Fungi Definition, Characteristics, Morphology, Importance, Thallus Organization.

Fungi Overview

Fungi are microscopic eukaryotic microorganisms that are of great practical and scientific interest to microbiologists. The appearance of fungi is very familiar to each of us. We have seen their growth on different foods such as orange, lemons, cheese, and the whitish-gray furry outgrowth on bread and jam, etc.

All of these are bodies of different fungi, therefore from the morphological characteristics, fungi are different types. Fungi is comprised of filamentous, multicellular bodies called molds and unicellular cells known as yeast. Apart from these there are other types of fungi known as slime molds, these are protozoan-like fungi.

Fungi can produce spores and they lack chlorophyll. They are mainly reproduced by both sexual and asexual methods.

There are about 1.5 million species of fungus, among them about 100,000 species of fungi are identified.

Fungi Definition

Definition of fungi According to Alexopoulos and Mims (1979);

Fungi are eukaryotic, spore-bearing, an achlorophyllous organism that generally reproduces sexually and asexually and whose usually filamentous, branched somatic structures are typically surrounded by cell walls containing chitin or cellulose, or both of these substances, together with many other complex organic molecules.

The currently modified definition of fungi is;

Fungi are eukaryotic organisms lacking plastids, with absorptive nutrition, reproducing both sexually and asexually by spores and hyphae surrounded by cell walls containing chitin and Beta-glucans and mitochondria with flattened cristae and peroxisomes.

• There are two major groups of organisms that comprise the fungi such as multicellular filamentous fungi known as molds and unicellular fungi known as yeast.
• All molds are fungi but all fungi are not mold.
• Yeast is a type of fungi that can not produce areal mycelium whereas mold can produce areal mycelium.
• The study of fungi is known as mycology, while the study of fungal disease is known as mycoses.

Characteristics of Fungi

1. Fungi are microscopic eukaryotic organisms.
2. They are chemoorganotrophic in nature.
3. In yeast, the fungal body or thallus consists of a single cell, while in molds the fungal body or thallus consists of 5-10 um branched filaments.
4. The thallus of yeast cells and mold is surrounded by a true cell wall.
5. The slime molds are a type of fungi that lacks a true cell wall.
6. The thallus of slime molds consists of a naked amoeboid mass of protoplasm.
7. Fungi lack chlorophyll.
8. The hyphal wall contains chitin, which is a polymer of N-acetylglucosamine. The structural rigidity maintained by the linkage between the sugars is like that of cellulose and peptidoglycan.
9. Some fungi show dimorphic characteristics, in which they exist in two forms (unicellular and multicellular form).
10. Fungi scatter themselves by discharging spores, usually by windblown.
11. The nucleus of fungi is enclosed by a nuclear membrane.
12. They lack the vascular system. Xylem and Phloem are absent.
13. The embryonic stage is absent in fungi.
14. Some fungi show a mutually beneficial symbiotic relationship with other organisms such as plants. This types of relationship with plants root are known as mycorrhiza. In this relationship, plants get benefits of more efficient minerals such as chiefly nitrates and phosphates while fungus get advantages by the sugars and other nutrients (such as lipids) translocated to the root by the plant.
15. Fungi contain different types of sexual and asexual spores which help them in reproduction such as Sexual spores are Oospores, Zygospores, Ascospores, Basidiospores, etc. and Asexual spores are Sporangiospores, Aplanospores, Zoospores, Conidia, etc.
16. They are non-motile in nature.
17. Fungi show a phenomenon known as alteration of generation. They contain both haploid and diploid stage.
18. The hyphae grow and form a network which known as mycelium.
19. The fungal cell membrane contains unique sterol and ergosterol.
20. They are heterotrophic in nature, which means they obtain their food and energy from organic substances, plant, and animal matters.
21. They mainly grow in an acidic environment, which means they can tolerate acidic pH.
22. They first digest the food and then ingest them, to perform this function they excreate different exoenzymes such as Lyases, Hydrolases, Transferase, Oxidoreductase, etc.
23. They store the foods in form of starch.
24. The Biosynthesis of chitin accomplished within the fungi.
25. Some fungi contain small nuclei with repetitive DNA.
26. In fungi, the nuclear envelope is not dissolved during the process of mitosis.
27. Nutrition in fungi – they are saprophytes (obtain energy from dead and decaying matters), or parasites (lives in a host, attack and kill) or symbionts (mutually beneficial).
28. The saprophytic fungi show optimal growth in  20-30°C while the parasitic fungi in (30-37)°C.
29. They grow slowly as compared to bacteria.
30. They exhibit both sexual and asexual means.
31. The sexual state is known as teleomorph (fruiting body) whereas the asexual state is known as anamorph (mold-like).
32. Some modes of Asexual reproduction are fragmentation, somatic budding, fission, asexual spore formation.
33. Some modes of Sexual method are gametic copulation, gamate-gametangium opulation, gametangium copulation, somatic copulation and Spermatization.
34. There are present several macroscopic fungi, which we can see by naked eyes such as Mold or mushrooms.
35. Examples of some fungi are Aspergillus, Blastomyces, Candida albicans, Pneumocystis jirovecii, Cryptococcus neoformans, Coccidioides, Histoplasma, etc.

Dimorphism of fungi

• Some pathogenic fungi of animals and humans exist as unicellular yeast within the host cell but when they have grown on a laboratory medium or in the soil they exist as a filamentous mold. This characteristic feature of fungi is known as dimorphism. Dimorphism is an inherited characteristic of some fungi.
• Some common example of human fungal pathogens which are shows the dimorphism are, Histoplasma, Sporothrix and Blastomyces.
• Some plant pathogenic fungi show the exact opposite dimorphic phenomenon. They appear in multicellular or filamentous mold form within the host cell while in laboratory medium they appear in the yeastlike form.
• Some examples of plant fungal pathogens are Taphrinales, and Ustilaginales.
• There are different factors which are influence the dimorphic characters of fungi, which means these factors trigger the conversion between two morphological forms. For example, from 20-25 to 37-degree centigrade temperature causes yeast-like growth in Paracoccidioids brasiliensis and Blastomyces dermatitidis.

Thallus organization of fungi

The majority of fungi contain a differentiated thallus which is made up of threadlike, tubular filaments, called the hyphae (singular; hyphe). Hyphe means web. The thallus of most fungi is differentiated into two parts such as vegetative part and reproductive part.

The vegetative part help in the absorption of nutrients while the reproductive parts involved in the formation of reproductive structures, such as thalli are known as eucarpic.

In some fungi, the thallus does not exhibit this differentiation and after a phase of vegetative growth, changes into one or more reproductive structures. Such thalli are known as holocarpic. The greek word holos mean entirely and karpos means fruit.

The hyphal network which consisting of the thallus is known as mycelium. The hypha is consists of a thin transparent, tubular filament, filled with a layer of cytoplasm.

I have already discussed the thallus organization of fungi in my previous article in detail, please follow this article for more detailed information, Thank you. Thallus organization of Fungi

Morphology/Structure of Fungi

There are different types of fungi which are range from the smallest unicellular fungi e.g. yeast to larger multicellular which has the ability to form hyphal threads or false roots. That’s why fungi are divided into different groups according to their morphologies.

Yeast Cell

• These are larger than most bacteria. The size of yeast is ranged from 1 to 5 um in width and from 5 to 30 um or more in length.
• Yeast can be found in egg-shaped, elongated shapes and some of them are also found in spherical shapes.
• They lack flagella, which means they are nonmotile.
• These are single celled fungi and can be found in soil, plants, animals, and in aquatic environments. 
• Yeast is eukaryotic, which means they contain organelles that are usually found in the cells of higher animals.
• An example of yeast is Saccharomyces cerevisiae, It contains membrane-bound nucleus, a vacuole, mitochondria, Golgi apparatus as well as the E.R (endoplasmic reticulum).
• The cell wall of Saccharomyces cerevisiae is consists of glucan (a polysaccharide compound) and mannoproteins.
• Yeast has a single, linear double-stranded DNA which is made up of several repeated sequences. 
• Yeast mainly reproduces by budding, where an outgrowth/bud occurs on the surface of the mother cell that eventually breaks off with its genetic material and grows to develop a new cell.

Molds

• These are the types of fungi that grown well in warmth and moist environment. They can be found on the surface of different food, from where it can get nutrients.
• Molds are multicellular organisms and can be seen by naked eyes.
• When they are observed under the microscope a numerous filaments (hyphae) are founds which are known as Mycelium.
• The thallus of molds is made up of two important parts known as the mycelium or mycelia and the spores. The spores are resistant, resting or dormant cell.
• Mycelium is referred to as a complex of several filaments which are known as hyphae (singular, hypha). New hyphae generally rise from a spore which on germination puts out a germ tube or tubes. These germ tubes elongate and the branch to form hyphae.
• The hyphae are about 5 to 10 um wide.
• The hyphae are made up of an outer tubelike wall which is enclosing a cavity, the lumen, which is loaded wilth protoplasm. 
• A double-layered membrane is present between the protoplasm and the wall known as plasmalemma, which surrounds the protoplasm. 
• The hyphal wall is made up of microfibrils which is consist of hemicelluloses or chitin: true cellulose found only in the walls of lower fungi. 
• Wall matrix material in which the microfibrils are embedded consists of proteins, lipids, and other substances.
• The microfibrils are embedded within the Wall matrix which is compoed of proteins, lipids, and other substances.
• The growth of a hypha mainly occurs near the tip. The main region of elongation occurs in the region just after the tip. 
• The young hypha separated into cells by a cross-wall which is developed by centripetal invagination (inward growth) from the existing cell wall. These cross-walls constrict the plasmalemma and grow inward to form generally an incomplete septum (plural, septa) that has a central pore which allows for protoplasmic streaming. Even nuclei may migrate from cell to cell in the hypha.

There are three forms of hyphae such as 

• Nonseptate, or coenocytic, these types of hyphae have no septa. 
• Septate with uninucleate cells
• Septate with multinucleate cells, Each cell contains more than one nucleus in each compartment.

There are two types of mycelia such as 

1. Vegetative: This type of mycelium enters within the medium to get nutrients; soluble nutrients are received through the walls. (Insoluble nutrients are first digested externally by secreted enzymes.) 
2. Reproductive: This type of mycelia is involved in spore production and usually extend from the medium into the air. The mycelium of a mold may be a loosely woven network or it may be an organized, compact structure, as in mushrooms.

Yeast-like Fungi

• These are a types of yeast that grow like normal yeast. The individual cells of Yeast-like fungi aggregates and forms a structure is known as pseudo hyphae (not a true hyphae).
• One example of this type of fungi is Candida. When it observed under a microscope, it shows the organism contain several layers which make up the cell wall.
• The  C. albicans wall made up of layers of mannoproteins, lipids, and a beta glucan, a chitin inner layer also present that strengthen the cell wall. 
• C. albicans appears in spherical or ovoid shape and size ranges between 4 to 8 micrometers.
• They also reproduced by the budding process. They create an elongated chain of cells as they continue dividing to form the pseudohyphae. 
• C. albicans is a type of polymorphic fungus that can be found in four types of morphology such as the yeast cell, pseudohyphae, hyphae as well as chlamydospores.
• They have varying appearances when they are viewed under the microscope. This depends on different conditions such as the availability of nutrition, pH, and temperature among others.

Importance of Fungi

1. The Mycorrhizal relationship of fungi benefits various plants in many ways. For instance, fungi get nutrients from plants and it provides phosphates which is readily available for the plant.
2. Some fungi function as biological insecticides, for example, Beauveria bassiana helps the farmers to control such pests as the emerald ash borer that can cause damage to plants.
3. Fungi are heterotrophic organisms hence they get their nutrients from dead organic matter. During the feed on dead organic matter, they decompose the complex animal and plant remains and breaking them down into simpler forms of chemical substances which are then returned to the soil. Therefore they increase the fertility of the soil.
4. In industrial fermentation, the saprophytic fungi are used for brewing beer, production of wine and antibiotics (penicillin).
5. Fungi become a suitable tool for phycologists, biochemists, biophysicists, geneticists, who find them a suitable subject for the study of some biological process.

Disadvantages of Fungi

1. Molds can cause food damage. They also release spores which are responsible for different health problems i.e. breathing difficulties. 
2. Molds also release toxins such as aflatoxin or myotoxins among others that can cause serious health problems to a human being.
3. There are different poisonous mushrooms that can cause death when consumed. 

References

1. Microbiology 5th Edition by Michael Joseph Pelczar 
2. https://www.biology-pages.info/F/Fungi.html#Characteristics_of_Fungi
3. https://microbenotes.com/characteristics-of-fungi/
4. https://www.microscopemaster.com/fungi.html