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.