Based on the source of carbon microorganisms are grouped as autotrophs and heterotrophs. Some can use CO2 as their sole source of carbon like plants and algae are termed as autotrophs. Others like some bacteria, fungi and actinomycetes utilize preformed organic compounds as carbon source and hence called heterotrophs. Most organisms that involve in decomposition of organic matter in soil are heterotrophs Fungi are saprophytic and depend on dead organic matter. Some fungi are parasitic on living plants and animals. The saprophytic and parasitic organisms are heterotrophs. Such of these heterotrophs that have elaborate requirements of specific nutrients like vitamins and growth promoting substances are called fastidious heterotrophs as they are not easily pleased or satisfied by ordinary nutrients available in nature.
The source of carbon for microbes is CO2 or carbohydrates. Autotrophs derive their entire requirement of C from CO2 while heterotrophs derive the carbon chiefly from carbohydrate. In nature, cellulose, hemicelluloses, starch, pectin, lignin etc serve as carbon sources. Amino acid, purine and pyrimidine bases, protein serve as a source of nitrogen. Phosphorus is obtained from the nucleotides, phytin etc. For cultivation of microorganisms in laboratory, media containing monosaccharides like glucose and disaccharides like sucrose are used as C sources. Peptone, Tryptone, inorganic salts like ammonium salts, potassium nitrate serve as nitrogen sources. Potassium dihydrogen phosphate and dipotassium hydrogen phosphate are commonly employed to serve as sources of phosphorus and also as a buffering agent.
Autotrophic bacteria have the simplest nutritional requirements as they can grow and reproduce in a mixture of inorganic compounds. They also possess an elaborate capacity to synthesize the carbohydrate, proteins, lipids, nucleic acids, vitamins and other complex substances of living cells. Photosynthesis is a normal autotrophic way of life and this occurs in plants, algae, photosynthetic bacteria and cyanobacteria. In this process, CO2 is reduced and converted to carbohydrate utilizing light. However, photosynthesis of plants, algae and cyanobacteria perform oxygen evolving photosynthesis by absorbing the reducing power from the photolysis of water. On the other hand, photosynthetic bacteria green and purple bacteria obtain the reducing power from a compound similar to water (H20) viz., H2S i.e. available in anoxygenic environment. The pigments and the light absorption also differ in these organisms.
Over all reaction of photosynthesis is,
2H20 + CO2 −−> (C2H0) x + O2 + H20 (Plants, algae, cyanobacteria)
2H2S+ CO2 −−> (C2H0) x + 2S + H20 (Photoautotroph bacteria)
Inorganic compounds like H2, H2S203 or the organic compounds lactate, succinate can be the source of reducing power instead of H2S.