Fungi are probably familiar to us all – the mushrooms and toadstools we see “sprouting” in the Spring and Autumn in our fields and woodlands. Although traditionally they are studied alongside bacteria as microorganisms, they are a very distinct form of life and occupy a separate kingdom away from plants, animals and prokaryotes. As a group they are extremely diverse with over 35,000 species and have a complex biology and genetics (Webster, 1985).
The mushroom or toadstool (there’s no real scientific distinction between the two) is a fruiting body which allows dispersal of spores but represents only a fraction of the whole organism. Most of the organism lives in the soil or substrate as thin filaments or “hyphae” which together forms the “mycelium”. Hyphae are typically 2-10µm in diameter and larger than bacteria. Other species do not form hyphae and exist as single cells (the yeasts) which tend to be substantially larger than bacterial cells.
The fungus grows by elongating and branching the hyphae from the tip and so is able to penetrate into the soil and dead and living plant tissues having a battery of digestive enzymes, allow the fungus to colonise wood and other plant materials. In this way the fungus is able to cover large areas. One square metre of soil can contain several kilometres of hyphae from a single organism. As the hyphae are able to move in three dimensions all the space within the soil can be potentially explored.
The fruiting body is a means of reproduction and is one of a number of hyphal aggregations. Others Other such as mycelial strands (e.g. the dry rot fungus Serpula lacrimans), sclerotia (e.g. the plant pathogen Claviceps purpurea) or rhizomorphs (e.g. the parasitic honey fungus, Armillaria melle), allow penetration into a dense substrate or act as survival structures with some food reserves.
There are five main groups of fungi which are differentiated on the bass of their reproduction and internal structure. The Zoosporic fungi are mostly found in water and as parasites of algae and insects with some species saprophytes in soil. Some can cause disease in plants such as damping-off in seedlings (Pythium). Zygomycotina reproduce asexually by non-motile spores which are carried on the wind or by animal vectors. Some are found in soil, dung and on fresh organic matter, whilst others are parasites of insects and arthropods, plants and on humans. Ascomycetes are probably the largest group of fungi and occur in a variety of habitats including; soil, dung, water, as saprophytes of plants & animals as well as plant and animal pathogens. Their hyphae are divided by a perforated septum and produce sexually produced spores in a sac or ascus (usually 8 spores) which are explosively ejected from the ascus. The basidiomycetes are perhaps the mostly saprophytes living on wood & dung but some cause significant damage to wood such dry rot whilst others are parasites of woody and herbaceous plants. The last group, the fungi imperfecti, have no known sexual reproductive stage. Members include Penicillium and Fusarium. They can be plant pathogens, found in soil or in water and some are can attack nematodes.
As fungi can tolerate more acidic and drier conditions than bacteria, they can dominate different environments such as forests where the soil can be acidic which generally do not favour bacteria and are able to recycle much of the woody tissue. Doing so, may suggest that many of the roles played by bacteria in an agricultural soil are being done by fungi. However, fungi have a greater need for oxygen and therefore will be out competed by bacteria in waterlogged environments.
In habitats where there is low level of nutrients, many species of form very close associations (mycorrhiza) with plants which can be beneficial to both fungus and plant. The plant receiving nutrients and protection against disease, which would otherwise be unobtainable without the association, and the fungus obtaining some nutrient from the plant.
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