Looking for a new biomarker for the earliest oxygenic photosynthesis on Earth

Abstract

From Archean to the early Palaeoproterozoic (~3.8-2.5 Ga), the deleterious short-wavelengthed ultraviolet radiations (UVRs), including 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 the Earth. Primitive photosynthetic microorganisms had to cope with this harsh surficial environment in order to harvest solar energy. It is known that some cyanobacterial species have developed sheath pigments on the cell exterior, such as scytonemin, to filter Ultraviolet A (UVA, 315-400 nm) as a living strategy. We investigated the properties of UVR absorbing by scytonemin and its structural derivatives, including two putative precursors and the oxidized/reduced transformations. We find that scytonemin and its derivatives have significant absorptions in the UVC region, which suggests the very surface the Earth could be habitable for Archean life with sheath pigments without the need for an atmospheric UVR shield.