Strategies of ultraviolet-B protection in microscopic algae

Different species of microalgae show a wide range of susceptibility to ultraviolet-B (UV-B) radiation. To identify factors responsible for the UV-B tolerance of some of the algae, we compared 8 species that are highly tolerant to UV-B to 8 species that are highly susceptible. The tolerant species contained substantial amounts of an acetolysis-resistant residue. The residue consists of sporopollenin, a biopolymer of variable chemical composition that occurs in the algal cell walls and absorbs UV-B radiation. The susceptible species contained little or no sporopollenin. We propose that sporopollenin provides protection to the tolerant species by screening the incident UV-B radiation. Previous studies showed that the mycosporine-like amino acids (MAA) also act as effective UV-B screens. Our data indicate that sporopollenin provides a constant protection while MAA are induced by radiation stress and occur with some delay. The tolerant species also differ from the susceptible species in their capacity to repair the reaction centers damaged by UV-B. The tolerant algae became vulnerable to UV-B when protein synthesis needed for repair was blocked by streptomycin. In the susceptible species, streptomycin had no effect during the UV-B stress. The repair deficiency in the susceptible species can be explained either by relatively less effective protein synthesis or by an inhibition of the protein synthesis by UV-B. In the tolerant species, the structures needed for protein synthesis are protected by UV-B screening of sporopollenin and MAA.