Fungi

Kingdom Fungi is composed of both unicellular eukaryotes (such as baker's yeast) and multicellular eukaryotes (such as mold and mushrooms). Fungi resemble both plants and animals, although they are more closely related to animals. Both fungi and plants have cell walls; however, fungal cell walls contain chitin whereas plant cell walls contain cellulose. Like animals, fungi are heterotrophs, meaning they cannot synthesize their food and must obtain nutrition from external sources.

Whereas animals eat organic materials to obtain nutrition, fungi digest externally by secreting digestive enzymes into the environment. These enzymes break down dead matter, or living plants or animals, and the fungi can then absorb the resulting dissolved molecules. Most fungi grow in terrestrial environments and thrive in damp areas. However, fungi can live in a wide variety of environments, including extremely hot deserts and aquatic habitats.

In ecosystems, fungi function as decomposers, organisms that break down organic matter. By decomposing organic matter and recycling nutrients, fungi play an important role in soil nutrient cycles. Saprotrophle fungi utilize non-living organic materials as sources of nutrients and are responsible for the biodegradation of organic materials, such as leaf litter, wood, and other plant debris. Such fungi play a vital role in recycling non-living organic materials into essential elements, particularly carbon and nitrogen. Unfortunately, the excellent digestive abilities of saprotrophic fungi means that these fungi can contaminate crops and food sources and destroy consumer goods manufactured from organic materials.

Importance of Fungi in Human Life

Although fungi can cause disease and agricultural losses, they are also an important part of our diet and daily life.

Mushrooms, particularly the Agaricus species, which includes button mushrooms, portobellos, as well as truffles, are Mushrooms, particularly the Agaricus species, which includes button mushrooms, portobellos, as well as truffles, are cultivated and consumed as a direct food source. Fungi are used as a leavening agent for bread (baker's yeast) and in the fermentation of wine and beer (brewer's yeast) as well as in soy sauce (Aspergillus). Fungi are also used as biological pesticides to control plant diseases and pests. Additionally, fungi are producers of useful biological agents, including antibiotics and enzymes used in detergents.

Basic Structure of Fungal Organisms

Most fungi are multicellular eukaryotes. Fungi have two morphological stages: reproductive and vegetative. The vegetative stage occurs first and involves rapid growth, whereas the reproductive stage follows and involves the development of a fruiting body, a multicellular structure that bears haploid cells. When in the reproductive stage, many fungi appear as mushrooms. The mushroom contains four major structures (Figure 17.1).

1. The cap, also known as the pileus, is the expanded, top part of the mushroom. It is thick and fleshy, rounded, and has a generally smooth surface.

2. The gills, also known as lamellae, are dark ridges located on he underside of the pileus that radiate out from the center.

The gills are very thin, delicate, and nearly black on mature mushrooms.

3. The stem, also known as the stipe, is attached to the bottom center of the pileus. It is white, fleshy, and usually smooth.

The stem lifts the pileus and the gills above the ground to assist with spore dispersal.

4. The ring, also known as the annulus, is joined to the upper end of the stem, just beneath the pileus, in mature mushrooms. The annulus, which is delicate and easily rubbed off, is a remnant of a covering enclosing the mushroom during early development. Thus, it has no functional significance in the mature mushroom.

When in the vegetative stage, fungi are arranged in a network of long, branching filaments called hyphae (singular: hypha).

In most fungi, the hyphae are divided into cells by septa (singular: septum). The septa are specialized internal cross-walls that have pores to allow cytoplasm, organelles, and sometimes nuclei to pass through them. The pores allow the fungal cells to communicate with each other. In fungi that lack septa, the entire fungus is essentially one multinuclear cell. Many fungal species have specialized hyphal structures designed for nutrient uptake. All hyphae are specialized for growth at their tips (Figure 17.2), a type of growth called apical growth.

Apical growth produces a mycelium (plural: mycelia), an interconnected network of hyphae (Figure 173). Mycelia can be underground or it can be visible to the naked eye on various surfaces, such as on damp walls and on spoiled food, where they are commonly called mold. Molds can spread rapidly because their thin hyphae grow at their tips to penetrate new food sources, such as rotting fruit or dead leaves.

Fungal Spores

Hyphae initially develop from spores, haploid cells that undergo mitosis to form a haploid multicellular organism. Fungi produce spores as part of both sexual and asexual reproduction. In asexual reproduction, the spores are genetically identical to the parent. In sexual reproduction, two haploid cells fuse, then the haploid nuclei fuse to form a diploid nucleus, and finally meiosis occurs to generate haploid spores.

In either case, the spores disperse and develop into mature fungi. Fungi that undergo sexual reproduction produce fruiting bodies for dissemination of sexual spores. The mushrooms you are most familiar with are the fruiting body formed above ground. They can also form below ground, as is the case with truffles.

Spores are not seeds. They do not have their own nutritional supply and must land in a suitable environment that provides nutrients. Unlike most plants, fungi are low to the ground, and their spores are not easily carried off by the wind. Fungi produce many spores, but only a few survive. To increase survival, fungi have evolved many mechanisms for dispersing spores, including releasing spores into flowing water, attracting insects to assist with transportation, and evolving spores that are forcibly ejected from the fruiting body.

Basidiomycota

The phylum Basidiomycota includes many edible mushrooms that you may be familiar with, particularly those mushrooms that are members of the genus Agaricus (Figure 17.4).

The basidium (plural: basidia) that this phylum is named for is the mushroom's microscopic spore-forming structure (Figure 17.5). Typically, each basidium produces four spores that remain attached until maturity. When the spores mature, they are forcibly discharged. In Basidiomycota that form fruiting bodies, the gills - the thin ridges on the underside of the mushroom cap - are formed from many basidia.

Ascomycota is the largest phylum of Fungi and includes brewer's and baker's yeast, Penicillium chrysogenum (the species of fungus that produces penicillin), and truffles (Figure 17.6). This phylum is characterized by its ascus (plural: asci), a microscopic, sac-like reproductive structure that arises during sexual reproduction. Nonmobile spores are formed within the ascus. Asci can arise singly, as is the case with baker's yeast, or they can be contained inside a fruiting structure or mushroom, as is the case with truffles and morels. The asci are contained within the mushroom, unlike the basidia of Basidiomycota mushrooms.

During asexual reproduction, conidia (singular: conidium), or asexual, nonmotile spores, form on conidiophores, specialized hyphae on which conidia are produced (Figure 177). The swollen tip of the conidiophore is called the vesicle and the phialide is the cell at the end of the conidiophore from which spores are produced.

Zygomycota

Zygomycota is a (former) phylum that includes fungi, such as black bread mold, and several species used for the industrial production of enzymes, such as lipase. Fungi that belonged to this phylum are now classified as either Mucoromycota or Zoopagomycota. Zygomycota primarily reproduce asexually. The black bread mold, Rhizopus stolonifer, is a terrestrial saprophyte from the phylum Zygomycota (Figure 17.8). Many fungi in this phylum have hyphae that are tubular, branched, and aseptate (Figure 179).

1. The rhizolds reach below the surface of the substrate on which the fungus is growing to release digestive enzymes and absorb nutrients.

2. The sporangiophores are long stalks above the surface that bear the clusters of spore-producing sporangia.

3. The stolons extend horizontally to connect groups of rhizoids and sporangiophores into a fungal network.