Archean bacteria living about 2.5 billion years ago needed to be able
withstand high levels of UV irradiation because of the lack of atmospheric
oxygen as a UV screen. Many early bacteria may have survived in a deep
marine environment where the water provided UV shielding. However, there is
evidence in the geological record of microbial mats and stromalites that
inhabited shallow water environments, which would require some other means
of UV protection.
Phoenix et al. placed cultures of cyanobacteria isolated from the Krisuvik
hot springs, Iceland, in a silicon- and iron-rich solution. Rims of iron-
enriched silica formed around the cells, and the cultures were then exposed
to UV irradiation. Cells continued to grow within the mineralized rims even
under intense UV. Thus, biomineralization may have helped the simplest
forms of life to thrive in the intense sunshine of the Archean. -- LR
Geology 29, 823 (2001).