For those fungi that produce them, the mushroom plays a similar role to a flower or a fruit in plants. Some part of each mature mushroom produces microscopic spores that are similar to pollen or seeds, sometimes numbering in the trillions [1]. The rest of the fungal organism typically lives in the soil, wood, or some other material and is composed of thread-like strands known as mycelium.
Many mushroom-producing species are important decomposers, particularly of wood. These species are often relatively easy to cultivate. Often, neither the mushroom nor the plant will grow without a mycorrhizal partner. Most mushroom-producing fungi are members of the phylums Basidiomycota or Ascomycota. The technical difference between these groups has to do with how the spores develop, which can be detected only by using a microscope.
Probably the best-known ascos are the highly prized morels genus Morchella , which typically fruit in the spring and are often mycorrhizal, or the true truffles genus Tuber , all of which are mycorrhizal. Most fungal hyphae are divided into separate cells by end walls called septa singular, septum. In most divisions like plants, fungal phyla are called divisions by tradition of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hyphae.
They are described as perforated septa. The hyphae in bread molds which belong to the division Zygomycota are not separated by septa. They are formed of large cells containing many nuclei, an arrangement described as coenocytic hyphae. Fungi thrive in environments that are moist and slightly acidic, and can grow with or without light.
They vary in their oxygen requirements. Most fungi are obligate aerobes, requiring oxygen to survive. Other species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes, meaning that they cannot grow and reproduce in an environment with oxygen.
Yeasts are intermediate: They grow best in the presence of oxygen but can use fermentation in the absence of oxygen. The alcohol produced from yeast fermentation is used in wine and beer production, and the carbon dioxide they produce carbonates beer and sparkling wine, and makes bread rise. Fungi can reproduce sexually or asexually.
In both sexual and asexual reproduction, fungi produce spores that disperse from the parent organism by either floating in the wind or hitching a ride on an animal. Fungal spores are smaller and lighter than plant seeds, but they are not usually released as high in the air. The giant puffball mushroom bursts open and releases trillions of spores: The huge number of spores released increases the likelihood of spores landing in an environment that will support growth [Figure 4].
Like animals, fungi are heterotrophs: They use complex organic compounds as a source of carbon rather than fixing carbon dioxide from the atmosphere, as some bacteria and most plants do. In addition, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet.
However, unlike most animals that ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order. Digestion precedes ingestion. First, exoenzymes, enzymes that catalyze reactions on compounds outside of the cell, are transported out of the hyphae where they break down nutrients in the environment. Then, the smaller molecules produced by the external digestion are absorbed through the large surface areas of the mycelium.
As with animal cells, the fungal storage polysaccharide is glycogen rather than starch, as found in plants. Fungi are mostly saprobes, organisms that derive nutrients from decaying organic matter. They obtain their nutrients from dead or decomposing organic matter, mainly plant material. Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily absorbable glucose molecules. Decomposers are important components of ecosystems, because they return nutrients locked in dead bodies to a form that is usable for other organisms.
This role is discussed in more detail later. Because of their varied metabolic pathways, fungi fulfill an important ecological role and are being investigated as potential tools in bioremediation. For example, some species of fungi can be used to break down diesel oil and polycyclic aromatic hydrocarbons.
Other species take up heavy metals such as cadmium and lead. The kingdom Fungi contains four major divisions that were established according to their mode of sexual reproduction. Polyphyletic, unrelated fungi that reproduce without a sexual cycle, are placed for convenience in a fifth division, and a sixth major fungal group that does not fit well with any of the previous five has recently been described.
Not all mycologists agree with this scheme. Rapid advances in molecular biology and the sequencing of 18S rRNA a component of ribosomes continue to reveal new and different relationships between the various categories of fungi. The traditional divisions of Fungi are the Chytridiomycota chytrids , the Zygomycota conjugated fungi , the Ascomycota sac fungi , and the Basidiomycota club fungi.
An older classification scheme grouped fungi that strictly use asexual reproduction into Deuteromycota, a group that is no longer in use. The Glomeromycota belong to a newly described group [Figure 5]. Many fungi have negative impacts on other species, including humans and the organisms they depend on for food.
Fungi may be parasites, pathogens, and, in a very few cases, predators. The production of enough good-quality crops is essential to our existence. Plant diseases have ruined crops, bringing widespread famine. Most plant pathogens are fungi that cause tissue decay and eventual death of the host [Figure 6].
In addition to destroying plant tissue directly, some plant pathogens spoil crops by producing potent toxins. Fungi are also responsible for food spoilage and the rotting of stored crops. For example, the fungus Claviceps purpurea causes ergot, a disease of cereal crops especially of rye.
The most common signs and symptoms are convulsions, hallucination, gangrene, and loss of milk in cattle. The active ingredient of ergot is lysergic acid, which is a precursor of the drug LSD. Smuts, rusts, and powdery or downy mildew are other examples of common fungal pathogens that affect crops. Aflatoxins are toxic and carcinogenic compounds released by fungi of the genus Aspergillus. Periodically, harvests of nuts and grains are tainted by aflatoxins, leading to massive recall of produce, sometimes ruining producers, and causing food shortages in developing countries.
Fungi can affect animals, including humans, in several ways. Fungi attack animals directly by colonizing and destroying tissues. Humans and other animals can be poisoned by eating toxic mushrooms or foods contaminated by fungi. In addition, individuals who display hypersensitivity to molds and spores develop strong and dangerous allergic reactions.
Fungal infections are generally very difficult to treat because, unlike bacteria, fungi are eukaryotes. Antibiotics only target prokaryotic cells, whereas compounds that kill fungi also adversely affect the eukaryotic animal host. They are usually visible on the skin of the animal. Fungi that cause the superficial mycoses of the epidermis, hair, and nails rarely spread to the underlying tissue [Figure 7]. These conditions are usually treated with over-the-counter topical creams and powders, and are easily cleared.
More persistent, superficial mycoses may require prescription oral medications. Systemic mycoses spread to internal organs, most commonly entering the body through the respiratory system. For example, coccidioidomycosis valley fever is commonly found in the southwestern United States, where the fungus resides in the dust. Once inhaled, the spores develop in the lungs and cause signs and symptoms similar to those of tuberculosis.
Histoplasmosis [Figure 7] c is caused by the dimorphic fungus Histoplasma capsulatum ; it causes pulmonary infections and, in rare cases, swelling of the membranes of the brain and spinal cord. Treatment of many fungal diseases requires the use of antifungal medications that have serious side effects.
Opportunistic mycoses are fungal infections that are either common in all environments or part of the normal biota. They affect mainly individuals who have a compromised immune system. Patients in the late stages of AIDS suffer from opportunistic mycoses, such as Pneumocystis , which can be life threatening. The yeast Candida spp. Fungi may even take on a predatory lifestyle.
In soil environments that are poor in nitrogen, some fungi resort to predation of nematodes small roundworms. Species of Arthrobotrys fungi have a number of mechanisms to trap nematodes. For example, they have constricting rings within their network of hyphae. The rings swell when the nematode touches it and closes around the body of the nematode, thus trapping it.
Unlike the mushroom, which pops up then passes away quickly, the mycelium persists, often for many years, extracting nutrients and sending up its annual crop of mushrooms. Slippery Jack - Suillus luteus XX. This mushroom has pores instead of gills under its cap King boletus - boletus edulis X.
Cortinarius violaceus XX - a striking purple mushroom. Scarlet waxy-cap - Hygrophorus puniceus XXX.
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