KINGDOM FUNGI: CHARACTERISTICS, CLASSIFICATION, REPRODUCTION, NUTRITION - BIOLOGY - 2025

The Fungi kingdom is the classification where fungi are grouped. This includes organisms such as bread and beer yeasts, smuts, fruit molds, and mushrooms, many of them with shapes and functions that are familiar to us as humans.

This is a very large group of organisms: initially it was thought that there were more than 100,000 species in this group and today it exceeds one million five hundred thousand. Thus, fungi, at least in terms of number of species, represent the second largest group after insects.

Photograph of the fungus belonging to the Basidiomycota division, Amanita muscaria (Image by Image PublicDomainPictures at www.pixabay.com)

It is important to note that the members of this kingdom comprise more than 90% of the terrestrial biomass added by prokaryotic and invertebrate organisms, which makes them the most abundant group of organisms on earth in terms of soil biomass.

Thus, fungi are widely distributed in the biosphere:

- Fungi can be found in Antarctic ice, on beaches and tropical forests, in Nordic landscapes and in mid-latitudes.

- They live on the ground, in water, on the surface of rocky mountains and even in sea water.

- They can be parasites of plants, fish, insects and large vertebrate animals such as mammals, that is, in all living organisms.

Characteristics of the Fungi kingdom

Fungi make up a complex group of organisms that, despite the many differences they may have among themselves, share some remarkable characteristics:

- are eukaryotic organisms

Unlike bacteria and archaea, but just like animals and plants, fungi are eukaryotic, that is, they have a nucleus and membranous systems that define other organelles inside their cells.

Although they are classified in a different group, many authors agree that the cells of fungi are more similar to those of animals than to those of plants, especially with respect to the characteristics of their internal organelles.

- They have a cell wall

The reason why fungi were classified together with plants has to do with the fact that their cells, although they do not have chlorophyll, are surrounded by a wall that protects them, as well as plant cells.

However, the differences in the composition of this wall are what separate them from the group of plants: plants have cell walls made up of a chemical compound called cellulose and fungi have cell walls made up of a different compound known as chitin.

It must be remembered that this compound, chitin, is the same material from which the exoskeletons of many insects and shellfish (arthropods, invertebrates) are made.

Photograph of a mushroom (Image by 272447 at www.pixabay.com)

- They are sessile and multicellular organisms

With some exceptions, most fungi are multicellular organisms, that is, they are made up of several cells that are somehow “connected” to each other.

In addition, they are sessile organisms, that is, just like plants, they cannot move and are always in the same place where they “take root”.

- They are heterotrophs

Plants are autotrophic organisms (they make their own food) and animals are heterotrophic organisms (they feed on other organisms).

Fungi are also heterotrophic, and many of them get the energy they need to live from decaying organic matter or waste from other organisms rather than other living organisms.

- They have hyphae and mycelia

Hypha and mycelium. Taken and edited from es.winner.wikia.com

All organisms classified within the group of fungi, that is, in the Fungi kingdom, have a very curious form of vegetative growth: they grow from the ends of filaments called hyphae, which can be grouped to form a "body", known as mycelium.

The mycelia are, therefore, the structures that are in charge of absorbing organic matter (food, which has been externally digested) from the environment that surrounds them.

The hyphae that make up these mycelia resemble filamentous strands. They are composed of "strips" of cells that are in contact with each other in such a way that the absorbed nutrients can flow from one to another without many obstacles.

The mycelium of a fungus, depending on the species, allows it to grow either on the ground, in water, on decaying tissues, on living tissues, etc.

Classification

Fungi are a monophyletic group, that is, their members have the same common ancestor. This group is composed of 7 phyla: Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Microsporidia, Glomeromycota, Ascomycota and Basidiomycota.

- Phylum Chytridiomycota

Saprophytic and parasitic organisms are grouped in this phylum that can also be unicellular or filamentous. They can form mycelia and reproduce asexually forming asexual spores. It has two classes: Chytridiomycetes and Monoblepharidomycetes.

- Phylum Blastocladiomycota

It is composed of fungi that are parasites of plants and animals and by some saprophytic fungi. It includes aquatic and terrestrial organisms, whose life cycles show alternation of generations between the haploid and diploid phases. It only contains one class: Blastocladiomycetes.

- Phylum Neocallimastigomycota

Organisms found in the digestive tract of many herbivorous animals belong to this group, so many of them are anaerobic (they live in the absence of oxygen, O2). Many produce asexual spores with one or two flagella.

Your cells, instead of having mitochondria, have hydrogensomes, organelles that are responsible for the formation of energy in the form of ATP. They also make up a single class: Neocallimastigomycetes.

- Phylum Microsporidia

This phylum includes parasitic fungi of animals and protist organisms. Since the phylogenetic relationships of this group have not been fully elucidated, this phylum is not subdivided into classes.

- Phylum Glomeromycota

These fungi are obligate mutualistic symbiotic fungi. The species belonging to this phylum are associated with the roots of many plants and establish symbiotic relationships with them. It is divided into three classes: Archaeosporomycetes, Glomeromycetes, and Paraglomeromycetes, and into four subphiles:

1. Mucoromycotina
2. Entomophthoromycotina
3. Zoopagomycotina
4. Kickxellomycotina

- Ascomycota Blade

Also known as "sac fungi", the organisms belonging to this phylum can be symbionts in lichens, they can be plant or animal parasites or saprophytes and they can be unicellular or filamentous.

They reproduce asexually by fission, budding, fragmentation, or spores. Their sexual reproduction occurs through meiospores formed in "sacs" called asci, which can assemble into structures or bodies (closed or open) called ascocarpus.

Some "cup mushrooms", "chair mushrooms" and truffles belong to this group. It is divided into the subphiles:

1. Taphrinomycotina
2. Saccharomycotina (where the yeasts are)
3. Pexixomycotina (many lichen-forming fungi)

- Phylum Basidiomycota

These fungi also have different forms of life and nutrition: in the group there are parasites of plants and insects and also saprophytes.

Many of these are filamentous fungi and can produce two types of mycelia: one with uninucleated cells (with a single nucleus) and others with dikaryotic cells (with two nuclei). They reproduce asexually by fragmentation or sporulation.

Their sexual reproduction can occur by fusion of hyphae or by fusion of two reproductive structures, one male and the other female.

It is one of the largest groups and includes the fungi containing rusts, blights, gelatinous fungi, mushrooms, globular fungi, stink fungi and "bird's nest" fungi, etc.

This phylum is subdivided into three other subphiles:

1. Pucciniomycotina
2. Ustilaginomycotina
3. Agaricomycotina

Reproduction in the Fungi kingdom

The way in which fungi reproduce is considerably variable and depends a lot on each species considered.

Some can reproduce by forming new colonies from fragments of their hyphae and others, on the other hand, are capable of producing fruiting bodies (such as mushrooms or mushrooms) that form spores.

To understand it better, let's keep in mind that in the Fungi kingdom there is both asexual reproduction and sexual reproduction.

Photograph of a mushroom (Image by enriquelopezgarre at www.pixabay.com)

Asexual reproduction in the Fungi kingdom

In fungi we could speak of a "simple" sexual reproduction and another a little more "complex". The most "simple" asexual reproduction has to do with fission, budding and fragmentation processes.

- Fragmentation occurs in some groups and has to do, as we mentioned, with the multiplication of colonies from fragments of the hyphae that form their mycelia.

- Budding is another form of asexual reproduction through which, in a cell, a kind of "papilla" arises that enlarges and then separates, forming an independent (but identical, that is, a clone) entity.

- Fission is characteristic of some unicellular fungi, such as certain yeasts, for example. It consists of the formation of a new cell from another that divides in half.

Microscopy of Saccharomyces cerevisiae, the yeast of bread and beer, a single-celled fungus that reproduces by budding (Source: Mogana Das Murtey and Patchamuthu Ramasamy / CC BY-SA (https://creativecommons.org/licenses/by-sa/ 3.0) via Wikimedia Commons)

Other more "complex" forms of asexual reproduction involve the formation of asexual spores, that is, spores that have been formed by mitotic or meiotic divisions (by mitosis or meiosis, respectively) and not by the fusion of two cells or sex gametes.

They are said to be more "complex" because the fungi that form these spores (mobile or immobile) generally develop more elaborate structures for this.

Sexual reproduction in the Fungi kingdom

Almost all the species that fall within this kingdom can reproduce sexually.

An important difference in the sexual reproduction of fungi from that of other organisms is that the nuclear membrane of their cells remains intact throughout the process (in other organisms it "dissolves" and forms again).

Plasmogamy, karyogamy, and meiosis

In fungi, sexual reproduction occurs in three sequential events. Initially, the diploid chromosomes (2n) are separated into two daughter cells, forming a haploid (n) stage.

Plasmogamy consists, then, in the fusion of two protoplasts that contain “compatible” nuclei. We call protoplast to everything that is contained within the cell wall: plasma membrane and cytosol with all its organelles.

This phase produces a cell with two haploid nuclei that have not fused, an event that occurs later and is known as karyogamy. Karyogamy gives rise to a diploid nucleus, genetically different from the previous two, within a cell that is now called a zygote.

There are many fungi that are always haploid and, therefore, the zygote is the only diploid cell in its entire life cycle. In addition, other fungi can remain with dikaryotic cells (with two nuclei) for a long time.

After karyogamy, meiosis follows, which is the process of cell division by which the genetic load or the number of chromosomes per cell is reduced, thus reestablishing the haploid “phase”. Haploid nuclei are usually in cells that develop into spores, meiospores.

«Rapid» diagram of the sexual reproduction of a fungus with a fruiting body (Source: Fungi_sessuate_reproduction.png: M.violante 10:22, 24 May 2006 (UTC) derivative work: DZadventiste / CC BY-SA (http: // creativecommons. org / licenses / by-sa / 3.0 /) via Wikimedia Commons, modified by Raquel Parada)

How are compatible haploid nuclei found?

Plasmogamy can be achieved through the production of specialized sex cells or gametes, which can be produced by sex organs called gametangia.

Some fungi put their gametangia in contact, allowing the nuclei of one (the male) to pass to the other (the female), but they do not produce sex cells. Other fungi fuse their gametangia in order to carry out plasmogamy.

The more advanced fungi, on the other hand, do not produce gametangia, but the vegetative hyphae exert sexual functions and fuse, exchanging nuclei.

Nutrition

We mentioned earlier that fungi are heterotrophic organisms. From this it is understood that, unlike plants, these living beings are unable to synthesize their own food from sunlight and carbon dioxide (CO2) present in the atmosphere.

The preferred source of energy and carbon for mushrooms is carbohydrates (although they obtain nitrogen from protein breakdown) and mushrooms are capable of absorbing and metabolizing various soluble carbohydrates, including glucose, xylose, fructose, sucrose, etc.

In addition, other more complex "insoluble" carbohydrates, such as cellulose and hemicellulose, lignin or starch, can also be broken down by the battery of digestive enzymes that fungi can produce.

As heterotrophic organisms, fungi can be classified according to the "way" they have to feed:

- Some are saprophytes or decomposers. Saprotrophy consists of the external digestion of decomposing organic matter (through the secretion of digestive enzymes) and then absorbing it through the “body” formed by the hyphae.

Photograph of the hyphae of a saprophytic fungus among the fallen leaves of a tree (Source: Sten / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

- Other fungi are parasites, so they obtain their food from the tissues of the body of which they are hosts, which can often be detrimental to the health of this.

- Other fungi are symbionts and coexist in the same "organism" with an alga, forming what is known as a lichen. The alga is photosynthetic (autotrophic) and the fungus is heterotrophic, so the pair has a feeding mode that we could consider "mixed".

Photograph of a lichen (source: Norbert Nagel, Mörfelden-Walldorf, Germany / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

- Some form structures called mycorrhizae, which are also part of a symbiotic relationship between the roots of a plant and a fungus. These exchange nutrients with their photosynthetic partner and provide it with certain benefits.

Scheme of the mutualistic relationship of a mycorrhizal fungus and a plant (Source: Nefronus / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons, modified by Raquel Parada)

References

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2. Encyclopaedia Britannica. (2020). Retrieved April 16, 2020, from www.britannica.com/science/fungus
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