Prokaryotes were largely responsible for creating the current properties of the atmosphere and the soil through billions of years of their activity. Today, they still affect the Earth and human life in many important ways.
Prokaryotes are involved in cycling important elements
Life on Earth is critically dependent on the cycling of chemical elements between organisms and the physical environments in which they live—that is, between the living and nonliving elements of ecosystems. Prokaryotes, algae, and fungi play many key roles in this chemical cycle.
Decomposition
The carbon, nitrogen, phosphorus, sulfur, and other atoms of biological systems all have come from the physical environment, and when organisms die and decay, these elements all return to them. The prokaryotes and fungi that carry out the decomposition portion of chemical cycles, releasing a dead organism’s atoms into the environment, are called decomposers.
Fixation
Other prokaryotes play important roles in fixation, the other half of chemical cycles, helping to return elements from inorganic forms to organic forms that heterotrophic organisms can use.
Carbon
The role of photosynthetic prokaryotes in fixing carbon is obvious. The organic compounds that plants, algae, and photosynthetic prokaryotes produce from CO2 pass up through food chains to form the bodies of all the ecosystem’s heterotrophs.
Ancient cyanobacteria are thought to have added oxygen to the Earth’s atmosphere as a by-product of their photosynthesis. Modern photosynthetic prokaryotes continue to contribute to the production of oxygen.
Nitrogen
Less obvious, but no less critical to life, is the role of prokaryotes in recycling nitrogen. The nitrogen in the Earth’s atmosphere is in the form of N2 gas. A triple covalent bond links the two nitrogen atoms and is not easy to break.
Among the Earth’s organisms, only a very few species of prokaryotes are able to accomplish this feat, reducing N2 to ammonia (NH3), which is used to build amino acids and other nitrogen-containing biological molecules.
When the organisms that contain these molecules die, decomposers return nitrogen to the soil as ammonia. This is then converted to nitrate (NO3–) by nitrifying bacteria, making nitrogen available for
plants. The nitrate can also be converted back into molecular nitrogen by denitrifiers that return the nitrogen to the atmosphere, completing the cycle.
To fix atmospheric nitrogen, prokaryotes employ an enzyme complex called nitrogenase, encoded by a set of genes called (“nitrogen fixation”) genes. The nitrogenase complex is extremely sensitive to oxygen and is found in a wide range of free-living prokaryotes.
In aquatic environments, nitrogen fixation is carried out largely by cyanobacteria such as Anabaena, which forms long chains of cells.
Because the nitrogen fixation process is strictly anaerobic, individual cyanobacteria cells may develop
into heterocysts, specialized nitrogen-fixing cells impermeable to oxygen. In soil, nitrogen fixation occurs in the roots of plants that harbor symbiotic colonies of nitrogen-fixing bacteria.
These associations include Rhizobium (a genus of proteobacteria; see figure ) with legumes, Frankia (an actinomycete) with many woody shrubs, and Anabaena with water ferns.
Prokaryotes may live in symbiotic associations with eukaryotes
Many prokaryotes live in symbiotic association with eukaryotes. Symbiosis refers to the ecological relationship between different species that live in direct contact with each other. The symbiotic association of nitrogen-fixing bacteria with plant roots is an example of mutualism, a form of symbiosis in which both parties benefit.
The bacteria supply the plant with useful nitrogen, and the plant supplies the bacteria with sugars and other organic nutrients . Many bacteria live symbiotically within the digestive tracts of animals, providing nutrients to their hosts. Cattle and other grazing mammals are unable to digest cellulose in the grass and plants they eat because they lack the required cellulase enzyme.
Colonies of cellulase-producing bacteria inhabiting the gut allow cattle to digest their food (see chapter 48 for a fuller account). Similarly, humans maintain large colonies of bacteria in the large intestine that produce vitamins—particularly B12 and K—that the body cannot make. Many bacteria inhabit the outer surfaces of animals and plants without doing damage.
These associations are examples of commensalism, in which one organism (the bacterium) receives benefits while the animal or plant is neither benefited nor harmed. Parasitism is a form of symbiosis in which one member (in this case, the bacterium) benefits and the other (the infected animal or plant) is harmed. Infection might be considered a form of parasitism.
Bacteria are used in genetic engineering
Because the genetic code is universal, a gene from a human can be inserted into a bacterial cell, and the bacterium produces a human protein. The use of bacteria in genetic engineering and it is a large part of modern molecular biology.
In addition to the production of pharmaceutical agents such as insulin, applying genetic engineering methods to produce improved strains of bacteria for commercial use holds promise for the future.
Bacteria are now widely used as “biofactories” in the commercial production of a variety of enzymes, vitamins, and antibiotics. Immense cultures of bacteria, often genetically modified to enhance performance, are used to produce commercial acetone and other industrially important compounds.
Bacteria can be used for bioremediation
The use of organisms to remove pollutants from water, air, and soil are called bioremediation. The normal functioning of sewage treatment plants depends on the activity of microorganisms. In sewage treatment plants, the solid matter from raw sewage is broken down by bacteria and archaea naturally present in the sewage. The end product, methane gas (CH4) is often used as an energy source to heat the treatment plant.
Biostimulation, that is, the addition of nutrients such as nitrogen and phosphorus sources has been used to encourage the growth of naturally occurring microbes that can degrade crude oil spills. This approach was used successfully to clean up the Alaskan shoreline after the crude oil spill of the Exxon Valdez in 1998.
Similarly, biostimulation has been used to encourage the growth of naturally occurring microbial flora in contaminated groundwater.
Current efforts include those concentrated on the use of endogenous microbes such as Geobacter to eliminate radioactive uranium from groundwater contaminated during the cold war. Chlorinated compounds released into the environment by a variety of sources are another serious pollutant.
Some bacteria can actually use these compounds for energy by performing reductive dehalogenation that is linked to electron transport, a process termed halorespiration.
Although still at the development stage, the use of such bacteria to remove halogenated compounds from toxic waste holds great promise.
References:
- www.ravenbiology.com
- https://pressbooks-dev.oer.hawaii.edu/biology/chapter/beneficial-prokaryotes
- https://en.wikipedia.org/wiki/Economic_importance_of_bacteria
- https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/11.13/
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