MICROBIOLOGY Study Guide - 2012

CHAPTER 11. ROLE OF MICROORGANISMS IN BIOGEOCHEMICAL CYCLES

11.2. THE CARBON CYCLE

Carbon is a structural element of all organic substances. The primary source of carbon is atmospheric CO₂, which makes up about 0.03% of the atmosphere. Plants, Algae, and cyanobacteria play an active role in the carbon cycle as organisms that fix CO₂ during Photosynthesis, alongside microorganisms that decompose the organic matter of dead plants and animals, releasing CO₂.

The carbon cycle (Fig. 36) begins with the fixation of CO₂ by green plants and autotrophic microorganisms. During photo- and METABOLISM/21.html">Chemosynthesis, sugars and other related Organic compounds are primarily formed. The bulk of fixed carbon in woody and herbaceous plants is stored in the form of polymeric CARBOHYDRATES, such as starch, Cellulose, Lignin, and pectic substances, which concentrate a significant portion of carbon. These substances are used by plants and photosynthetic microorganisms for energy, and are consumed by humans and animals, which release carbon as CO₂ through Respiration. The carbon released during the decomposition of dead PLANT AND ANIMAL matter is oxidized to CO₂ and also returned to the atmosphere.

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Fig. 36. Diagram of the carbon cycle

Heterotrophic microorganisms play a leading role in returning carbon to the atmosphere. They decompose organic compounds through respiration and Fermentation. Carbohydrates and alcohols are the most readily accessible of these. However, natural environments (such as soil and Water) contain far more recalcitrant high-molecular-weight compounds. The decomposition of these substances begins with Hydrolysis, yielding simpler compounds whose subsequent transformation occurs via respiration or fermentation.

Starch is decomposed by microorganisms exhibiting amylolytic activity. Under aerobic conditions, starch is oxidized via The Tricarboxylic Acid Cycle into CO₂ and H₂O. The ability of microorganisms to break down starch is utilized in the alcohol, baking, and textile industries (for desizing textiles).

Pectic substances (from Greek pektos — congealed, curdled) are plant Cell wall Heteropolysaccharides based on galacturonic acid, with side chains containing arabinose, galactose, and rhamnose. Pectins are abundant in fruits, berries, and ROOT/tuber vegetables. The decomposition of pectic substances begins with their hydrolysis by the enzyme pectinase. Under aerobic conditions, this process can be carried out by B. subtilis, B. mesentericus, and others; under anaerobic conditions, by C. pectinovorum and C. felsineum. High pectolytic activity is characteristic of Molds such as Aspergillus niger, Penicillium glaucum, Mucor mucedo, Cladosporium herbarum, Sclerotinia libertiana, and others. The retting of flax and hemp relies on the METABOLIC ACTIVITY OF pectin-degrading microorganisms. Active strains serve as producers of pectolytic Enzymes used for clarifying fruit and vegetable juices. Microorganisms with active pectinases are pathogens causing various fruit and vegetable diseases, including Bacteria of the genus Ervinia, and Fungi of the genera Fusarium, Botrytis, and Aspergillus.

Cellulose (from Latin cellula — a small cell) is the most widespread plant cell wall substance, representing a polysaccharide composed of chains of 100 to 200 glucose molecules. Both anaerobic and aerobic breakdown of cellulose are distinguished. In nature, these processes occur in soil, manure, aquatic ecosystems, and the digestive tract of herbivores. Anaerobic cellulose-degrading bacteria include Ruminococcus flavefaciens found in the rumen of ruminants.

Aerobic oxidation of cellulose is carried out by bacteria of the genus Cytophaga, discovered by S. N. Winogradsky. Cellulolytic activity is also exhibited by Pseudomonas fluorescens var. cellulosa, myxobacteria of the genera Myxococcus, Archangium, and Polyangium, actinomycetes, and fungi of the genera Fusarium, Botrytis, and Rhizoctonia. Some fungi cause wood decay: some develop on living trees, while others attack harvested timber.