A microbial mat is a multi-layered sheet of 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 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 , 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 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.
- Structure 1.1
- Types of environment colonized 1.2
- Ecological and geological importance 1.3
Role in the history of life 2
- The earliest mats 2.1
- Photosynthesis 2.2
- Origin of eukaryotes 2.3
- Life on land 2.4
- The earliest multicellular "animals" 2.5
- The Cambrian substrate revolution 2.6
- Current status 2.7
- How microbial mats help paleontologists 3
- Industrial uses 4
- See also 5
- Notes 6
- References 7
- External links 8
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.
A microbial mat consists of several layers, each of which is dominated by specific types of 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 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
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- Cambrian substrate revolution
- Evolutionary history of life
- Sippewissett Microbial Mat
- Ediacaran type preservation
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.
- 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 20 - 30 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.
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:
How microbial mats help paleontologists
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.
 the depths of the oceans, where burrowing activity today is at a similar level to that in the shallow coastal seas before the revolution. rocky "floors" which the burrowers cannot penetrate; 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
The Cambrian substrate revolution
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 earliest multicellular "animals"
Microbial mats from ~ provide the first evidence of life in the terrestrial realm.
Life on land
- 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.
The time at which eukaryotes first appeared is still uncertain: there is reasonable evidence that fossils dated between and represent eukaryotes, but the presence of steranes in Australian shales may indicate that eukaryotes were present . 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:
Origin of eukaryotes
Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms: the photosynthesis mechanisms of both phytoplankton, which forms the basis of most marine food chains.
 — for example anaerobic  Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms), 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.2 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 (H
 It is estimated that the appearance of oxygenic photosynthesis increased biological productivity by a factor of between 100 and 1,000. All photosynthetic
 The last major stage in the evolution of microbial mats was the appearance of
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.
 Heterotrophic scavengers would have accompanied the photosynthesizers in their migration out of the "hydrothermal ghetto". The earliest photosynthesis may have been powered by
It is generally thought that 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.
 The earliest mats were probably small, single-species
Microbial mats are among the oldest clear signs of life, as microbially induced sedimentary structures (MISS) formed 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 . 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
Role in the history of life
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.
Microbial mats use all of the types of metabolism and feeding strategy that have evolved on Earth — anoxygenic and oxygenic predation and detritivory).
Ecological and geological importance
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.