Algal mat

Algal mats are one of many types of microbial mat that forms on the surface of water or rocks. They are typically composed of blue-green cyanobacteria and sediments. Formation occurs when alternating layers of blue-green bacteria and sediments are deposited or grow in place, creating dark-laminated layers. Stromatolites are prime examples of algal mats. Algal mats played an important role in the Great Oxidation Event on Earth some 2.3 billion years ago. Algal mats can become a significant ecological problem, if the mats grow so expansive or thick as to disrupt the other underwater marine life by blocking the sunlight or producing toxic chemicals. Cyanobacteria found in sedimentary rocks indicate that bacterial life began on Earth during the Precambrian age. Fossilized cyanobacteria are commonly found in rocks that date back to Mesoproterozoic. Cyanobacteria are photoautotrophs in nature; they convert carbon dioxide and sunlight into food and energy via photosynthesis. Some species are also able to fix atmospheric nitrogen and convert it into the biologically-usable form of nitrate or nitrite. This gives them competitive advantage over other organisms that may be limited by the shortage of biologically available nitrogen. The cyanobacteria colonies contain two types of cells, the regular cells with chlorophyll carrying out the photosynthesis, and heterocysts which fix the nitrogen. These heterocysts have thick walls and lack chlorophyll, both of which limits their exposure to oxygen, the presence of which inhibits nitrogen fixation. For the same reason, fixation may also be limited to nighttime when the light-dependent reactions of photosynthesis are shut down, minimizing oxygen production. Stromatolites are alternating layers of cyanobacteria and sediments. The grain size of sediment portion of stromatolites is affected by the depositional environment. During the Proterozoic, stromatolites' compositions were dominated by micrite and thinly laminated lime mud, with thicknesses no greater than 100 microns.