Microbial mat

A microbial mat is a multi-layered sheet or biofilm of microbial colonies, composed of mainly bacteria and/or archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. 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 microorganisms. In many cases some of the bacteria 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, 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 allowed mats to proliferate outside of these environments by utilizing a more widely available energy source, sunlight. 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 microorganisms 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, and hyper-saline and brackish lagoons. They are found also 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 may also be referred to as algal mats and bacterial mats. 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 microorganisms, mainly bacteria. Although the composition of individual mats varies depending on the environment, as a general rule the by-products of each group of microorganisms 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 microorganism 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 generally depend on their phylogeny, 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, while the lowest layers are generally dominated by anaerobic sulfate-reducing bacteria. Sometimes there are intermediate layers inhabited by facultative anaerobic bacteria. For example, in hypersaline ponds near Guerrero Negro 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 sulfur 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, which tangle and thus increase the colonies' structural strength, especially if the filaments have sheaths.
This combination of slime and tangled threads attracts other microorganisms 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 microorganisms 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 ; organic and inorganic respiration and fermentation ; autotrophy and heterotrophy.
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 and biostromes 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, uranium, copper, silver and gold deposits.

Role in the history of life

The earliest mats

Microbial mats are among the oldest clear signs of life, as microbially induced sedimentary structures 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, and the evolutionary split between bacteria and archea may also have occurred around this time.
The earliest mats may have been small, single-species biofilms of chemotrophs that relied on hydrothermal vents to supply both energy and chemical "food". Within a short time the build-up of dead microorganisms 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 chemical energy from light, evolved shortly after . However an isotope analysis suggests that oxygenic photosynthesis may have been widespread as early as. 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 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 microorganisms 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 could have been separated into oxidized and reduced layers.