Carbon Cycle: Degradation of Organic Compounds, Carbon Dioxide Fixation

 Carbon cycle is the process of degradation of complex organic compounds and fixation of carbon dioxide.

Carbon is the most Important element in the structure of a cell.

• 40-50% of a cells dry-weight is carbon.
• This carbon comes from CO2, or organic carbon.

Carbon cycle

I].Organic Carbon Formation :  

• Plants, algae and photosynthetic bacteria fix CO2 into organic compounds through photosynthesis.
• Atleast, half the carbon present in earth is fixed mainly marine photosynthetic bacteria namely by Prochlorococcus, Synechococcus and Diatoms.

The other examples of CO2 transformation are by :

1. Autotrophic bacteria as per the following reaction : CO2 + 4H ➞ (CH2O)x + H2O
2. Heterotrophic microorganisms can fix CO2 by following reaction : CH3COCOOH + CO2 ➞ HOOC-CH2 CO.COOH. Pyruvic acid is converted into Oxaloacetic acid
3. Plant organic carbon is converted Into animal organic carbon when animals feed on plant.

• Deposition of all this organic carbon occurs in soil.
• Decomposition of organic compounds from soll occurs by microbial processes.
• Microbial mineralisation in aerobic conditions results into complete oxidation of these compounds with major end products CO2 and H2O.
• Under anaerobic condition Incomplete degradation of organic compounds produce CH4, H2, various organic acids and alcohols.
• CH4 is formed by Methanobacterium, Methanococcus, Methanosarcina and Clostridium spp.
• CH4 can be oxidised to CO2 by two rare species of Pseudomonas and Methylomonas. Thus, by the activity of microbes the immobilised organic carbon is mineralised to CO2.
• The plant and animal organic compounds are of different types. All these compounds are degraded and mineralised differently by different microorganisms.
• The organic constituents of plants are divided into the following different categories :

• Degradation of each substance is done by different microorganisms.

II]. Cellulose Degradation :

• Cellulose is present in the cell-wall of plant cell. It is a linear polymer of D-glucose linked by  β-1, 4, linkage. 
• A molecule of cellulose consists of 1900 to 10,000 units of glucose. 
• It is degraded by bacteria and fungi.
• In the first step cellulose is converted into cellobiose by enzyme cellulase.
• Cellobiose is then converted Into glucose by the enzyme β-glucosidase.
• Glucose is then converted into CO2 and H2O by enzyme systems of many microbes during catabolism.
• Cellulose is converted into Cellobiose by enzyme cellulase.
• Cellobiose is converted into Glucose by enzyme β-glucosidase
• Glucose Catabolise into CO2 + H2O and other products and energy.

• Factors affecting decomposition of cellulose :

1. Decomposition is faster in presence of nitrates.
2. Temperature of decomposition is between 5-56°C.
3. Presence of CO2 is necessary.
4. Presence of moisture is necessary.
5. Neutral to alkaline pH is needed.
6. Presence of organic substances increase the rate of decomposition.
7. The examples of aerobic microbes that decompose cellulose with the production  CO2 and H20 are as under :

Becteria

Achromobacter, Cellfalcicura, Ceilulomonas, Cellvibrio, Cytophaga, Pseudomonas, Bacillus, Micromonospora and Streptomyces.

Fungi

Alternaria, Aspergillus, Fusarium, Rhizopus, Penicillium, etc.

• Anaerobic organisms produce organic acids, alcohol CO2, H2 by cellulose decomposition. Clostridium, Bacteroids, Ruminococcus, etc. belong to this group.

III]. Hemicellulose Degradation :

• Hemicellulose is a polymer of pentose sugar especially of xylose and arabinose linked by β-1, 4 linkage. Enzyme responsible for its degradation is hemicellulase.
• The mechanism of degradation is not clearly understood.
• Final end products are CO2 and H2O.
• The organisms that degrade hemicellulose are:
• Bacteria - Bacillus, Pseudomonas, Cytophaga
• Fungi - Alternaria, Fusarium, Aspergillus, Rhizopus, Helminthosporium etc.

IV]. Lignin Degradation :

• Lignin is a polymer of aromatic alcohol and is highly resistant to degradation. 
• Lignin is a very complex molecule. Assaying and purification of lignin fraction from soil is difficult. 
• The end-product of lignin degradation are vanillin and vanilic acid.
• These compounds are formed very slowly, but can be oxidized as soon as they are formed.
• Microorganism responsible for lignin degradation are Clavaria, Hypholoma, Agaricus, other basidomycetes, Streptomyces, etc.

V). Pectic Substances Degradation :

• Pectin is a polymer of methy D-galactouronate
• It is degraded by enzymes protopectinase, polygalactouronase and pectin methyl esterase.
• The end-product of pectin degradation is galacturonic acid.
• The microorganisms involved are Bacillus, Clostridium, Pseudomonas, Fusarium, etc.

VI]. Humus :

• When plant and animal residue decompose in soil, the product formed is called Humus.
• It is soft, spongy, amorphous dark coloured substance made up of residual organic matter which is not capable of further degradation by microorganisms.
• It consists of heterogenous group of substances having an unknown chemical structure.
• It has no definite composition.
• Humus plays important role in soil fertility. It improves texture of soil by binding soll particles together.
• It has many types of functional groups and therefore it is a good buffering agent.
• It increases soil fertility by providing conditions favourable for growth of plants and microorganisms.
• Thus, humus is considered to be a store house of nutrients which may be available slowly to the living forms present In soil.