Microbial mats are representatives of the oldest ecosystems
on Earth, but are still around! In these pages, learn more about
microbial mats and their mineralized counterparts, stromatolites.
An interactive Flash feature demonstrates biogeochemical cycles
operating inside microbial mats.
WHAT ARE MICROBIAL MATS?
Photosynthetic microbial mats are a type of microbial
mat that obtains its energy from sunlight.
On the early Earth, before higher multicellular organisms evolved, photosynthetic microbial mats literally covered the planet, and microbes were the only life forms. Later, as higher plants evolved, photosynthetic mat communities had to compete for sunlight and nutrients. The evolution of animals brought further pressures because mats are a nutritious food.
So, although microbial mats are trying to grow almost everywhere almost all of the time, it is only in special places the microbes have the opportunity to grow the large amounts of biomass comparable to what was found on early Earth. Examples of places where microbial mats are found in modern times include extremely productive salt marshes and high energy carbonate beaches such as in the Bahamas. In many cases, microbial mats flourish in so-called "extreme environments" where the extreme temperatures, dryness or saltiness act to exclude plants and animals. These include: hypersaline (very salty) bodies of water, hot springs, dry temperate deserts and the cold dry environment of Antarctica.
To summarize, it is generally thought that microbial mats can be observed in these places because there are fewer competing plants or grazing organisms that can inhabit these extreme environments and interfere with the relatively slow-growing microbial mats.
WHAT ARE STROMATOLITES?
Stromatolites, formerly defined as: "laminated organo-sedimentary
structures formed by the trapping and binding, and/or precipitation
of minerals by microorganisms", can be thought of as microbial
mats in which a rock-like layer of either sand or precipitated
minerals is also present. Fossil stromatolites constitute our
earliest and most pervasive record of life on Earth. Although
many stromatolites are fossils, there are a number of locations
on the modern-day Earth where stromatolites are still forming.
Examples of Stromatolites in the Fossil Record:
Stromatolite, Rossport Group, 1300 million years old, Ontario, Canada This flat laminated structure was constructed approximately 1300 million years ago by a cyanobacterial microbial mat community which grew in a shallow, hypersaline coastal marine environment. Higher concentrations of silica in the darker layers have protected the organic matter from oxidation.
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Stromatolite, Chuar Group, 760 million years old, Arizona
This dome-shaped, laminated structure was constructed approximately 760 million
years ago by a cyanobacterial microbial mat community which grew in a shallow,
subtidal coastal marine environment. Subtle differences in mineral content cause
the individual growth layers to stand out when the rock is subjected to weathering..
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Stromatolite, Morrison Formation, 150 million years old, New Mexico
This is a vertical cross section of a stromatolite that was constructed by cyanobacteria and green algae in a continental, alkaline lake. This 150 million-year-old stromatolite was found in the Mesozoic-age Morrison Formation, which is better known for its dinosaur remains. Note the laminated texture of the stromatolite, each layer of which marks the growth site of an ancient microbial mat.
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Modern Stromatolite, Alkaline Evaporitic Lake, Nevada
This structure consists of numerous internal columns which were constructed by
cyanobacteria growing in an evaporitic lake. The growth habit of the microbial
communities has been imparted upon the carbonate structure, thereby creating a
"biogenic fabric" which is a characteristic of this particular bacterial community.
Stromatolites still grow today in alkaline and saline lakes where the adverse
chemistry of the lake waters have suppressed the populations of algae and grazing
animals.
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Stromatolite, Belt Supergroup, 1350 million years old, Glacier Park, Montana
This stromatolite once grew in a shallow carbonate platform in water depths of a few meters, about 1350 million years ago. It was part of an extensive subtidal and intertidal reef and mudflat complex that, despite the absence of plants and animals, bore striking resemblance to modern coastal environments.
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Stromatolite, Transvaal Supergroup, 2500 million years old, Barberton Mountains, Transvaal, South Africa
This stromatolite developed as part of an enormous 2500 million-year-old carbonate
platform complex that included extensive reef, forereef and backreef components.
This platform complex reveals that, despite the absence of corals or other multicellular
reef-forming organisms, these ancient stromatolitic reefs resembled their modern
counterparts in virtually all other aspects What Next?
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HOW DO MICROBIAL MATS WORK?
Microbial Mat Biogeochemical Cycling
All life forms require energy to grow and reproduce, and that
energy must somehow be extracted from the environment. Humans,
for example, breathe air and eat food. When the organic matter
in the food combines chemically with the oxygen in the air,
energy is released. Where did the oxygen and the organic matter
come from? Both were made by other organisms, so that our survival
is dependent upon the activity of other life forms. Microbial
mats function in the same way as a complex food web in which
each organism both depends and is depended on by other members
of the community. Mats are remarkable in this regard because
the organisms that live there constitute an amazing array of
energy harvesting strategies. In fact, virtually every way that
nature has to "make a living" occurs in microbial mats, all
within a few millimeters! This diagram (modified from Fenchel
and Findlay, 1995) illustrates the basics of how a microbial
mat food web works. Click
here to see what role different organisms play in the overall
function of the mat. (requires flash plugin)
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