Algal mat

Algal mat is one of many types of microbial mat formed on the water surface or on the surface of rocks. It is made out of blue-green cyanobacteria and sediments. It is formed by having alteration layers between the blue-green bacteria and sediments, creating dark-laminated layers. Stromatolites are prime examples of algal mat. Algal mats played an important role in the Great Oxidation Event on Earth some 2.3 billion years ago. Overpopulation of algal mat may be an ecological problem, when mats disrupt the other underwater marine life by blocking the sunlight.

Cyanobacteria found in sedimentary rocks shows that the bacteria started life on Earth during the Pre-Cambrian age. Fossilized cyanobacteria are commonly found in rocks that date back to Mesoproterozoic. Cyanobacteria are photoautotroph in nature; meaning they convert carbon dioxide and absorbed sunlight into food and energy via photosynthesis. They are also able to fix atmospheric nitrogen and convert it into the biologically-usable form (Paerl, Pinkney and Steppe, 2000). 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 nitrogen. Heterocysts have thick walls and lack chlorophyll, both of which limits the exposure of heterocysts to oxygen which presence inhibits nitrogen fixation. For the same reason nitrogen fixation may be limited to night time when with the shutdown of photosynthesis in the photosynthetic cells, oxygen is no longer produced and therefore does not interfere with nitrogen fixation.

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, the stromatolite’s compositions were dominated by micrite and thinly laminated lime mud, with thickness no more than 100 microns. Modern stromatolites are characterized by their thicker and irregular laminations due to coarser grain size. Stromatolites trap sediment particles when the particles come to a rest from wave agitation. Trapping is separate process where filaments of bacteria traps the particle, provided the angle of the filaments are still within the limits before the grain rolls off due to overcoming the friction of the film. The same authors found that the length of the filaments played an important role in deciding the grain size trapped. Many of these bacterial mats are found in extreme environments because of evolution of oxygen and competition. Paerl, Pinkney and Steppe (2000) commented that these bacterial mats were marked by geochemical areas, such as volcanism and tectonics. They favour harsh environments that are either nutrient-depleted or have high salinity levels. In another article, the authors mentioned the autotrophic lifestyle of the bacteria enabled them to thrive in variety of regions with harsh surroundings. Stromatolites can be found in places with ranging temperature such as in the marine, limnic and soil

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