Many cyanobacteria (blue-green algae) produce a yellowish pigment in the sheaths surrounding their cells. We have shown that the pigment (scytonemin) is an effective sunscreen, shielding the organism from the damaging effects of ultra-violet (UV) radiation. It is that this protective system evolved during the early periods of life on earth (i.e. the Precambrian) when UV radiation was higher. Over the next three years we will continue to try understand the chemical nature of this pigment, the biochemical steps in its production, how its quantity is regulated, and to what extent it may work in harmony with other UV-absorbing substances within the cells (i.e. mycosporines). We will experimentally determine the importance of these mycosporine-like compounds with those that do not, and in fact try to determine what other means are used to survive under high UV radiation by those species lacking sunscreens. This research is particularly relevant now when stratospheric ozone depletion is threatening to increase damaging levels of UV at the earth's surface.