A microbial mat is a multi-layered sheet of microorganisms,
mainly bacteria
and archaea.
Microbial mats grow at interfaces between different types of
material, mostly on submerged or moist surfaces, but a few survive in deserts. They
colonize environments ranging in temperature from –40°C to 120°C. A few are
found as endosymbionts of animals.
Although only a few centimetres thick at most, microbial
mats create a wide range of internal chemical environments, and hence generally
consist of layers of microorganisms that can feed on or at least tolerate the
dominant chemicals at their level and which are usually of closely related
species. In moist conditions mats are usually held together by slimy substances secreted by the
micro-organisms, and in many cases some of the micro-organisms form tangled
webs of filaments which make the mat tougher. The best known physical forms are
flat mats and stubby pillars called stromatolites,
but there are also spherical forms.
Microbial mats are the earliest form of life on Earth for
which there is good fossil
evidence, from 3,500
million years ago, and
have been the most important members and maintainers of the planet's ecosystems.
Originally they depended on hydrothermal
vents for energy and chemical "food", but the development of photosynthesis
gradually liberated them from the "hydrothermal ghetto" by providing
a more widely available energy source, sunlight, although initially the
photosynthesizing mats still depended on the diffusion of chemicals emitted by
hydrothermal vents. The final and most significant stage of this liberation was
the development of oxygen-producing photosynthesis, since the main chemical
inputs for this are carbon dioxide and water.
As a result microbial mats began to produce the atmosphere
we know today, in which free oxygen is a vital component. At around the same time they may
also have been the birthplace of the more complex eukaryote
type of cell, of which all multicellular
organisms are composed. Microbial mats were abundant on the shallow seabed
until the Cambrian substrate revolution, when
animals living in shallow seas increased their burrowing capabilities and thus
broke up the surfaces of mats and let oxygenated water into the deeper layers,
poisoning the oxygen-intolerant micro-organisms that lived there. Although this
revolution drove mats off soft floors of shallow seas, they still flourish in
many environments where burrowing is limited or impossible, including rocky
seabeds and shores, hyper-saline and brackish lagoons, and are found on the
floors of the deep oceans.
Because of microbial mats' ability to use almost anything as
"food", there is considerable interest in industrial uses of mats,
especially for water treatment and for cleaning up pollution.
Description
Microbial mats have also been referred to as "algal mats" and
"bacterial
mats" in older scientific literature. They are a type of biofilm that is
large enough to see with the naked eye and robust enough to survive moderate
physical stresses. These colonies of bacteria form
on surfaces at many types of interface, for example between water and the sediment or
rock at the bottom, between air and rock or sediment, between soil and
bed-rock, etc. Such interfaces form vertical chemical
gradients, i.e. vertical variations in chemical composition, which make
different levels suitable for different types of bacteria and thus divide
microbial mats into layers, which may be sharply defined or may merge more
gradually into each other. A variety of microbes are able to transcend the
limits of diffusion by using "nanowires" to shuttle electrons from
their metabolic reactions up to two centimetres deep in the sediment - for
example, electrons can be transferred from reactions involving hydrogen sulfide
deeper within the sediment to oxygen in the water, which acts as an electron acceptor.
The best-known types of microbial mat may be flat laminated
mats, which form on approximately horizontal surfaces, and stromatolites,
stubby pillars built as the microbes slowly move upwards to avoid being
smothered by sediment deposited on them by water. However there are also
spherical mats, some on the outside of pellets of rock or other firm material
and others inside spheres of sediment.
Structure
A microbial mat consists of several layers, each of which is
dominated by specific types of micro-organism,
mainly bacteria.
Although the composition of individual mats varies depending on the
environment, as a general rule the by-products of each group of micro-organisms
serve as "food" for other groups. In effect each mat forms its own food chain,
with one or a few groups at the top of the food chain as their by-products are
not consumed by other groups. Different types of micro-organism dominate
different layers based on their comparative advantage for living in that
layer. In other words they live in positions where they can out-perform other
groups rather than where they would absolutely be most comfortable — ecological
relationships between different groups are a combination of competition and
co-operation. Since the metabolic capabilities of bacteria (what they can
"eat" and what conditions they can tolerate) generally depend on
their phylogeny
(i.e. the most closely related groups have the most similar metabolisms), the
different layers of a mat are divided both by their different metabolic
contributions to the community and by their phylogenetic relationships.
In a wet environment where sunlight is the main source of
energy, the uppermost layers are generally dominated by aerobic
photosynthesizing
cyanobacteria
(blue-green bacteria whose color is caused by their having chlorophyll),
while the lowest layers are generally dominated by anaerobic sulfate-reducing bacteria. Sometimes
there are intermediate (oxygenated only in the daytime) layers inhabited by facultative anaerobic bacteria. For example,
in hypersaline ponds near Guerrero Negro (Mexico) various kind of mats were
explored. There are some mats with a middle purple layer inhabited by photosynthesizing
purple bacteria. Some other mats have a white layer inhabited by chemotrophic sulfide-oxidizing
bacteria and beneath them an olive layer inhabited by photosynthesizing green sulfur bacteria and heterotrophic
bacteria. However, this layer structure is not changeless during a day: some
species of cyanobacteria migrate to deeper layers at morning, and go back at
evening, to avoid intensive solar light and UV radiation at mid-day.
Microbial mats are generally held together and bound to
their substrates by slimy extracellular polymeric substances
which they secrete. In many cases some of the bacteria form filaments
(threads), which tangle and thus increase the colonies' structural strength,
especially if the filaments have sheaths (tough outer coverings).
This combination of slime and tangled threads attracts other
micro-organisms which become part of the mat community, for example protozoa, some of
which feed on the mat-forming bacteria, and diatoms, which
often seal the surfaces of submerged microbial mats with thin, parchment-like
coverings.
Marine mats may grow to a few centimeters in thickness, of
which only the top few millimeters are oxygenated.
Types of environment colonized
Underwater microbial mats have been described as layers that
live by exploiting and to some extent modifying local chemical
gradients, i.e. variations in the chemical composition. Thinner, less
complex biofilms
live in many sub-aerial
environments, for example on rocks, on mineral particles such as sand, and
within soil. They have
to survive for long periods without liquid water, often in a dormant state.
Microbial mats that live in tidal zones, such as those found in the Sippewissett salt marsh, often contain a
large proportion of similar micro-organisms that can survive for several hours
without water.
Microbial mats and less complex types of biofilm are found
at temperature ranges from –40°C to +120°C, because variations in pressure
affect the temperatures at which water remains liquid.
They even appear as endosymbionts
in some animals, for example in the hindguts of some echinoids.
Ecological and geological importance
Microbial mats use all of the types of metabolism and
feeding strategy that have evolved on Earth — anoxygenic and oxygenic photosynthesis;
anaerobic and aerobic chemotrophy (using chemicals rather than sunshine as a
source of energy); organic and inorganic respiration and fermentation (i..e converting food into
energy with and without using oxygen in the process); autotrophy
(producing food from inorganic compounds) and heterotrophy
(producing food only from organic compounds, by some combination of predation and
detritivory).
Most sedimentary rocks and ore deposits have grown by a reef-like build-up
rather than by "falling" out of the water, and this build-up has been
at least influenced and perhaps sometimes caused by the actions of microbes. Stromatolites,
bioherms (domes or columns similar internally to stromatolites) and biostromes
(distinct sheets of sediment) are among such microbe-influenced build-ups.
Other types of microbial mat have
created wrinkled "elephant skin" textures in marine sediments,
although it was many years before these textures were recognized as trace
fossils of mats. Microbial mats have increased the concentration of metal
in many ore deposits, and without this it would not be feasible to mine them —
examples include iron (both sulfide and oxide ores), uranium, copper, silver
and gold deposits.
Role in the history of life
History of life
The earliest mats
Microbial mats are among the oldest clear signs of life, as microbially induced
sedimentary structures (MISS) formed 3,480 million years ago have
been found in western Australia. At that early stage the mats'
structure may already have been similar to that of modern mats that do not
include photosynthesizing bacteria. It is even possible that
non-photosynthesizing mats were present as early as 4,000 million years ago. If so,
their energy source would have been hydrothermal
vents (high-pressure hot springs around submerged volcanoes), and
the evolutionary split between bacteria and archea may also have occurred around this time.
The earliest mats were probably small, single-species biofilms of chemotrophs
that relied on hydrothermal vents to supply both energy and chemical
"food". Within a short time (by geological standards) the build-up of
dead micro-organisms would have created an ecological
niche for scavenging heterotrophs, possibly methane-emitting
and sulfate-reducing organisms that would
have formed new layers in the mats and enriched their supply of biologically
useful chemicals.
Photosynthesis
It is generally thought that photosynthesis,
the biological generation of energy from light, evolved shortly after 3,000 million years ago.
However an isotope analysis suggests that oxygenic
photosynthesis may have been widespread as early as 3,500 million years ago.The
eminent researcher into Earth's earliest life, William Schopf, argues that, if
one did not know their age, one would classify some of the fossil organisms in
Australian stromatolites from 3,500 million years ago as cyanobacteria,
which are oxygen-producing photosynthesizers.There are several different types
of photosynthetic reaction, and analysis of bacterial DNA indicates that
photosynthesis first arose in anoxygenic purple
bacteria, while the oxygenic photosynthesis seen in cyanobacteria
and much later in plants
was the last to evolve.[16]
The earliest photosynthesis may have been powered by infra-red
light, using modified versions of pigments whose original function was to detect
infra-red heat emissions from hydrothermal vents. The development of
photosynthetic energy generation enabled the micro-organisms first to colonize
wider areas around vents and then to use sunlight as an energy source. The role
of the hydrothermal vents was now limited to supplying reduced metals into the
oceans as a whole rather than being the main supporters of life in specific
locations. Heterotrophic scavengers would have accompanied the
photosynthesizers in their migration out of the "hydrothermal
ghetto".
The evolution of purple bacteria, which do not produce or
use oxygen but can tolerate it, enabled mats to colonize areas that locally had
relatively high concentrations of oxygen, which is toxic to organisms that are
not adapted to it. Microbial mats would have been separated into oxidized and
reduced layers, and this specialization would have increased their
productivity. It may be possible to confirm this model by analyzing the isotope
ratios of both carbon and sulfur in sediments laid down in shallow water.
The last major stage in the evolution of microbial mats was
the appearance of cyanobacteria, photsynthesizers which both produce and
use oxygen. This gave undersea mats their typical modern structure: an
oxygen-rich top layer of cyanobacteria; a layer of photsynthesizing purple
bacteria that could tolerate oxygen; and oxygen-free, H2S-dominated
lower layers of heterotrophic scavengers, mainly methane-emitting and sulfate-reducing
organisms.
It is estimated that the appearance of oxygenic
photosynthesis increased biological productivity by a factor of between 100 and
1,000. All photosynthetic reactions
require a reducing agent, but the significance of oxygenic
photosynthesis is that it uses water as a reducing agent, and water is much more plentiful
than the geologically produced reducing agents on which photosynthesis
previously depended. The resulting increases in the populations of
photosynthesizing bacteria in the top layers of microbial mats would have
caused corresponding population increases among the chemotrophic
and heterotrophic
micro-organisms that inhabited the lower layers and which fed respectively on
the by-products of the photosynthesizers and on the corpses and / or living
bodies of the other mat organisms. These increases would have made microbial
mats the planet's dominant ecosystems. From this point onwards life itself
produced significantly more of the resources it needed than did geochemical
processes.
Oxygenic photosynthesis in microbial mats would also have
increased the free oxygen content of the Earth's atmosphere, both directly by
emitting oxygen and because the mats emitted molecular hydrogen (H2),
some of which would have escaped from the Earth's atmosphere before it could
re-combine with free oxygen to form more water. Microbial mats thus played a
major role in the evolution of organisms which could first tolerate free oxygen
and then use it as an energy source. Oxygen is toxic to organisms that are not
adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted
organisms[— for example anaerobic fermentation produces a net yield of
two molecules
of adenosine triphosphate, cells' internal
"fuel", per molecule of glucose, while aerobic respiration produces a net yield of 36.
The oxygenation of the atmosphere was a prerequisite for the
evolution of the more complex eukaryote type of cell, from which all multicellular
organisms are built.
Cyanobacteria have the most complete biochemical
"toolkits" of all the mat-forming organisms: the photosynthesis
mechanisms of both green bacteria and purple bacteria; oxygen
production; and the Calvin cycle, which converts carbon
dioxide and water into carbohydrates and sugars. It is likely
that they acquired many of these sub-systems from existing mat organisms, by
some combination of horizontal gene transfer and endosymbiosis
followed by fusion. Whatever the causes, cyanobacteria are the most
self-sufficient of the mat organisms and were well-adapted to strike out on
their own both as floating mats and as the first of the phytoplankton,
which forms the basis of most marine food chains.[14]
Origin of eukaryotes
The time at which eukaryotes
first appeared is still uncertain: there is reasonable evidence that fossils
dated between 1,600 million years ago and 2,100 million years ago
represent eukaryotes, but the presence of steranes in Australian shales may indicate
that eukaryotes were present 2,700 million years ago. There
is still debate about the origins of eukaryotes, and many of the theories focus
on the idea that a bacterium first became an endosymbiont of an anaerobic
archean and then fused with it to become one organism. If such endosymbiosis
was an important factor, microbial mats would have encouraged it. There are two
possible variations of this scenario:
- The boundary between the oxygenated and oxygen-free zones of a mat would have moved up when photosynthesis shut down at night and back down when photosynthesis resumed after the next sunrise. Symbiosis between independent aerobic and anaerobic organisms would have enabled both to live comfortably in the zone that was subject to oxygen "tides", and subsequent endosymbiosis would have made such partnerships more mobile.
- The initial partnership may have been between anaerobic archea that required molecular hydrogen (H2) and heterotrophic bacteria that produced it and could live both with and without oxygen.
Life on land
Microbial mats from ~1,200 million years ago provide
the first evidence of life in the terrestrial realm.
The earliest multicellular "animals"
The Ediacara biota are the earliest widely accepted
evidence of multicellular "animals". Most Ediacaran
strata with the "elephant skin" texture characteristic of microbial
mats contain fossils, and Ediacaran fossils are hardly ever found in beds that
do not contain these microbial mats. Adolf
Seilacher categorized the "animals" as: "mat
encrusters", which were permanently attached to the mat; "mat
scratchers", which grazed the surface of the mat without destroying it;
"mat stickers", suspension feeders that were partially embedded in
the mat; and "undermat miners", which burrowed underneath the mat and
fed on decomposing mat material.
The Cambrian substrate revolution
In the Early Cambrian, however, organisms began to burrow
vertically for protection or food, breaking down the microbial mats, and thus
allowing water and oxygen to penetrate a considerable distance below the
surface and kill the oxygen-intolerant micro-organisms in the lower layers. As
a result of this Cambrian substrate revolution, marine
microbial mats are confined to environments in which burrowing is non-existent
or negligible: very harsh environments, such as hyper-saline lagoons or
brackish estuaries, which are uninhabitable for the burrowing organisms that
broke up the mats; rocky "floors" which the burrowers cannot
penetrate; the depths of the oceans, where burrowing activity today is at a
similar level to that in the shallow coastal seas before the revolution.
Current status
Although the Cambrian substrate revolution opened up new
niches for animals, it was not catastrophic for microbial mats, but it did
greatly reduce their extent.
How microbial mats help paleontologists
Most fossils preserve only the hard parts of organisms, e.g.
shells. The rare cases where soft-bodied fossils are preserved (the remains of
soft-bodied organisms and also of the soft parts of organisms for which only
hard parts such as shells are usually found) are extremely valuable because
they provide information about organisms that are hardly ever fossilized and
much more information than is usually available about those for which only the
hard parts are usually preserved. Microbial mats help to preserve soft-bodied
fossils by:
- Capturing corpses on the sticky surfaces of mats and thus preventing them from floating or drifting away.
- Physically protecting them from being eaten by scavengers and broken up by burrowing animals, and protecting fossil-bearing sediments from erosion. For example the speed of water current required to erode sediment bound by a mat is 30 - 20 times greater than the speed required to erode a bare sediment.
- Preventing or reducing decay both by physically screening the remains from decay-causing bacteria and by creating chemical conditions that are hostile to decay-causing bacteria.
- Preserving tracks and burrows by protecting them from erosion. Many trace fossils date from significantly earlier than the body fossils of animals that are thought to have been capable of making them and thus improve paleontologists' estimates of when animals with these capabilities first appeared.
Industrial uses
The ability of microbial mat communities to use a vast range
of "foods" has recently led to interest in industrial uses. There
have been trials of microbial mats for purifying water, both for human use and
in fish
farming,and studies of their potential for cleaning up oil spills.
As a result of the growing commercial potential, there have been applications
for and grants of patents
relating to the growing, installation and use of microbial mats, mainly for
cleaning up pollutants and waste products.
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