Fungi are eukaryotic organisms, generally with a cell wall. Most of them have a filamentous and multicellular thallus (a body made up of cells that do not differentiate into separate and distinct tissues and organs). The filaments, called hyphae, form the mycelium. In the most evolved fungi (ascomycetes and basidiomycetes), hyphae can aggregate to produce specialized structures for the dispersion of reproductive propagules (fruiting bodies). A limited number of fungi occurs as single cells that reproduce by budding or fission (yeasts).

Reproduction in fungi can occur through two different ways: asexual reproduction and sexual. The first one is important for the spreading of the species, the second one guarantees genetic variability and leads to the differentiation of resistance spores. For many fungi both modes are known (sometimes simultaneously present); for others sexual reproduction is not known (mitosporic fungi or deuteromycetes) and, in some cases, not even asexual reproduction is known (sterile mycelia).

The asexual reproduction can simply occur by vegetative propagation (fragmentation of the mycelium) or through the production of spores very variable in shape and size. Reproduction leads to the formation of haploid meiospores, characteristics of each phylum.

All fungi are heterotrophic, feeding by absorption thanks to the action of extracellular enzymes. Enzymes break down carbohydrates, lipids and proteins, which are then absorbed through the walls of the hyphae. They obtain the organic substances necessary for their growth through saprotrophism, parasitism and mutualistic symbiosis.

The largest organism on our planet is a fungus, belonging to the species Armillaria solidipes, which in Oregon (USA) covers an area of more than 2400 acres (equivalent to 1665 soccer fields), with an estimated age of about 2200 years.

Systematics

Fungi belong to the Eukaryota domain and to the Opisthokonta group, which includes also animals. Indeed, contrary to what most people believe, fungi, sharing a common ancestor with animals, are closer to animals than to plants.

Fungi play crucial roles for the survival of our planet and in recent years, we have learned to use them in numerous biotechnological applications. Changes in the composition and functioning of fungal communities can greatly influence the diversity, health and productivity of different natural environments.

Saprotrophic fungi, together with bacteria, are involved in the decomposition process of organic substances and in the cycle of elements, both in natural environments, and in the agrarian ones, where they are the main mineralizing organisms. They are able to degrade almost all organic compounds, even the most recalcitrant natural polymers such as lignin, chitin and keratin.

The same enzymes that fungi use to degrade lignin can be used to remove most of the pollutants in the environment using bioremediation techniques: fungi and their enzymes are studied for the removal of persistent pollutants such as oil and derivatives, PCBs , IPA, synthetic dyes, etc. Since environmental pollution is one of the most current emergencies, the use of fungi in bioremediation will become increasingly important to improve the quality of water and soils.

On the other hand, such a strong degradative capability against a wide variety of substrates makes fungi among the most important agents of bio-deterioration for manufactured goods (textiles, leather goods, wooden objects, paper, photographic films, etc.) and for all foodstuffs intended for human or animal consumption. In this regard, it is worth mentioning the ability of numerous fungal species to produce very toxic substances, known as mycotoxins, which represent one of the most relevant and worrying aspects of food contamination.

Fungi are fundamental for the balance of ecosystems because they are able to create a vast network of mutualist or pathosist interactions with plants, animals and bacteria. Of great importance for our planet are the mutualist symbiosis between fungi and photosynthetic organisms: lichens and mycorrhizae.

Lichens are associations between a fungus and a photosynthetic partner, a green alga or more rarely a cyanobacterium. About 20,000 species of lichens are known. They are pioneer organisms able to colonize extreme environments inaccessible to the majority of living beings. They are important as a food source for animals. For example they represent up to 95% of reindeer feeding and, according to scholars, the manna mentioned in the Bible (Exodus 16, v 13-15) was a lichen.

Lichens, besides being important bioindicators (especially of air quality), are studied for their ability to synthesize secondary metabolites useful in different industrial sectors, first of all the cosmetic industry.

Fungi growing in close association with the roots of both herbaceous and arboreal plants form the so-called mycorrhizae, essential for the evolution of life on our planet. These symbioses, indeed, allowed the colonization of lands by plants and are still indispensable because they favour plant nutrition, in particular the absorption of phosphorus and nitrogen, they protect from the presence of toxic compounds (e.g. heavy metals), they make the plant more resistant to stresses (especially drought) and often provide a useful barrier against pathogens of the root system. Mycorrhizae also create networks between plants, making plant communities more resilient to stresses.

To date, about 6000 fungal species are known to form mycorrhizae with more than 240,000 plant species (95% of the total).

Concerning the fungi-animals interaction, many fungi are used as food by various animals (from ruminants to microarthropods).

About 80% of the thousands of species of microarthropods that live in forests are fungivorous, i.e. they feed mainly or exclusively on fungi. Many specific symbioses between fungi and animals are also known: the leafcutter ants cultivate the mycelium of some fungi (Leucocoprinus and Leucoagaricus) inside the nest creating the so-called "ants gardens", to produce food resources.

A particularly important symbiosis the one established in the ruminant rumen involveing some Chytridiomycota, obligate anerobic fungi that represent the first degraders of the ingested plant material. Fungi are therefore essential to the nutrition of ruminants.

A particular group of fungi (about 150 species) evolved a system for predating soil nematodes through real traps. Since nematodes are important plant pathogens, these fungi can be used in biological control.

Fungi are the main (70-80%) pathogens of both herbaceous and arboreal plants causing considerable damage to the world economy with crop losses that can reach 20-50%.

Moreover, fungi make unusable a significant portion of the foodstuffs for human and / or animal consumption due to the production of mycotoxins (e.g. aflatoxin, ochratoxin, etc.), very toxic and often carcinogenic metabolites, active even at very low concentrations and heat-resistant.

Plant parasitic fungi can have negative effects on the landscape and the environment, can affect the tourist-recreational function of forest formations and can reduce the level of safety in urban or anthropized contexts, significantly increasing the risk of tree crashes.

Some fungi characterized by a high lignivorous capacity (e.g. Serpula lacrimas) can determine structural problems on beams.

On the other hand, a rather low number of fungi is able to attack humans and other warm-blooded animals. Most of them are opportunistic pathogens, who live as saprotrophs in nature, but can cause infections, especially in immunosuppressed individuals, following inhalation of spores or penetration through wounds. In human clinics, fungal infections are an increasingly important problem due to the continuous increase in chronic immunodeficiency patients for whom fungi are the leading cause of death (about 80% of AIDS patients die from pneumonia caused by Pneumocystis carinii).

Some fungi (Tinea capitis, T. corporis, T. pedis, etc.) may cause in humans and in animals, such as dogs and cats, infections to the skin, hair and nails (onychomycosis).

Airborne fungi (mainly in the form of various types of propagules) can cause serious allergies and respiratory diseases (rhinitis, asthma) or produce toxic metabolites resulting in different symptoms (e.g. sick building syndrome).

A very limited number of macromycetes, if ingested, can cause different types of poisoning and, in some cases, even lead to death (e.g .Amanita phalloides).

Not to be forgotten that the most important pandemics of the animal world in recent years have been caused by fungi: the white nose syndrome caused the death of millions of bats, especially in the American continent; the chitridiomycosis of amphibians is, instead, implicated in the global decline of amphibians.

Fungi definitely dominate the scene of biotechnologies and their use is expected to grow exponentially.

The peculiar lifestyle of fungi is based on the secretion of enzymes to digest the nutrients outside; the same enzymes are nowadays exploited in many biotechnological sectors to provide products of common use.

In addition to the food industry, in which selected strains have been used for a very long time to give particular aromas to cheeses and sausages, fungi contribute to our daily life through the production of primary metabolites such as fatty acids (above all polyunsaturated fatty acids such as arachidonic acid, used to lower cholesterol and triglyceride levels in the blood, but also as a supplement for newborns, as it promotes brain and retinal development), organic acids (first of all citric acid), secondary metabolites of pharmaceutical interest (e.g. antibiotics) and enzymes (e.g. cellulases, amylases, lipases, proteases).

Fungi have many biotechnological applications that affect us in our daily lives. Think that alcoholic fermentation, used for the production of alcoholic beverage and to produce a biofuel such as bioethanol, is carried out by yeasts. The type of yeast used and the raw materials give rise to an infinite variety of products.

Another very important molecule produced by fungi is citric acid, one of the most used additives in the food and pharmaceutical field as pH buffer acidulant, preservative. The production of citric acid by fermentation (previously extracted from lemons), is one of the oldest and most consolidated industrial productions: it began in 1919 in Belgium and then in 1923 in the USA by Pfizer.

One of the main biotechnological applications of fungi consists in their use as biological control agents in order to contrast pests or more generally plant pathogens. Fungi can be active against plant pathogens based on one or more of the following action mechanisms: i) competing for the niche or for nutrient substrate; ii) synthesizing active antibiotics; iii) parasitizing them. Several biological products that can prevent the infection by plant pathogens are currently available. Some products are even capable of treating a plant after infection, as in the case of healing chestnut cancer based on the phenomenon of exclusive hypovirulence.

Other examples of biotechnological applications of fungi:

Textile and tanning industry
Fungal cellulases are used in the treatment of “stone-washed” denim and in the “biofinishing” of fabrics (cellulases and proteases) to remove non-aggregated fibers or “bioscouring” to remove lignin residues. Fungal enzymes are applied in the finishing and in the functionalization of both natural and synthetic fiber fabrics.

Detergent industry
Almost all detergents are supplemented with fungal enzymes (proteases, lipases, oxidative enzymes) that favour the removal of stains even at low temperatures or in alkaline conditions.

Starch hydrolysis
Fungal amylases are used for the production of high fructose syrups used in the confectionery industry.

Cheese production
Fungal proteases have been used for many years in the dairy industry as an alternative to bovine chymosin. The use of bovine chymosin produced through heterologous expression in yeasts and filamentous fungi is increasingly widespread today. Moreover, in all the blue cheeses (gorgonzola, stilton, roquefort) some mushrooms (Penicillium roquefortii) are inoculated in the cheese paste to produce enzymes (proteases and lipases) that act on the texture of the cheese and on the organoleptic properties. For some cheeses (brie, camembert) fungi (Penicillium camembertii) are, instead, inoculated on the surface.

Production of salami
Specific fungi (Penicillium nalgiovense) are sprayed on the salami surface to promote maturation and improve organoleptic properties, through the production of proteases and lipases.

Production of fermented foods
Many Asian fermented foods such as soy sauce and tempeh, miso, ang-kak, are produced through the use of filamentous fungi.

Beer production
Amylases, glucanases and proteases are used to improve barley extraction during the "mashing" process before fermentation.

Food industry
Maceration enzymes (mixture of pectinases, cellulases and hemicellulases) are used to favour the extraction phase during the pressing of fruits and olives for the production of oil, fruit juices, etc. Pectinases are also widely used in the clarification of wine and fruit juices to remove pulp and peel residues and to modify the organoleptic qualities of the product.

Bakery products
Xylanases are used to increase the volume of the dough and to promote leavening.

Pharmaceutical industry
Different enzymes of fungal origin can be used to modify sterols and other precursors during the preparation of semisynthetic drugs, for example 6-amino-penicillanic acid (6-APA), a semisynthetic precursor of penicillin. Other products of pharmaceutical interest are cyclosporine (immunosuppressants), statins (hypocholesterolemics), alkaloids (used to achieve vasodilation, lowering of blood pressure and to induce contractions of smooth muscles), steroids (anti-inflammatories, immunosuppressive, diuretic, contraceptive, etc.), antitumor and immunomodulators.

Fungi are a valuable food not only for many animals, but also for us! Some of the finest mushroom can be harvested only in the field while others (champignon, pleurotus, black truffle, etc.) can be easily grown.

Mushrooms have many nutritional properties (they have a high protein and fibres content and do not contain cholesterol!). Often they also contain pharmacologically active substances that have always been used in traditional oriental medicine (medical mushrooms).