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

During the Precambrian, ultraviolet (UV) radiation reaching the Earth's sur face, 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 at mosphere. High UV flux would have been damaging to photosynthetic organisms exposed to solar radiation. Nevertheless, fossil evidence indicates that c yanobacteria-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, Scyt onemin, a cyanobacterial extracellular sheath pigment, strongly absorbs WC radiation. Exposure to high-irradiance conditions caused cells to synthesiz e scytonemin and resulted in decreased WC inhibition of photosynthetic carb on uptake. It was further demonstrated that scytonemin alone was sufficient for substantial protection against WC damage. This represents the first ex perimental demonstration of biological protection against UVC radiation in cyanobacteria, These results suggest that scytonemin may have evolved durin g the Precambrian and allowed colonization of exposed, shallow-water and te rrestrial habitats by cyanobacteria or their oxygenic ancestors.