A microbial mat is a thick layer of microorganisms, mainly bacteria and archaea, that forms a biofilm. These mats grow where different materials meet, such as on wet or underwater surfaces. Some also live in deserts, and a few live inside other animals.

Although microbial mats are usually only a few centimeters thick, they create many different chemical conditions inside. Because of this, they often have layers of microorganisms that can live in or tolerate the chemicals around them. These layers usually contain closely related species. In wet areas, the mats are held together by slimy substances made by the microorganisms. Some bacteria create tangled webs of filaments, making the mat stronger. The most common shapes are flat mats and rounded pillars called stromatolites, but other shapes, like spheres, also exist.

Microbial mats are the oldest life forms with strong fossil evidence, dating back 3,500 million years. They were important for Earth’s ecosystems. At first, they relied on energy from hydrothermal vents, but later, photosynthesis allowed them to use sunlight as an energy source. The development of oxygen-producing photosynthesis was especially important because it used carbon dioxide and water as main ingredients.

This process helped create Earth’s current atmosphere, which contains oxygen. Around the same time, microbial mats may have contributed to the formation of more complex eukaryotic cells, which make up all multicellular organisms. Until the Cambrian period, microbial mats were common on shallow seabeds. However, animals that burrowed into the seabed disrupted the mats, letting oxygen into deeper layers and harming oxygen-intolerant microorganisms. Although this change reduced mats in shallow seas, they still thrive in places where burrowing is rare, such as rocky seabeds, shores, and salty lagoons. They also live on the ocean floor in deep areas.

Because microbial mats can use many substances as food, scientists are interested in using them for industrial purposes, such as treating water and cleaning up pollution.

Description

Microbial mats are also called algal mats or bacterial mats. They are a type of biofilm that can be seen without a microscope and can survive some physical stress. These bacterial colonies grow on surfaces where two different materials meet, such as where water touches sediment or rock, where air meets rock or sediment, or where soil meets bedrock. These meeting places create chemical changes from top to bottom, which allow different types of bacteria to live in different layers of the mat. These layers may have clear edges or may blend together slowly. Some microbes can move electrons from deep within the sediment to the water above by using "nanowires," which help transfer electrons up to two centimeters deep. For example, electrons from reactions involving hydrogen sulfide in the sediment can reach oxygen in the water, which acts as an electron acceptor.

The best-known types of microbial mats are flat, layered mats that form on horizontal surfaces and stromatolites, which are short, pillar-like structures built as microbes slowly move upward to avoid being covered by sediment carried by water. Other types of mats include spherical mats, some that form on the outside of rock or firm material and others that form inside spheres of sediment.

A microbial mat has several layers, each dominated by specific types of microorganisms, mainly bacteria. While the makeup of each mat depends on its environment, the waste products of one group of microbes often serve as food for another group. This creates a food chain within the mat, with one or a few groups at the top because their waste is not used by others. Different microbes dominate different layers based on their ability to thrive in those conditions. They live where they can outcompete other groups, even if it is not the most comfortable place for them. These relationships involve both competition and cooperation. Since the metabolic abilities of bacteria depend on their phylogeny (how closely related they are), the layers of a mat are divided both by their different roles in the community and by their relatedness.

In wet environments where sunlight is the main energy source, the top layers are usually filled with aerobic photosynthesizing cyanobacteria (blue-green bacteria that have chlorophyll, which gives them their color), while the bottom layers are often filled with anaerobic sulfate-reducing bacteria. Some layers in between may be oxygenated only during the day and are inhabited by facultative anaerobic bacteria. For example, in very salty ponds near Guerrero Negro, Mexico, some mats have a purple middle layer with photosynthesizing purple bacteria, while others have a white layer with sulfur-oxidizing bacteria and an olive layer with green sulfur bacteria and heterotrophic bacteria. However, the layer structure changes during the day, as some cyanobacteria move deeper at morning and return to the surface at evening to avoid strong sunlight and UV radiation.

Microbial mats are held together and attached to surfaces by slimy substances they produce. In many cases, some bacteria form filaments (thin threads) that tangle together, increasing the strength of the mat, especially if the filaments have tough outer coverings.

This combination of slime and tangled threads attracts other microorganisms, such as protozoa, which eat some of the bacteria forming the mat, and diatoms, which often cover the surfaces of submerged mats with thin, paper-like layers.

Marine microbial mats can grow up to a few centimeters thick, with only the top few millimeters containing oxygen.

Underwater microbial mats live by using and sometimes changing local chemical gradients (differences in chemical makeup). Thinner, simpler biofilms live in many environments above water, such as on rocks, sand, and in soil. These biofilms must survive long periods without liquid water, often in a dormant state. Mats in tidal zones, like those in the Sippewissett salt marsh, often contain microbes that can survive several hours without water.

Microbial mats and simpler biofilms are found in environments ranging from –40 °C to +120 °C, as water remains liquid at different temperatures depending on pressure.

They even live inside some animals, such as in the hindguts of certain echinoids.

Microbial mats use all types of metabolism and feeding strategies that exist on Earth, including oxygenic and anoxygenic photosynthesis, aerobic and anaerobic chemotrophy (using chemicals instead of sunlight for energy), organic and inorganic respiration and fermentation (converting food into energy with or without oxygen), autotrophy (producing food from inorganic compounds), and heterotrophy (producing food only from organic compounds through predation or consuming dead material).

Most sedimentary rocks and ore deposits have formed through a reef-like buildup rather than by settling out of water. This buildup has been influenced or caused by microbial activity. Stromatolites, bioherms (dome- or column-shaped structures similar to stromatolites), and biostromes (flat layers of sediment) are examples of such microbial-influenced formations. Other mats have created wrinkled "elephant skin" textures in marine sediments, which were later recognized as trace fossils. Microbial mats have also increased the concentration of metals in many ore deposits, making mining these deposits possible. Examples include iron (sulfide and oxide ores), uranium, copper, silver, and gold deposits.

Role in the history of life

Microbial mats are some of the oldest signs of life on Earth. These structures, called microbially induced sedimentary structures (MISS), were found in western Australia and are about 3,480 million years old. At that time, the mats may have had a structure similar to modern mats that do not include photosynthesizing bacteria. It is possible that non-photosynthesizing mats existed as early as 4,000 million years ago. If true, their energy source would have been hydrothermal vents, which are hot springs under the ocean near underwater volcanoes. The evolutionary split between bacteria and archaea may also have occurred around this time.

The earliest mats may have been small, single-species biofilms of chemotrophs, which rely on hydrothermal vents for energy and chemical "food." Soon after, the buildup of dead microorganisms created a place for scavenging heterotrophs, such as methane-emitting and sulfate-reducing organisms, to live. These organisms formed new layers in the mats and added more useful chemicals to them.

Photosynthesis, the process of using light to create energy, is generally thought to have evolved after 3,000 million years ago. However, isotope analysis suggests that oxygenic photosynthesis, which produces oxygen, may have been common as early as 3,500 million years ago. There are different types of photosynthesis, and DNA analysis shows that it first appeared in anoxygenic purple bacteria. Oxygenic photosynthesis, seen in cyanobacteria and later in plants, evolved much later.

The earliest photosynthesis may have used infrared light, with pigments originally used to detect heat from hydrothermal vents. This allowed microorganisms to move farther from vents and use sunlight as an energy source. Hydrothermal vents then mainly supplied reduced metals into the oceans instead of supporting life in specific areas. Heterotrophic scavengers likely followed photosynthesizers as they moved away from vents.

The evolution of purple bacteria, which do not use oxygen but can tolerate it, allowed mats to grow in areas with higher oxygen levels. These areas were toxic to many organisms but safe for purple bacteria. This led to mats having layers with different oxygen levels.

The last major stage in microbial mat evolution was the appearance of cyanobacteria, which produce and use oxygen. This created the modern structure of underwater mats: a top layer of oxygen-rich cyanobacteria, a middle layer of oxygen-tolerant purple bacteria, and a bottom layer of oxygen-free, hydrogen sulfide-dominated heterotrophic scavengers.

Oxygenic photosynthesis increased biological productivity by 100 to 1,000 times. All photosynthesis requires a reducing agent, but oxygenic photosynthesis uses water, which is more abundant than other reducing agents. This led to more photosynthesizing bacteria in the top layers of mats, which in turn supported more chemotrophic and heterotrophic microorganisms in the lower layers. These increases made microbial mats the dominant ecosystems on Earth. From this point, life produced more resources than geochemical processes.

Oxygenic photosynthesis also increased the amount of free oxygen in Earth's atmosphere. This oxygen allowed organisms to evolve that could use oxygen for energy. Oxygen is toxic to many organisms but greatly improves the efficiency of those adapted to it. For example, anaerobic fermentation produces 2 molecules of ATP per glucose molecule, while aerobic respiration produces 36. The rise in oxygen was essential for the evolution of eukaryotic cells, which form all multicellular organisms.

Cyanobacteria have the most complete biochemical systems among mat-forming organisms, including photosynthesis mechanisms from green and purple bacteria, oxygen production, and the Calvin cycle, which converts carbon dioxide and water into sugars. They likely gained these systems through horizontal gene transfer or endosymbiosis. Cyanobacteria were well-suited to live independently as floating mats or as phytoplankton, which form the base of marine food chains.

The time when eukaryotes first appeared is still unclear. Fossils from 1,600 to 2,100 million years ago suggest eukaryotes existed, and steranes in Australian rocks may indicate they were present 2,700 million years ago. Theories about eukaryote origins often involve endosymbiosis between bacteria and archaea. Microbial mats may have helped this process by creating environments where aerobic and anaerobic organisms could coexist.

Microbial mats from about 1,200 million years ago are the first evidence of life on land. The Ediacara biota are the earliest widely accepted evidence of multicellular animals. Most Ediacaran fossils are found in rock layers with microbial mats, which Adolf Seilacher classified into groups like "mat encrusters" and "undermat miners."

In the Early Cambrian period, organisms began burrowing into microbial mats for protection or food, breaking them apart and allowing water and oxygen to reach deeper layers.

Use of microbial mats in paleontology

Most fossils show only the hard parts of living things, such as shells. This means they give clear information only about those hard parts. However, when soft-bodied fossils are preserved—like the remains of animals without hard parts or the soft parts of animals that usually have hard parts, such as shells—these are very valuable. Microbial mats help preserve soft-bodied fossils in several ways:

• They trap the remains on their sticky surfaces, stopping them from floating or moving away.
• They protect the remains from being eaten by scavengers or broken apart by animals that dig in the ground. They also protect sediments with fossils from being worn away by water. For example, water moving at a speed 20 to 30 times faster than usual is needed to erode sediment that is held together by a microbial mat.
• They stop or slow down decay by blocking harmful bacteria from reaching the remains and by creating conditions that make it hard for these bacteria to survive.
• They help protect footprints and burrows left by animals from being worn away. Many trace fossils, such as these footprints or burrows, are much older than the body fossils of animals that are believed to have made them. This helps scientists better understand when animals with these abilities first appeared.

Industrial uses

Microbial mat communities can use many different types of "foods," which has increased interest in using them for industrial purposes. Scientists have tested microbial mats to clean water for people to drink and in fish farms. They have also studied how well they can help clean up oil spills. Because of their growing usefulness, companies have applied for and received patents related to growing, placing, and using microbial mats. These patents mostly focus on using microbial mats to remove pollution and waste.