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Competition Between Fungi

Introduction

Even though soil is oligotrophic, up to 106 fungal propagules per g may be isolated from soil. Even higher densities can be found in substrates rich in organic matter. It is easy to argue that fungi compete in these circumstances, though as fungi may become quiescent as food becomes scarce, competition is more likely to be highly variable over time in these environments. Competition studies are common in botany and zoology, and a vast body of research applies to fungi. After all, fungi require organic energy to survive, and by occupying space, they may increase their potential access to food.

However, interacting organisms need not compete if their food requirements differ. One of the organisms may be the source of food for the other, or they may share resources in some way. Further, we may not be aware of the competitive interaction between two different organisms. The interactions between different organisms are varied and often complex, especially so, when they are small and the act of study influences the outcome.

Fungi have a variety of requirements for growth and reproduction. LINK Though organic energy is of overriding importance to fungi, space, water, other nutrients and oxygen are also important. To continue through time, a fungus must reduce the effect of potential competitors or utilise effective competitive mechanisms.

Mechanisms of Competition

The basic mechanisms used by microbes to compete include:

  1. Rapid recovery, growth and sporulation.
  2. Use of inhibitors
  3. Negation of inhibitors
  4. Special niche

Probably all fungi use each of these factors to a greater or lesser extent. The “sugar” fungi respond to presence of simple sugars with explosive growth, and reproduction, followed by a rapid decline. On dung, the first fungi to appear are Zygomycotina such as Pilobolus and Pilaira LINK with rapid rates of growth. These simple fungi rapidly utilise simple organic molecules, they rapidly produce sporangia and then they disappear. If they are cultured on artificial media, you often find that they can utilise complex carbohydrates, but they grow slowly on complex carbon because they lack all the necessary enzymes, or the enzymes are excreted to a limited extent.

Fruiting bodies of Zygomycotina are commonly replaced on dung by filamentous Ascomycotina such as Ascobolus or Podospora. These fungi excrete enzymes capable of degrading cellulose and other more complex carbohydrates. They also produce some antibiotics which either deter or inhibit some fungal competitors, such as the Zygomycotina. They grow at about the same rate, and they form their more complex fruiting bodies some time after the Zygomycotina.

The next stage in this succession of fruiting bodies is commonly Coprinus. This genus of gilled fungi is in the Basidiomycotina. Coprinus is a very effective competitor. It is assumed that the fungus produces an array of antagonists, for once a population is established, Zygomycotina and Ascomycotina are rarely found to fruit in the immediate vicinity of Coprinus on the dung. LINK Coprinus is able to degrade lignin, a very complex carbohydrate, and access the associated cellulose, a source of energy that is unavailable to its ascomycetous competitors. LINK

However, even Coprinus may be overtaken by other Basidiomycotina. Nidularia is commonly found on cow dung in paddocks. It would appear that Nidularia is able to tolerate the antagonists excreted by Coprinus, and environmental conditions that are unsuitable for continuation of Coprinus. Thus, Nidularia has the digestive capabilities of Coprinus, the array of enzymic capabilities, plus the ability to function in more stressful conditions. It has a special niche.

These interactions suggest succession of fungal populations in any particular substrate as the organic carbon, nitrogen, iron, water and other requirements for growth are removed. However, they do not indicate the mechanisms underlying the competitive interaction. Studies in culture plates indicate some of these mechanisms. They can be summarised as indirect and direct interactions.

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Hyphal Interactions

Indirect interactions would be where one fungus shows evidence of the presence of the other without any intervening chemical or stimulus. For example, one fungus may remove all nutrients from the zone between two fungi thus preventing the other from entering it. An indirect interaction would be particularly important where a resource is in very short supply, and one of the competitors has a high demand for the resource. Where the resource is abundant, then we would expect to see the hyphae of both fungi intermingling without apparent interaction.

Direct interactions can take place in the absence of any contact between the competitors. The production of inhibitors is widespread in the fungi, and these are expressed in culture. Many fungi produce secondary metabolites which either inhibit or kill competing fungi, some distance away. Antibiosis is often specific to groups of species, indicating that in nature, this interaction may be widespread. Antibiotics may be used to capture resources already occupied by a competitor or to secure a resource that may be under threat from a competitor.

Antibiotics may be volatile or nonvolatile compounds. The latter may be accumulated in media where they may induce autotoxicosis. These fungi are often seen growing in concentric rings on agar. Each new ring of the fungi arises from hyphae that have emerged from the agar beyond the zone of staling materials.

Several fungi have been found to produce a range of antibiotics, each produced under specific conditions. The specific action in nature of the diverse array of molecules is unclear in most cases, though their action can be understood in anthropocentric terms. For instance, soil fungi such as Trichoderma, Penicillium and Aspergillus produce a diverse range of antibiotics. Specific compounds, however, appear to have specific targets. Plectasin is a defensin (peptide) with antibacterial activity. Aflatoxins have mammalian toxicity. Trichodermin may have broad antifungal activity.

Many compounds appear to have such broad potential activity that their role in the survival of the producer is unclear. Some disrupt DNA and protein synthesis, others disrupt ribosomal activity or the cytoskeleton, making them extremely broad and general toxins. Frequently, a fungus known to produce one important secondary metabolite with bioactivity, will have a diverse array of secondary metabolites with unknown activity (and benefit) to the producer.

The production of antibiotics has been exploited widely in human health, penicillin is perhaps the most commonly used antibiotic produced by fungi. LINK

Contact phenomena are also important mechanisms of competition. The most obvious example of interaction is where the hyphae of one lyse the hyphae of a second fungus on contact. Often times, the period immediately prior to lysis is characterised by a period where the antagonist overgrows the other fungus. The hyphae appear to become intertwined, and then the antagonist takes over the resource. Sometimes, lysis is accompanied by antibiotics.  The cell lyses  in advance of hyphal contact. The target fungus is affected in a number of ways: cells burst, proteins are coagulated, compartments are vacuolated, or cytoplasm is withdrawn from the growing hyphal tip.

The antagonist may also coil around the target hyphae. Direct penetration is possible, though this is uncommon for necrotrophs. In some cases, the colonising fungus may form a biotrophic association with its host. In cases where the host has an asexual life cycle, establishment of the parasitic biotroph may lead to a long-lived association. For instance, fungi have been isolated from viable spores of Glomalean fungi, and in some instances, the Glomalean fungus has been maintained in long-term pot culture with the biotrophic hyperparasite.

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Conclusion

Fungi inevitably interact with one another. Competitive interactions may result from depletion of resources or requirement for the same resource. Mechanisms include direct inhibition of the competitor. Death of the inhibitor without contact, or death of the competitor following contact are possible consequences. Partitioning of resources may also take place. Though not a result of direct competition, biotrophic colonisation of another fungus may have evolved from a competitive interaction.

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References

Dix NJ & Webster J 1995 Fungal Ecology. Chapman Hall.

Griffin DH 1994 Fungal Physiology. Wiley. Ch 9.

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Fungal Succession in Leaf Litter

A number of fungi commonly found on the surface of leaves have the capacity to survive extremely low water potential for short periods of time. Germinated spores of Alternaria alternata Cladosporium cladosporioides, Helminthosporium sativum and Stemphylium botryosum survived several hours at 40% humidity, though the longer the germ tube, the shorter the period of survival. These fungi are commonly found on and in leaves. They are also the fungi that are initially isolated from senescent leaves after they have fallen to the ground.

These species, along with the other common phyloplane fungi Aureobasidium pullulans, Epicoccum purpurascens and the yeast Rhodotorula spp, have rather poor degradative capacity. In fact, they are rapidly replaced by more aggressive fungi soon after the leaf reaches the ground. The common soil fungi Aspergillus, Fusarium, Gliocladium, Penicillium and Trichoderma quickly colonise fallen leaves and are widely believed to be responsible for its degradation. These fungi have cellulolytic capacity and tolerance of the phenols common in leaves. However, they cannot degrade leaf litter in isolation, and rates of degradation are greater in more complex fungal (arthropod and plant) communities. The reasons for synergistic increases are unclear. Presumably, the fungi preferentially degrade different carbon sources and release breakdown products that benefit the other fungi.

Finally, in deep litter, a range of basidiomycetes appear. The populations associated with each type of litter is specific. The basidiomycetous fungi can degrade the more complex organic molecules, tolerate the specific plant secondary metabolites and they have a variety of specific activities which enable them to suppress the competing fungi. These fungi fruit within and upon the litter as the resources and environment dictate.

The succession of fungi in leaves is clear: from common leaf fungi to common soil fungi to specific litter fungi.

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