Mould Ecology

Fungi

All fungi are initially microscopic in size with most spores ranging in size from 2 to 20 µm (Gravesen et al., 1994). There is an enormous number of fungi with over 6000 genera (Gravesen et al., 1994), 69,000 species described (Levetin, 1995) and an estimated total of around 1.5 million species (Hawksworth, 1991), many of which are still not described (Exeter & Stirling, 1995). Many of the visible manifestations of fungi are commonly known and include yeasts, mould growth, mildew, large mushrooms, puffballs and bracket fungi and are all the result of the convergence of millions of individual fungal units coming together to create larger structures (Levetin, 1995).

Fungi are eukaryotic organisms and share many basic characteristics of genetic makeup with humans, but belong to a kingdom that is distinct from plants and animals (Levetin, 1995; Deacon, 1997) as they differ from all others in their behaviour and cellular organisation (Deacon, 1997). As a result of this, the study of fungi (mycology) sits uneasily between microbiology and botany. Many of the techniques to study the prokaryotic bacteria are applicable to eukaryotic fungi but differentiating between their reproductive structures is more similar to those studied in the green plant world (Jennings & Lysek, 1999).

Growth

Fungi are diverse in structure and vegetative organisation but have three major growth forms, which are mycelial (with a network of hyphae and referred to as moulds); as single rounded cells or dichotomously branched chains of cells attached to a food source by tapering rhizoids; or as unicellular yeasts, which produce daughter cells either by budding or by binary fission (Dix & Webster, 1995; Deacon, 1997). Some fungi are dimorphic (with two shapes) and can alternate between a yeast-type growth phase and a mycelial growth phase (Deacon, 1997). Most fungi are characteristically mycelial heterotrophs with absorptive nutrition and reproduction by spores (Flannigan, 1994b; Deacon, 1997).

The fungal filament, the hypha, is the key unit of most fungi and they normally aggregate into a fungal mat or mycelium (Jennings & Lysek, 1999), which becomes visible and is commonly known as mould. Hyphae have no cross walls that divide them into elements (apart from the porous septa) and this continuity of their protoplasm makes them unique among walled cells (Jennings & Lysek, 1999). Hyphae are able to transport nutrients across great distances from rich sources to sites where the same nutrients and water are in short supply. This allows the hyphae to continue to extend to new nutrient sources and/or to differentiate into reproductive structures (Jennings & Lysek, 1999).

The filamentous fungi (those with hyphal growth) are close to the perfect organism. They can digest organic substances regardless of whether they are solid, liquid or gaseous; from living or dead organisms; and can do it with a high efficiency and virtually without temporal and spatial limits. In addition, they have a great ability to quickly adapt to new substances and use them as nutrients (Jennings & Lysek, 1999). Furthermore, fungi have a high degree of environmental adaptability due to the three normal mechanisms of hybridization, genetic mutation and environmentally induced non genetic variation, and an additional two mechanisms peculiar to fungi, which are parasexual recombination and the labile system provided by heterokaryosis (Park, 1968). Because of these abilities, the most limiting factors to growth for most fungi are temperature and water availability (Levetin, 1995).

Nutrients

To obtain nutrients for energy and cellular synthesis, fungi produce extra-cellular enzymes (exo enzymes) that can digest complex organic compounds into smaller molecules, which can then be absorbed (Levetin, 1995; Deacon, 1997). Fungal enzymes are able to degrade almost all natural material (Wanner et al., 1993; Jennings & Lysek, 1999). When a suitable substrate becomes available, fungi are capable of limitless growth and will only be stopped by the exhaustion of the substrate (Jennings & Lysek, 1999). This can be seen in the mycelium of the fungus Armillaria bulbosa, which grows in forest soil and can yield amongst the largest biomass of any single living organisms (Jennings & Lysek, 1999).

Fungi are particularly notable for their ability to grow at low water availability. They have a lower water activity requirement than other microorganisms (Stetzenbach, 1997). Most can grow below the permanent wilting point of plants, and can even thrive in conditions of extreme water stress such as on stored grains (by Aspergillus) (Deacon, 1997). They also have a remarkable ability to transport water along hyphae over great distances in order to maintain growth (Jennings & Lysek, 2001).

Reproduction

Most fungal spores are adapted for airborne dispersal and the remainder are specialised for dispersal by vectors, such as water, or by insects (Levetin, 1995). Spores are used by fungi as a method of dispersion to gain access to new nutrient sources (Jennings & Lysek, 1999). Spores are always microscopic and range from less than 2 µm to more than 100 µm (Levetin, 1995). Hyphal fragments also serve as propagules and can be readily found in atmospheric samples (Levetin, 1995).

The formation of reproductive spores results from either sexual and/or asexual processes at different stages in the life cycle (Levetin, 1995). Fungal spores contain at least one and sometimes many cells and normally many nuclei (Jennings & Lysek, 2001). Spores differ greatly in size, shape colour, and their method of formation (Levetin, 1995). The trigger for spore development depends on particular nutrients or environmental conditions (Levetin, 1995) such as the naturally occurring daily rhythms in light, temperature and humidity (Lysek, 1984; Moore-Landecker, 1990).

Metabolites

Fungi, like all living organisms, share the same fundamental primary metabolism and have the same chemical process mechanisms involving four classes of primary metabolites, which are proteins, nucleic acids, carbohydrates, and lipids (Jarvis, 1994). However, fungi also produce secondary metabolites, which are a diverse range of compounds (Deacon, 1997). There are hundreds of known or suspected mycotoxins but in practice the list could expand to include thousands (Jarvis, 1994). Some have become useful to humans and these include many antibiotics, some mycotoxins, and alkaloids (Levetin, 1995).

The vast numbers of secondary metabolites that are produced by fungi do not have a recognised role in the maintenance of life (Moss, 1984). The primary function of secondary metabolites that are highly biologically active appears to be as a defence (or antagonism) against competitors (Jarvis, 1994). An important example is mycotoxins. These secondary metabolites are idiosyncratic and are often highly restricted to a group or even a single species (Jarvis, 1994). One type of mycotoxin is aflatoxin, produced by fungi that rot foodstuffs (Aspergillus). Aflatoxins are among the most potent carcinogens known to humans and have been implicated in liver cancer (Deacon, 1997; Gravesen, 2000).

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