Nutritional Groups of Fungi

According to their mode of nutrition, fungi are divided into saprotrophic, parasitic (biotrophic) and mycorrhizal (mycosymbiotrophic).

Fallen leaves, needles, branches, snags and windthrown trees in forests, parks and even gardens and fields, or valuable timber from trees that grew for decades, sometimes even centuries, left too long unattended, dead herbaceous cover, excrement and small animal organisms – all this is food for saprotrophs.

Saprotrophs, as active sanitation workers of the environment, play an enormous role in nature. It has been calculated that every year from 50 to 100 billion tonnes of organic matter is synthesised in various ecosystems (forests, fields, waters, etc.), most of it of plant origin. In its decomposition, especially of lignin and cellulose, the main role belongs to fungi. In the biogeocoenosis, fungi are decomposers and users of organic and inorganic compounds, and they are food for plants, bacteria, invertebrates and small vertebrates. Saprotrophs are the most widespread group of fungi in nature. They were the first fungi on earth. Many of them are edible.

Parasitic fungi feed on the juices of living organisms (plants, animals). They live in the bodies of plants, animals and even humans. These include rusts, scabs, smuts, powdery mildews, downy mildews, etc. These fungi cause particularly great damage and losses to agriculture. Not infrequently, parasites cover the fertile part of the fruiting body of ceps, Russula and other mushrooms – the hymenophore – with a yellowish, greenish, white or greyish layer (mould). In killing their victim, they not only take its life, but also its chance to disperse spores and leave offspring.

These are cannibal fungi that destroy their own kin. Or perhaps this is nature's design – the self-regulation of fungi. Edible mushrooms attacked by a parasite lose their nutritional value, become unfit for consumption and sometimes become poisonous. Even the nimble house fly cannot escape the snares of these fungi, which envelop its body as if with down and glue it to the window pane. This most often happens in autumn. A similar fate befalls many insect species living in the forest, meadow or farmstead.

Never mind catching a fly or some other insect – even mighty trees, even century-old oaks, cannot withstand an attack by a parasitic fungus. A scratch on the root collar, trunk or a branch, or some other injury, is for the honey fungus an open gateway to the heart of the oak. Who would think that the honey fungus rhizomorphs – cords capable of jumping from one tree to another – hidden under the forest litter, penetrate between the bark and the wood and, slowly but relentlessly, fell the tree more terribly than a woodcutter. Not one or two, but clusters, troops, hundreds of fruiting bodies adorned with yellowish caps arouse no distrust or suspicion; on the contrary, with their edible qualities they awaken sympathy, which is why the fungus is even affectionately called "kelmutis" ("little stump one"), almost "gerutis" ("good little one").

As the years pass, although the oak will put up a fierce resistance, one day it will be defeated and, helped by a gust of wind, will crash to the ground. On it, replacing one another, ever more fungi will settle, and the "benefactor" honey fungus, becoming a saprotroph, will not leave either but will further multiply its troops, until finally, having sucked out everything useful, they will pass the crumbs on to the grass, which, growing lush, will seem to hurry to hide all traces of the tragedy.

Mycorrhizal fungi, unlike saprotrophs or parasites, are quite different: they live in friendly coexistence with plants, especially trees and shrubs, form mycorrhiza (a symbiosis of fungus and plant) and not only do not harm each other, but even help each other and are mutually beneficial.

Using the radioactive isotope method, it has been established that fungi obtain from trees carbohydrates – organic nutrients they need very much – and in exchange, as it were, help the tree to supply itself more easily with water and mineral substances (N, P, Ca, K, Na). Of course, as in all cases of exchange, signs of parasitism have been observed not only on the part of the fungus, but also on the part of the tree. However, this does not overshadow the essence of this mutually beneficial phenomenon.

At present more than 2000 plant species have been recorded that seek close association with fungi, and a considerable proportion maintain even very close and beneficial relationships. Some sources indicate that as many as 80 percent of plants tend to form mycorrhiza.

Mycorrhiza can be of several types: endomycorrhiza (internal), when the mycelium settles in the fine rootlets or root hairs between the cells without changing either the internal or the external morphological features of the rootlet or root hair; and ectomycorrhiza (external), when the mycelium wraps around them from the outside (as if the rootlet or root hair were wearing a frayed glove or a felted sock) but without penetrating inside them, thereby increasing the absorbing surface of the root hairs and fine rootlets of almost all the coniferous and broadleaved trees and some shrubs growing in Lithuania by more than several or even 10 times. Of course, because of the hyphal sheath that forms, the morphological features of the fine rootlets and root hairs change somewhat. Depending on the fungal species, the hyphal sheath formed around the rootlet or root hair has its own distinctive structure, surface pattern (texture) and colour. It is believed that 5–6 thousand fungal species form ectomycorrhiza, and it is of particularly great importance to trees and to all natural forest ecosystems in the temperate zone.

Micromycetes are the allies of herbaceous plants (mostly through endomycorrhiza), while macromycetes are the allies of woody plants. This is a very interesting phenomenon in the life of fungi and plants. English scientists noticed that when the leaves of a birch were stripped, mushrooms stopped growing under it, but when the birch leafed out again, carbohydrate production in the tree resumed and the mushrooms began to grow again.

Later still they established that fungi need not only carbohydrates obtained through the roots, but also other growth-promoting substances. It has also been observed that tree roots, while absorbing nutrients, release their own phytopathogenic "agents" into the rhizosphere that are harmful to soil microorganisms and sometimes even to another tree growing nearby. Or perhaps this is how mycorrhizal fungi serve their host – by destroying unwanted organisms. It has been established that mycorrhizal fungi do not leave their benefactor host from early spring to late autumn, even for many years in a row. Depending on the age of the tree and the composition of the soil, the fungal species gradually change too. Which of them – the tree or the fungus – is more faithful and more useful to the other is for now known only to themselves.

An observant mushroom picker knows that some mushrooms grow only under certain trees, others only under other trees, or that the same mushrooms grow in both coniferous and broadleaved forests. It has also been observed that at the base of the stem of a mycorrhizal mushroom (for example, a cep) there are fewer loose mycelial hyphae than in a saprotrophic one – some kind of Clitocybe, which is sometimes not even easy to pull out.

Mycorrhizal fungi are very faithful to their host plant. When it dies, the fungus withdraws too, most often giving way to saprotrophs. There are more forms of mycorrhiza. This phenomenon has recently been studied intensively; elucidating it as fully as possible is very important from both a theoretical and a practical point of view, especially for foresters planting new forest areas. It is gratifying that ectomycorrhiza is also being studied in Lithuania – at the Institute of Botany.

It has been noticed that the mode of nutrition of many fungi is not strictly defined. For example, the variegated Tricholoma, when growing in a broadleaved forest, lives as a saprotroph, and in a coniferous one as a mycorrhizal fungus. The oyster mushroom and the velvet shank cover stumps, branches and rotting trunks in clusters, but given the chance, like the common honey fungus, they readily attack living trees, for example the soft, juicy wood of limes and alders. Or the "good little" common honey fungus in forests, where there is a wider choice, more often makes use of its saprotrophic properties, whereas in parks, and especially in gardens, on the contrary, it more often reveals its true parasitic way of life. It is believed that saprotrophic, biotrophic and mycorrhizal fungi inherited this way of life from their ancestors.