The soil microorganisms bring out decomposition of the organic matter by producing multiple enzymes to obtain energy and simultaneously recycle nutrient in the ecosystem. Plant take up carbon dioxide (CO2) and water in presence of sunlight (radiant energy) and synthesize carbohydrate via process known as photosynthesis (conversion of radiant energy into chemical energy). The chemical energy is stored as starch or lipid. Energy liberated in form of Adenosine triphosphate (ATP) during metabolism (glucose oxidation) is stored in cells and is used when cells require it for various function and biosynthetic pathway.
The rate of reaction is accelerated by enzymes which are protein. These enzymes break the biomolecule into their monomer unit, thus the bioconversion of the biomolecules takes place. Enzymes are highly specific in their action on substrate. Diverse group of microorganisms degrade varied organic compound by releasing enzyme specific for that substrate.
- Carbohydrate (cellulose, hemicellulose, pectin, lignin etc.) is decomposed into oligosaccharide and monosaccharide units such as glucose etc.
- Proteins on decomposition will break into peptide and then into amino acid
- Nucleic acid – polynucleotide will decompose to nucleotide and later into purine, pyramidines, phosphoric acid and sugars
- Lipids (fats, oils, waxes and sterols) on decomposition disintegrate into fatty acid, glycerol and into short hydrocarbon chain
Various component of carbohydrate when decomposed results into monosaccharide units which is used by the soil microorganism to fulfill their requirement of carbon and energy.
Cellulose is present as the most abundant plant matter. It is the main component of the plant cell. Plant cell has primary cell wall, middle lamella and secondary cell wall. Cellulose is found in both primary and secondary cell wall and is a straight chain polymeric carbohydrate of β (1-4) linked glucose unit which is insoluble in water. Cellulose has both the crystalline as well as amorphous structure. The depletion of amorphous form of cellulose is rather rapid as compared to the crystalline structure by the microorganisms. Cellulose, hemicellulose or lignin molecules are linked together by a hydrogen bond. Cellulose molecules arrange regularly, form a bundle and determine the framework of the cell wall (Chen 2014). The gathered cellulose chain into microfibril are complex structure stabilized by intra as well as intermolecular hydrogen bond. Rigid cellulose fibril are embedded in amorphous polymer. Cellulose was first separated by Anselme Payen (1838, 1839) by treating wood successively by nitric acid and sodium hydroxide solution. The chemical formula of cellulose is (C6H10O5)n where n is the degree of polymerization and represents the number of glucose unit. Cell wall consists of cellulose, hemicellulose and lignin in the ratio of 4:3:3 (Chen 2014). Utilization of cellulose biomass requires, cellulolytic microorganisms secreting cellulolytic enzymes, hydrolysis of the biomass and fermentation of the resultant sugar to desired product by cellulolytic microorganisms.
Bioconversion of cellulose to soluble sugar is catalyzed by a group of enzyme called cellulases. Cellulolytic enzyme has been produced by range of microorganisms. Fungi are the main cellulase producing microorganisms though bacteria and actinomycetes also show cellulolytic activity. Cellulase is a complex of three enzymes that hydrolyzes the cellulose to oligosaccharides, cello-biose and glucose. The prime three enzymatic activities found are:
- Endoglucanases or 1,4-β-D-glucan-4-glucanohydrolases (EC 22.214.171.124)
- Exoglucanases, including 1,4-β-D-glucan glucanohydrolases (cellodextrinases) (EC 126.96.36.199) and 1,4-β-D-glucan cellobiohydrolases (cellobiohydrolases) (EC 188.8.131.52)
- Β-glucosidases or β-glucoside glucohydrolases (EC 184.108.40.206)
- Endoglucanase cut randomly at internal amorphous sites in the cellulose chain generating oligosaccharide of varied length and ultimately the new chain ends
- Exoglucanase attacks reducing or non-reducing ends of cellulose chain liberating glucose (glucanohydrolases) or cellobiose (cellobiohydrolase)
- β-Glucosidase hydrolyze cellobiose to glucose
The β-1, 4-glucosidic bonds between glucosyl residue can be hydrolyzed by cellulase and this distinguishes it from other glycoside hydrolases (Lynd et al., 2002). O-Glycosyl hydrolases is a group of enzymes hydrolyzing the glycosidic bond between carbohydrates or between a carbohydrate and a non-carbohydrate moiety exhibiting a range of substrate specificities (Henrissat et al., 1998). The bioconversion of the biomolecule fulfills the energy requirement of the soil microbe as well as leads to the mineralization in soil.
Chen, H. 2014 Chemical Composition and Structure of Natural Lignocellulose in “Biotechnology of Lignocellulose: Theory and Practice” Chemical Industry Press, Beijing and Springer Science Business Media Dordrecht Chapter 2, 25-71
Henrissat, B., Teeri, T.T. and Warren, R. A. J. 1998 A scheme for designating enzymes that hydrolyse the polysaccharides in the cell walls of plants FEBS Lett. 425(2):352-354
Lynd, L. R., Weimer, P. J., Zyl van, W. H. and Pretorius, I. S. 2002 Microbial Cellulose Utilization: Fundamentals and Biotechnology Microbiol Mol. Biol. Rev. 66(3): 506–577
Payen, A. 1838 Memoire sur la composition du tissu propre des plantes et du ligneux, Comptes Rendus, 7: 1052-1056
Payen, A. 1839 Composition de la matière ligneuse, Comptes Rendus, 8: 51-53