#945;-tocopherol (13 pmol (10(4) cell)-1) showed a marked decrease during the exponential growth phase (4 pmol (10(4) cell-1) by day 3). The initial content of &#946;-carotene (3 pmol (10(4) cell)-1) did not show significant differences over time in cells exposed to UVBR. Two mycosporine-like amino acids (MAAs) were identified: porphyra-334 and shinorine. Cellular concentrations of MAAs increased significantly on days 2 and 3, and exposure of the algae to UVBR significantly enhanced this value. The relative importance of MAAs concentration was significant (P < 0.05) in relation to the &#945;-tocopherol content. A positive correlation was shown between cellular MAAs concentration and growth rate. Our results suggest that photoprotection against UV-induced damage is characterized by short-term consumption of &#945;-tocopherol and longer-term synthesis of MAAs. The UVBR damage/repair ratio during long-term exposure involves the combined action of several endogenous factors within the cell, with MAAs synthesis being the most effective factor related to photoprotection.