Carbon source: Any nutrient that can provide carbon for the growth and reproduction of microorganisms is called carbon source. The carbon sources required by microorganisms can be inorganic carbon (carbon-containing inorganic substances such as carbon dioxide/sodium bicarbonate) and organic carbon (carbon-containing organic substances such as sugars/fatty acids/petroleum). Autotrophic bacteria (such as nitrifying bacteria) can fix CO2 in the air by themselves to synthesize their own organic matter and use inorganic carbon sources. Heterotrophic bacteria (such as lactic acid bacteria) use sugars to produce lactic acid, and the carbon source is organic carbon. The distinction between autotrophic and heterotrophic depends on whether the organism can synthesize inorganic matter into organic matter by itself.