Cyanobacteria, isolated from the Krisuvik hot spring, Iceland, were mineralized in an iron-silica solution and irradiated with high levels of ultraviolet light. Analysis of the rates of photosynthesis, chlorophyll-a content, and phycocyanin autofluorescence revealed that these mineralized bacteria have a marked resistance to UV compared to nonmineralized bacteria. Naturally occurring sinters composed of iron-silica biominerals collected from the Lysúholl hot spring, and made into wafers of 150–250 μm thickness, also provided cyanobacteria with an effective UV screen. Analysis of the UV-absorbing capacity of these wafers showed that they absorbed an order of magnitude more UV than photosynthetically active light (required for photosynthesis). From these results, it is evident that both natural and experimental biomineralization provide bacteria with an effective UV screen through the passive precipitation of iron-enriched silica crusts. The UV-shielding capacity of iron- bearing silicate biominerals may have been important for early life forms. We propose that the biomineralization of Archean bacteria similarly provided protection from the high-intensity UV present at that time, and hence allowed colonization and bacterial diversification of shallow-water environments.