Where can microbial mats be found? Mats develop in marine, lacustrine, desert (hot and cold), and thermal environments. They can be found from the tropics to the poles where they may be the only biological communities. Intertidal sand and mudflats, hypersaline lagoons and ponds, polar lakes and soils, desert soils, thermal vents and springs, and alkaline lakes have all been found to possess microbial mat communities. All of these environments may be considered some form of 'extreme' environment. They all are characterized by some sort of harsh physical and/or chemical environmental factor or combination of factors that limits the type of life that may inhabit that area. For example, hypersaline ponds are characterized by really high salt content. Normal seawater salinity is measured at about 35 practical salinity units (PSU). Some hypersaline lakes may reach beyond 250 PSU, or more than seven times the amount of salt in the ocean. That's salty!! Another characteristic of manyenvironments where mats develop is that the availability of reduced nitrogen sources limits.

It should be noted that whether or not an environment is considered extreme has primarily been based on the physiological tolerances of the so-called 'higher' organisms, the eukaryotes. In other words, an extreme environment constitutes a place where humans are not likely to be found. However, what we consider extreme is often a set of conditions that some organisms find ideally suited to their lifestyle. So, in general we may describe an extreme environment as one that is more selective for prokaryotic growth. Under environmentally extreme conditions, prokaryotic structural simplicity provides advantages over greater structural complexity that has evolved in the eukaryotes. Together with the increased structural complexity of eukaryotes, their increased cell size and volume can lead to increased sensitivity to osmotic (high salt), light, nutrient, temperature (heat and cold), and desiccation stresses. Whereas, the relative structural simplicity and small size of prokarytoic cells helps confer increased resistance to harsh conditions.

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