The Fungi

As we saw in the introduction to this section on microbial diversity, fungi were for many years classified along with bacteria, algae and the slime moulds (Chapter 9) as members of the Kingdom Plantae. As recently as the 1960s it was possible to find fungi being discussed under this heading, but in more recent times there has been universal agreement that they should be assigned their own kingdom. This is because fungi differ from plants in two quite fundamental respects:

• plants obtain energy from the sun, fungi do not

• plants utilise CO2 as a carbon source, fungi do not.

To use the terminology we introduced in Chapter 4, plants are photoautotrophs, whereas fungi are chemoheterotrophs. In fact it now seems on the basis of molecular evidence that fungi are more closely related to animals than they are to plants!

We may define true fungi as primarily terrestrial, spore-bearing organisms, lacking chlorophyll and having a heterotrophic, absorptive mode of nutrition. Some 80 000 species are known and it is thought possible that at least a million more remain to be described! True fungi are a monophyletic group; that is, they are all thought to descend from a common ancestor, some 550 million years ago.

Fungi are of great importance economically and socially, and may have beneficial or detrimental effects. Many fungi, particularly yeasts, are involved in industrial fermentation processes (Chapter 17). These include, for example, the production of bread and alcohol, while other fungi are essential to the cheese-making process. Many antibiotics, including penicillin, derive from fungi, as does the immunosuppressive drug cyclosporin. Along with bacteria, fungi are responsible for the decomposition and reprocessing of vast amounts of complex organic matter; some of this is recycled to the atmosphere as CO2j while much is rendered into a form that can be utilised by other organisms (Chapter 16). The other side of this coin is seen in the activity of fungi that degrade and destroy materials of economic importance such as wood, paper and leather, employing essentially the same biochemical processes. Additionally, some fungi may cause disease; huge damage is caused to crops and other commercially valuable plants, while a number of human diseases, particularly of the skin and scalp, are also caused by fungi.

(b)

Septate

Coenocytic

Figure 8.1 Hyphae and mycelia. (a) Individual hyphae branch and aggregate to form a mycelium. (b) Hyphae may or may not contain cross-walls (septa)

Septate

Coenocytic

Figure 8.1 Hyphae and mycelia. (a) Individual hyphae branch and aggregate to form a mycelium. (b) Hyphae may or may not contain cross-walls (septa)

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