Macrophytes were analysed for UVR absorbing, Mycosporine-like amino acids (MAAS) by, HPLC. The beneficial effect of the sunscreen substances on avoiding, UVR stress was studied by measuring, variable chlorophyll fluorescence of photosystem 11. Red algal species from the eulittoral contain several MAAS, which are assumed to function as UV sunscreens. In contrast, deep-water red algae, as well as green and brown algae, lack MAAs or exhibit only trace concentrations. The contents of all MAAs correlates with the biogeoraphic region indicating that the lower the natural solar irradiance the less MAAs are synthesised and accummulated, i. e. the red algal samples from Spain exhibited tip to 2-fold hicher MAA contents compared to species from cold-temperate and polar water. Additional experiments indicate that formation of MAA's is induced species specifically, by PAR, UVAR or UVBR and that MAA's seem to especially prevent photodamage of photosynthesis. The results show - beside the negative impact of UV radiation oil metabolism and growth - the strong, capacity of MAAs to protect algae against UV stress. However, this protective capacity is limited in species front deeper waters and in microscopic developmental stages.