Role of Bacteria and Fungi in Decomposition

Role of Bacteria and Fungi in Decomposition: Decomposers are saprophytes which feed on faeces and dead organisms. Many bacteria and fungi are decomposers/ they secrete enzymes onto their food. These enzymes break down complex organic compounds of the food like carbohydrates and proteins in simpler components with the release of energy.

Decomposers absorb only a small amount of energy and nutrients thus produced for their own use. Most of the energy is lost as heat and the remaining materials are released into the surrounding soil, air and water.

The inorganic compounds released during decomposition include gases like CO2, H2S and water vapours and salts like nitrates, sulphates, phosphates and potassium ions. These are reused by green plants during photosynthesis and other synthetic processes.

Nitrogen Fixation

Another important function performed by bacteria is nitrogen fixation. Nitrogen fixation is the conversion of molecular nitrogen (N2) of atmosphere to soil nitrogen in the form of nitrites and nitrates, thus increasing soil fertility.

“Nitrogen is and important component of protein”

Nitrogen fixation by bacteria

Free-living nitrogen fixers

Bacteria of this type called nitrifying bacteria use carbohydrates from the soil, humus and atmospheric N2 to make nitrates. These nitrates are used by plants.

Symbiotic nitrogen fixers

Bacteria of this type live inside the roots of leguminous plants (e.g. peas, beans) where they form tiny swellings called root nodules. Bacteria obtain both protection and carbohydrate from plant cells and in return release nitrates into the plant tissues and the soil. An association of this kind in which both organisms get benefit by living together is a type of symbiosis and is called mutualism. (fig 20.3)

Ecological importance

Fungi have great ecological impact. They are very important as decomposers and symbionts. Fungi, along with saprobic bacteria, play vital role in the recycling of inorganic nutrients in the ecosystem. Without their activity all the essential nutrients would soon become locked up in the mounds of dead animals, plants, would be unavailable for use by organisms, and life would cease. Mycorrhizal fungi improve the growth of plants with which they are associated. 95% of all kinds of vascular plants have this association.

Lichens growing on rocks break them, setting stage for other organisms during the course of ecological succession. Lichens are very good bioindicators of air quality as they are very sensitive to pollution. Some fungi are also used for bioremediation ( degrading/ removing environmental poisons/ pollutants by organisms).

Commercial importance

Fungi cause economic gains as well as losses.

Economic gains due to fungi

Certain fungi are edible. About 200 species of mushrooms (e.g. Agaricus sp), morels (e.g. Morchella esculenta), truffles (underground fruiting bodies some

Fig. 8.13 Edible fungi (a) A common morel (morchella esculenta). (b) the truffles (tuber species) are underground fruiting bodies that people find with the help of trained dogs or pigs.

Ascomycetes, e.g. tuber sp) are common edible fungi. Beware of poisonous mushrooms called the toadstools, such as death cap/death angel (amanita) and jack-O, lantern mushroom (fig. 8.14).

Fig. 8.14 a: poisonous mushroom Jack-O’ lantern (omphalotus olearius) whose gills glow in the dark. B: amanita, another common poisonous mushroom.

Reindeer moss (a lichen, not a moss) is used as food for reindeers and some other large animals in arctic/subarctic/boreal regions.

1. Certain fungi are used in food industry. Because of their fermenting ability, yeasts (saccharomyces cerevisiae) are used in the production of bread and liquor. Penicillium species are used for giving flavor, aroma and characteristic colour to some cheese. Some species of Aspergillus are used for fermenting/producing soya sauce and soya paste from soya bean. Citric acid is also obtained from some Aspergillus species.
2. Some fungi are source of antibiotics and some other drugs. Penicillin, first antibiotic to be ever discovered (by A. Fleming-1928) is obtained from penicillium notatum. Lovastatin is used fro lowering blood cholesterol; cyclosporine obtained from a soil fungus is used in organ transplantation for preventing transplant rejection; and ergotine to relieve one kind of headache migraine. Griseofulvin is used to inhibit fungal growth.
3. Some natural dyes obtained from lichens are used in textile industry.
4. Yeasts are heavily used in genetic/molecular biological research because of their rapid generation and rapidly increasing pool of genetic and biochemical information. Yeast were the first eukaryotes to be used by genetic engineers. In 1983, a functional artificial chromosome was made in Saccharomyces cerevisiae. The same yeast was the first eukaryote whose genomic sequence was completely studied in 1996. Yeasts are also being investigated for production of some hormones. Pink bread mold Neurospora has also been used for genetic research.

Economic losses due to fungi

Fungi are responsible for many serious plant diseases because they produce several enzymes that can breakdown cellulose, lignin and even cutin. All plants are susceptible to them. Extensive damages due to rusts and smut diseases of wheat, corn (Fig. 8.15) and rice prompted mass displacement, and starvation to death of many people.

Powdery mildews (on grapes, rose, wheat etc), ergot of rye, red rot of sugar cane, potato wilt, cotton root rot, apple scab, and brown rot of peaches, plums, apricots and cherries are some other common plant diseases caused by fungi.

Fig. 8.15:Plant pathogenic fungus. Corn smut on an ear of sweet corn is caused by Ustilago maydis.

1. Fungi also cause certain animal diseases. Ringworm and athlete’s foot are superficial fungal infections caused by certain imperfect fungi. Candida albicans, a yeast, causes oral and vaginal thrush (candidiasis or candidosis). Histoplasmosis is a serious infection of lungs caused by inhaling spores of a fungus which is common in soil contaminated with bird’s feces. If infection spreads into blood stream and then to other organs (which is very occasional), it can be serious and even fatal. Aspergillus fumigatus causes aspergillosis, but only in persons with defective immune system such as AIDS, and may cause death. Some strains of Aspergillus produce one of the most carcinogenic (cancer-causing) mycotoxins (toxins produced by fungi), called aflatoxins. Aspergillus contaminates improperly stored grains such as peanuts and corn etc. milk, eggs and meat may also have small traces of aflatoxins. Any moldy human food or animal forage product should be discarded. Ergotism is caused by eating bread made from purple ergot-contaminated rye flour. The poisonous material in the ergot causes nervous spasm, convulsion, psychotic delusion and even gangrene.
2. Saprobic fungi are not only useful recyclers but also cause incalculable damage to food, wood, fiber, and leather by decomposing them. 15-50% of world’s fruit is lost each year due to fungal attack. Wood-rotting fungi destroy not only living trees but also structural timber. Bracket/shelf fungi (Fig. 8.16) cause lot of damage to stored cut lumber as well as stands of timber of living trees.

A pink yeast (Rhodotorula) grows on shower curtains and other moist surfaces.

Fig. 8.16:This shelf fungus is parasitizing a tree. These are important decomposers of wood.