What are stromatolites and how do they relate to astrobiology?
Stromatolites are rocky structures built by bacteria over time. Their structure consists of layers of different types of bacteria. Usually, the top-most species must make use of oxygen and sunlight while the middle layer bacteria can use oxygen if available, but can also survive without it. The bottom-most species of bacteria thrives in darkness and cannot use oxygen at all. As sediments (rock, sand, mud) are deposited on top of the stromatolites, the bacteria migrate up through the sediments to keep their preferred depth. Over time, the layers of sediment left behind are distinct and observable to the naked eye. However, living stromatolites are relatively rare today, primarily because the colonies of bacteria that comprise them are eaten by other organisms before larger structures can be built. In fact, before living stromatolites were discovered, fossilized stromatolites were heavily debated as to whether they were of biological origin or not. Consequentially, some of these ancient
