What Is Fungus? What Are Fungi?

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Main Category: Biology / Biochemistry
Also Included In: Infectious Diseases / Bacteria / Viruses;  Dermatology
Article Date: 21 Jul 2009 - 0:00 PDT

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Fungi (Singular: fungus) are classified within their own kingdom - The Kingdom Fungi, while some are in The Kingdom Protista. A fungus is neither a plant nor an animal. It is similar to a plant, but it has no chlorophyll and cannot make its own food like a plant can through photosynthesis. They get their food by absorbing nutrients from their surroundings.

Kingdom Fungi includes mushrooms, rusts, smuts, puffballs, truffles, morels, molds, and yeasts, and thousands of other organisms and microorganisms. They range from microscopic single-celled organisms, such as yeast, to gigantic multicellular organisms.

Many fungi play a crucial role in decomposition (breaking things down) and returning nutrients to the soil. They are also used in medicine, an example is the antibiotic penicillin, as well as in industry and food preparation.

For a long time fungi were classified as plants, mainly because of their similar lifestyles - both are seen to grow in soil and are sessile (permanently attached; not moving). Plant and fungal cells both have a cell wall, while cells from the animal kingdom don't. Fungi are thought to have diverged from the plant and animal kingdoms about one billion years ago.

What is Mycology?

Mycology is the study of fungi - it is a branch of biology. A mycologist studies fungi's genes, biochemical properties, their use to us as a source of food, their hallucinogenic, poisonous and pathogenic (ability to cause disease) properties. It was not until the 16th century, when the microscope was developed, that mycology became a well established science.

What is the difference between a mushroom and a toadstool?

Most reputable scientific reference sources indicate that there is no scientific difference. People tend to refer to toadstools as the toxic (poisonous) ones and mushrooms as the edible ones. However, many mushrooms are poisonous too. A number of non-scientefic dictionaries state that a toadstool is an inedible mushroom. So, the safest answer is "There is no scientific difference, but people refer to toadstools as the inedible or toxic ones." The word toadstool is commonly used in children's stories to indicate a poisonous or colorful mushroom.

Where do fungi exist?

Fungi exist in various habitats, including deep down in the ocean, lakes, rocks, deserts, very salty environments, and areas of extremely high or low temperatures. Some can prevail even after being exposed to intense UV and cosmic radiation as one would encounter during space travel. During the 13 years the Mir space station was in orbit, a great deal of equipment was continuously being damaged by mutated fungi that had been breeding in the space station. At first technicians were puzzled and thought the problems must have been due to faulty workmanship. The majority of fungi live on land.

Fungi and bacteria are the main decomposers of organic matter in virtually all ecosystems on Earth.

Taxonomists have classified approximately 70,000 types of fungi. Experts say there are many more - possibly 1.5 million. Fungi used to be classified according to their shape, structure, biological and biochemical characteristics. Advances in DNA sequencing have helped extend the classification of different species of fungi. Taxonomy is the classification of organisms.

How do fungi feed?

Although fungi are similar to plants in many ways, they do not have chlorophyll, the green pigment that enables plants to make their own food with the aid of sunlight (photosynthesis). Fungi release digestive enzymes that decompose things around them, turning them into food. The fungus then absorbs the dissolved foods through the walls of its cells.

Fungi have adapted various ways of doing this:

• Parasitic fungi - several species of fungi exist as parasites, feeding on live hosts, which might be animals, plants or even other fungi. Some of these parasitic fungi damage our crops, sicken farm animals, and harm or completely destroy trees. Dutch elm disease, caused by the fungus Ophiostoma ulmi destroyed hundreds of millions of elm tress worldwide. The rice blast fungus Magnaporthe oryzae can devastate rice crops.
  
  The following fungi can cause serious diseases to humans: aspergilloses, candidoses, coccidioidomycosis, cryptococcosis, histoplasmosis, mycetomas, and paracoccidioidomycosis. An example is vaginal yeast infection.


• Saprobes or saprophytes - these break down dead organisms and substances that contain organic compounds and feed on them when they have rotted. Humans welcome saprobes and also fear them. They are useful decomposers of organic material, but also damage wood products and spoil our food. When ships used to be made of wood they were often rendered unusable by wood-digesting saprobes (polypores).


• Symbiosis - this is when one living thing builds up a relationship with another for the mutual survival of both. Some fungi form mycorrhizae which enhance a plant-root's capacity to absorb nutrients. The plant synthesizes nutrients the fungus needs and exchanges these nutrients for minerals the fungus absorbs from the soil - i.e. the plant and the fungus trade nutrients.
  
  Some leaf-cutting ants eat nothing but a type of fungi that lives in their nests. The fungi live on nothing but the leaves the ants carry in for them. If the ant starved the fungi and killed them the ant would have no food and would die; if the fungi found a way of poisoning the ants and killing them off, the fungi would have no food and would die. They both depend on each other for survival.

The structure of fungi

The majority of fungi - except for the one-celled oranisms - are composed of hyphae; threadlike tubular filaments. Hyphae is the plural of hypha.

A hypha has a rigid wall around it generally made of chitin. The outer skeletons (exoskeletons) of insects are also made of chitin.

The hyphae may be partitioned by dividing cross walls called septate hyphae - and are called septate hyphae, while those without cross walls are called nonseptate, or coenocytic hyphae.

The cells of all species of fungi contain cytoplasm - a mixture of nutrients and fluids. The cytoplasm flows inside the hyphae and nourishes any part that requires it.

The tips of a hypha grow by elongation and branch out to form an interwoven mat known as the mycelium. As the mycelium gets bigger it may produce structures (fruiting bodies) that contain spores.

The fruiting bodies generally grow above the soil or other surfaces so that the spores can blow in the wind and spread. The mycelium is generally beneath the surface of whatever the fungus is decomposing. The umbrella-like structure of a mushroom is its fruiting body and is typically above the surface of whatever animal, plant or substance it is decomposing, while its mycelium is below the surface.

How do fungi reproduce?

Most fungi reproduce by making spores. A puffball may contain trillions of spores.

• Sexual reproduction - fungi generally undergo a reproductive cycle that includes the production of sexual spores. A sexual spore contains one nucleus that has set of chromosomes; just half of the total set of the fungal-cell chromosomes - they are haploid. Human sperm and eggs are haploid; they contain 23 chromosomes each, half of the 46 that exist in human cells. Some spores contain two or more nuclei.
  
  When a spore germinates it eventually develops into a mycelium that produces fruiting bodies with sexual spores - and so the reproductive cycle starts all over again.


• Asexual reproduction - asexual spores may be produced directly from the hypha in some fungi - without the need for fruiting bodies. The spores then germinate and produce additional mycelium, which spreads rapidly. Experts say this allows more rapid dispersal than sexual reproduction.


• The dikaryon stage - There are two mating strains of hyphae which exist in the mycelium - the plus and the minus strain. They both look the same, but are different. Sexual reproduction occurs when the plus and minus strains fuse. Their nuclei will remain separate during the initial stages - this intermediate stage is called the dikaryon. Dikaryon means a pair of associated but unfused haploid nuclei of a fungus cell capable of participating in repeated cell division as separate entities before their eventual fusion - i.e. two nuclei, each with one half of the chromosome pairs, participating in cell division, but with nuclei not fusing yet, before the nuclei eventually fuse.
  
  With some species the dikaryon stage may last for several years, while with others it may be just a question of weeks. Eventually the two nuclei fuse and become one nucleus with the pairs of chromosomes joined up (two sets containing half the total chromosomes each), forming a diploid cell.
  
  The diploid cell then divides producing daughter cells with half the parent cell's genetic material - this process is called meiosis. Usually four genetically unique haploid spores are produced, and the cycle restarts. This form of procreation using genetically different spores helps fungi adapt more effectively to novel diseases and environmental changes. If all the fungi were genetically identical they could all be destroyed by a single disease or a significant environmental change.


• Fragmentation - in some types of fungi the hyphae fragment, with each fragment developing into a new separate organism. With the single cell of yeast, a bump forms on the cell which eventually breaks off and ultimately becomes a new yeast cell.

How are fungi classified?

The classification of fungi has long been a subject of controversy among experts. Pier Antonio Micheli, an Italian botanist, was the first to describe fungi in scientific terms we are used to today. He classed them as plants. For a long time the study of fungi (Mycology) was a subdivision of botany.

Robert H. Wittaker (USA, 1920-1980) a vegetation ecologist introduced a 5-Kingdom taxonomy, granting fungi equal status with animals and plants. The 5-Kingdom taxonomy included:

• Kingdom Animalia (animal kingdom)


• The Kingdom Plantae (plant kingdom)


• Kingdom Fungi (fungi kingdom)


• Kingdom Protista (types of eukaryotic organisms; containing complex structures enclosed within membranes)


• Kingdom Monera (types of microscopic single-celled organisms whose genetic material is loose in the cell, instead of being held in the cell's nucleus)

Today some countries, such as the USA use a 6-Kingdom system:

• Kingdom Animalia
• Kingdom Plantae
• Kingdom Fungi
• Kingdom Protista
• Kingdom Archaea
• Kingdom Bacteria
Archaea are single-cell microorganisms with no nucleus or other microscopic organ-like parts (organelles).

while many British, Irish and Australasian scientists use a 5-Kingdom system:

• Kingdom Animalia
• Kingdom Plantae
• Kingdom Fungi
• Kingdom Protista
• Kingdom Prokaryota/Monera

Current mycologists say that as some slime molds, mildews and water molds have similar feeding stages to amoebas, plus some other qualities, they should be part of a separate kingdom called Kingdom Protista, while others talk about Kingdom Stremenopila for water molds and downy mildews that have no chitin, as well as some other characteristics.

Kingdoms are divided into phila

Today, the main criteria for fungus classification is the type of spores and fruiting bodies it produces. All Kingdoms are divided into phyla (singular: phylum). A phylum is a primary division of a kingdom. Fungi are commonly divided into four broad phyla:

• Chytridiomycota (Chytrids) - these are microscopic fungi and are mostly found in freshwater or damp/drenched soil. The majority of chytrids are parasites of algae and animals. Some are saprobes (living on decomposed organic matter). A chytrid called Batrachochytrium dendrobatidis causes disease in amphibians. Scientists have identified over 800 species of chytrids.


• Zygomycota - these mold that forms on bread and many other foods. The mold has tiny black dots (sporangia) which produce sporangiospores - asexual spores. Many types of zygomycota are insect and spider parasites.


• Ascomycota - there are about 33,000 identified species of ascomycetes that feed on both living and dead matter. Ascomycetes are primary decomposers of plants. They are also important causes of plant and human diseases. They are used in brewing, baking, winemaking, and as sources of therapeutic drugs. They are also used in the production of some cheeses, such as Camembert, Roquefort and Stilton.


• Basidiomycota - Both edible and poisonous mushrooms are types of basidiomicetes. They cause devastating plant diseases. Basidiomicetes are commonly found in birds' nests. Most decomposition of living or dead wood that occurs in forests as well as inside man-made structures is caused by basidiomicetes. Leaf ants live exclusively on a type of basidiomicetes, as do some termites.
  
  Cryptococcosis, a human infection caused by inhaling a basidiomicetes fungus called Cryptococcus neoformans, commonly infects patients with weakened immune systems, such as those with HIV/AIDS, or people receiving medications that suppress their immune system. Infection is usually limited to the lungs, but can spread to other parts of the body, causing meningitis.

What do humans use fungi for?

• Fungi as a food or used in food preparation
  
  Humans have been eating fungi since before we started walking on our hind legs. Today we eat an enormous variety of edible fungi, including truffles, mushrooms, quorn, shitake and hundreds others.
  
  We also use fungi, e.g. yeast, in food manufacturing. Yeast is needed for the fermentation of wine, beers and other alcoholic drinks. We add yeast to dough to make the bread rise when we bake it.


• Fungi used in industry and agriculture
  
  Fungi are used in the production of ethanol. They are used extensively to produce industrial chemicals, such as citric acid, gluconic acid, malic acid, and biological detergents. The production of many deliberately faded garments would be much more difficult without using fungi. They are used in bioremediation - the detoxification of polluted water or soil.
  
  Fungi are also used in agriculture for pest control and to protect crops from diseases.


• Fungi used in medicine
  
  
  • Antibiotics - Many types of antibiotics come from fungi, such as penicillin (Penicillium chrysogenum), cephalosporin (Acremonium) , and griseofulvin (Penicillium griseofulvin). Penicillin works by destroying the wall of a bacterial cell. Fungal antibiotics are extensively used for treating tuberculosis, syphilis, and leprosy, to name but a few diseases.
  
  
  • Chemotherapy - Antibacterial chemotherapy uses fungi. Lentilan, a drug used in cancer treatment, is sourced from the shiitake mushroom.
  
  
  • Immunosuppressants - Cyclosporin is a medication that transplant patients take to suppress their immune system. It is produced by the fungus Beauveria nivea and significantly lowers the risk of transplanted organ refection. Many successful transplant procedures could not have occurred without this drug.
  
  
  Laboratory studies indicate that Agaricus blazei may stimulate the human immune system and could have major implications for human health - especially in the treatment of many cancers.
  
  US researchers reported that a rapid production of therapeutic human drugs using modified mushrooms may help mount a quicker response to various public health problems.
  
  The reishi mushroom (Ganoderma lucidum) and green tea may both enhance the body's immune functions and hold the potential for treatment and prevention of many types of cancer, scientists from Pharmanex BJ Clinical Pharmacology Center in Beijing reported.
  
  Extracts of the mushroom Phellinus linteus may halt breast cancer growth , according to scientists from the Methodist Research Institute in Indianapolis, USA.


• Traditional Chinese Medicine includes several types of mushrooms, such as Agaricus, Ganoderma, and Cordyceps among its various therapies.

Fungi that cause human diseases - pathogenic fungi

Mycosis is a disease caused by a fungus (plural: mycoses). Fungi that cause diseases are called pathogenic fungi. Mycoses can be divided into four broad groups; these groups are classified according to how deeply the fungus penetrates into the body:

• Superficial mycosis
  
  The fungus does not penetrate at all. It is present only on the surface of the skin or on the patient's hair. This may include various types of ringworm (tinea), including athlete's foot, scalp ringworm, and body (skin) ringworm.
  
  Candida albicans is a yeast that causes candidiasis. As a superficial mycosis candidiasis usually affects the vagina of females or the mouth of both males and females. It exists normally in the vagina or gastrointestinal tract, but may multiply rapidly, especially if the person is ill or has a weakened immune system. Taking antibiotics may sometimes result in rapid reproduction of Candida albicans. Fungi that obtain nutrients from keratinized material are called dermatophytes. Dermatophytes cause superficial and cutaneous mycosis (below).


• Cutaneous mycosis
  
  The infection is generally limited to the nonliving keratinized layers of skin, hair, and nails - no living tissue is invaded. Also includes tinea (ringworm), as well as candidiasis of the skin, mucus membranes and nails (Candida albicans and related species).


• Subcutaneous mycosis
  
  The fungi reach below the skin and infect subcutaneous (area just below the skin), connective and bone tissue. These infections are usually chronic (long-lasting) and often occur when the skin is pierced or wounded, allowing the fungi to enter, usually in the form of vegetable matter. These infections may be hard to treat and often require removal of damaged skin (debridement). Subcutaneous mycoses are much more common in the tropics.
  
  Sporotrichosis caused by Sporothrix schenckii is an example of subcutaneous mycosis. Sporotrichosis used to be common in Europe, but is very rare there today. It is more common in Australia, South Africa and in the warmer regions of the Americas. Infection may occur as a result of an insect bite, a thorn prick or a scratch from any sharp object. Infection risk is higher among florists, farmers, gardeners and people who work with hay and moss.


• Systemic mycosis
  
  This is a fungal infection that may reach any part of the body, including the brain and heart, as well as the bloodstream. The fungi usually enter via the lungs, gastrointestinal tract or intravenously.
  
  An example of systemic mycosis is cryptococcosis, which is caused by Cryptococcus neoformans, a type of yeast. It can cause subacute or chronic meningitis. The fungi enter the body by inhalation. Infected patients may also develop lung infections. Cryptococcus neoformans may be found in pigeon droppings.

Written by Christian Nordqvist
Copyright: Medical News Today
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