What is fungi?
The fungi are a large and successful groupof organisms of about 80 000 named species. They range in size from unicellularyeast to large toadstools, puffballs and stinkhorns, and occupy a very widerange of habitats, both aquatic and terrestrial.
They are also of major importance for therole that they play in the biosphere, and for the way in which they beenexploited by humans for economic and medical purposes.
Fungi include the numerous moulds whichgrow on damp organic matter (such as bread, leather, decaying vegetation anddead fish), the unicellular yeasts which are abundant on the sugary surfaces ofripe fruits and many parasites of plants. The latter cause some economicallyimportant 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.
Read: Differences between fungi and algae
Table for classification & characteristics of kingdomfungi
| 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. Itconsist of a mass of fine tubular branching threads called hyphae (singular; hypha), the whole mass being called a mycelium. Each hypha has a thin rigidwall whose chief component is chitin, a nitrogen-containing polysaccharidewhich is also found as a structural component in the exoskeletons ofarthropods.
The haphae are not divided into true cells.Instead, the protoplasm is either continuous or interrupted at intervals bycross-walls called septa whichdivide the haphae into compartments similar to cells. Unlike normal cell walls their formation isnot a consequence of nuclear division, and a pore normally remains at theircentre allowing protoplasm to flow between compartments.
Each compartment may contain one, two ormore nuclei, which are distributed at more or less regular intervals along thehyphae. Hyphae having cross-walls are called septate, as in Penicillium. Hyphaelacking cross-walls are called non-septate(aseptate) as in Mucor.
Within the cytoplasm the usual eukaryoteorganelles are found, such as mitochondria, Golgi apparatus, endoplasmicreticulum, ribosomes and vacuoles. In the older parts, vacuoles are large andcytoplasm is confined to a thin peripheral layer. Sometimes hyphae aggregate toform more solid structure such as the spore-producing bodies of the mushrooms.The yeasts are unusual in being unicellular fungi and therefore lack thetypical hypha structure e.g. Saccharomyces
Penicilliumspecies form blue, green and sometimes yellowmoulds, common, for example, on bread and decaying fruit. The mycelium forms acircular colony of small diameter with septate hyphae and the spores givecolour to the colony. Penicillium reproduceasexually by means or spores called conidia.These are found at the tip of special hyphae called conidiophores. They are notenclosed in a sporangium, but are naked and free to be dispersed as soon asthey mature.
Mucor is a genus which includes a number ofwell-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 areaseptate and profusely branching. It produces spores in spherical sporangia borne on very long stalksknown as sporangiophores. These arenumerous in the more mature part of the mycelium and resemble a collection ofpins; hence Mucor is often referredto as pin mould. Sporangia are clearly visible using the lower power of themicroscope. 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 ofMucor lack cross-walls.
Rhizopusis very similar to Mucor. Some hyphae, called stolons,are arch-shaped and produce tufts of short, root-like hyphae at their tips. Twoor more sporangiophores grow from the same point, unlike Mucor where sporangiophores occur singly.
Yeasts are unicellular, saprotrophic fungi. They occur widely in natureand are particularly common on the sugary surfaces of ripe fruits. The bloom ongrapes, for example, is due to yeast. The fermentation (anaerobic respiration)of the sugars by yeast produces alcohol, a fact made use of by humans forthousands of years and which forms the basis of the wine and brewingindustries. Under appropriate conditions yeast cells multiply rapidly bybudding, a form of asexual reproduction. Yeast cells show the usual eukaryoticfeatures.
Nutrition in fungi
Fungi are heterotrophic, that is theyrequire an organic source of carbon, in addition, they require a source ofnitrogen, usually organic such as amino acids; inorganic ions such as K+ andMg2+; trace elements such as Fe, Zn and Cu; and organic growthfactors such as vitamins. The exact range of nutrients required, and hencesubstrates on which they are found, is variable.
The nutrition of fungi can be described asabsorptive because they absorb nutrients directly from outside their bodies. Thisis in contrast to animals, which normally ingest food and then digest it withintheir bodies before absorption takes place. With fungi, digestion, ifnecessary, is performed by the fungus secreting enzymes out of its body on to itsfood.
Fungi obtain their nutrients assaprotrophs, parasites or mutualists. In this respect they are like mostbacteria.
Saprotrophs
Saprotrophic organisms feed on dead organicmaterial. Fungal saprotrophs produce a variety of digestive enzymes, namelycarbohydrates (digest carbohydrates) such as amylases (digest starch), lipases(digest lipids) and proteases (digest proteins), they can utilise a wide rangeof substrates. For example the Penicilliumspecies form green and blue moulds on substrates such as soil, dampleather, bread and decaying fruit.
Thehyphae of saprotrophic fungi are usually chemotrophic, that is, they growtowards certain substrates in response to chemicals diffusing from thesesubstrates.
Fungal saprotrophs usually produce large numbersof light, resistant spores. This allows efficient dispersal in other foodsources. Examples are Mucor, Rhizopus andPenicillium.
Saprotrophic fungi and bacteria togetherform the decomposers which are essential in the recycling nutrients. Especiallyimportant are the few that secrete the enzymes cellulose and ligase, whichbreak down cellulose and lignin respectively. Cellulose and lignin (a complexchemical found particularly in wood) are important structural components ofplant cell walls, and the rotting of wood and other plant remains is achievedpartly by decomposers secreting cellulose and ligase.
Some fungal saprotrophs are of economicimportance, such as saccharomyces (yeast) used in brewing and bread making and Penicillium which is used in medicine.
Parasites
Fungal parasites may be facultative orobligate, and more commonly attack plants than animals. Obligate parasites donot normally kill their hosts, whereas facultative parasites do and then livesaprotrophically off the dead remains. Obligate parasites include the powderymildews, downy mildews, rusts and smuts, which attack cereals and many othercrops.
Once inside the plant, hyphae normally growwithin cells. Facultative parasites normally produce enzymes called pectinaseswhich digest the middle lamellae between cells and cause ‘soft rot’ of thetissue, reducing it to a mush. Subsequently, they may invade cells and killthem with the aid of cellulose which digests the cell walls. Cell constituentsmay be absorbed directly or digested by secretion of further fungal enzymes.This type of attack is shown by Pythium (thecause of ‘damping off’ of seedlings) and phytophthora(the cause of potato blight), both of which belong to a group, theOomycota, now regarded as ancestral to fungi and classified in the kingdomprotoctista.
An example of facultative parasite whichinfects humans is the yeast Candidaalbicans. This is a normal and usually harmless part of the surface or gutmicro flora of about 5% of the adult human population. However, particularly ifbalance of natural microorganisms living on or in the body is disturbed withthe use of antibiotics or prolonged use of steroid drugs (which have theside-effect of suppressing the body’s immune system), the yeast may grow out ofcontrol and become pathogenic, that is cause disease. It causes a conditionknown as thrush (candidiasis orcandidosis). Damp conditions are needed and it can infect the mouth (oralthrush) and vagina. The latter is associated with increased vaginal dischargeand there may be itching or soreness on passing urine. It is very common, butnot serious and can be controlled with antifungal drugs.
Mutualism (symbiosis)
Two important types of mutualistic unionare made by fungi, namely lichens and mycorrhizas. Lichens are associationsbetween fungi and green algae or blue-green bacteria. Lichens are commonlyencrusted on exposed rocks and trunks of trees; they also hang from trees inwet forests. It is believed that the alga contributes organic food fromphotosynthesis, while the fungus is protected from high light intensity and isable 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 afungus and a plant root. The fungus absorbs mineral salts and water which passto the plant, and in return receives organic products of photosynthesis.
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