The fungi are a large and successful group of organisms of about 80 000 named species. They range in size from unicellular yeast to large toadstools, puffballs and stinkhorns, and occupy a very wide range of habitats, both aquatic and terrestrial.
They are also of major importance for the role that they play in the biosphere, and for the way in which they been exploited by humans for economic and medical purposes.
Fungi include the numerous moulds which grow on damp organic matter (such as bread, leather, decaying vegetation and dead fish), the unicellular yeasts which are abundant on the sugary surfaces of ripe fruits and many parasites of plants. The latter cause some economically important diseases of crops, such as mildews, smuts and rusts.
A few fungi are parasites of animals, but these are less significant in this aspect than bacteria.
The study of fungi is called mycology (mykes, mushroom). It forms a branch of microbiology because many of the handling techniques used, such as sterilising and culturing procedures, are the same as those used with bacteria.
Classification and characteristics of fungi
As you must have learnt in your basic classification classes, fungi are eukaryotes that lack chlorophyll, and are therefore heterotrophic, like animals. However, they have rigid cell walls and are non-motile, like plants, but modern classifications place them in a separate kingdom. Their classification and characteristics is summarised in the table below.
The two largest and most advanced groups are the Ascomycota and the Basidiomycota. Structure and nutrition of fungi are discussed in more detail below.
Table for classification & characteristics of kingdom fungi
|General characteristics Heterotrophic nutrition because they lack chlorophyll and are therefore non-photosynthetic. They can be parasites, saprotrophs or mutualists. Nutrition is absorptive; digestion takes place outside the body and nutrients are absorbed directly. Digestion does not take place inside the body, unlike animals.Rigid cell walls containing chitin as the fibrillar material. Chitin is a nitrogen-containing polysaccharide, very similar in structure to cellulose. Like cellulose it has high tensile strength. It therefore gives shape to the hyphae and prevents osmotic bursting of the cells.Body is usually a mycelium, a network of fine tubular filaments called hyphae. These may be septate (have cross-walls), e.g. Penicillium, or aseptate (no cross-walls), e.g. Mucor. If carbohydrate is stored, it is usually as glycogen, not starch.Reproduce by means of spores.They are non-motile|
|CLASSIFICATION OF FUNGI|
|Phylum Zygomycota||Phylum Ascomycota||Phylum Basidiomycota|
|Asexual reproduction by conidia or sporangia containing spores||Asexual reproduction by conidia. No sporangia||Asexual reproduction by formation of spores. Sporangia not common|
|Non-septate hyphae and large well-developed branching mycelium||Septate hyphae||Septate hyphae|
|e.g. Rhizopus stolonifer, common bread mould, a saprotrophs Mucor, common moulds, saprotroph||Penicillium and Aspergillus, saprotrophic moulds Saccharomyces (yeast), unicellular saprotrophs Erysiphe, obligate parasites causing powdery mildews, e.g. of barley||Agaricus campestris, field mushroom, saprotroph|
Structure of fungi
The body structure of fungi is unique. It consist of a mass of fine tubular branching threads called hyphae (singular; hypha), the whole mass being called a mycelium. Each hypha has a thin rigid wall whose chief component is chitin, a nitrogen-containing polysaccharide which is also found as a structural component in the exoskeletons of arthropods.
The haphae are not divided into true cells. Instead, the protoplasm is either continuous or interrupted at intervals by cross-walls called septa which divide the haphae into compartments similar to cells. Unlike normal cell walls their formation is not a consequence of nuclear division, and a pore normally remains at their centre allowing protoplasm to flow between compartments.
Each compartment may contain one, two or more nuclei, which are distributed at more or less regular intervals along the hyphae. Hyphae having cross-walls are called septate, as in Penicillium. Hyphae lacking cross-walls are called non-septate (aseptate) as in Mucor.
Within the cytoplasm the usual eukaryote organelles are found, such as mitochondria, Golgi apparatus, endoplasmic reticulum, ribosomes and vacuoles. In the older parts, vacuoles are large and cytoplasm is confined to a thin peripheral layer. Sometimes hyphae aggregate to form more solid structure such as the spore-producing bodies of the mushrooms. The yeasts are unusual in being unicellular fungi and therefore lack the typical hypha structure e.g. Saccharomyces
Penicillium species form blue, green and sometimes yellow moulds, common, for example, on bread and decaying fruit. The mycelium forms a circular colony of small diameter with septate hyphae and the spores give colour to the colony. Penicillium reproduce asexually by means or spores called conidia. These are found at the tip of special hyphae called conidiophores. They are not enclosed in a sporangium, but are naked and free to be dispersed as soon as they mature.
Mucor is a genus which includes a number of well-known moulds. It is common in soil and may also be found growing on bread. It forms more or less circular colonies when grown on agar. Its hyphae are aseptate and profusely branching. It produces spores in spherical sporangia borne on very long stalks known as sporangiophores. These are numerous in the more mature part of the mycelium and resemble a collection of pins; hence Mucor is often referred to as pin mould. Sporangia are clearly visible using the lower power of the microscope. Mucor grows rapidly and can cover a petri dish in 3 days at 20oC. Internally, its hyphae have the same typical eukaryotic structure as Penicillium except that the hyphae of Mucor lack cross-walls.
Rhizopus is very similar to Mucor. Some hyphae, called stolons, are arch-shaped and produce tufts of short, root-like hyphae at their tips. Two or more sporangiophores grow from the same point, unlike Mucor where sporangiophores occur singly.
Yeasts are unicellular, saprotrophic fungi. They occur widely in nature and are particularly common on the sugary surfaces of ripe fruits. The bloom on grapes, for example, is due to yeast. The fermentation (anaerobic respiration) of the sugars by yeast produces alcohol, a fact made use of by humans for thousands of years and which forms the basis of the wine and brewing industries. Under appropriate conditions yeast cells multiply rapidly by budding, a form of asexual reproduction. Yeast cells show the usual eukaryotic features.
Nutrition in fungi
Fungi are heterotrophic, that is they require an organic source of carbon, in addition, they require a source of nitrogen, usually organic such as amino acids; inorganic ions such as K+ and Mg2+; trace elements such as Fe, Zn and Cu; and organic growth factors such as vitamins. The exact range of nutrients required, and hence substrates on which they are found, is variable.
The nutrition of fungi can be described as absorptive because they absorb nutrients directly from outside their bodies. This is in contrast to animals, which normally ingest food and then digest it within their bodies before absorption takes place. With fungi, digestion, if necessary, is performed by the fungus secreting enzymes out of its body on to its food.
Fungi obtain their nutrients as saprotrophs, parasites or mutualists. In this respect they are like most bacteria.
Saprotrophic organisms feed on dead organic material. Fungal saprotrophs produce a variety of digestive enzymes, namely carbohydrates (digest carbohydrates) such as amylases (digest starch), lipases (digest lipids) and proteases (digest proteins), they can utilise a wide range of substrates. For example the Penicillium species form green and blue moulds on substrates such as soil, damp leather, bread and decaying fruit.
The hyphae of saprotrophic fungi are usually chemotrophic, that is, they grow towards certain substrates in response to chemicals diffusing from these substrates.
Fungal saprotrophs usually produce large numbers of light, resistant spores. This allows efficient dispersal in other food sources. Examples are Mucor, Rhizopus and Penicillium.
Saprotrophic fungi and bacteria together form the decomposers which are essential in the recycling nutrients. Especially important are the few that secrete the enzymes cellulose and ligase, which break down cellulose and lignin respectively. Cellulose and lignin (a complex chemical found particularly in wood) are important structural components of plant cell walls, and the rotting of wood and other plant remains is achieved partly by decomposers secreting cellulose and ligase.
Some fungal saprotrophs are of economic importance, such as saccharomyces (yeast) used in brewing and bread making and Penicillium which is used in medicine.
Fungal parasites may be facultative or obligate, and more commonly attack plants than animals. Obligate parasites do not normally kill their hosts, whereas facultative parasites do and then live saprotrophically off the dead remains. Obligate parasites include the powdery mildews, downy mildews, rusts and smuts, which attack cereals and many other crops.
Once inside the plant, hyphae normally grow within cells. Facultative parasites normally produce enzymes called pectinases which digest the middle lamellae between cells and cause ‘soft rot’ of the tissue, reducing it to a mush. Subsequently, they may invade cells and kill them with the aid of cellulose which digests the cell walls. Cell constituents may be absorbed directly or digested by secretion of further fungal enzymes. This type of attack is shown by Pythium (the cause of ‘damping off’ of seedlings) and phytophthora (the cause of potato blight), both of which belong to a group, the Oomycota, now regarded as ancestral to fungi and classified in the kingdom protoctista.
An example of facultative parasite which infects humans is the yeast Candida albicans. This is a normal and usually harmless part of the surface or gut micro flora of about 5% of the adult human population. However, particularly if balance of natural microorganisms living on or in the body is disturbed with the use of antibiotics or prolonged use of steroid drugs (which have the side-effect of suppressing the body’s immune system), the yeast may grow out of control and become pathogenic, that is cause disease. It causes a condition known as thrush (candidiasis or candidosis). Damp conditions are needed and it can infect the mouth (oral thrush) and vagina. The latter is associated with increased vaginal discharge and there may be itching or soreness on passing urine. It is very common, but not serious and can be controlled with antifungal drugs.
Two important types of mutualistic union are made by fungi, namely lichens and mycorrhizas. Lichens are associations between fungi and green algae or blue-green bacteria. Lichens are commonly encrusted on exposed rocks and trunks of trees; they also hang from trees in wet forests. It is believed that the alga contributes organic food from photosynthesis, while the fungus is protected from high light intensity and is able to absorb water and mineral salts. The fungus can also conserve water, enabling some lichens to grow in dry conditions where no plants exist.
A mycorrhiza is an association between a fungus and a plant root. The fungus absorbs mineral salts and water which pass to the plant, and in return receives organic products of photosynthesis.