Fungi

Fungi **Michael "Fun Guy" Broder**

**Fungi** (Kingdom Fungi) are eukaryotic detritivores that recycle vital chemical nutrients back through an ecosystem by means of decomposing organic material such as dead organisms, rotting wood, feces etc. This process helps return key nutrients to the soil to be assimilated by various plant species. Fungi have many uses in our modern society, for, in addition to the aforementioned ecological roles, fungi are also used in diverse ways by humans. Humans eat fungi, add them to dough to make bread rise, and culture them for purposes of producing antibiotics and other pharmaceuticals. Fungi are also important in plants because they grow on the roots of plants, increasing surface area throught stuctures called mycorrhyzae, thereby increasing the aborption of nutrients (EG). Fungi differ from most other eukaryotes in their nutritional mode, structure, reproduction, and growth patterns. Genetic studies have shown that animals, not plants, are the closest relatives of fungi.


 * **Table of Contents** ||
 * Diagnostic Characteristics of Fungi ||
 * Fungi Anatomy ||
 * Fungi Reproduction ||
 * The Fungus Among Us ||
 * Fungi Habitats ||
 * Transport of Materials in Fungi ||
 * Environmental Adaptations of Fungi ||

Diagnostic Characteristics of Fungi

Fungi are the principle decomposers that keep the biosphere supplied with nutrients essential for plant growth. Without such organisms, essential chemical elements would be unavailable in organic material and would not be cycled in an ecosystem. As important decomposers of organic matter, fungi penetrate the tissues and cells of dead plant and animal material by secreting exoenzymes that break up polymers. All fungi are detritovores that feed on organic material by decomposing it. Although some fungi reproduce only asexually while others do so solely sexually, fungi are all common in that they contain spore-producing structures and they release spores during reproduction. In addition, fungi are distinguished by a cell wall of glucans and chitin, the combination of which is found in no other organism (SW 19). The study of fungi is called mycology (SW 19).  Fungi Anatomy

Except for unicellular yeast, the bodies of fungi are constructed of thin, tubular filaments called **hyphae**. An individual hypha is composed of a tubular wall surrounding the plasma membrane, which bounds the cytoplasm, the entire contents of the cell. These hyphae form a densely branched network within fungi called the **mycelium**, also known as the feeding system of a fungus. The mycelia are hard to notice, for they are below ground. The tubular structure of mycelium provides an extensive surface area that allows the fungus to practice **absorption**, the uptake of small nutrient molecules by an organism's own body. Most fungi are multicellular organisms with each hypha divided into cells by cross-walls known as **septa**. Septa generally have pores large enough to allow such cell organelles as ribosomes, mitochondria, and even nuclei to move from cell to cell within the hypha. Cell walls of fungi are composed of a different material than those of plants. Instead of cellulose as the main component, **chitin**, a strong and flexible nitrogenous polysaccharide, is the primary constituent of the cell walls of fungi. Some fungi, such as //cyenocytic// fungi, differ in the structure of their hyphae in that they do not contain septa; rather, these fungi consist of a continuous cytoplasmic mass with thousands of nuclei. ​  This picture illustrates the basic anatomy of fungi. (AW)

Fungi Reproduction

Fungi practice reproduction by releasing **spores**, haploid cells that divide mitotically once established, generating a multicellular organism without fusing with another cell, that are produced either sexually or asexually. A single fungus can release trillions of spores which are then carried by wind or water and eventually germinate to produce mycelia only if the land in a moist environment with sufficient nutrients. The miniscule spores that are released by all fungi permeate the air and colonize and infiltrate a substrate initiating the process of decomposition. Since spores can be carried long distances, many fungi are able to disperse over wide geographic ranges. In many fungi with sexual reproductive cycles, the union of partners occurs in two distinct stages called **plasmogamy** and **karyogamy**. First comes **plasmogamy**, the fusion of the cytoplasm of cells from two individual fungus when their mycelia come together. In between the stages of plasmogamy and karyogamy, the hybrid mycelium exists as a heterokaryon (meaning different nuclei), a mycelium formed by the fusion of two hyphae that have genetically different nuclei. In a heterokaryon, the haploid nuclei, those containing only one set of chromosomes, remain separate, and the different nuclei stay in separate parts of the same mycelium (it is a mosaic in terms of genotype and phenotype). A mycelium that is **dikaryotic** (meaning two nuclei) has two haploid nuclei per cell, one from each parent. The following stage, **karyogamy**, is the fusion of the haploid nuclei contributed by the two parent fungi. In most fungi, the structures formed by karyogamy are the only diploid, containing two sets of chromosomes, stages in the life cycle. Meiosis restores the haploid condition before reproductive structures of the mycelium produce and release spores.It is estimated that a third of all fungi reproduce by different modes of propagation; for example, reproduction may occur in two well-differentiated stages within the life cycle of a species, the [|teleomorph]and the [|anamorph](11DO). Also, some fungi reproduce using parasexual processes, which is initiated by anastomosis between a hyphae and plasmogamy of the cells (13 SC).

This diagram illustrates the generalized life cycle of fungi: (VK 7)



Fungi illustrating it's asexual reproduction organs. (EK)

The Fungus Among Us

There are more than 100,000 species of fungi currently known; however, mycologists, biologists who study fungi, estimate that there are approximately 1.5 million species worldwide. Fungi are classified in four phyla: **Chytridiomycota**, **Zygomycota** , **Ascomycota** , and
 * Basidiomycota**.


 * [[image:http://www.davidlnelson.md/Cazadero/CazImages/Chytridiomycota.jpg width="114" height="92" caption="Chytrids (HL 21)"]] ||
 * Chytrids (HL 21) ||
 * Chytrids** are mainly aquatic. While all other forms phyla of fungi are characterized by an absence of flagellated cells, ones with a cellular appendage used for locamotion, chytrids have uniflagellated spores called zoospores. These fungi possess an absorptive mode of nutrition and contain cell walls made of chitin. Most chytrids form coenocytic hyphae, ones without septa, although some are unicellular. Supported by molecular evidence is the hypothesis that chytrids are the most primitive fungi, meaning that they belong to a lineage that diverged earliest in the phylogeny of fungi.While most Chytrid Fungi are harmless, some species are capable of infecting and killing invertebrates and vascular plants (6 AN). //Batrachochytrium dendrobatidis// is a Chytrid species described to infect amphibians and oftentimes cause their death through the disease Chytridiomycosis. (6 AN)


 * [[image:http://media-2.web.britannica.com/eb-media/23/59223-004-8654DD58.jpg width="113" height="120" caption="Rhizopus stolonifer (21 HL)"]] ||
 * Rhizopus stolonifer (21 HL) ||
 * Zygote fungi** are mostly terrestrial and live either in soil or on decaying plant or animal material. A very important group of zygomycota forms **mycorrhizae**, mutualistic relationships with the roots of plants. The hyphae of zygote fungi are for the most part coenocytic, with septa found only where reproductive cells are formed. A common zygomycete (suffix -mycete means fungus) is black bread mold, Rhizopus stolonifer, who's horizontal hyphae spread out over food, penetrate it, and absorb its nutrients. **Zygosporangia** are multinucleate structures, first heteokaryotic with many haploid nuclei from the two parents, then with many diploid nuclei after karyogamy. Zygosporangia are resistant to freezing and drying and are metabolically inactive; however, when conditions become more favorable they release haploid spores that colonize a new area.


 * [[image:http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lectures/fungi/morel_small.jpg width="100" height="148" caption="Morel (21 HL)"]] ||
 * Morel (21 HL) ||
 * Sac fungi**, ascomycetes, exist in a variety of terrestrial and aquatic habitats. Many are important saprobes, organisms that act as decomposers by absorbing nutrients from dead organic matter, especially of plants. About half of sac fungi species live in mutualistic relationships with algae to form lichens. The one feature that defines Ascomycota is the production of sexual spores in saclike **asci** . Asci are cases, containing spores, that may have an opening at one end. (9 AL) These spores can be dicharged by forcefully squeezing the asci. (9 AL) Most sac fungi species bear the spore-forming asci in fruiting bodies, **ascocarps** , which are visible to the naked eye. This phylum of fungi reproduces using asexual spores that that are produced at the tips of specialized hyphae called conidiophores and dispersed via the wind. Unlike Zygomycota, the spores formed in sac fungi are not formed in sporangia; rather, they are "naked" spores known as **conidia** . Ascomycota are characterized by a longer heterokaryotic stage, associated with the formation of ascocarps, than that of Zygomycota.


 * [[image:http://www.catoctinmountain.com/galleries/fungi/orangeshelffungus030916web.jpg width="169" height="131" caption="Shelf fungi (21 HL)"]] ||
 * Shelf fungi (21 HL) ||
 * Club fungi**, Basidiomycota, include mushrooms, shelf fungi, puffballs, and rusts. This phylum of fungi is important in the decomposition of wood and other plant material. Basidiomycota also includes mycorrhiza-forming mutualists and parasites that obtain nutrients from plants. Rusts and smuts are very destructive plant parasites, while shelf fungi are the best at decomposing the abundant component of wood known as the polymer lignin. The life cycle of club fungi include a long-lived dikaryotic mycelium stage. Sometimes, in response to external stimuli, this mycelium reproduces sexually by producing fruiting bodies known as **basidiocarps** , which are analogous to the ascocarps of sac fungi. The numerous basidia of a basidiocarp produce sexual spores. Asexual reproduction is much more common in ascomycetes than in basidiomycetes. By concentrating growth in the hyphae of a mushroom, a basidiomycete mycelium can erect the basidiocarp in just a few hours. Such a mushroom may release a billion spores, which drop from the basidia-containing cap and are blown away. Some basidiomycota form fairy rings, slowly expanding circles of the fruiting bodies of the fungus, in fields. As the old hyphae die, they release nitrogen into the soil promoting lush growth. Most fairy ring-forming species cause no harm to the grass, however several species have dense hyphae that prevent water from reaching the grass’ roots, creating rings of dead grass followed by the rich growth (DPOD 11).

Fungi Habitats

Because the diverse forms of fungi perform different tasks and occupy a variety of niches, they are bound to have many different habitats. Sac fungi alone inhabit marine, freshwater, and terrestrial habitats. As mentioned in the above section, chytrids are mainly aquatic while zygote and club fungi are primarily terrestrial organisms. Yeasts are unicellular fungi that inhabit liquid or moist habitats, including the sap of plants and the tissues of animals. Lichens, symbiotic association of millions of alga held in a mesh of fungal hyphae, are abundant on newly cleared rock and soil surfaces, such as lava flows and burned forests. Lichens can survive arid climate, though they grow slowly, and severe cold. Fungi are commonly found in woodlands due to all the rotting wood and leaf litter. Woodlands provide a large range of dead organic matter for fungi to feed on (8 JSun). In a woodland environment, fungi will often create cavities that provide shelter to various forest-dwelling animals (CS 15).

Transport of Materials in Fungi

Fungi are heterotrophs that acquire their nutrients by the process of **absorption**, the uptake of small organic molecules from the surrounding substrate by an organism's own body. A fungus's digestion is performed externally when the organism secretes powerful hydrolytic enzymes, **exoenzymes**, into its food. The enzymes decompose complex molecules to simpler ones that can be absorbed and used by fungi for growth and reproduction. By externally breaking down organic matter for fungi to absorb, they are inadvertently decomposing the matter. A fungus's mycelium, or network of subterranean hyphae, serves as the feeding network of a fungus by providing an extensive surface area that suits the absorptive nutrition of fungi. A fungal mycelium grows at a rapid rate, sometimes adding around 1 kilometer of hyphae each day as it expands throughout a food source. Since a fungus concentrates the majority of its energy and resources on such expansion of the hyphae, such fast growth is only possible as a result of proteins and other materials synthesized in the mycelium being channeled by cytoplasmic streaming to the tips of the elongating hyphae.

Environmental Adaptations of Fungi

Ever since organisms began to occupy land, the terrestrial communities that resulted depended on fungi as detritovores and symbionts. The diversity of fungi that now pervades every corner of Earth may have evolved through adaptive radiation when life began to colonize land. Molecular evidence supports the hypothesis that the four phylums of fungi are **monophyletic**, pertaining to a taxon derived from a single ancestral species that gave rise to no species in any other taxa. The fact that chytrids, representing the oldest ancestry of fungi, have tail-like flagella indicates that the earliest ancestors of the modern fungi phyla were aquatic flagellated organisms. It is very likely that fungal flagella were lost as Zygomycota became adapted for life on land. Many of the differences among the three remaining phyla of fungi that adapted to life on land represent different solutions to the problem of terrestrial reproduction and dispersal. When placed in low nitrogen environments, fungi develop structures resembling nooses and/or sticky knobs that are used to trap and degrade nematodes and other small animals. Some fungi also form symbiotic relationships with arthropods, prokaryotes, and plants (including algae). (AS 21) Some fungi, in Antarctica, highly adapted for extreme weather conditions. These fungi can endure extreme weather, osmotic conditions, and radiation. Because of this, some even hypothesize that the fungi are the best suited eukaryotes for outer space (17T2). Despite their large habitat areas, many fungi appear to be limited in their range by geographic factors, such as oceans and other large bodies of water, and relationships with other organisms--a species of fungus may be restricted by the range of its host species or one with which it shares a symbiotic relationship. (AR 18)

Review Questions

1) Fungi have the ability to live in marine, terrestrial, and freshwater habitats. Which phyla of fungi has the ability to live in all three habitats? (CH) 2) What are the two stages of the sexual reproductive cycles in fungi that involve the union of partners? (J. Stein) 3)What characteristics differentiate the fungal and plant kingdoms? (7RM) 4) How do the three different phyla of fungi represent different solutions to terrestrial reproduction (JE)? 5) Describe how a fungus obtains food. How does food affect the growth of the fungus? (KA) __

Sources: 1. Campbell, Neil A, and Jane B. Reece.__ __. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002. 2. Allan, Pessier. "Chytrid Fungus." Amphibian Ark__. 24 Oct. 2009 < [|http://www.amphibianark.org/] 3. "Natural Perspective: Sac Fungi (Phylum Ascomycota)." //Natural Perspective//. Web. 25 Oct. 2009. < [|http://www.perspective.com/nature/fungi/ascomycota.html]>. 4. [|http://images.google.com/imgresimgurl=http]: //kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb2pg20_files/image004.jpg&imgrefurl= [|http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb2pg20.htm&usg=]//(7 VK) 5. Deacon, Jim. “The Microbial World: Basidiomycota: Activities and Lifestyle.” 26 Oct. 2009 < [|http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/basidio.htm]>. 6. Blackwell, Meredith, Rytas Vilgalys, Timothy Y. James, and John W. Taylor. "Fungi." Tree of Life Web Project //. 10 Apr. 2009. Web. 1 Nov. 2009. < [|http://tolweb.org/Fungi/2377]>. 7. Salinger, Richard. "Evolution and adaptation of fungi at boundaries of life." //Science Direct //[Minnesota] 1999: 56-58. Print. 8. "Fungus."// Wikipedia.org//. Wikipedia, the free encyclopedia. Web. 4 Nov. 2009. < [|http://en.wikipedia.org/wiki/Fungus]>. 9. "Fungi."// Hand specialist Hand Surgery David Nelson San Francisco Marin//. Web. 04 Nov. 2009. . 10. http://tolweb.org/fungi

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