Inferred ancestry of scytonemin biosynthesis proteins in cyanobacteria indicates a response to Paleoproterozoic oxygenation

Protection from radiation damage is an important adaptation for
phototrophic microbes. In the case of cyanobacteria, surface, shallow
water, and peritidal environments are especially exposed to long-
wavelength ultraviolet (UVA) radiation. Several groups of cyanobacteria
within these environments are protected from UVA damage by the production
of the pigment scytonemin. Paleontological evidence of cyanobacteria in
UVA-exposed environments from the Proterozoic, and possibly as early as
the Archaean, suggests a long evolutionary history of radiation protection
within this group. We show that phylogenetic analyses of enzymes in the
scytonemin biosynthesis pathway support this hypothesis, and reveal a deep
history of vertical inheritance of this pathway within extant
cyanobacterial diversity. Referencing this phylogeny to cyanobacterial
molecular clocks suggests that scytonemin production likely appeared
during the early Proterozoic, soon after the Great Oxygenation Event. This
timing is consistent with an adaptive scenario for the evolution of
scytonemin production, wherein the threat of UVA-generated reactive oxygen
species becomes significantly greater once molecular oxygen was more
pervasive across photosynthetic environments.