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Fungi as Decomposers

Features of Fungi-Decomposers

Zygomycetes (Mucorales) as Decomposers

Basidiomycetes as Decomposers

Wood Decomposition

 


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Fungi as Decomposers

Some fungi the parasitic and mutualistic symbionts, obtain their nutrients from living organisms. Others, however, are saprotrophs, and obtain their nutrients from dead organisms. These latter fungi are the main group of organisms responsible for recycling the components of dead plants; the decomposition of the remains of aniamls and microorganisms (including fungi and bacteria) is accomplished mainly by bacteria.

This activity of fungi is essential for the continuation of life on earth. The carbon cycle involves the fixation of atmospheric carbon dioxide into organic molecules by photosynthesis; the fungi play an important part in degrading these molecules and thereby replenishing the carbon dioxide of the atmosphere.

Without degradative processes, life on earth would probably come to an end after a few decades because of the accumulation of plant remains and lack of free atmospheric carbon dioxide for photosynthesis. The degradation of plant remains is also important in the cycling of other elements, particularly nitrogen, phosphorus and potassium, which are incorporated into insoluble plant components such as cell walls. Microbes absorb these as plant material decays and release them in inorganic form into the soil whence they can used again by the plants.

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Features of Fungi-Decomposers

Fungi are better equipped for bringing about the decay of insoluble plant remains than are bacteria, both through their physical form and mode of growth and their enzyme capabilities and metabolism.

Their growth form enables them to penetrate plant tissues. The hyphae of of an appropriate diameter (5-20 Ám) to grow inside plant cells and, particularly those specialized to decay wood, inside the tubular cells that are a major component of wood. Apical growth from a mycelium that is firmly attached to a substratum enables the hyphae to grow into tissues, generating physical pressure which together with local enzymic lysis can perforate woody cell walls. The mycelium of those fungi, mainly Ascomycetes and Basidiomycetes, that are able to degradde refractory plant remains is itself often long-lived and capable of cellular differentiation and the development of multihyphal structures such as mycelial strands and rhizomorphs.

Just as species differ in their morphology, so also their enzymic equipment is characteristic of particular species and groups. Although all are heterotrophic at least for carbon, they vary in their capacity for using particular substrates. Extracellular hydrolyases are produced in wide variety, and almost any naturally-occurring macromolecules can be degraded by some fungi.

The soluble products of extracellular enzyme action, and other small molecules, may enter hyphae by diffusion replacing those utilized by metabolism, but more usually they are actively transported across the cell membrane by specific active transport systems. It is characteristic of the fungi that these transport systems have a very high affinity for their transported substances, permitting rapid uptake. This also faciliates scavenging activities by fungi under starvation conditions, since nutrients can be concentrated in the mycelium from very low external concentrations.

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Zygomycetes (Mucorales) as Decomposers

The only truly saprotrophic Zygomycetes are in the Mucorales. These include fungi which seem to be specialized for growth in habitats such as dungs and ripe fruit which provide soluble nutrients. Most Mucorales can break down starch. Extracellular proteases and lipases as well as amylases are often produced. Only a few species are cellulytic or chitinlytic. They depend on the chance appearance of suitable substrates, which are soon exhausted.

Many Mucorales are found acting as both primary and secondary saprotrophic sugar fungi. Primary saprotrophic sugar fungi are those species which act as pioneer colonizers of sugary resources; this way of life is characterized by rapid growth, spoulation and spore germination which enable the fungus to avoid competition by arriving on such resources before potential competitors. Rhizopus stolonifer is a common mould making a primary attack on over-ripe fruit.

 

Secondary saprotrophic sugar fungi are also confined to soluble substrates but obtain these by growing close to other fungi which release them. Mucor hiemalis may be found on rotting woodapparently deriving nutrients at second hand from the activities of wood rotting Basidiomycetes.

Because of their coenocytic, aseptate mycelium, the peripheral growth zones that have been measured for Zygomycetes are generally larger than those found in septate fungi. This may be why these fungi are capable of rapid extension over a substratum, often of the order of 20 mm per day, compared with many higher fungi extending at about 1 mm per day.

The ability of Mucorales to break down starch, fats and proteins ,and their capacity for limited fermentative metabolism, is used in various oriental food fermentations. One example is the Indonesian food tempeh, made by inoculating cooked soybeans with spores of Rhizopus obligosporus and incubating it under near-anaerobic conditions for 24 hours.

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Basidiomycetes as Decomposers

Morphologically and biochemically, the saprotrophic members of the Basidiomycetes are well adapted to living on cellulose and lignocellulose. Their vegetative mycelium is long-lived. Asexual sporulation is much less common than in Ascomycetes, but very great numbers of sexually generated basidiospores are produced once a fruit body has been formed.

Basidiomycetes are usually among the last fungi to colonize a fresh substrate such as timber; for example the succession of fungi isolated from wooden posts sunk in soil begins with soft rot Ascomycetes and the penetration of the wood by the mycelium of Basidiomycetes only occurs some months later.

A Basidiomycete colony in nature can become a large entity measured in metres, with considerable diversification of function. Such elaboration would not be possible without continuity of food supply to maintain growth. Lignocellulose is a long-lasting substance and one which these fungi may colonize with little competiton since they are able to degrade lignin. Together with certain insects, mainly beetles, ants and termites, the fungi are responsible for the removal of woody remains from ecosystems.

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