Glossary Item
A carbon source is the compound from which an organism obtains carbon for synthesizing cellular components and deriving energy. It can be organic, such as sugars or fatty acids, or inorganic, such as carbon dioxide.
Explanation
Microorganisms require carbon to build biomolecules like proteins, nucleic acids, lipids and carbohydrates. The type of carbon source influences metabolic pathways and growth. Heterotrophs assimilate organic carbon compounds, oxidizing them to generate ATP and using intermediate metabolites to build biomass. Many bacteria and fungi preferentially use simple sugars like glucose because they enter glycolysis easily, while others can utilize complex substrates like cellulose or hydrocarbons after secreting enzymes that break them down. Autotrophs, including cyanobacteria and nitrifying bacteria, fix inorganic carbon from CO2 or bicarbonate using the Calvin–Benson cycle or other pathways, converting it into organic molecules. Chemoautotrophs derive energy from oxidation of inorganic compounds but still fix carbon dioxide. Selecting an appropriate carbon source in culture media is critical for isolating and cultivating microbes, as availability affects gene expression, enzyme production and secondary metabolism. Carbon limitation or excess can trigger physiological responses such as sporulation, biofilm formation or production of storage compounds like polyhydroxyalkanoates.
Common Carbon Sources in Media
In laboratory culture, carbon sources are chosen based on the organism’s nutritional profile. Glucose is the most widely used monosaccharide, supporting rapid growth of enteric bacteria and yeast. Sucrose and lactose are disaccharides frequently included in differential media to test fermentation abilities. Organic acids such as acetate and citrate serve as carbon sources for Acetobacter and some Gram-negative rods. Ethanol or glycerol support certain yeast and methylotrophs. Complex natural extracts like peptone and yeast extract provide a mixture of amino acids and peptides that supply both carbon and nitrogen. Autotrophic media omit organic carbon and instead provide inorganic salts and a supply of CO2. A suitable carbon source ensures balanced growth and influences metabolic end products.
Related Terms: heterotroph, autotroph, metabolism, energy source, nutrient