II. FUNGI AS SAPROBES                                              TOP

A. Beneficial Activities of Saprobic Fungi

1. Decomposition of Waste

In total numbers, most fungi are saprobic. It is in the decomposition of waste that fungi serve their most important role. What if we did not have fungi? To get a full understanding of their benefit to us, fungal ecologist tell us that in a serene pasture (Fig. 3-1), were it not for the fungi, these cows could not be seen within six years.

Literally, without fungi, “we would be up to our armpits” and the earth could not function were it not for the fungi dissolving and absorbing the leftovers. Saprobic fungi are also of great benefit in the production of drugs, foods and beverages, and wild mushrooms with medicinal and food value.

1. Decomposition of Waste:

Species of Aspergillus and Penicillium are involved in the decomposition of more kinds of substrates than any other groups of fungi. Aside from foods, fabrics, and other organic substrates, these fungi have been isolated from many peculiar habitats. Several species are tolerant to copper and sulphur and can grow on high concentrations of CuS04 and H2SO4. They are often found on electro-plating equipment and contaminants of laboratory chemicals. Several species of both genera can tarnish lenses of optical equipment and others are found in many unexpected habitats.  Hundreds of other fungi, like species of Aspergillus and Penicillium, can grow on a wide array of substrates. This is both good and bad.

a. Decomposition of Cellulose & Wood products. 

Cellulose is the most abundant polymer on earth . Estimates are that there exist at any one time approximately 1400 billion tons of cellulose on earth; 28 billion tons being produced each year (Markham & Bazin, 1991, Handbk. Appl Mycol. 1:379-424). An equal amount of cellulose, fortunately, is decomposed each year. Fungi are responsible for decomposing more than 80% of the cellulose formed annually.

Fungi & bacteria are rich in cellulolytic enzymes, i.e. those that can digest cellulose. Fungi, however, have an advantage over bacteria in being filamentous and able to invade tissues more quickly, and forming wind-blown spores that are able to invade substrates a great distance away. A large number of fungi have potent cellulolytic enzyme systems. They include species of Trichoderma, Phanerochaeta, Chaetomium, Penicillium, Fusarium, Pythium, Mortierella, and Agaricus. These represent most of the major groups of fungi.

Approximately 1/3 of the earth is covered by forest. In nature, much cellulose is protected from decomposition by the impregnation of  lignin. Lignin is a heterogenous, three-dimensional polymer of oxyphenylpropanoid units. It impregnates the xylem (i.e. woody) tissue of plants and adds rigidity to these structural cells. It serves also as a good preservative against many fungi. Many wood-rotting fungi, however, are able to break down lignocelluloses. Most of these fungi attack dead trees, but species of such groups as Armillaria, Heterobasidium, Ganoderma, and other wood-rotting fungi can invade healthy trees. Some start to rot trees while they are still standing.

Six major groups of fungi are involved in soft rot, white rot, and brown rot of wood. White rot occurs when the chemical compounds are bleached out of woody tissues by enzymes that break down pectin, cellulose, and lignin. White rots are most common in  angiosperms, i.e. broad leaf trees.  Whereas, brown rot results whenever cellulose only is broken down and the lignin is untouched, leaving wood discolored and in cuboid blocks. Brown rots are more common on  conifer trees, i.e. those trees with needles and cones. Lastly, soft rots occur under very moist conditions and as the cellulose and pectins are broken down, simple sugars are formed, leaving a mucilaginous condition. Some common wood rotting fungi include species of the following :

White rot: Armillaria (Fig. 3-2), Daedaleopsis (Fig. 3-3), Ganoderma (Fig. 3-4), Heterobasidium, Lentinus (Fig. 3-5), Pleurotus, and Stereum, and Xylaria.

Fig. 3-2. Armillaria is not only a wood- rotter but is edible.

Fig. 3-3. Daedaleopsis, an easily recognized genus because of its labyrinthoid pores.

Fig. 3-4. Ganoderma zonatum causes a dieback of palms.

Fig. 3-5. Lentinus edodes, the popular shiitake mushroom, is a white rot fungus.

Lignin degradation is done by white rot fungi, and paper mills and similar manufacturers may be able use these fungi in the clean up of pollutants discharged from these facilities.

  Brown rot: Inonotus (Fig. 3-6), Laetiporus (Fig. 3-7), and Fomes.

Fig. 3-6. Inonotus hispidus is a common brown rot fungus.

Fig. 3-7. Laetiporus sulphureus is one of the few edible bracket fungi.

Soft rot: Pholiota, Meripilus.

For more information on wood decay fungi see Gilbertson & Ryvarden (North American Polypores, Vol. 1, 1986, Vol.2, 1987, Fungiflora, Olso, Norway).

Leaf litter makes up a sizeable portion of the forest biomass. Several “tiny little mushrooms” are well adapted for breaking down leaf litter. They include species of Mycena (Fig. 3-8), Marasmius (Fig. 3-9), Collybia, and Clitocybe.

b. Decomposition of Biological and Chemical Waste.

Most of you are too young to remember a scene that was shown around the world; I’m speaking of the Love Canal near Buffalo, NY (Fig. 3-10). 

Fig. 3-10. A street built over a toxic waste dump, Love Canal.

This was a dumpsite that the US military used during WWII to dump toxic waste. In later years it was filled over and eventually turned into a housing development. Several years later, after alarming rates of cancer, premature births, birth defects, and infant deaths occurred, we come to realize the great harm such waste can cause. More than 20,000 tons of toxic wastes were dumped in these sites. In 1978, President Carter declared it a federal emergency at Love Canal. People were relocated and massive clean-up efforts began. Worldwide industrialization has resulted in the production of hostile environments for a wide range of organisms. There is a steadily increasing number of synthetic chemicals, insecticides, herbicides, and environmental pollutants. If it is possible to degrade them by various fungi or bacteria, nature’s balance can be maintained. If not, we have problems! 

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During degradation, four things may happen: (1) fungi may use the pollutant directly as a nutrient source; (2) the fungus may not use the pollutant as an energy source but may transform it; (3) molecules in the toxin may combine with metabolites of the fungus and be detoxified as the fungus grows on other compounds in the environment; and (4) the fungus may absorb the pollutant without causing an ill effect on the fungus. For additional information, see Nansdan and Raisuddin, Hdbk. Appl. Mycol. I. 1992

Aromatic compounds: Microorganisms can transform aromatic compounds in a number of ways. A monooxygenase enzyme system formed by species of Rhizopus, Cunninghamella, Trichosporon, Fusarium (Fig. 3-11), Aspergillus (Fig. 3-12), and others enables them to decompose aromatic compounds such as aniline, benzene, napthaline, and toluline. 

Fig. 3-11. Fusarium oxysporum. A) colony growing on agar media. B) Macrosporidia and mirosporidia.

Fig. 3-12. Species of Aspirgillus form their spores on distinct vesicles.

Isolates of Penicillium (Fig. 3-13; Fig. 3-14), Paecilomyces, Beauvaria, and Verticillium were shown to metabolize n-dodecylbenzene from petroleum. 

Fig. 3-13. Penicillium  growing on oranges.

Fig. 3-14. Scanning electron micrograph of Penicillium conidiophores and spores.

A number of compounds with esters, i.e. plastizers, insecticides, and herbicides may be detoxified by hydrolases produced by certain species of Penicillium.

 Hydrocarbons represent an enormous energy source that enables us to have a very high standard of living. At present, pollution of both land  and sea by hydrocarbons is reaching critical levels. Certain fungi are adapted to utilizing hydrocarbons and their use in degrading oil spillage is actively being investigated. In the use of such fungi, nitrogen and phosphorus are often limited in the marine environment and compounds with these minerals must be added when oil spills are “seeded” with fungi. Among filamentous fungi, the extracellular metabolites of  Cladosporium resinae have been studied most extensively. In the early days of testing jet airplanes in the 1940’s, many crashes occurred because of an accumulation of this fungus in the fuel pumps and filters. The black yeast, Aureobasidium pullulans, is able to attack certain polyvinyls and has been a problem in the building industry (Fig. 3-15; Fig. 3-16).          

Fig. 3-15. Discoloration of tile caused by the growth of Aurobasidium pullulans beneath the tile.

Fig. 3-16. Mycelium and chlamydospores of Aurobasidium pullulans.  

Effluents from Tanneries and Pulp Mills: Discharge from tanneries has complex organic and chemical wastes (Fig. 3-17). Species of Aspergillus, Penicillium, Fusarium, Chaetomium Rhizoctonia, and Trichoderma (Fig. 3-18) have been tested in degrading tannery waste. 

Heavy Metals: Among the inorganic pollutants of the environment, heavy metals are the most obnoxious toxins released from a variety of industries. The presence of heavy metals even in trace quantities can be toxic to fish, crabs, and other aquatic organisms. The application of fungal biomass to  decontaminate metal-polluted water has been tested. Species of Aspergillus, Penicillium, Rhizopus, Trichosporon, and others have an innate ability to absorb high levels of various metals without damage to themselves (Nandan & Raisuddin. 1992. Handb. Appl. Mycol. 4:931-961).

c. Fungal Degradation of Pesticides.

 Pesticides are used extensively to protect food and fiber used by man and other animals. There are more than 2500 different pesticides, the most common of which are fungicides, herbicides, and insecticides. The fate of pesticides is of great concern to us today because of their lingering effect in the environment. Already throughout several Midwestern states, alarming levels of herbicides and other toxic agricultural chemicals have made it into the aquifer and are contaminating vital water sources. Biodegradation is probably the major, or only, means to detoxify water and soils with residual pesticides. Most available evidence indicates that microbial activity is the main route by which chemicals are degraded in our environment. This is where fungi can play a role.

Degradation of Fungicides: Fungicides are those compounds used to control or eradicate fungi. Common fungicides contain quinines, organometallics, phenols, and aliphate compounds. Species of Aspergillus, Penicillium, and Trichoderma can utilize high levels of organo-mercurial compounds and species of Aspergillus, Cladosporium, Glomerella, Rhizoctonia, and several yeasts such as Saccharomyces and Hansenula can detoxify organo-sulphurs. These are two of our most common groups of fungicides.

Degradation of Herbicides: Herbicides are compounds used to eradicate noxious or toxic weeds. Phenylamides, s-triazines, and thiocarbomates are used most extensively as agricultural herbicides. Species of  Penicillium, Fusarium, and Pullularia can use the compounds in certain herbicides as nutrient.

Degradation of Insecticides: Chlorinated hydrocarbons have been banned from use in the U.S. There has been an extensive amount of research in the past two decades on the removal of residual  chlorinated hydrocarbons such as DDT, toxaphene, aldrin, dieldrin, and chloridane from the environment. Again, species of Aspergillus, Fusarium, and Trichoderma have been shown to be able to utilize these compounds as a carbon source. Surprisingly, a number of white rot species of Pleurotus, Polyporus, and Phanerochaete have also been shown to be effective in utilizing these compounds. Organophosphates such as Malathion, Guthion, and Parathion can be utilized by Aspergillus niger, Penicillium notatum, and Trichoderma viride. It is interesting that the same species of  Penicillium that is used to make penicillin may also be effective in breaking down organophosphates! (Lindley et al., 1992. Handb. Appl. Mycol. 4:905-929).