UV-protectants in cyanobacteria
Cyanobacteria are the largest group of Gram-negative photosynthetic prokaryotes on earth and have a cosmopolitan distribution. As cyanobacteria are believed to have originated in the Precambrian era at a time when the ozone shield was absent, they presumably faced high fluxes of UV radiation, which must have acted as an evolutionary pressure leading to the selection for efficient UV radiation protecting mechanisms. Tolerance of cyanobacteria to intense sunlight as well as UV radiation might have contributed to their success during early stages of colonization. The synthesis of UV-absorbing/screening compounds is an important mechanism to prevent UV-induced photodamage. In cyanobacteria photoprotectants such as mycosporine-like amino acids (MAAs) and scytonemin strongly absorb in the UV-A and/or UV-B region of the spectrum, and thus play an important role in allowing these organisms to grow and survive in habitats exposed to strong irradiation.