Mode of Nutrition of Fungi

 1.5 Fungal nutrition

            We have already discussed that fungi are heterotrophic organisms. Like other green plants and some photosynthetic bacteria, fungi cannot prepare their foods. Fungi are depended on other sources like dead and decaying organic materials to draw their nutrition. Remember again that like animals, fungi are heterotrophic organisms. They use complex organic compounds as a source of carbon, rather than fix carbon dioxide from the atmosphere as do some bacteria and most plants. In addition, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet. They decompose the proteins and other materials to get their nitrogen. Ammonium salts, nitrites, and nitrates are the source of inorganic nitrogen. Fungi also require phosphates, potassium, and magnesium as their inorganic requirements. Like other organisms, vitamins and growth factors are also needed for their growth. While some fungi can synthesize their vitamin, others depend on external supply. The common vitamins required for their growth are thiamine, biotin, pyridoxine, riboflavin, etc.

    However, unlike most animals, which ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order: digestion precedes ingestion. First, exoenzymes are transported out of the hyphae, where they process nutrients in the environment. Then, the smaller molecules produced by this external digestion are absorbed through the large surface area of the mycelium. As with animal cells, the polysaccharide of storage is glycogen rather than the starch found in plants.

    Depending upon the mode of nutrition fungi may be classified into four important groups- i) Saprophytes, ii) Parasites, iii) Symbiotic, and iv) Predacious. Let us see details about these nutritional groups 

    1.5.1 Saprophytic fungi- Have you seen any rotten vegetables, bread, old chapatti, etc. in your kitchen? Have you observed mycelial growth on such rotten organic materials? Let us see what are these! The fungi which live on dead or decaying organic matter of animals and plants' origins are called saprophytes. The majority of fungi are saprophytes. They grow upon dead organic matter such as rotten vegetables, moist wood, moist leather, jams, jellies, plant debris, cow dung, moist bread, and many such materials. Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily-absorbable glucose molecules. The carbon, nitrogen, and other elements are thus released into the environment.

    Good examples of such saprophytes fungi are Mucor, Saprolegnia, Rhizopus, Penicillium, Aspergillus, Agaricus, etc.

        Facultative saprophytes- Some parasitic fungi which usually live in living hosts may adopt to the saprophytic mode of life at times and they are known as facultative saprophytes (Example – Taphrina).

 1.5.2 Parasitic fungi - Do you have any experience of skin disease ever? Or have you seen diseases on rice, potato, wheat plants, etc.? Let us discuss such organisms which cause these!

            The fungi which grow on the tissues of living animal or plant hosts are called parasites. Such fungi absorb their food requirements from the hosts in different ways.

            To absorb the foods from the host body, fungi use mycelia. Mycelia penetrates the host tissue and draw their food. The fungus having the mycelia outside the host is called ectoparasite and the fungus having the mycelia embedded in the host tissues is called endoparasite. In ectoparasites, certain cushion-like appressoria develop on the surface of the host. From each appressorium, a peg-like structure develops which penetrates the host epidermal cells giving rise to branched or unbranched absorbing organs called haustoria.

            Houastoria may also be formed by the mycelia of endoparasites. They vary in shape and may be small rounded button-like or convoluted (Example Peronospora) or highly branched (Example Erysiphae).

           In some fungi (Example Puccinia) mycelia remain confined to the pustules and do not spread into the host tissues. Such fungi are called localized fungi. Again some fungi, mycelia are spread in the whole plant body (Example Ustilago). This type of fungi are called systemic fungus.

           The mycelia that spread in the spaces between the cells are called intercellular mycelia and those that enter the host tissues are called intracellular mycelia. Intercelualr mycelia bear haustoria.    

            Based on the life spent, parasitic fungi are two types

           a) Obligate parasites- They live only on living hosts and they cannot live on dead organic matter. (Example- Puccinia, Peronospora, Melanospora, etc.)

          b) Facultative parasites –Some fungi usually lead a saprophytic mode of life living on dead and decaying organic matter, but under certain conditions, they parasitize suitable hosts. Such fungi are called facultative parasites. (Example- Pythium, Fusarium).

Some parasitic fungi live in the human body and cause skin disease. Ringworm is an example of a parasitic fungal infection (Example- Trichophyton, Microsporum).

1.5.3 Symbiotic fungi

        Do you know the meaning of symbiosis? We human beings are living with mutual help and benefit. Do you imagine that a plant species can also live with the mutual benefits of others? Let us see such a relationship in plants-

            Some fungi live in close associationships with other plants with mutual cooperation and benefit.

This mutualistic association is called symbiosis and the participants are known as symbionts.I think you know about lichen. This lichen is a common and good example to understand the symbiotic association ships of fungi. In this association, two partners, one is fungi and one is algae. They together form an individual plant. Here algal partner provides carbohydrate food prepared through photosynthesis and the fungal partner provides protection and inorganic elements such as water, phosphorous, etc from the substratum to the algal partner. This way they get benefited from each other and make symbiotic associations.

        Mycorrhiza is another example of symbiotic associationships of fungi. Here fungi live in the roots of higher plants.  In this symbiosis, the fungus gets its organic requirements from the plant by absorption and in return provides minerals to the plant. It is found that mycorrhizal fungi help in phosphate solubilization.

Such mycorrhizae are of two types –

i) Ectomycorrhiza, and

ii) Endomycorrhiza

i) Ectomycorrhiza – When the fungus grows on the surfaces of the roots it is called ectomycorrhiza/

ii) Endomycorrhiza – When the fungus grows inside the root tissues or inside the host is called endomycorrhiza. These are also called vesicular–arbuscular mycorrhiza (VAM) due to the presence of large vesicles and arbuscles within the plant cell.  

1.5.4 Predacious fungi

     There are some fungi that have developed mechanisms to capture small animals like eelworms, rotifers, and protozoa. Such fungi are called predacious fungi.

Several species of fungi in the genera Arthrobotrys, Dactylella, and Dactylaria are predacious in nature. When a nematode comes near the fungus, it rapidly utilizes a constrictions ring around the nematode and holds it captive while the hyphae sink haustoria into the body of the victim. Whole eelworms are present, and the hyphae of the fungus produce loops that are stimulated to swell rapidly and close the opening. When an eelworm passes through the loop it rolls against its inner surface.

      It is assumed that the amount of osmotically active material in the ring cells increases greatly as a result of stimulating and causing water to enter the cells increasing their turgor pressure. The ring cells swell rapidly and the ring closes around the eelworm which is thus held tightly in the trap.

      Some predacious fungi also secrete a sticky substance on the surface of their hyphae to which a passing small can stick. Haoutorium-like hyphae then grow into the body of the organism and absorb food ultimately causing the death of the organism.

 Check Your Progress

a. Define nutrition?

b. What are the nutritional methods of fungi?

c. What is the obligate parasite mode of fungal nutrition?

d. What are the symbiotic associationships of fungi?

e. How do fungi draw their nutrition from the host body?