The algae presumably do not release proteases and therefore must remain in the parts of the structure, which are composed of only calcium carbonate. Boring patterns reflect in part the shape and behaviour of the boring microorganisms and in part the structural properties of the shell itself Che et al. In the black pearl oyster, the phototrophic endoliths dominated the external prismatic region of the shell whereas the interior nacreous region was attacked mainly by heterotrophs Che et al.
Infection always begins in the oldest part of the shell. We expect that endoliths play important roles in the global calcium carbonate cycle and other ecological processes especially in aquatic ecosystems and may be significantly impacted by global climate change. Endoliths actively contribute to the biodegradation of geological substrates and skeletons and shells of dead animals composed of calcium carbonate. They have been implicated as a causative agent of shell diseases in live molluscs, corals and other phyla of invertebrate animals Golubic ; Kohlmeyer ; Che et al.
How old are fungi?
Rates of microbial bioerosion of experimental blocks cut from live skeletons of the coral Porites have been estimated at sites along the Great Barrier Reef by Kiene and Hutchings and Tribollet The results suggested that endoliths have a significant impact on the overall calcium budget of coral reef ecosystems, but rates were not measured in skeletons or shells of other species or in other ecosystems Tribollet A thorough understanding of the ecology of endoliths awaits further investigation into blue carbon research.
Historically, endolithic microorganisms have been very poorly studied primarily because they are difficult to detect and correctly identify with standard laboratory procedures and their ecological properties are difficult to study in vivo with present technology. Endoliths can be observed often growing in vivo on the surface of calcareous structures with the light microscope, but they must be grown in the laboratory in culture to see all stages of their life cycles.
They can only be observed growing inside calcareous structures with the light microscope in cast resins or double embedded preparations or with scanning and transmission electron microscopes as chemically fixed specimens using specialised techniques. The specialised techniques for observing endoliths have been reviewed in detail by Golubic et al.
A method using immunofluorescence has been designed to detect thraustochytrids inside host cells Raghukumar and Lande It might be possible to use similar immunofluorescence techniques for studies with other endolithic species. The natural history of parasitic and saprotrophic endolithic boring microorganisms and the skeletons and shells of the host animals which they inhabit has been well recorded in the fossil record.
The significance of endolithic microbial ecosystems in both aquatic and terrestrial ecosystems in general has been reviewed in detail by Walker and Pace Fossilised fungal structures have been reported from a variety of mineral substrates including Devonian Rhynie Chert, as fossil lichen mycobionts in stromatolites, in Djebel-Onk phosphorites, Triassic silicified rock, Bitterfield amber and Tertiary Dominican amber see Burford et al.
Various types of endoliths, including fungi, have been found in marine shells in the Late Ordovician and Middle Devonian volcanic rocks, while microborings have also been found in early Cambrian phosphatic and phosphatised fossils Taylor et al. Fossilised microorganisms have also been observed in drilled cores and dredged samples from the ocean floor, with a majority of these findings representing fungi Schumann et al.
These fungi existed in a symbiotic-like relationship with two types of chemolithotrophic prokaryotes, which appeared to use the structural framework of the mycelium for their growth Bengtson et al. Early fossil records of eukaryotes, including fungi and primitive plants in terrestrial ecosystems, appear to have come from the Ordivician period Heckman et al.
However, it has been postulated that they may have occurred earlier, during the Precambrian, as lichens Heckman et al. Fungi do not preserve well in the fossil record, suggesting the possibility of an earlier, unrecorded history Heckman et al. It has been suggested that mycorrhiza-driven weathering may have originated more than MYA, and that it subsequently intensified with the evolution of trees and mycorrhizas Quirk et al.
Some of the free-living MCF also referred to as rock-inhabiting fungi are slowly growing melanised Ascomycetes especially suited to colonising rocks in arid environments. Because these organisms often form early diverging groups in the Chaetothyriales and Dothideomyceta, the ancestors of these two lineages were suggested to be rock-inhabitants.
It was deduced that the rock-inhabiting fungi in the Dothideomyceta evolved in the late Devonian, much earlier than those in the Chaetothyriales, which originated in the middle Triassic, both periods correlating with an expansion of arid landmasses. It was proposed that the paleoclimate record provided a good explanation for the diversification of fungi subject to abiotic stresses and adapted to life on rocks Gueidan et al. Rocks and minerals represent a vast reservoir of elements and compounds, many of which are essential to life and which must be released in specific soluble forms which can be assimilated by the biota Burford et al.
These include essential metals as well as nutrients like phosphate. Fungi have been found associated with a wide range of rock types including limestone, soapstone, marble, granite, sandstone, andesite, basalt, gneiss, dolerite, amphibolite and quartz, even from most harsh environments, e. It is likely that fungi are ubiquitous members of the microbiota of all rocks, occurring over a wide range of geographical and climatic zones Burford et al.
Free-living and symbiotic fungi are therefore associated with elements besides O that account for over Fungal activities in rock and mineral transformations can therefore lead to increased mobility of such elements, and other minor crustal components, as well as the formation of secondary mineral products. Fungi can play a role in the dissolution of common minerals including carbonates, phosphates and silicates and less common compounds including oxides and oxalates Gadd ; Gadd et al.
Lichens are a symbiosis of a fungus with either a cyanobacterium or a trebouxian green algae see earlier and actively digest rock surfaces. The fine structure and chemistry of the shells of several species of molluscs have been studied in detail, particularly in the pearl oyster Pinctada fucata Takeuchi and Endo and the abalone Haliotis asinina Marie et al.
The primary structure, origin and evolution of shell matrix proteins of molluscs have been reviewed by Marin et al. Oysters have two complex layers in their shells: the nacreous and the prismatic layers Sudo et al. Both layers are composed from microlaminate composites of calcium carbonate crystals aragonite in the nacreous and calcite in the prismatic layer. The major macromolecules are a complex mixture of structural or matrix proteins and glycoproteins which determine the framework of each shell layer. The matrix proteins are secreted by shell-forming tissue in the mantle of molluscs.
Calcium carbonate crystals grow within the matrix. Modern corals are a broadly defined group of anthozoan Cnidaria, which grow in shallow tropical and semi-tropical waters in the upper photic zone Veron All such corals are a symbiosis between the coral animal scleractinian or stony corals in the Class Anthozoa and a dinoflagellate alga known as zooxanthellae in the genus Symbiodinium , which live in the endoderm inner layer of the animal Veron Modern research has shown that the Symbiodinium species are remarkably diverse, comprising at least eight distinct genetic clades A—H Wham and Lajeunesse , and more are being discovered regularly.
Scleractinian corals probably evolved about MYa from rugose corals; and they lay down a skeleton of calcium carbonate aragonite spicules in a process, which is driven by light and dependent on photosynthesis of the zooxanthellae. Corals can broadly be divided into branching and massive forms. The skeletons of corals are finely sculptured and each species has a unique structure by which it can be identified. Massive corals, such as species of Porites , can grow into very large structures, often roughly globose and several meters in diameter.
Such massive corals have a thin outer veneer of living coral tissue that encloses an inner mass of largely dead aragonite material. Commonly photosynthetic endoliths occur in this aragonite material. These species of algae are major colonisers Ralph et al. In addition, other eukaryotic algae or cyanobacteria can be present in lesser abundance forming distinctive outer layers. Deeper within the coral, inner layers of anoxygenic photosynthetic bacteria are evident by their characteristic absorption spectra. The light absorbing pigments are bacteriochlorophylls rather than chlorophylls, and these bacteria presumably live in the kind of spectral radiation reaching these depths, which is enriched in infra-red radiation that the prokaryotic bacteria but not eukaryotic algae can absorb.
The endolithic microorganisms found inside the skeletons of massive corals fall into the class of organisms that broadly is described as lithoclastic, i. However, there are other calcifying organisms on coral reefs, most notably the green alga Halimeda spp and its relatives. These green algae lay down aragonite, that ultimately forms into grains of calcium carbonate, which constitute over half of the coral sand of lagoon floors Perry et al.
In addition, the calcite skeletons of calcifying red algae generate a large amount of the calcium carbonate of coral reefs Anthony et al. Much less is known about the lithoclastic processes that are responsible for breaking down the green and red algal products. Presumably, endoliths are partly involved as well as surface or interfacial bioeroders. In addition to those already mentioned, it is becoming increasingly apparent that many other microorganisms on coral reefs interact with calcium carbonate skeletons. Nearly a decade ago, Moore et al. This free-living photosynthetic alga lies on the evolutionary path to non-photosythetic apicomplexans such as the malaria pathogen and is therefore a bridge between photosynthetic phytoplankton, such as the dinoflagellate Symbiodinium , and non-photosynthetic, parasitic apicoplexans.
The interaction of environmental factors associated with global climate change such as temperature increase, ocean acidification, eutrophication, changes in salinity etc. Diaz-Pulido et al. It is well established that global warming of the oceans has led to bleaching and the death of corals.
This was first established by Hoegh-Guldberg and recently confirmed beyond doubt by recent events on the Great Barrier Reef, Australia Hughes et al. Under severe and prolonged stress, the coral animals starve, and the entire coral bleaches completely and finally dies. Bleaching or loss of colour is caused by loss of photosynthetic pigments. The trigger is connected with inhibition of photosynthesis in the zooxanthellae of the affected coral Jones et al. In addition, the increase of greenhouse gases in the atmosphere has led to ocean acidification whereby the upper layers of the oceans have decreased in pH.
This has led to a decline in the rate of calcification in corals and other calcifiers on coral reefs. Thus, the effects of global warming and ocean acidification on coral reefs are very serious. Fungal colonisation of rock and mineral substrates can result in physical and biochemical effects which are influenced by substrate chemistry and mineralogy. The presence of weatherable minerals such as feldspars and clays may increase susceptibility to attack Warscheid and Braams Typical transformation mechanisms involve physical and biochemical processes that are generally interlinked Gadd Physical mechanisms include penetration by the hyphae along points of weakness, or direct tunnelling or boring, especially in weakened or porous substrata Jongmans et al.
Tunnels may also result after fungal exploration of pre-existing cracks, fissures and pores in weatherable minerals and formation of a secondary mineral matrix of the same or different chemical composition as the substrate, e. This can result in the fissures and cracks becoming cemented with mycogenic minerals, and after death and degradation of fungal hyphae, tunnels are left within the minerals. There is some debate as to the relative significance of fungal boring or tunnelling as compared to penetration through pores and points of weakness.
However, it would seem unusual if fungi were not capable of direct boring, a feature found in many groups of microorganisms Cockell and Herrera Fungi possess the properties of filamentous apical growth, cell turgor pressure and the ability to dissolve minerals that make it possible for fungi develop such a pattern of growth.
Tunnelling by fungi has been observed clearly for some natural biogenic minerals such as ancient ivory Pinzari et al. Weakening of a mineral lattice can also occur through wetting and drying cycles and expansion or contraction of the biomass. Lichens can cause mechanical damage due to penetration by their anchoring structures, composed of fungal hyphae Chen et al.
Biochemical weathering of rock and mineral substrates occurs through excretion of hydrogen ions, CO 2 , organic acids, siderophores and other metabolites, for example, resulting in pitting, etching or dissolution Gadd Some organic metabolites affect dissolution by complexing with constituent metals, permitting removal of the mineral in a mobile form.
Biogenic organic acids are very effective in mineral dissolution and are one of the most damaging agents affecting mineral substrates Gadd Of the suite of organic acids produced by fungi, oxalate is of major significance through metal complexation and dissolution effects as well as causing physical damage by formation of secondary metal oxalates expanding in pores and fissures Gadd et al. Citric and gluconic acid are other significant fungal metabolites. This appears to result from mechanical destruction caused by cell turgor pressure and extracellular polysaccharide production rather than the acid dissolution caused by many other fungi Marvasi et al.
These fungi may also form casual mutualistic associations with algae in rock crevices Gorbushina Endoliths which bore into solid substrates possess specialised filaments which actively penetrate readily soluble substrates, most commonly composed of calcium carbonate, such as calcareous rocks, shells of molluscs and corals, skeletal fragments and sand grains. When microbial endoliths actively penetrate calcarious substrates, they leave characteristic boring traces on the surface.
It is thought that endoliths release substances, possibly organic acids as discussed previously , which can dissolve calcium carbonate. Microbial parasites must bore through the matrix proteins to penetrate mollusc shells presumably by excreting extracellular proteases. These proteases inhibitors were active against some serine and cysteine proteases. Endoliths are important components of the marine calcium carbonate cycle because they actively contribute to the biodegradation of shells of dead animals composed of calcium carbonate and calcareous geological substrates.
They have been implicated as a causative agent of shell diseases in live corals, molluscs and other invertebrate animals which have shells composed of calcium carbonate Golubic ; Kohlmeyer ; Che et al. Endolithic microorganisms have important roles as saprotrophs in bio-erosion of many calcium carbonate substrates, as parasites on the production of commercially important animal species, regulate biodiversity in marine ecosystems, and they respond to environmental factors which are involved significant components of global climate change.
Heterotrophic endoliths can destroy the shells of animal species living in marine ecosystems or bioerode dead shells buried in the sediment Kendrick et al. The dissolution of calcium carbonate in bioerosion causes the release of carbon dioxide into the marine environments, which increases acidification. Calcareous substrates contain large amount of carbon.
Therefore, heterotrophic endoliths are key players in the marine calcium carbonate cycle.
The total amount of global calcareous substrates in sediments in the ocean has not been accurately estimated. However, as carbon dioxide from bioerosion of calcium carbonate in the ocean eventually enters the atmosphere, large losses in calcareous substrates in carbon sinks would be expected to result in increased heat retention by the atmosphere, increasing global mean temperatures. Contributors include more than highly qualified scientists and 43 Nobel Prize winners. Botany Fungus-like microorganisms of the Oomycota. BR The biflagellate water molds, downy mildews, and other microorganisms classified as members of the Oomycota also termed Oomycetes have long puzzled taxonomists.
See also: Algae ; Eukaryotae ; Fungal genomics ; Fungi ; Mycology Regardless of the exact taxonomic scheme, conclusive phylogenetic and morphological analyses have indicated a number of features that distinguish these microorganisms from fungi. You may already have access to this content.
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Download the flyer Get Adobe Acrobat Reader. These fungi break down organic matter of all kinds, including wood and other types of plant material. Wood is composed primarily of cellulose, hemicellulose, and lignin. Lignin is a complex polymer that is highly resistant to degradation, and it encrusts the more readily degradable cellulose and hemicellulose. Fungi are among the few organisms that can effectively break down wood, and fall into two main types—brown and white rot fungi.
Brown rot fungi selectively degrade the cellulose and hemicellulose in wood, leaving behind the more recalcitrant lignin. The decayed wood is brown in color and tends to form cubical cracks due to the brittle nature of the remaining lignin Fig. Brown rot residues make up 'humus' in temperate forest soils and are important for mycorrhizal formation see the following paragraph for information on mycorrhizal fungi , moisture retention, and for sequestering carbon.
Brown rot residues are highly resistant to decomposition and can remain in the soil for up to years. White rot fungi are more common than brown rot fungi; these fungi degrade cellulose, hemicellulose, and lignin at approximately equal rates. The decayed wood is pale in color, light in weight, and has a stringy texture Fig. White rot fungi are the only organisms that can completely degrade lignin. An important group of fungi associated with plants is mycorrhizal fungi. Mycorrhiza means 'fungus root', and it refers to a mutually beneficial association a type of symbiosis between fungi and plant roots.
There are seven major types of mycorrhizal associations, the most common of which is the arbuscular mycorrhizae, involving members of phylum Glomeromycota associated with roots of most major groups of plants. Another common type of association is ectomycorrhizae formed between forest trees and members of phyla Basidiomycota and Ascomycota. In this association, the fungus forms hyphae around host root cortical cells—the "Hartig net"— and a sheath of hyphae around the host roots called a "mantle.
A valuable group of ectomycorrhizal fungi are truffles, members of phylum Ascomycota that form underground fruiting bodies. Figure 6. Lichens are examples of a symbiotic association involving a fungus and green algae or less frequently Cyanobacteria. The lichen thallus is composed mostly of fungal hyphae, usually with the alga or cyanobacterium confined to discrete areas of the thallus.
In lichens, reproductive structures of the fungus are often conspicuous, for example disc- or cup-like structures called apothecia Fig. The fungus obtains carbohydrates produced by photosynthesis from the algae or cyanobacteria, and in return provides its partner s with protection from desiccation and ultraviolet light.
Lichens grow in a wide range of habitats on nearly every continent. Think about an inhospitable place, and there's probably a lichen that grows there—on bare rocks, sidewalks, grave stones, the exoskeletons of some insects, and even on cars that remain for a long time in one place! Figure 7. Some fungi are hidden inside their plant hosts; these are endophytes , defined by their presence inside asymptomatic plants. All plants in natural ecosystems probably have some type of symbiotic association with endophytic fungi Rodriguez et al.
Endophytic fungi have been shown to confer stress tolerance to their host plant, for example, to disease, herbivory, drought, heat, salt and metals. The clavicipitaceous endophytes in the genus Neotyphodium phylum Ascomycota are among the best studied. These fungi produce alkaloid compounds that protect the grass host from insects that would otherwise feed on them; endophyte-infected turfgrass seed is sold commercially for seeding lawns and other types of grassy recreational areas. Unfortunately, livestock such as sheep, cattle, llamas and horses also are negatively affected by toxins produced by endophytes when they eat infected grass.
Afflicted animals develop symptoms including tremors and jerky or uncoordinated movements. Let's now consider the role of fungi as plant pathogens. Plant pathogenic fungi are parasites, but not all plant parasitic fungi are pathogens. What is the difference between a parasite and a pathogen? Plant parasitic fungi obtain nutrients from a living plant host, but the plant host doesn't necessarily exhibit any symptoms. In this sense, endophytic fungi discussed in the preceding paragraph are plant parasites because they live in intimate association with plants and depend on them for nutrition.
Plant pathogenic fungi are parasites and cause disease characterized by symptoms. Biotrophic fungal pathogens obtain nutrients from living host tissues, often via specialized cells called haustoria that form inside host cells Fig. Necrotrophic pathogens obtain nutrients from dead host tissue, which they kill through the production of toxins or enzymes. Most biotrophic fungi have fairly narrow host ranges—they are specialized on a limited number of plant hosts.
Necrotrophic fungi can be either generalists, growing on a wide range of host species, or specialized on a restricted range of hosts. Some plant pathogenic fungi change the way that their hosts grow, either by affecting the level of growth regulators produced by the plant, or by producing growth regulators themselves. Examples of changes in plant growth caused by plant pathogenic fungi include cankers , galls , witches' broom , leaf curl and stunting. Figure 8. We can further divide plant pathogenic fungi by the stage of the plant host that is attacked, for example, seeds, seedlings, or adult plants, and by what part of the plant is affected—roots, leaves, shoots, stems, woody tissues, fruits or flowers.
A group of fungi including species of Fusarium , Rhizoctonia and Sclerotium cause seed rot and infect plants at the seedling stage. These pathogens can attack a wide range of plants. Often, seedling pathogens cause damping-off symptoms because they occur in wet soils.
Many of the same fungi that kill seedlings can also infect the roots of mature plants and cause root and crown rot diseases. Infection often occurs through wounds, and results in lesions or death of part or all of the root system and crown. Some common root rots of trees are caused by members of phylum Basidiomycota in the genera Armillaria and Heterobasidion.
Armillaria spp. Species of Heterobasidion survive as saprotrophs in dead tree stumps and roots, but can also infect living hosts through root contact.
Marine Fungi and Fungal-like Organisms
These fungi cause decay in the roots and crown; infected trees become weakened and die, or may blow over in high winds. Wood rot fungi, most of which are also members of Basidiomycota, infect trees through wounds, branch stubs and roots, and decay the inner heartwood of living trees. Extensive decay weakens the tree, and reduces the quality of wood in trees harvested for timber see the discussion of "white rot" and "brown rot" fungi above. Vascular wilt pathogens kill their host by infecting through the roots or through wounds and growing into the xylem, where they produce small spores that get carried upward until they are trapped at the perforated ends of the xylem vessels.
The spores germinate and grow through the pores. The fungus is transported throughout the plant in this manner. The first symptom of vascular wilt is a loss of turgidity in the plant leaves, often on one side of the plant or a single branch. If the stems of infected plants are cut open, vascular discoloration is evident.
Among the important vascular wilt fungi are Fusarium oxysporum , Verticillium albo-atrum and V. One of the most famous vascular wilts is Panama disease of bananas, caused by Fusarium oxysporum forma specialis f. This fungus nearly wiped out banana production in Latin America in the early twentieth century. Most bananas that were being grown for export were a single cultivar, 'Gros Michel', which turned out to be highly susceptible to Panama disease.
There is no effective method for controlling Panama disease and it rapidly spread throughout banana plantations around the world. The banana industry was saved by the discovery of the cultivar 'Cavendish' that is resistant to the strain of Panama disease that killed 'Gros Michel'. Leaf spot pathogens infect through natural plant openings such as stomates or by penetrating directly through the host cuticle and epidermal cell wall. In order to penetrate directly, fungi produce hydrolytic enzymes—cutinases, cellulases, pectinases and proteases—for breaking down the host tissue.
Alternatively, some fungi form specialized structures called appressoria sing. Turgor pressure builds up in the appressorium, and in combination with an infection peg , mechanical force is exerted to breach the host cell walls. Once inside the plant leaf, the fungus must obtain nutrients from the cells, and this is often accomplished by killing host cells necrotrophs. Death of host cells is evident as an area of dead cells called a lesion Fig.
Figure 9. Many leaf-spotting fungi produce toxins that kill host cells and this often produces a lesion surrounded by a yellow halo Fig.
If enough of the leaf surface is killed, or if the infected leaves drop prematurely, the plant's ability to produce photosynthates is severely impaired. Figure Returning to bananas, another devastating disease of this host is black leaf streak, or black Sigatoka , caused by Mycosphaerella fijiensis. Unlike the root-infecting pathogen that causes Panama disease, the black Sigatoka pathogen can be controlled by applications of a protective fungicide to banana leaves.
American chestnut trees were once a prominent hardwood tree in the eastern U. Cankers develop when the pathogen kills the phloem and vascular cambium in a woody host. If the canker encircles the trunk or branch of a tree, that plant part will die. The canker-causing fungus can often be identified based on the fruiting bodies that form in the canker. In contrast to cankers, galls result from abnormal growth of a plant, usually due to an increase in cell size and cell division.
Although galls are often associated with insect pests, some fungal pathogens induce galls; two common examples are the black knot pathogen , Apiosporina morbosa on Prunus spp. Gymnosporangium is a type of rust fungus. Rust fungi are biotrophic pathogens—they infect, grow, and sporulate in living plant tissue. Even though biotrophs require living host tissue for their growth and reproduction, they can be devastating pathogens by reducing the photosynthetic surface and increasing water loss in the host plant. Rust fungi attack a wide range of plants, and often require two, unrelated hosts in order to complete their life cycles.
Rust fungi are so-named because of the abundant orange spores that are formed on plants that are infected by these fungi; infected plants often look as though they are rusting. One historically important rust fungus is black stem rust of wheat, a disease that was well known to the ancient Romans. Black stem rust , caused by Puccinia graminis f. Since the barberry host is required for the pathogen to complete its life cycle, early control measures in the United States and Canada were aimed at eliminating this host, not the economically important host, wheat. We now know that this method of eradication was of limited success because rust spores can be carried long distances—for example, from northern Mexico to the U.
Now let's take a closer look at fungi and the types of structures that they form. A key characteristic of fungi that has contributed to their successful exploitation of diverse ecological niches is the formation of a filamentous thallus called the mycelium. A mycelium is composed of branching, microscopic tubular cells called hyphae Fig.
The fungal cell wall in the Kingdom Fungi is composed of chitin and glucans in Ascomycota, Basidiomycota and Chytridiomycota as well as chitosan and other components in Zygomycota Kirk et al. Hyphae can have cross walls called septa , or lack cross walls nonseptate; aseptate; coenocytic. The type of hyphae— septate or aseptate —is characteristic of specific groups of fungi. In fungi that form septate hyphae, there are perforations at the septa, called septal pores, which allow the movement of cytoplasm and organelles from one compartment to the next.
The type and complexity of the septal pore is characteristic of specific groups of fungi. Hyphae grow from a germinating spore or other type of propagule, and these are described in more detail in the section "Fungal Reproduction. As a result of apical growth, hyphae are relatively uniform in diameter, and mycelium that grows in an unimpeded manner forms a circular colony on solid substrates that support fungal growth; agar, a gelatinous material derived from seaweed, amended with different types of nutrients is commonly used to grow fungi in culture Fig.
Some fungi grow exclusively or mostly as yeasts , defined as single-celled fungi that reproduce by budding or fission. In contrast to apical growth that is characteristic of hyphae, yeasts exhibit wall growth over the entire cell surface, often resulting in a nearly spherical cell Fig. There are also fungi that can switch between mycelial growth and yeast-like growth, dependent upon the environmental conditions.
The ability to grow in different forms is called dimorphism, and is exhibited by some members of phyla Ascomycota, Basidiomycota and Zygomycota. Most of the organelles present in fungal cells are similar to those of other eukaryotes. Fungi have been found to possess between 6 and 21 chromosomes coding for 6, to nearly 18, genes. Genome sizes range from 8.
Many fungi Ascomycota have a life cycle that is predominantly haploid, while others Basidiomycota have a long dikaryotic phase. Fungi frequently reproduce by the formation of spores. A spore is a survival or dispersal unit, consisting of one or a few cells, that is capable of germinating to produce a new hypha. Unlike plant seeds, fungal spores lack an embryo, but contain food reserves needed for germination. Many fungi produce more than one type of spore as part of their life cycles.
Fungal spores may be formed via an asexual process involving only mitosis mitospores , or via a sexual process involving meiosis meiospores. The manner in which meiospores are formed reflects the evolutionary history and thus the classification for the major groups phyla of fungi. Many fungi produce spores inside or upon a fruiting body.
Many people are familiar with the mushroom, a type of fruiting body produced by some Basidiomycota. You may recognize other fungal fruiting bodies such as puffballs, or shelf fungi. These are examples of large, conspicuous fruiting bodies, but there is an even greater diversity of microscopic fruiting bodies produced by various fungi. What all fruiting bodies have in common is that they produce spores and provide a mechanism for dispersing those spores.
Fruiting bodies will be discussed in more detail within the fungal groups. Many fungi are able to reproduce by both sexual and asexual processes. Sexual and asexual reproduction may require different sets of conditions e. In some fungi, two sexually compatible strains must conjugate mate in order for sexual reproduction to occur. The terms ' anamorph ' and ' teleomorph ' are used to convey the asexual and sexual reproduction morphological types, respectively, in a particular fungus.
The concept of anamorph and teleomorph is a confusing one for many students, as we are not accustomed to thinking about organisms with such reproductive flexibility. For a more thorough discussion of anamorph and teleomorph, refer to Alexopoulos et al. Examples of meiospores—spores that are the products of meiosis—include ascospores see Ascomycota and basidiospores see Basidiomycota. Ascospores are formed inside a sac-like structure called an ascus Fig. An ascus starts out as a sac of cytoplasm and nuclei, and by a process called "free cell formation" Kirk et al.
Ascospores vary in size, shape, color, septation, and ornamentation among taxa. Basidiospores are formed on a basidium Fig. Basidiospores vary in size, color and ornamentation depending upon the taxonomic group. More information on dispersal of ascospores and basidiospores can be found below. Examples of mitospores are conidia sing. Another type of asexual propagule produced by fungi in several different phyla is the chlamydospore. Conidia are formed from a modified hypha or a differentiated conidiogenous cell of the fungus.
Conidiogenous cells can be formed singly on hyphae, on the surface of aggregated hyphal structures, or within different types of fruiting bodies. Fruiting bodies inside which conidia are formed are pycnidia and acervuli. Sporodochia and synnemata are examples of fruiting bodies on which conidia are formed.
Conidia are produced primarily by Ascomycota, although some Basidiomycota are capable of producing them as well. Sporangiospores are asexual propagules formed inside a globose or cylindrical sporangium by a process involving cleavage of the cytoplasm. Sporangiospores are thin-walled, one-celled, hyaline or pale-colored, and are usually globose or ellipsoid in shape. One to 50, sporangiospores may be formed in a single sporangium. When mature, sporangiospores are released by breakdown of the sporangial wall, or the entire sporangium may be dispersed as a unit. Sporangiospores are produced by fungi in phyla Chytridiomycota and Zygomycota, as well fungal-like Oomycetes see section "Fungal-like Organisms Studied by Plant Pathologists and Mycologists".
A zoospore is a microscopic, motile propagule, approx. Zoospores are produced by one group of true Fungi Chytridiomycota , and by fungal-like organisms in Kingdom Straminipila and some slime molds see section "Fungal-like Organisms Studied by Plant Pathologists and Mycologists".
Two types of flagella are known—the whiplash flagellum, which is directed backward, and the tinsel flagellum, which is directed forward. The tinsel flagellum is only present in members of Kingdom Straminipila and does not occur in true fungi. The length of time zoospores are able to swim is determined by their endogenous energy reserves—zoospores cannot obtain food from external sources—and environmental conditions.
Zoospores may exhibit chemotaxis—movement in response to a chemical gradient, e. At the end of its motile phase, the zoospore undergoes a process called encystment in which it either sheds or retracts the flagella and produces a cell wall. The encysted zoospore, called a cyst, may germinate directly by the formation of a germ tube , or indirectly by the emergence of another zoospore. Zoospores are formed inside a sac-like structure called a zoosporangium by a process involving mitosis and cytoplasmic cleavage—similar to the formation of sporangiospores in sporangia.
Depending upon the taxonomic group, zoospores emerge from the zoosporangium through breakdown of the zoosporangial wall, through a preformed opening in the wall covered with a cap called an operculum that flips back, or by a gelatinous plug that dissolves. Chamydospores are survival propagules formed from an existing hyphal cell or a conidium that develops a thickened wall and cytoplasm packed with lipid reserves.
The thickened cell walls may be pigmented or hyaline, and chlamydospores develop singly or in clusters, depending upon the fungus. Chlamydospores are passively dispersed, in most instances when the mycelium breaks down. Chlamydospores are formed by many different groups of fungi and are often found in aging cultures. Sclerotia sing. Sclerotia contain food reserves, and are a type of survival propagule produced by a number of fungi in phyla Ascomycota and Basidiomycota; in some fungi, such as Rhizoctonia solani , they are the only type of propagule produced, whereas in fungi such as Claviceps purpurea and Sclerotinia sclerotiorum , they are overwintering structures that can germinate directly, or give rise to structures in which the meiospores are formed.
The characteristics and diversity of the major phyla of true Fungi will be briefly described. Selected representatives of the different phyla are introduced and, in many instances, illustrated. A generalized life cycle also is presented for each phylum that illustrates when plasmogamy cell fusion , karyogamy nuclear fusion and meiosis occur relative to each other, and the types of structures involved in these events.
For more detailed information on members of Kingdom Fungi, recommended reading is provided at the end of this article. Phylum Ascomycota is the largest group of fungi, with approximately 33, described species in three subphyla—Taphrinomycotina, Saccharomycotina, and Pezizomycotina. Members of this phylum reproduce sexually or meiotically Fig. Many species of Ascomycota also or exclusively produce spores through an asexual or mitotic process; these spores, called conidia , exhibit a wide range of size, shape, color and septation among the different fungi in which they are formed.
Conidia and ascospores are usually produced at different times of year, if ascospores are formed in the lifecycle. The existence of many Ascomycota having sexual and asexual states that are separated in time and space has long confused those new to mycology and plant pathology. The asexual states of Ascomycota are especially important to the plant pathologist because they are more commonly encountered than the sexual state, and must be identified for control, quarantine, or other purposes.
Fungi that reproduce only via asexual means have been given various designations including deuteromycetes, fungi imperfecti, mitosporic fungi, conidial fungi, and anamorphic fungi. Subphylum Taphrinomycotina includes fungi that, with one known exception, do not form fruiting bodies—as examples, the fission yeast Schizosaccharomyces Fig.
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Subphylum Saccharomycotina contains approximately species of yeasts, most of which live as saprotrophs in association with plants and animals, but also including a small number of plant and animal pathogens Suh et al. Asci are formed naked Fig. Yeasts traditionally have been important in the production of beer, wine, single cell protein and baker's yeast, but their role in industry has expanded to the production of citric acid, fuel alcohol, and riboflavin Kurtzman and Sugiyama Saccharomyces cerevisiae Fig.
In , S. Subphylum Pezizomycotina is the largest group in the phylum, with more than 32, identified species that occupy a wide range of ecological niches, occurring as saprotrophs, parasites and mutualists with plants, animals and other fungi. Three different types of asci occur in this subphylum, prototunicate, unitunicate and bitunicate.
Prototunicate asci release ascospores by breakdown of the ascus wall, whereas in the unitunicate and bitunicate asci, the ascospores are forcibly discharged. Bitunicate asci have an inner wall that balloons out from the outer wall prior to ascospore discharge, and in unitunicate asci the wall layers do not separate from each other.
A wide range of fruiting bodies are formed by members of subphylum Pezizomycotina, including cleistothecia , chasmothecia , apothecia , perithecia and pseudothecia. Stromata , hardened masses of hyphae on or in which perithecia or pseudothecia are formed, occur in some members of this subphylum. Cleistothecia sing. Common fungi that produce cleistothecia include the teleomorphic sexual states of Aspergillus and Penicillium Fig. Species of Aspergillus are important in the production of fermented foods and beverages, including soy sauce, miso and rice wine sake.
Some species of Aspergillus infect animals, causing a disease known as aspergillosis, and others produce mycotoxins. Aflatoxin is a potent carcinogen produced by A. The U. Food and Drug Administration established a strict limit of 20 parts per billion on aflatoxin levels in food, and the U.
Penicillium species are also used in food production. For example, the blue veins in Roquefort and Gorgonzola cheeses are due to the growth and sporulation of particular species of Penicillium Fig. The antibiotic penicillin, the "wonder drug" of the 20 th century, is produced by strains of P. Other species of Penicillium, such as P. The term is now used to refer to the fruiting bodies of the powdery mildew fungi Fig. Apothecia sing. Apothecia-forming fungi are also called "cup fungi" or discomycetes.
Some important groups of plant pathogens that form apothecia include species of Monilinia brown rot of peach; Figs. Perithecia sing. Most fungi producing perithecia also have unitunicate asci and are classified in Sordariomycetes , one of the largest classes of Ascomycota with more than 3, described species Zhang et al. These fungi have also been called pyrenomycetes. Members of this group are common in nearly all ecosystems, where they occur as saprotrophs, endophytes of plants, or pathogens of plants, animals and other fungi.
A large number of economically important plant pathogens belong to Sordariomycetes, including those that cause anthracnose diseases Glomerella cingulata , blasts Magnaporthe oryzae , rice blast pathogen , blights Cryphonectria parasitica , chestnut blight , ergot Claviceps purpurea , and Fusarium head blight scab of small grains Gibberella zeae.
Pseudothecia sing. Asci form in locules openings inside vegetative fungal tissue called ascostroma ; this group has been called loculoascomycetes, but is now placed in class Dothideomycetes. Other characteristics of Dothideomycetes include the formation of bitunicate asci, and many members of this group produce darkly pigmented, multiseptate asospores or conidia. Similar to the Sordariomycetes, members of Dothideomycetes occur in a wide range of habitats as saprotrophs and associate with plants as pathogens, endophytes and growing on the surface of plants as epiphytes Schoch et al.
Examples of well-known plant pathogens belonging to this group include Venturia inaequalis apple scab; Fig.
Marine Fungi: the missing tile in the Ocean Biodiversity mosaic
Most of the lichen-forming members of Ascomycota belong in class Lecanoromycetes. This is the largest class of fungi, with over 13, described species Miadlikowska et al. Most of the members of this class produce apothecial fruiting bodies Figs. The lichen thallus produces a wide range of secondary metabolites that are of biological and ecological importance Miadlikowska et al. The lichen thallus is able to grow under a range of adverse conditions and some can survive for hundreds of years.
Lichens are found in a wide range of habitats from the Arctic to Antarctic, including some species that can grow in aquatic and marine environments Webster and Weber Phylum Basidiomycota represents the second largest phylum of fungi, with nearly 30, described species.
Related Marine Fungi and Fungal-like Organisms
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