The following text was taken from
http://www.physicscentral.com/pictures/2006/algae.html
Ingenious Algae
The alga Calyptrolithophora papillifera is encased in a shell of calcite crystals with a twolayer structure (visible on oblique face). Calculations show that this protective covering
reflects ultraviolet light.
Image Credit: J. Young/Natural History Museum, London
Many of the oceans' algae have evolved natural "sunscreens" as
protection from the sun's ultraviolet rays. But researchers recently showed
that geometrical arrangements of tiny calcite crystals surrounding certain
algae achieve the same effect as chemical sunscreens, reflecting
ultraviolet radiation while allowing longer wavelengths through for
photosynthesis. This natural "photonic" device parallels similarly ordered
structures that generate the colored patterns of butterfly wings and may
lead to new designs for technological devices that manipulate light.
Text courtesy of Physical Review Focus.
Read more about this research at http://focus.aps.org/story/v18/st9.