Although often inconspicuous, fungi occur in every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles and in many food webs.
Many fungi are important as partners in symbiotic relationships with other organisms, as mutualists, parasites, or commensalists, as well as in symbiotic relationships that do not fall neatly into any of these categories. One of the most critically important of these relationships are various types of mycorrhiza, which is a kind of mutualistic relationship between fungi and plants, in which the plant's roots are closely associated with fungal hyphae and other structures. The plant donates to the fungus sugars and other carbohydrates that it manufactures from photosynthesis, while the fungus donates water and mineral nutrients that the hyphal network is able to find much more efficiently than the plant roots alone can, particularly phosphorus. The fungi also protect against diseases and pathogens and provide other benefits to the plant. Recently, plants have been found to use mycorrhizas to deliver carbohydrates and other nutrients to other plants in the same community and in some cases can make plant species that would normally exclude each other able to coexist in the same plant community. Such mycorrhizal communities are called "common mycorrhizal networks". Over 90% of the plant species on Earth are dependent on mycorrhizae of one type or another in order to survive, and it is hypothesized that the presence of terrestrial fungi may have been necessary in order for the first plants to colonize land.
In soil, microorganisms which reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) help in the transformation of molecular dinitrogen gas as their source of nitrogen, converting it to nitrogenous compounds in a process known as nitrogen fixation. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of the gut flora in the large intestine can help prevent the growth of potentially harmful microbes.
The ability of bacteria to degrade a variety of organic compounds is remarkable. Highly specialized groups of microorganisms play important roles in the mineralization of specific classes of organic compounds. For example, the decomposition of cellulose, which is one of the most abundant constituents of plant tissues, is mainly brought about by aerobic bacteria that belong to the genus Cytophaga. This ability has also been utilized by humans in industry, waste processing, and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Some beaches in Prince William Sound were fertilized in an attempt to facilitate the growth of such bacteria after the infamous 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil.
Bacteria, often in combination with yeasts and molds, are used in the preparation of fermented foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt. Using biotechnology techniques, bacteria can be bioengineered for the production of therapeutic drugs, such as insulin, or for the bioremediation of toxic wastes.
"Friendly bacteria" is a term used to refer to those bacteria that offer some benefit to human hosts, such as Lactobacillus species, which convert milk protein to lactic acid in the gut. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion). Other bacteria that are helpful inside the body are many strains of E. coli, which are harmless in healthy individuals and provide Vitamin K.
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