Fungi

The fungal kingdom includes all the organisms that are commonly known as fungi (a large group of multi-cellular organisms that also includes edible mushrooms), yeast (unicellular organisms responsible for fermentation) and mildews (organisms of microscopic size, such as the green-coloured Penicillium). This kingdom is composed of heterotrophic organisms, because they feed themselves by absorbing organic matter from the environment. Fungi produce molecules, the so-called digestive enzymes, which are used to degrade the organic matter so that it can be absorbed. Fungi are mainly multi-cellular organisms composed of masses of threads and living virtually anywhere. A group of fungi, commonly known as yeast, includes unicellular organisms, such as, for instance, Saccaromyces cerevisiae. Some fungi are parasites of animals, plants, protista and even of other fungi, while others are saprobic (they live by decomposing dead organic matter) and are essential for the degradation of the matter. Some species have developed complex symbioses with photosynthetic organisms, as in mycorrhizas or lichens.

The reproduction

Fungi consist of one division only, the Eumycota, which is divided into four classes: zygomycetes, ascomycetes, basidiomycetes (which includes most edible mushrooms) and deuteromycetes. They are divided by their cellular features and sexual reproduction methods, which occur through the recombination of genetic material. Most fungi reproduce alternately through asexual and sexed methods.

Classification

At present, fungi are classed into four groups, including: zygomycetes, ascomycetes, basidiomycetes and deuteromycetes. Zygomycetes: fungi living in the soil or on decomposing animal or vegetal organic material. Their reproduction is asexual and occurs by scattering spores, which are produced in special structures (black bulb-type sporangia). If the spores are formed in a different type of sporangium (zygosporangium), a sexed cycle occurs. This group includes the “black mildew” which develops on fruits, vegetables and many baked products…

Why are not plants?

Fungi come in a wide variety of forms and life cycles. As a consequence, it is very hard to classify them. In the past, fungi were classed as plants and only after in-depth studies the features that distinguished them from both vegetal organisms and all the other eukaryote organisms were discovered, so that they could be considered as belonging to a completely different kingdom.

How many species?

Little is known about the first fungi to have appeared on Earth; these organisms must have derived from protista. These originated the first fungi, whose fossil finds date back to approximately 400 million years ago. At present, about 100,000 species of fungi are known, and it is estimated that at least 200,000 are still to be identified. The evolutionary links between the main classes of zygomycetes, ascomycetes and basidiomycetes are not clear yet, while the class deuteromycetes includes all the species in which no sexed reproduction cycle has ever been observed.

The mycelium

Yeast and some fungi are unicellular organisms, but most species are multi-cellular organisms, composed of masses of filaments known as hyphae, which altogether compose the mycelium or body of the fungus. This structure differs according to the function that the organism has to serve. Saprobic fungi have special hyphae, called rhizoids, which anchor the mycelium to the substrate, while some parasitic fungi develop hyphae, called haustoria, which invade the cells of the host organism.

Distribution in the environment

Fungi are spread anywhere and can live on their own or as parasites of other eukaryote organisms. Some species can tolerate adverse conditions, such as, for instance, extreme temperatures: some live even at 5-6 degrees below zero, others at temperatures above 50 degrees. Mildews are microscopic fungi that look like a white, grey, green or black dusty coating. They are usually visible on such foods as bread, jam and fruits in the form of a blue or green coating.

Saprobic fungi

Fungi and bacteria are the main decomposers of organic matter; the activity of these organisms is as essential for the endless functioning of the earth’s ecosystems as that of the producers of nutrients. As they feed on dead organic matter, saprophytic fungi decompose it into simple molecules that go back into the soil and can be reused by plants and all other organisms. Their ability to demolish organic matter has been used by man, in particular to destroy foods and clothes (cotton and leather items).

Symbiont fungi

Many fungi are involved in close and long associations known as symbiotic associations, which are mutually beneficial to both organisms. Two of these associations, lichens and mycorrhizas, have enabled some photosynthetic organisms to colonise deserted environments. Lichens are the combination of a fungus with a green alga or cyanobacteria; they are the first colonisers of bare rocky areas, they survive even when dry, they need light, air and mineral salts…

The parasitic species transmit diseases and form relations with other organisms, in which the fungus benefits from the association and damages the host organism (human beings and plants). Such fungi as Endothia parasitica, Ceratocystis ulmi, Puccinia sparganioides, Puccinia graminis are parasites of plants, while fungi of the genus Aspergillus or Candida albicans carry infections to the human organisms.

Plant parasites. Devastating effects have been observed in the forests due to the spread of two different diseases: the “powdery mildew”, that affects chestnut trees (Endothia parasitica), and the “dry disease”, that affects elms (Ceratocystis ulmi). Not to mention the damages caused by such fungi as rusts and smuts to the harvests of the whole world. Some rusts, such as Puccinia sparganioides, need several host plants to complete their life cycle: in temperate climates, they spend the winter on the spartina, while in spring they produce small spores that the wind carries away to make them sprout on the new leaves of the nearby ashes. The “boil smut” damages the crops, even if Mexicans consider it a delicacy.

Human parasites. About one hundred of the thousands of known species of fungi are pathogenic to man, causing infections that are called mycoses. A large part of mycoses is transmitted through inhalation, ingestion or the infection of skin wounds and attack the skin, hairs, nails and mucosa. When inhaled, the spores of the genus Aspergillus transmit a serious lung infection, Aspergillosis, while the mycoses of the skin, hairs and nails are called tineas. The mouth, the digestive and the reproductive systems can be infected by the genus Candida albicans; in new-born children, this infection is called thrush and causes white patches on the mouth mucosa.

Pharmaceutical uses

From a pharmaceutical point of view, mushrooms are extremely interesting. Fungi have recently helped to produce other innovative and important drugs, such as cyclosporin, an anti-rejection substance that has aided the development of organ-transplant surgery over the last few years. Penicillin, the first antibiotic ever, is made from a fungus of the genus Penicillium, a green mildew, belonging to deuteromycetes.

Let’s discover Penicillin

In the early 20th century, the micro-organisms that cause the most important infectious diseases in man had already been discovered. Up to that time, the control of the spread of such diseases relied on prevention, in the form of vaccines (against smallpox, rabies and anthrax) and disinfectants. Chemicals, such as antibiotics, that could kill the micro-organisms without damaging the infected individual, began to be discovered and used.

Lichens

Lichens are ecologically important because they are sensitive to the atmospheric pollutants, and this is why they are called bio-indicators. The presence of some substances, such as sulphur dioxide, in the air produces changes in the growth of the lichen. The quality of the environment can be assessed by studying the growth of these organisms. Lichens are organisms that colonise an environment that has just settled.