Hippocampus Biology: Characteristics of Fungi

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Let’s examine some of the typical fungal characteristics. We must remember, however, that the fungi are a highly diverse group of organisms, so there will be exceptions to almost every generalization.

The growing body of most fungi is called a mycelium. The root of this word—MYC—means fungus. We’ll see that root again. A mycelium is made up of a mass of threadlike structures called hyphae. Hyphae have tough walls surrounding their plasma membranes and cytoplasm. Fungal cell walls, unlike those of plants, aren’t made of cellulose. Instead, they contain the carbohydrate polymer called chitin. A form of chitin is also a component of the external skeletons of insects and their relatives.

Most hyphae are divided into cells by cross-walls called septa. A septum doesn’t usually completely separate two cells. Pores in a septum allow cytoplasm, organelles, and even nuclei to travel between cells. Some fungi lack septa. Their hyphae are tubes of continuous cytoplasm containing many nuclei, along with other organelles.

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Fungal hyphae grow into their food source. The fine, threadlike hyphae provide a huge surface area for the efficient absorption of nutrients. Many species of fungi are saprobes. They obtain their carbon and energy from dead organic matter, thereby causing its decay. Many species of fungi are particularly well adapted to break down wood.

Together with bacteria, saprobic fungi have a very important role in the environment, because they release nutrients from organic matter to be reused by other organisms.

Of course, fungi don’t distinguish between the wood of a fallen tree and the wood of a canoe. And the fruit on our table is just as nutritious to fungi as fruit that has fallen in a forest or jungle. The role of fungi as decomposers extends throughout the living world, right into our homes.

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Other species of fungi have adapted for life in close association with organisms of other species. These fungi are symbionts. Symbiosis, the association of the two organisms, can benefit both the fungus and the other species. An important example of such a relationship is the close association of some species of fungi with plants.

Most plants have symbiotic fungi associated with them.

The association of a root of a plant with the mycelium of a fungus is called a mycorrhiza. In mycorrhizae, the fungal cells are in direct contact with the cells of the plant root; in some cases, the hyphae even penetrate the plant’s cells. The fungus helps the plant absorb minerals and keep water near the roots. In exchange, the fungus obtains organic nutrients from the plant.

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When a fungus benefits at the expense of its partner in a symbiotic relationship, the fungus is a parasite. A parasitic fungus obtains nutrients from its host, and usually harms the host in the process. Parasitic fungi are responsible for many plant diseases.

Dutch elm disease and chestnut blight have devastated populations of elm and chestnut trees in North America. Many plants, including important crops, are attacked by parasitic fungi, which cause diseases known variously as rusts, smuts, scabs, rots, and wilts.

Some fungi are parasites of animals, including humans. Several species of fungi can cause serious lung infections. The lung diseases histoplasmosis and Pneumocystis pneumonia are particularly dangerous to people with impaired immune systems, such as those with the disease AIDS. Fungi also cause a variety of skin diseases, like athlete’s foot and ringworm.

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Although fungi are generally multicellular organisms, some species of fungi are unicellular for part or all of their life cycle. Unicellular fungi are called yeasts. They are widespread in wet environments, and are saprobes, symbionts, or parasites, like other fungi.

All yeasts can reproduce asexually, by simple cell division, or by the process of budding, in which a small daughter cell pinches off from the parent cell. Some yeasts can reproduce sexually, forming complex spore-bearing structures that help in classifying the organism.

Yeasts are important in food preparation, such as the leavening of bread and the fermentation of beverages. Strains of the species Saccharomyces cerevisiae, the most important baker’s and brewer’s yeast, have also been widely studied in the laboratory as a model of eukaryotic genetics.

Some yeast species can cause human disease. Yeasts of the genus Candida are widespread in the environment and live on the external surfaces of every person. Sometimes the natural balance between this yeast and its human host can become disturbed, and the disease candidiasis results. The disease can vary from a mild skin irritation to a serious infection of the internal organs.

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Some species of fungi have adapted to a symbiotic lifestyle as components of lichens. You may have seen a lichen as a colorful crusty growth on a rock. Other lichens have the form of a leaf or tiny shrub. Lichens aren’t single organisms. They’re the result of a symbiosis between a fungus and a photosynthetic microorganism.

The phototroph is either a green alga or a type of photosynthetic eubacterium called a cyanobacterium. A lichen is identified by its fungal component, which can’t live independent of its phototrophic partner. The complex relationship between fungus and phototroph may not be completely balanced. Evidence indicates that the fungal component may take advantage of its partner.

Lichens can endure extreme conditions, and can be found on mountaintops, newly formed volcanic rock, and in the Arctic and Antarctic. But many lichens live in temperate and tropical habitats as well. The fungal and photosynthetic parts of a lichen can reproduce independently, but a lichen can also form special dispersal fragments containing one or a few photosynthetic cells surrounded by fungal hyphae. Dispersal fragments can be carried great distances by the wind.

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Most fungi disperse their offspring by releasing spores. Most fungal spores, and the nuclei of most growing fungal mycelia, are haploid. When a fungus is growing rapidly and food is plentiful, it’s more likely to reproduce asexually. The fungus produces fruiting structures that release spores genetically identical to the parent. The spores may be transported great distances by the wind.

Sexual reproduction can occur when individual organisms of different mating types encounter each other. Mating type is a genetic characteristic analogous to the distinction of male and female in plants and animals. But a fungal species may have more than two mating types, and individuals of different mating types usually have no obvious morphological differences. Since hyphae of the same mating type cannot mate, self-fertilization is prevented.

In sexual reproduction, the hyphae of the mating fungi join and their cytoplasms fuse. A distinctive characteristic of fungi is that in many cases the two haploid nuclei do not join immediately. Instead, the fungal hyphae may exist in a dikaryotic stage. The word “dikaryotic” has roots meaning “two nuclei.” The pairs of nuclei may coexist for a long time after the mating, before the haploid nuclei eventually fuse. The diploid stage, which may be brief, is followed by a meiosis step that leads to the formation of genetically unique haploid spores.