In this article we will discuss about the carbon cycle with the help of diagram.
Carbon is the most important element in the biological system and constitutes about 50% of all living organisms. Carbon dioxide present in the atmosphere or dissolved in water is the ultimate source of organic carbon compounds occurring in nature; its complete cycle is schematically represented in Fig. 30.3.
The cycle of carbon in nature comprises of two main processes:
(i) The conversion of oxidized form of carbon into reduced organic form by photosynthetic organisms, and
(ii) Restoration of original oxidized form through mineralization of the organic form by the micro-organisms.
1. Conversion of Oxidized form of Carbon (CO2) into Reduced Organic Form:
CO2 is reduced into organic carbon compounds mainly by the process of photosynthesis. Photosynthetic algae and higher plants are the most important agents of carbon dioxide fixation. In the ocean the major plant forms that fix carbon are the free floating microscopic algae called phytoplanktons. They are estimated to fix annually about 1.2 x 1010 tons of carbon.
Nearly 1.6 x 1010 tons of carbon is said to be fixed annually by photosynthetic terrestrial plant life. Besides, autotrophic and heterotrophic bacteria are also capable of synthesizing organic matter from inorganic carbon. In addition to the occurrence of photosynthesis among microorganisms, the latter also represent the example of CO2 fixation into organic compounds which are as follows.
(i) The carbon dioxide represents the sole source of carbon for autotrophic bacteria. The latter fix CO2 to carbohydrates by a reduction reaction.
CO2 + 2H2 → (CH2O)x + H2O
(ii) Heterotrophic bacteria fix carbon dioxide commonly.
2. Restoration of Original Oxidized Form (CO2) through Mineralization of the Organic Form:
One can consider three different modes through which the organic matter is mineralizes and the CO2 is released in the atmosphere.
(i) Process of respiration,
(ii) Accidental (forest fire) and intentional (fuel) burning.
(iii) Decomposition of organic matter by microorganisms.
The process of respiration in plants and animals, and the accidental and intentional burning of plants and their parts result in the breakdown of organic carbon compounds releasing carbon dioxide in the atmosphere.
Decomposition of Organic Matter by Microorganisms:
The organic carbon compounds that eventually are deposited in the soil are degraded by the activities of microorganisms which are mainly the bacteria and fungi. The CO2 is released into the air and soil.
(i) Cellulose Decomposition:
Cellulose is the most abundant organic material in plants. It is readily attacked by many species of fungi and bacteria.
The process of cellulose decomposition to carbon dioxide can be summarized in the form of following reactions:
The fungi which decompose cellulose in soil are mainly Trichoderma, Aspergillus, Penicillium, Fusarium, Chaetomium, Verticillium. Rhizoctonia, Myrothecium, Merulius, Pleurotus, Fomes, etc.
The bacteria that bring about cellulose decomposition in soil consist mainly of the species of Clostridium, Cellulomonas, Streptomyces, Cytophaga, Bacillus, Pseudomonas, Nocardia, Micromonospora, Sporocytophaga, Polyangium, Cellfalcicula, etc.
(ii) Hemicellulose Decomposition:
Hemicelluloses are the polymers of simple sugars such as pentoses, hexoses and uronic acid. The decomposition of hemicelluloses by microorganisms takes place through the agency of extracellular enzymes called hemicellulases.
The fungi that degrade hemicelluloses in soil are examplified by Chaetomium, Aspergillus, Penicillium, Trichoderma, Fusarium, Humicola, etc. Bacillus, Pseudomonas, Cytophaga, Vibrio, Erwinia, Streptomyces, Actinomyces, etc. are the bacteria that degrade hemicelluloses in soil.
(iii) Lignin Decomposition:
Lignin is the third most abundant constituent of the plants. It is highly resistant of microbial degradation. However, certain fungi (examplified by Aspergillus, Penicillium, Fusarium, Lenzites, Clavaria, Polyporus, etc.) and bacteria (examplified by Streptomyces, Nocardia, Flavabacterium, Xanthomonas, Pseudomonas, Micrococcus, etc.) are known to degrade lignin at slow rates.