Microorganisms might be able to deal with UVR before the emergence of a ozonosphere

Abstract

The surficial environment of early Earth (~3.8-2.5 Ga) was characterized by deleterious short-wavelengthed ultraviolet radiations (UVRs), where Ultraviolet C (UVC, 200-280 nm) and most of Ultraviolet B (UVB, 280-315 nm) were able to penetrate the atmosphere and reached the surface of oceans and land. As an important part of early biosphere, primitive photosynthetic microorganisms had to cope with this harsh surficial environment for the sake of harvesting solar energy. At present, the life on Earth are protected from UVR by an ozonosphere shield. However, the ecological distribution of photosynthesis in Archean before the formation of an effective ozonosphere remains unclear. It is known that some cyanobacterial species have developed sheath pigments on the cell exterior, such as scytonemin, to filter UVR as a living strategy. In this study, ab initio calculations were carried out to investigate the properties of UVR absorbing by scytonemin and its structural derivatives, including two putative precursors and the oxidized/reduced transformations. The results indicate that that scytonemin and its derivatives have significant absorptions in the UVC region, which further suggest the oceanic and terrestrial surfaces of Earth could be habitable for Archean life with sheath pigments before the formation of an atmospheric UVR shield.