It is a treasure trove of truly astonishing organisms, including some incredibly old individuals. A living fungus recently found in America, was already years old at the time of Christ's birth two thousand years ago! What are fungi? Fungi are extraordinary organisms which are neither plants, nor animals. They are in a Kingdom all of their own.
What do they look like? The main body of a fungus is usually composed of enormous numbers of fine, branching threads, called hyphae , which together form a tangled mat or web.
This is the body of the fungus and it is called a mycelium. It is usually hidden from view. How do they reproduce? The familiar mushrooms which we may cook and eat are the fruiting bodies of the fungus. Each fruiting body or more correctly, 'sporophore' produces millions of tiny structures called spores. The spores are so small that, depending on the type of fungus, you could fit between and on the head of a pin!
A new organism classified as a vertebrate, for example, will be commonly understood to have a spine composed of vertebrae. For scientists, taxonomic groups are touchstones of understanding: a foundation upon which to build new knowledge. This metaphor communicates the fundamental importance of taxonomy, but it implies a stability that taxonomic classification lacks.
Classifying fungi as plants has led to some curious events. This pairing of fungi with plants is a present problem: misclassification matters because how we classify organisms affects how we understand, support financially and culturally and engage with them.
Why were fungi ever considered plants? Today, we know that fungi are not plants, but the botanical history of fungi provides an interesting perspective on our scientific biases, on how we classify organisms and how these impact our collective knowledge. Taxonomic classifications are in constant flux, as we refine our understanding of the incredible diversity that surrounds us.
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.
The fungus extends specialized hyphae that can penetrate the body of the worm and slowly digest the hapless prey. Fungi play a crucial role in the balance of ecosystems. They colonize most habitats on Earth, preferring dark, moist conditions. They can thrive in seemingly hostile environments, such as the tundra, thanks to a most successful symbiosis with photosynthetic organisms, like lichens.
Fungi are not obvious in the way that large animals or tall trees are. Yet, like bacteria, they are major decomposers of nature. With their versatile metabolism, fungi break down organic matter that is insoluble and would not be recycled otherwise.
Food webs would be incomplete without organisms that decompose organic matter and fungi are key participants in this process. Decomposition allows for cycling of nutrients such as carbon, nitrogen, and phosphorus back into the environment so they are available to living things, rather than being trapped in dead organisms.
Fungi are particularly important because they have evolved enzymes to break down cellulose and lignin, components of plant cell walls that few other organisms are able to digest, releasing their carbon content.
Fungi are also involved in ecologically important coevolved symbioses, both mutually beneficial and pathogenic with organisms from the other kingdoms.
Mycorrhiza , a term combining the Greek roots myco meaning fungus and rhizo meaning root, refers to the association between vascular plant roots and their symbiotic fungi. Somewhere between 80—90 percent of all plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant.
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