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Dung Fungi

Introduction

Faeces commonly include various fungi. Apart from microbial colonists of the Gastro Intestinal Tract (GIT), the mycota of faeces includes fungi accidentally consumed, contaminants settling from air, and fungi that are specifically adapted to dung as a resource. The latter group are called Dung fungi (or more correctly coprophilous fungi).

Dung fungi survive passage through the GIT, germinate and grow in the freshly deposited dung. They form adhesive spores that are actively and violently discharged. LINKThe spores are deposited on vegetation, and are then consumed by herbivores to complete the cycle.

Dung is a very rich medium for fungal growth. It consists of the remains of plant material plus the microbiota associated with its digestion. Much of the material contains readily available carbohydrate in addition to cellulose and lignin. The material is complex, and includes fatty acids, vitamins and amino acids. In addition, initially, the waste is moist, and the pH is close to neutral. Dung is an ideal medium for microbial growth.

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The Fungi

The fungi found growing in dung include those that have passed through the animal, and saprotrophs that land after deposition of the faeces. However, most studies of the ecology of dung have noted that the fungi that fruit on the surface consist of a group of recognisable taxa commonly including

Zygomycota and related: Pilobolus, Pilaira, Mucor, Phycomyces and Piptocephalis

Ascomycota: Ascobolus, Saccobolus, Sordaria, Podospora, Chaetomium

Basidiomycota: Coprinus and Nidulariales

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Order of Fruiting

If you incubate freshly collected herbivorous dung in warm humid conditions, a succession of fruiting structures may be observed. Commonly, the first fungi to sporulate are members of the Zygomycota. These are followed by members of Ascomycota and then Basidiomycota.

For instance, using fresh horse dung from the field, incubated in large Petri dishes, we will see Pilobolus appearing within 2 days in summer in Sydney. Ascobolus will apear within 5 days and Coprinus initials within 10 days. Size of the fragment of dung, and warmth and moisture characteristics influence the rate of appearance and the fungi that appear.

Cooler temperatures and drier conditions will slow the rate of sporulation. Further, some animals from specific locations of native vegetation carry quite different groups of fungi. Rabbits, bandicoots and rats have smaller pellets, and these may produce some unusual fungi. Even when dung comes from the same source, and contains the same fungi, the mycobiota that sporulate can be manipulated by altering the size of particle and environmental conditions.

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Energy Required

The order of appearance of sporophores is related to several factors. Apart from environmental conditions, aspects such as energy requirements for sporulation, complexity of fruiting body, and competition have been postulated. Fruiting bodies of Pilobolus and other Zygomycota are simple, and require relatively small input of organic energy. Thus the fruiting bodies of Pilobolus should be formed rapidly when energy is abundant and environmental conditions are appropriate.

This hypothesis is based on the notion that Zygomycota can only utilise readily available nutrients; they lack the necessary array of enzymes to digest complex carbohydrate. Thus these 'sugar' fungi will take up nutrients rapidly and then die back as the nutrients are depleted. They will be replaced by fungi that utilise more complex carbohydrate.

The second postulate is that interspecific interactions determine the order of appearance of sporocarps. Dung is a rich source of nutrients. Fungi that grow will be in strong competition soon after germination of their spores. Fungi will be competing with each other, and the various invaders colonising the resource. Competition will be intense, and we might expect all the aspects of a competitive interaction to be seen in dung. LINK

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Specificity

The ecology of dung fungi is still unclear. While some specificity between animals and fungi, and environments and fungi are indicated in surveys, the reason for any specificity is unclear. Further, arboreal animals such as possums and koalas also host dung fungi. These animals feed above ground. The fungi must disperse enormous distances to the leafy canopy from faeces on the ground.

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Spore Release

Release of spores has received considerable attention. The fungi generally expel their spores or sporocarps towards light. In Pilobolus, the release mechanism can target extremely small gaps between surrounding plants. LINK The mechanism controlling this accuracy is a light sensor surrounding the base of the sporangium. When light falls evenly on the sensor, the direction of the sporangium remains unchanged. As the light falls progressively on one side, the sporangium is progressively redirected such that the sensor is shaded.

Similar mechanisms regulate the direction of release of spores of Ascomycota. In perithecia and apothecia, the head of the ascus is pushed above the surface of the ascocarp or through the ostiole of the perithecium. The ascus is directed towards the light where spores are directed during release. By directing spore release, the fungi expel spores towards the gap between plants. Thus the spores are then dispersed away from the fruiting body and towards fresh plant material.

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Conclusion

Dung fungi are among a variety of fungi found growing on faeces. Dung fungi have a close relationship with their herbivorous hosts. Their life cycle is closely adapted to using dung: elements of dispersal, reproduction and growth offer insights into a complex interaction.

Reference

Bell, A (2005) An Illustrated Guide to the coprophilous Ascomycetes of Australia. CBS, Utrecht, The Netherlands.

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