Everything about Fungus totally explained
A
fungus is a
eukaryotic organism that's a member of the
kingdom Fungi . The fungi are
heterotrophic organisms possessing a
chitinous
cell wall. The majority of species grow as
multicellular filaments called
hyphae forming a
mycelium; some fungal species also grow as single
cells. Sexual and asexual reproduction of the fungi is commonly via
spores, often produced on specialized structures or in
fruiting bodies. Some species have lost the ability to form specialized reproductive structures, and propagate solely by
vegetative growth.
Yeasts,
molds, and
mushrooms are examples of fungi. The fungi are a
monophyletic group that's
phylogenetically clearly distinct from the morphologically similar
slime molds (
myxomycetes) and
water molds (
oomycetes). The fungi are more closely related to
animals than
plants, yet the discipline of
biology devoted to the study of fungi, known as
mycology, often falls under a branch of
botany.
Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as
symbionts of plants, animals, or other fungi. They perform an essential role in all ecosystems in decomposing
organic matter and are indispensable in
nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or molds. Many fungal species have long been used as a direct source of food, such as mushrooms and
truffles and in
fermentation of various food products, such as
wine,
beer, and
soy sauce. More recently, fungi are being used as sources for
antibiotics used in medicine and various
enzymes, such as
cellulases,
pectinases, and
proteases, important for industrial use or as active ingredients of
detergents. Many fungi produce
bioactive compounds called
mycotoxins, such as
alkaloids and
polyketides that are toxic to animals including humans. Some fungi are used
recreationally or in traditional ceremonies as a source of
psychotropic compounds. Several species of the fungi are significant
pathogens of humans and other animals, and losses due to
diseases of
crops (for example,
rice blast disease) or food
spoilage caused by fungi can have a large impact on human
food supply and local economies.
Etymology and definition
The
English word
fungus is directly adopted from the
Latin fungus, meaning "mushroom", used in
Horace and
Pliny. This in turn is derived from the
Greek word
sphongos/σφογγος ("sponge"), referring to the
macroscopic structures and morphology of some mushrooms and molds and also used in other languages (for example, the
German Schwamm ("sponge") or
Schwammerl for some types of mushroom).
Diversity
Fungi have a worldwide distribution, and grow in a wide range of habitats, including
deserts. Most fungi grow in terrestrial environments, but several species occur only in aquatic habitats. Fungi along with
bacteria are the primary
decomposers of organic matter in most if not all terrestrial
ecosystems worldwide. Based on observations of the ratio of the number of fungal species to the number of plant species in some environments, the fungal kingdom has been estimated to contain about 1.5 million species. Around 70,000 fungal species have been formally described by taxonomists, but the true dimension of fungal diversity is still unknown. Most fungi grow as thread-like filaments called
hyphae, which form a
mycelium, while others grow as single cells. Until recently many fungal species were described based mainly on morphological characteristics, such as the size and shape of spores or fruiting structures, and
biological species concepts; the application of
molecular tools, such as
DNA sequencing, to study fungal diversity has greatly enhanced the resolution and added robustness to estimates of diversity within various taxonomic groups.
Importance for human use
Human use of fungi for food preparation or preservation and other purposes is extensive and has a long history:
yeasts are required for
fermentation of
beer,
wine and
bread, some other fungal species are used in the production of
soy sauce and
tempeh.
Mushroom farming and
mushroom gathering are large industries in many countries. Many fungi are producers of
antibiotics, including
β-lactam antibiotics such as
penicillin and
cephalosporin. Widespread use of these antibiotics for the treatment of bacterial diseases, such as
tuberculosis,
syphilis,
leprosy, and many others began in the early 20th century and continues to play a major part in anti-bacterial
chemotherapy. The study of the historical uses and sociological impact of fungi is known as
ethnomycology.
Cultured foods
Baker's yeast or
Saccharomyces cerevisiae, a single-cell fungus, is used in the baking of
bread and other wheat-based products, such as
pizza and
dumplings. Several yeast species of the genus
Saccharomyces are also used in the production of
alcoholic beverages through
fermentation. Mycelial fungi, such as the
shoyu koji mold (
Aspergillus oryzae), are used in the brewing of
Shoyu (
soy sauce) and preparation of
tempeh.
Quorn is a high-protein product made from the mold,
Fusarium venenatum, and is used in
vegetarian cooking.
Other human uses
Fungi are also used extensively to produce industrial chemicals like
lactic acid,
antibiotics and even to make stonewashed
jeans. Several fungal species are ingested for their
psychedelic properties, both
recreationally and religiously (see main article,
Psilocybin mushrooms).
Mycotoxins
Many fungi produce compounds with
biological activity. Several of these compounds are
toxic and are therefore called
mycotoxins, referring to their fungal origin and toxic activity. Of particular relevance to humans are those mycotoxins that are produced by moulds causing food spoilage and poisonous mushrooms (see below). Particularly infamous are the
aflatoxins, which are insidious
liver toxins and highly
carcinogenic metabolites produced by
Aspergillus species often growing in or on grains and nuts consumed by humans, and the lethal
amatoxins produced by mushrooms of the genus
Amanita. Other notable mycotoxins include
ochratoxins,
patulin,
ergot alkaloids, and
trichothecenes and fumonisins, all of which have significant impact on human food supplies or animal
livestock.
Mycotoxins belong to the group of
secondary metabolites (or
natural products). Originally, this group of compounds had been thought to be mere byproducts of
primary metabolism, hence the name "secondary" metabolites. However, recent research has shown the existence of
biochemical pathways solely for the purpose of producing mycotoxins and other natural products in fungi. Mycotoxins provide a number of
fitness benefits to the fungi that produce them in terms of physiological adaptation, competition with other microbes and fungi, and protection from
fungivory. These fitness benefits and the existence of dedicated biosynthetic pathways for mycotoxin production suggest that the mycotoxins are important for fungal persistence and survival.
Edible and poisonous fungi
Some of the best known types of fungi are the
edible and the
poisonous mushrooms. Many species are commercially raised, but others must be harvested from the wild.
Agaricus bisporus, sold as button mushrooms when small or Portobello mushrooms when larger, are the most commonly eaten species, used in salads, soups, and many other dishes. Many Asian fungi are commercially grown and have gained in popularity in the West. They are often available fresh in grocery stores and markets, including
straw mushrooms (
Volvariella volvacea),
oyster mushrooms (
Pleurotus ostreatus),
shiitakes (
Lentinula edodes), and
enokitake (
Flammulina spp.).
There are many more mushroom species that are
harvested from the wild for personal consumption or commercial sale.
Milk mushrooms,
morels,
chanterelles,
truffles,
black trumpets, and
porcini mushrooms (
Boletus edulis) (also known as king boletes) all demand a high price on the market. They are often used in gourmet dishes.
For certain types of
cheeses, it's also a common practice to inoculate milk curds with fungal spores to foment the growth of specific species of
mold that impart a unique flavor and texture to the cheese. This accounts for the
blue colour in cheeses such as
Stilton or
Roquefort which is created using
Penicillium roqueforti spores. Molds used in cheese production are usually non-toxic and are thus safe for human consumption; however, mycotoxins (for example, aflatoxins,
roquefortine C, patulin, or others) may accumulate due to fungal spoilage during cheese ripening or storage.
Many mushroom species are toxic to humans, with toxicities ranging from slight digestive problems or
allergic reactions as well as
hallucinations to severe organ failures and death. Some of the most deadly mushrooms belong to the genera
Inocybe,
Cortinarius, and most infamously,
Amanita. The latter genus includes the destroying angel
(A. virosa) and the death cap
(A. phalloides), the most common cause of deadly mushroom poisoning. The false morel (
Gyromitra esculenta) is considered a delicacy by some when cooked, yet can be highly toxic when eaten raw.
Tricholoma equestre was considered edible until being implicated in some serious poisonings causing
rhabdomyolysis.
Fly agaric mushrooms (
A. muscaria) also cause occasional poisonings, mostly as a result of ingestion for use as a
recreational drug for its
hallucinogenic properties. Historically Fly agaric was used by
Celtic
Druids in Northern Europe and the
Koryak people of north-eastern
Siberia for religious or shamanic purposes. It is difficult to identify a safe mushroom without proper training and knowledge, thus it's often advised to assume that a mushroom in the wild is poisonous and not to consume it.
Fungi in the biological control of pests
In agricultural settings, fungi that actively compete for nutrients and space with, and eventually prevail over,
pathogenic microorganisms, such as bacteria or other fungi, via the
competitive exclusion principle, or are
parasites of these pathogens, may be beneficial agents for human use. For example, some fungi may be used to suppress growth or eliminate harmful plant pathogens, such as insects,
mites,
weeds,
nematodes and other fungi that cause diseases of important
crop plants. This has generated strong interest in the use and practical application of these fungi for the
biological control of these agricultural pests.
Entomopathogenic fungi can be used as
biopesticides, as they actively kill insects. Examples of fungi that have been used as
biological insecticides are
Beauveria bassiana,
Metarhizium anisopliae,
Hirsutella spp,
Paecilomyces spp, and
Verticillium lecanii. Endophytic fungi of grasses of the genus
Neotyphodium, such as
N. coenophialum produce
alkaloids that are toxic to a range of invertebrate and vertebrate
herbivores. These alkaloids protect the infected grass plants from herbivory, but some endophyte alkaloids can cause poisoning of grazing animals, such as cattle and sheep. Infection of grass cultivars of
turf or
forage grasses with isolates of the grass endophytes that produce only specific alkaloids to improve grass hardiness and resistance to herbivores such as insects, while being non-toxic to livestock, is being used in
grass breeding programs.
Bioremediation
Certain fungi, in particular 'white rot' fungi, can degrade insecticides, herbicides, pentachlorophenol, creosote, coal tars, and heavy fuels and turn them into carbon dioxide, water, and basic elements. Research has recently discovered that fungi can be used to lock uranium into mineral form.
Ecology
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. As decomposers, they play an indispensable role in
nutrient cycling, especially as
saprotrophs and
symbionts, degrading organic matter to inorganic molecules, which can then re-enter anabolic metabolic pathways in plants or other organisms.
Symbiosis
Many fungi have important
symbiotic relationships with organisms from most if not all
Kingdoms. These interactions can be mutualistic or antagonistic in nature, or in case of commensal fungi are of no apparent benefit or detriment to the host.
With plants
Mycorrhizal symbiosis between
plants and fungi is one of the most well-known plant-fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in some kind of mycorrhizal relationship with fungi and are dependent upon this relationship for survival. The mycorrhizal symbiosis is ancient, dating to at least 400 million years ago. It often increases the plant's uptake of inorganic compounds, such as
nitrate and
phosphate from soils having low concentrations of these key plant nutrients. In some mycorrhizal associations, the fungal partners may mediate plant-to-plant transfer of carbohydrates and other nutrients. Such mycorrhizal communities are called "common mycorrhizal networks".
Lichens are formed by a symbiotic relationship between
algae or
cyanobacteria (referred to in lichens as "photobionts") and fungi (mostly various species of ascomycetes and a few basidiomycetes), in which individual photobiont cells are embedded in a tissue formed by the fungus. As in
mycorrhizas, the photobiont provides sugars and other carbohydrates, while the fungus provides minerals and water. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism.
With insects
Many insects also engage in
mutualistic relationships with various types of fungi. Several groups of ants cultivate fungi in the order
Agaricales as their primary food source, while
ambrosia beetles cultivate various species of fungi in the bark of trees that they infest. Termites on the African Savannah are also known to cultivate fungi.
As pathogens and parasites
However, many fungi are parasites on plants, animals (including
humans), and other fungi. Serious fungal pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the
rice blast fungus
Magnaporthe oryzae, tree pathogens such as
Ophiostoma ulmi and
Ophiostoma novo-ulmi causing
Dutch elm disease, and
Cryphonectria parasitica responsible for
chestnut blight, and plant-pathogenic fungi in the genera
Fusarium,
Ustilago,
Alternaria, and
Cochliobolus;
Histoplasma, and
Pneumocystis. Several pathogenic fungi are also responsible for relatively minor human
diseases, such as
athlete’s foot and
ringworm. Some fungi are
predators of
nematodes, which they capture using an array of specialized structures, such as constricting rings or adhesive nets.
Nutrition and possible autotrophy
Growth of fungi as
hyphae on or in solid substrates or single cells in aquatic environments is adapted to efficient extraction of nutrients from these environments, because these growth forms have high
surface area to volume ratios. These adaptations in
morphology are complemented by
hydrolytic enzymes secreted into the environment for digestion of large organic molecules, such as
polysaccharides,
proteins,
lipids, and other organic substrates into smaller molecules. These molecules are then absorbed as nutrients into the fungal
cells.
Traditionally, the fungi are considered
heterotrophs, organisms that rely solely on carbon fixed by other organisms for
metabolism. Fungi have
evolved a remarkable metabolic versatility that allows many of them to use a large variety of organic substrates for growth, including simple compounds as
nitrate,
ammonia,
acetate, or
ethanol. Recent research raises the possibility that some fungi utilize the pigment
melanin to extract energy from
ionizing radiation, such as
gamma radiation for
"radiotrophic" growth. It has been proposed that this process might bear some similarity to photosynthesis in plants, Fungi are unusual among the eukaryotes in having a cell wall that, besides
glucans (for example, β-1,3-glucan) and other typical components, contains the
biopolymer chitin.
Many fungi grow as thread-like filamentous microscopic structures called
hyphae, and an assemblage of intertwined and interconnected hyphae is called a
mycelium. Hyphae can be septate, for example, divided into hyphal compartments separated by a
septum, each compartment containing one or more
nuclei or can be
coenocytic, for example, lacking hyphal compartmentalization. However, septa have pores, such as the
doliporus in the basidiomycetes that allow cytoplasm, organelles, and sometimes nuclei to pass through. In some cases, fungi have developed specialized structures for nutrient uptake from living hosts; examples include
haustoria in plant-parasitic fungi of nearly all divisions, and
arbuscules of several
mycorrhizal fungi, which penetrate into the host cells for nutrient uptake by the fungus.
Macroscopic structures
Fungal mycelia can become visible macroscopically, for example, as concentric rings on various surfaces, such as damp walls, and on other substrates, such as spoilt food (see figure), and are commonly and generically called
mould (
American spelling,
mold); fungal mycelia grown on solid
agar media in laboratory
petri dishes are usually referred to as colonies, with many species exhibiting characteristic macroscopic growth morphologies and colours, due to spores or
pigmentation.
Specialized fungal structures important in
sexual reproduction are the
apothecia,
perithecia, and
cleistothecia in the ascomycetes, and the
fruiting bodies of the basidiomycetes, and a few ascomycetes. These reproductive structures can sometimes grow very large, and are well known as
mushrooms.
Morphological and physiological features for substrate penetration
Fungal hyphae are specifically adapted to growth on solid surfaces and within substrates, and can exert astoundingly large penetrative mechanical forces. The
plant pathogen,
Magnaporthe grisea, forms a structure called an
appressorium specifically designed for penetration of plant tissues, and the pressure generated by the appressorium, which is directed against the plant
epidermis can exceed 8
MPa (80
bars). The generation of these mechanical pressures is the result of an interplay between physiological processes to increase intracellular
turgor by production of
osmolytes such as
glycerol, and the morphology of the appressorium.
Reproduction
Reproduction of fungi is complex, reflecting the heterogeneity in lifestyles and genetic make up within this group of organisms.
Most fungi have both a
haploid and
diploid stage in their life cycles. In all sexually reproducing fungi, compatible individuals combine by cell fusion of vegetative hyphae by
anastomosis, required for the initiation of the sexual cycle. Ascomycetes and basidiomycetes go through a
dikaryotic stage, in which the
nuclei inherited from the two parents don't fuse immediately after cell fusion, but remain separate in the hyphal cells (see
heterokaryosis).
In ascomycetes, dikaryotic hyphae of the
hymenium form a characteristic
hook at the hyphal septum. During
cell division formation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments. An
ascus (plural
asci) is then formed, in which
karyogamy (nuclear fusion) occurs. These asci are embedded in an
ascocarp, or fruiting body, of the fungus. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. The ascospores are disseminated and germinate and may form a new haploid mycelium. The most commonly known basidiocarps are mushrooms, but they may also take many other forms (see Morphology section).
In zygomycetes, haploid hyphae of two individuals fuse, forming a
zygote, which develops into a
zygospore. When the zygospore germinates, it quickly undergoes
meiosis, generating new haploid hyphae, which in turn may form asexual
sporangiospores. These sporangiospores are means of rapid dispersal of the fungus and germinate into new genetically identical haploid fungal colonies, able to mate and undergo another sexual cycle followed by the generation of new zygospores, thus completing the lifecycle.
Spore dispersal
Both asexual and sexual spores or sporangiospores of many fungal species are actively dispersed by forcible ejection from their reproductive structures. This ejection ensures exit of the spores from the reproductive structures as well as travelling through the air over long distances. Many fungi thereby possess specialized mechanical and physiological mechanisms as well as spore-surface structures, such as
hydrophobins, for spore ejection. These mechanisms include, for example, forcible discharge of ascospores enabled by the structure of the ascus and accumulation of
osmolytes in the fluids of the ascus that lead to explosive discharge of the ascospores into the air. The forcible discharge of single spores termed
ballistospores involves formation of a small drop of water (
Buller's drop), which upon contact with the spore leads to its projectile release with an initial acceleration of more than 10,000
g. Other fungi rely on alternative mechanisms for spore release, such as external mechanical forces, exemplified by
puffballs. Attracting insects, such as flies, to fruiting structures, by virtue of their having lively colours and a putrid odour, for dispersal of fungal spores is yet another strategy, most prominently used by the
stinkhorns.
Other sexual processes
Besides regular sexual reproduction with meiosis, some fungal species may exchange genetic material via
parasexual processes, initiated by anastomosis between hyphae and
plasmogamy of fungal cells. The frequency and relative importance of parasexual events is unclear and may be lower than other sexual processes. However, it's known to play a role in intraspecific hybridization and is also likely required for hybridization between fungal species, which has been associated with major events in fungal evolution.
Phylogeny and classification
For a long time
taxonomists considered fungi to be members of the
Plant Kingdom. This early classification was based mainly on similarities in lifestyle: both fungi and plant are mainly
sessile, have similarities in general
morphology and growth habitat (like plants, fungi often grow in soil, in the case of
mushrooms forming conspicuous
fruiting bodies, which sometimes bear resemblance to plants such as
mosses). Moreover, both groups possess a
cell wall, which is absent in the
Animal Kingdom. However, the fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged approximately one billion years ago. Many studies have identified several distinct morphological, biochemical, and genetic features in the Fungi, clearly delineating this group from the other kingdoms. For these reasons, the fungi are placed in their own kingdom.
Physiological and morphological traits
Similar to animals and unlike most plants, fungi lack the capacity to synthesize organic carbon by chlorophyll-based
photosynthesis; whereas plants store the reduced carbon as
starch, fungi, like animals and some bacteria, use
glycogen for storage of
carbohydrates. A major component of the cell wall in many fungal species is the nitrogen-containing
carbohydrate,
chitin, also present in some animals, such as the
insects and
crustaceans, while the plant cell wall consists chiefly of the carbohydrate
cellulose. The defining and unique characteristics of fungal cells include growth as
hyphae, which are microscopic filaments of between 2-10 microns in diameter and up to several centimetres in length, and which combined form the fungal
mycelium. Some fungi, such as yeasts, grow as single ovoid cells, similar to unicellular
algae and the
protists.
Unlike many plants, most fungi lack an efficient
vascular system, such as
xylem or
phloem for long-distance transport of water and nutrients; as an example for
convergent evolution, some fungi, such as
Armillaria, form rhizomorphs or
mycelial cords, resembling and functionally related to, but morphologically distinct from,
plant roots.
Some characteristics shared between plants and fungi include the presence of
vacuoles in the cell, and a similar pathway in the biosynthesis of
terpenes using
mevalonic acid and
pyrophosphate as
biochemical precursors; plants however use an additional terpene biosynthesis pathway in the
chloroplasts that's apparently absent in fungi. Ancestral traits shared among members of the fungi include
chitinous cell walls and heterotrophy by absorption.
Similar to plants, fungi produce a plethora of
secondary metabolites functioning as defensive compounds or for
niche adaptation; however, biochemical pathways for the synthesis of similar or even identical compounds often differ markedly between fungi and plants.
Evolutionary history
The first organisms having features typical of fungi date to, the Proterozoic. However, fungal fossils don't become common and uncontroversial until the early Devonian, when they're abundant in the Rhynie chert. For much of the
Paleozoic Era, the fungi appear to have been aquatic, and consisted of organisms similar to the
extant Chytrids in having flagellum-bearing spores. The early fossil record of the fungi is fragmentary, to say the least. The fungi probably colonized the land during the
Cambrian, long before land plants. For some time after the
Permian-Triassic extinction event, a fungal spike, originally thought to be an extraordinary abundance of fungal spores in
sediments formed shortly after this event, suggested that they were the dominant life form during this period—nearly 100% of the
fossil record available from this period. However, the relative proportion of fungal spores relative to spores formed by algal species is difficult to assess, the spike didn't appear world-wide, and in many places it didn't fall on the Permian-Triassic boundary.
Analyses using
molecular phylogenetics support a
monophyletic origin of the Fungi.
There is no unique generally accepted system at the higher taxonomic levels and there are constant name changes at every level, from species upwards. However, efforts among fungal researchers are now underway to establish and encourage usage of a unified and more consistent
nomenclature.
The Blastocladiomycota were previously considered a taxonomic clade within the Chytridiomycota. Recent molecular data and ultrastructural characteristics, however, place the Blastocladiomycota as a sister clade to the Zygomycota, Glomeromycota, and Dikarya (Ascomycota and Basiomycota). The blastocladiomycetes are fungi that are saprotrophs and parasites of all eukaryotic groups and undergo sporic meiosis unlike their close relatives, the chytrids, which mostly exhibit zygotic meiosis.
Members of the Glomeromycota are fungi forming arbuscular mycorrhizae with higher plants. Only one species has been observed forming zygospores; all other species solely reproduce asexually. The symbiotic association between the Glomeromycota and plants is ancient, with evidence dating to 400 million years ago.
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