The following points highlight the seven main animal and plant organic compounds degraded by microorganisms. The organic compounds are: 1. Proteins 2. Lipids and Starch 3. Chitin 4. Mucopeptide 5. Cellulose 6. Hemicellulose 7. Lignins.
Organic Compound # 1. Proteins:
In nature, proteins are often complexed with polysaccharides or tannins and are most resistant to decay. Fibrous proteins with many cross links, such as keratin, are very resistant to microbial attack though most actinomycetes (e.g., Streptomyces) and some fungi (e.g., Penicillium, Keratinomyces) can degrade them. Proteins have great nutritional advantage over microorganisms because they contain both carbon and nitrogen.
Proteins present in the body of the organisms when left in the soil become still complex forming lignoprotein complex, protein-clay complex and protein-uronide complex which are very resistant to microbial degradation. These complexes are the chief constituents of humus in soil.
Organic Compound # 2. Lipids and Starch:
There is surprisingly little information on the biodegradation of lipids and starch in the natural environment but these common constituents of organisms are readily utilized by bacteria and fungi in the laboratory. Under anaerobic conditions, only some bacteria such as Clostridium can cause their degradation in natural environment.
Organic Compound # 3. Chitin:
Chitin is an important source of carbon and is degraded quite quickly unless it is protected by tanned proteins. It is a polymer of N-acetylglucosamine and thus contains excess nitrogen which is mineralized in aerobic environments.
Biodegradation of this compound is brought about mostly by actinomycetes (e.g., Streptomyces) and other bacteria (e.g., Pseudomonas, Bacillus, Clostridium). In acid environments fungi such as Mortierella may play major role in chitin degradation as they are less sensitive to low pH than are most bacteria.
Organic Compound # 4. Mucopeptide:
Mucopeptide (also called peptidoglycan or murein) is a distinctive polymer in bacterial cell walls and is composed of N-acetylglucosamine and N-acetylmuramic acid. It is sometimes not a major component of the walls by weight but, considering the wide distribution of bacteria and their high biomass in some environments, its breakdown is significant to the carbon balance in nature.
Much is known about the enzymes concerned with bacterial lysis under laboratory conditions but very little is known about the breakdown in natural environments. It is probable that the mucopeptide is degraded mostly by autolysis through Mycobacterium and some Bacillus species; these species can cause degradation of these complex compounds to some extent.
Organic Compound # 5. Cellulose:
Cellulose is a polymer of D-glucopyranose. The biodegradation of this compound has been the subject of many investigations because it is the major constituent of plant cell walls and therefore of the insoluble carbon added to the carbon cycle, and also because it is widely used by man as textiles, paper and as a component of timber.
Microorganisms that degrade cellulose can produce an enzyme called “cellulase” which catalyses the hydrolysis of the polymer to the dimer cellobiose. The latter is hydrolysed by the enzyme “cellobiase” to glucose which is absorbed by the decomposer or enters the soluble carbon pool.
The microorganisms that carry out this breakdown vary with the environment. Under aerobic conditions a wide range of fungi e.g., Chaetomium, Stacliybotrys, Trichoderma and Penicillium are important; some bacteria, e.g., Cytophaga, Bacillus, Pseudomonas may be the major decomposers of cellulose in aquatic environments.
Some bacteria, e.g., Clostridium can degrade cellulose anaerobically and are therefore important in waterlogged soils and in deep water sediments. In Indian conditions, certain fungi like species of Aspergillus, Memnoniella, Trichothecium and Ascotricha have been found to be active decomposers of cellulosic materials in nature.
Organic Compound # 6. Hemicellulose:
Hemicelluloses are low-molecular weight polysaccharides occurring abundantly in plant cell walls. The hemicellulose-degrading microorganisms belong to all major fungal groups; the most important fungi that degrade hemicelluloses are species of Alternaria, Aspergillus, Penicillium, Chaetomium, Fusarium, Glomerella and Trichoderma.
The degradation of hemicelluloses involves hydrolysis of the complex polymer to simpler units by the act of mainly three types of enzymes:
(i) Endo-enzymes that randomly break the bonds between building blocks in the polymer;
(ii) Exo-enzymes that cleave either a dimer or monomer from the end of the polysaccharide chain; and
(iii) Glycosidase enzymes that hydrolyse the oligomers or disaccharides resulting in simple sugar or uronic acid.
Organic Compound # 7. Lignins:
Lignins, a major cell-wall constituent characteristic of woody tissues, are thought to be polymers of p-hydroxyphenylpropanes and are characteristically difficult to be degraded either chemically or biologically.
There are, however, some fungi, e.g., common mushrooms and toadstools and bracket fungi on trees, and some bacteria, e.g., actinomycetes which are capable of degrading lignins into low molecular weight aromatic and aliphatic products.
These lignicolous microorganisms produce lignolytic enzymes known as “lignases” which are responsible for catalysing the degradation of these complex compounds. Much work is in progress throughout the world on microbial degradation of lignins on account of obvious reasons.