Wednesday, 16 December 2020

Carbon Cycle: Degradation of Organic Compounds, Carbon Dioxide Fixation

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

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 mineralization 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.

• CH2 is formed by Methanobacterium, Methanococcus, Methanosarcina and Clostridium spp.

• CH4 can be oxidized to Co2 by two rare species of Pseudomonas and Methylomonas. Thus, by the activity of microbes the immobilized organic carbon is mineralized to CO2.

• The plant and animal organic compounds are of different types. All these compounds are degraded and mineralized differently by different microorganisms.

• The organic constituents of plants are divided into the following different catagories :

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.

• 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 purfications 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 methyl
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.

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