Microbial mat
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.
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.
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.
[7][8] Microbial mats are generally held together and bound to their substrates by slimy extracellular polymeric substances which they secrete.
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.
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.
[15] 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 (by geological standards) 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.
[15] It is generally thought that photosynthesis, the biological generation of chemical energy from light, evolved shortly after 3,000 million years ago (3 billion).
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.
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.
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.
[18] Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms[17] — 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.
[22] 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.
[2] There are two known variations of this scenario: Microbial mats from ~1,200 million years ago provide the first evidence of life in the terrestrial realm.
[26] 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 microorganisms in the lower layers.
[27] 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.
There have been trials of microbial mats for purifying water, both for human use and in fish farming,[31][32] and studies of their potential for cleaning up oil spills.
