Apart from the emission of ozone-destroying chemicals another threat to the ozone layer is the increasing level of greenhouse gases. Thus, climate change could prolong ozone depletion and thereby increase ultraviolet-B radiation in Antarctica for many years. In the intertidal, benthic marine diatoms were exposed for PAR (P, 400-700 nm), PAR+UV-A (PA, 320-700 nm) and PAR+UV-A+UV-B (280-700 nm) and left to grow on ceramic tiles for up to 3-4 months during two seasons, respectively. No UV effects on biomass, cell number or number of species were observed. In addition, mechanistic effects of enhanced ultraviolet radiation was studied on semi-natural assemblages isolated from 5-7 m depth. Diatoms were settled on white sand and exposed to different time treatments of P, PA and PAB in the laboratory. Photosynthetic efficiency (Fv/Fm) was measured after exposure and after recovery of different time periods (e.g. 10, 20, 40, 60 min and 24 hrs) in dim white light. The UV-B intensity was 0.8 W m-2. Photosynthetic efficiency was significantly reduced after 4 h exposure to PA and PAB while longer exposure of 8 and 16 hours produced only small additional effect on Fv/Fm. This suggests acclimation to UVR. Efficient recovery of photosynthetic efficiency was already measured after 10 min (80% of control) and full recovery was observed after 24 hours. No UV effects were found for DNA damage. Some effects on photosynthetic pigments were observed. The ratio of the xanthophylls diatoxanthin to diadinoxanthin was affected by the UV treatments and this effect increased with exposure time. The results show that Antarctic benthic diatoms can tolerate UVR but the mechanism for this tolerance is still to be elucidated.