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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 of .
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 filamentation which make the mat tougher. The best known physical forms are flat mats and stubby pillars called , 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 . 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.
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.
In a wet environment where sunlight is the main source of energy, the uppermost layers are generally dominated by Aerobic organism photosynthesis 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.Lucas J. Stal: Physiological ecology of cyanobacteria in microbial mats and other communities, New Phytologist (1995), 131, 1–32 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.Garcia-Pichel F., Mechling M., Castenholz R.W., Diel Migrations of Microorganisms within a Benthic, Hypersaline Mat Community, Appl. and Env. Microbiology, May 1994, pp. 1500–1511 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.Bebout B.M., Garcia-Pichel F., UV B-Induced Vertical Migrations of Cyanobacteria in a Microbial Mat, Appl. Environ. Microbiol., Dec 1995, 4215–4222, Vol 61, No. 12
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 filamentation (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 microorganisms which become part of the mat community, for example protozoa, some of which feed on the mat-forming bacteria, and , 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.
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 in some animals, for example in the hindguts of some .
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, (domes or columns similar internally to stromatolites) and (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 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.
The earliest mats may have been small, single-species of that relied on hydrothermal vents to supply both energy and chemical "food". Within a short time (by geological standards) the build-up of dead microorganisms would have created an ecological niche for scavenging , possibly methanogen and sulfate-reducing organisms that would have formed new layers in the mats and enriched their supply of biologically useful chemicals.
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.
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 microorganisms 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 would have 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 likely 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 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 and . 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 .
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