The kingdom of fungi (Fungi) comprises about 150,000 described species, but it is estimated that there are between 1.5 and 3 million species on our planet. The fungi are eukaryotic organisms, i.e. their cells contain a well-defined cell nucleus and in most cases also mitochondria. Fungi are chemoorganoheterotrophic, as are animals. I.e. they use organic carbon compounds both as an energy source and as a carbon source. A unique property of fungi is that the cell wall consists of a mixture of glucans and chitin. Glucans are also found in plants and chitin is the exoskeleton of arthropods, but no other organisms than fungi have a cell wall, which consists of a mixture of these substances.
Like bacteria and other living organisms, fungi are hierarchically divided into different taxonomic categories with increasing inclusion ---> genus ---> family ---> order ---> class ---> phylum ---> domain (or kingdom ). For fungi and also certain bacteria there are subcategories, e.g. subkingdom, which then ends up between kingdom and phylum, as well as subphylum, subclass and so on.
The classification of fungi has been changed several times as more knowledge becomes available and is likely to be revised further in the near future. The fungi are for the moment divided into 8 different phyla: Ascomycota (sac fungi), Basidiomycota (club fungi), Blastocladiomycota (blastoclads), Chytridiomycota (chytrids), Glomeromycota (arbuscular mycorrhizal fungi), Microsporidiomycota, Neocallimastigomycota and Zygomycota (conjugating fungi). The latter phylum will probably be divided into two different phyla: the Mucoromycota and Zoopagomycota. The phylum Microsporidiomycota may not even belong to the kingdom of Fungi.
The figure shows a simplified picture of fungal reproduction by spores, which can be asexual or sexual. Plasmogamy involves fusion of cytoplasm from two cells and karyogami involves fusion of two cell nuclei in the same cell. - Click on the image to enlarge it.
Fungi multiply by budding or by spores and the spores can be sexual or asexual. The spores thus generally do not have the same function as spores in spore-forming bacteria. Sexual reproduction involves two special stages, plasmogamy and karyogamy. In plasmogamy, the cytoplasm fuses from two different hyphae, which may come from two different fungal individuals (heterotallic fungi) or from the same fungal individual (homotallic fungi). These hyphae will then contain two cell nuclei, which then fuse together through karyogamy and transition to a diploid stage occur. After meiosis, haploid spores can then be produced.
One reason why there is such great diversity among fungi is that they have managed to adapt to most environments, which occur on our planet. Humans benefit greatly from fungi thanks to:
- Some are considered delicious.
- They can be used for flavor enhancement (e.g. Penicillium roquerorti).
- Production of various substances (e.g. citric acid and lactic acid).
- Production of enzymes (e.g. lipases, amylases and invertases).
- Production of antibiotics and antifungals.
- Fungi help certain plants (e.g. trees) to absorb inorganic molecules from the soil by a process termed mycorrhizal symbiosis.
- They are very important decomposers of organic material in nature and in gardens.
However, mushrooms are not only good because:
- They can cause diseases in animals, plants and humans.
- They can break down organic material in the form of e.g.timber.
- They can destroy food and produce mycotoxins.