Decomposition of Water Soluble and Insoluble Organic Substances

After reading this article you will learn about the decomposition of water soluble and water insoluble organic substances.

Decomposition of Water Soluble Organic Substances:

In-spite of having variation in the composition of different organic compounds the end products of decomposition under aerobic conditions is more or less similar.

Sugars and other water soluble organic nitrogenous compounds are subjected to decomposition first as depicted below:

Water soluble organic nitrogenous substances (e.g. protein, amides etc.) are subjected to break-down as more plant available forms NH₄ and NO₃ (inorganic nitrogen forms) through the process of ammonification and nitrification and also forms gaseous nitrogenous compounds. The mineralization process includes both ammonification and nitrification processes (conversion of organic compounds to inorganic forms of nitrogen).

Ammonification:

In this process organic nitrogenous compounds transform to NH₄⁺ form (inorganic) by enzymic hydrolysis through some intermediate steps as follows:

Organic nitrogenous compounds → polypeptides → amino acids → NH₃ or salts of NH₄⁺ e.g. proteins.

Nitrification:

In this process, the so formed ammonia or ammonium salts are converted to nitrate (NO₃^–) form of inorganic nitrogen through intermediate nitrite (NO₂^–) formation as follows:

Decomposition of Water Insoluble Organic Substances:

There are various insoluble organic compounds which are present in plant and animal residues. Some important compounds are proteins, cellulose, hemicellulose, starch, fats and waxes, lignin, lipids, cysteine, cystine.

Protein:

The protein molecule is composed of a long chain of amino acids having general structure H₂N CHRCOOH where R may be a hydrogen atom, a single methyl group, a short carbon chain, or a cyclic structure. In a protein molecule, amino acids are linked together by peptide bonds (CO—NH).

Enzymes proteases attack the protein molecule and hydrolyze the peptide bonds and release free amino acids via peptones and peptides. Proteases are of two general types.

(i) Exopeptidases:

That hydrolyze peptide bonds near the end of the amino acid chain and it can cleave only the simple peptides and

(ii) Endo-peptidases:

That can hydrolyze bonds at a distance from the terminal end of the chain and it is also called proteinases because it can hydrolyze both proteins and peptides.

Cellulose:

Cellulose is a carbohydrate composed of glucose units bound together in a long, linear chain by (3-linkages at carbon atoms 1 and 4 of the sugar molecule. A number of polysaccharides (xylans. polyuronides etc.) are also associated with the cellulose of the plant cell wall. The polysaccharides that are structurally linked with the cellulose of the cell wall are called cellulosans.

Both starch and cellulose are polymers of the same building block (glucose) but their structural patterns are different. Due to structural differentiation, starch is subjected to ready microbial attack whereas cellulose compounds are more resistant to microbial decomposition and enzymatic breakdown.

Several micro-organisms are capable of decomposing cellulose in absence of O₂, the most common species being Clostridium. Some species are specific in their requirement for cellulose. The main end products are CO₂, H₂, ethanol and various organic acids. Methane (CH₄) is a byproduct produced by bacteria (not by Clostridium) that metabolize the organic acids liberated during the primary stage.

Initially cellulose enzyme hydrolyses and splits the long chain cellulose molecule to form cellobiose units and then cellobiose enzyme hydrolyzes cellobiose with the formation 2 glucose molecules. From the glucose, formation of simple products likes CO₂ and H₂O (under aerobic conditions) and organic acids and alcohols (under anaerobic conditions).

Hemicelluloses:

Hemicelluloses are water-insoluble polysaccharides. During break-down of hemicelluloses, the production of single soluble sugars is found. Hemicelluloses attack first by micro-organisms than that of cellulose compounds. The enzymes for the break-down of hemi cellulose compounds are hemicelluloses.

Starch:

Starch is the polymer of glucose and it serves the plant as a storage product, and as such it is the major reserve carbohydrate, plant starches usually contain two components, amylose and amylopectin.

The amylose has a linear structure built up of 200 to 500 or more glucose units linked together by an α-1, 4-glucosidic bonding. In amylopectin the individual glucose units are bound together by α-1, 4-linkages, but the molecule is branched and has side chains attached through α-1, 6 glucosidic linkages.

Starches decompose at a faster rate than that of celluloses and hemicelluloses. Various microorganisms like bacteria, fungi and actinomycetes have the capacity to hydrolyze starch.

There are two enzymes—α-amylase and β-amylase that are responsible for the breakdown of starch (amylose and amylopectin). Both the enzymes act upon the 1,4 linkage, but the hydrolysis is retarded once the amylopectin branch point is encountered.

The process of breakdown of starch is given below:

Lignin:

Woody type of plants contributes large amounts of lignin. The lignin content of young plants is relatively low, but the quantity increases as the plant matures. The decomposition of lignin proceeds either in presence or in the absence of O₂. The rate and extent of lignin decomposition is affected by temperature, availability of nitrogen, 00 and by constituents of the plant residue undergoing decay.

Most fungi can decompose lignin. Lignin is probably depolymerized to give simple aromatic substances such as vanillin and vanillic acid or possibly other methoxylated aromatic structures. The enzyme system is undoubtedly extra cellular and converts the lignin to more available form for subsequent digestion.

In the final stages, the remaining methoxyls are removed with the formaltion of hydroxy-benzene derivatives which are ultimately cleared to give low molecular weight organic acids and a partial or complete methoxyl loss liberate simple aromatic compounds.

With regard to the decomposition mechanism of aromatic compounds it may take place with the formation of an oxygenated derivative of benzene.

As for example the oxidation scheme of phenanthrene is given below:

In this way phenanthrene compound is converted to a simpler compound pyro catechol having high reaction capacity. However, the complete utilization by micro-organisms of phenols and other aromatic compounds as a source of energy is only possible after benzene ring fission and the conversion of a cyclic compound into an aliphatic compound.

The microbial oxidation leading to benzene ring fission takes place as follows:

(a) Oxidation occurs at the site of double bond between partly oxidised carbon atoms (COH).

(b) Fission of the benzene ring takes place at three places simultaneously as for an example, the oxidation of pyro catechol results three molecules of oxalic acids as follows:

So in this way lignin undergoes decomposition with the formation of simple aromatic compounds which either take part in a condensation reaction with other substances to form a primary molecule of humus substances or are subjected to further microbial attack and decomposed by ring fission into aliphatic compounds (low molecular weight organic acids). Complete oxidation of lignin gives rise to CO₂ and H₂O.

Organic compounds carrying Sulphur. There are various organic nitrogenous substances also carry sulphur (e.g. cysteine, cystine etc). These organic substances break down by the action of different micro-organisms through the process of mineralization and convert those sulphur bearing compounds into inorganic sulphur forms which are easily available to the plants.

The final transformation carried out by the sulphur-oxidizing organisms is shown below:

The organisms involved obtain energy by the transfer and bring (SO₄^2-).

Organic Compounds carrying Phosphorus:

There are various organic substances which carry phosphorus like phospholipid, phosphoproteins, nucleic acids and phytin, inositol hexaphosphate etc. Out of total soil phosphorus, organic phosphorus fraction occupies a large proportion.

All these organic phosphatic compounds are attacked by different micro-organisms and cause mineralization of those compounds. Due to such mineralization process, organic phosphorus compounds are transformed into inorganic combinations of phosphorus.

Phosphatase enzymes play a major role in the mineralization of organic phosphates in soil. They are a broad group of enzymes that catalyze the hydrolysis of both esters and anhydrides of phosphoric acid. There exists in soil a wide range of micro-organisms which through their phosphatase activities have the capability of mineralizing or dephosphorylating all known organic phosphates of plant origin.

Phosphatase activity of a soil is due to the combined functioning of the soil micro flora and any free enzymes present. It has been suggested that if the C: Pi (inorganic phosphorus) ratio is 200: 1 or less, mineralization of phosphorus will occur.

Again, if the C: P ratio is 300: 1 or more, immobilization of phosphorus will occur i.e. the native soil phosphorus will be taken up by soil micro-organisms and thereby decrease the concentration of available phosphorus in the soil. The final inorganic forms of phosphorus generally arises due to mineralization are H₂PO₄^– and HPO₄^2- (depending on soil pH). Both of these forms are available to plants.