A stromatolite is an accreted mass of microorganisms which have grown so closely together that they form a mat. Stromatolites typically form in layers, creating a characteristic stratified look, and they are a topic of intense interest among geologists. Until the 20th century, the only evidence of stromatolites was in fossil form, and scientists presumed that these unique biofilms were extinct. However, in Shark Bay, Australia, a colony of living stromatolites was discovered, rocking the scientific world.
Typically, the microorganisms in a stromatolite are cyanobacteria, also known as blue-green algae. Living in the form of a colony is highly advantageous to these organisms, and numerous examples of microorganisms living in similar colonies can be found all over the world. A single stromatolite can take thousands of years to form, layer by layer, and it may appear in a variety of shapes; many stromatolites look like columns, but they can also form in discs, pyramids, and rings like giant misshapen donuts.
Cyanobacteria thrive where other organisms cannot survive, and some scientists believe that cyanobacteria and stromatolites are probably responsible for the modern survivable atmosphere. These organisms generate energy by using carbon dioxide and sunlight, and the byproducts are oxygen and calcium carbonate, also known as lime. The presence of huge amounts of stromatolites on Earth probably helped fill the atmosphere with oxygen, while the generation of lime ensured that they would be fossilized for people to look at in the future.
Modern living stromatolite colonies are typically found in areas with highly saline and very warm water, mimicking the conditions which stromatolites survived in during the Precambrian era. These mats of bacteria are very fragile, which may explain why the modern colonies have been found in remote areas, far from the reach of humans and grazing animals which could damage them. The Shark Bay colony has actually been designated a World Heritage Site, recognizing its uniqueness.
In fossilized form, a stromatolite has distinctive flaky layers of material, which often form in hills and hummocks. It is also common to find deposits of various minerals between the layers, providing clues to when they formed and what conditions were like. Many fossilized stromatolites are quite colorful, with bands of rich color between more dully colored layers, and they are popular collector's items.