The continuing degradation of the Earth's ozone layer by atmospheric pollutants has generated concern about the impact of increased solar ultraviolet-B radiation (UV-B) on aquatic ecosystems. UV-B is a small (less than 1% of total energy) but highly active component of the solar spectrum that can penetrate to biologically significant depths in lakes and oceans. It has the potential to cause wide-ranging effects, including mutagenesis, chronic depression of key physiological processes, and acute physiological stress that may result in death. There are major uncertainties at present about the appropriate time scales and bioassay protocols for assessing such effects. Algal and cyanobacterial cells have four lines of defence against the toxic effects of UV-B. Some species avoid UV exposure by their choice of habitat or by migration strategies. Many species produce sunscreening pigments that filter out UV wavelengths; mycosporine-like amino acids are an especially important and ubiquitous class of such compounds. Most cells have a variety of defences against the toxic end products of UV radiation, such as radical scavenging by carotenoid pigments and superoxide dismutase. Finally, most cells have at least some ability to identify and repair the UV damage of DNA and other biomolecules. There is a large interspecific variability in the extent of each of these defence strategies. Continuing ozone depletion is not likely to cause an abrupt collapse of photosynthetic production, but may result in subtle, community-level responses that could ultimately impact on higher trophic levels.Key words: Arctic, Antarctic, photosynthesis, UV radiation, UV-B, ozone, atmospheric pollutants.