Scytonemin, a cyanobacterial sheath pigment, protects against UVC radiation: Implications for early photosynthetic life

During the Precambrian, ultraviolet (UV) radiation reaching the Earth's surface, including UVC wavelengths (190-280 nm), was considerably higher than present because of the lack of absorbing gases (e.g. O-2 and O-3) in the atmosphere. High UV flux would have been damaging to photosynthetic organisms exposed to solar radiation. Nevertheless, fossil evidence indicates that cyanobacteria-like ancestors may have evolved as early as 3.5 x 10(9) yr ago, and were common in shallow marine habitats by 2.5 x 10(9) years ago, Scytonemin, a cyanobacterial extracellular sheath pigment, strongly absorbs WC radiation. Exposure to high-irradiance conditions caused cells to synthesize scytonemin and resulted in decreased WC inhibition of photosynthetic carbon uptake. It was further demonstrated that scytonemin alone was sufficient for substantial protection against WC damage. This represents the first experimental demonstration of biological protection against UVC radiation in cyanobacteria, These results suggest that scytonemin may have evolved during the Precambrian and allowed colonization of exposed, shallow-water and terrestrial habitats by cyanobacteria or their oxygenic ancestors.