Heterotrophs's most suitable carbon source: organic matter such as sugar and protein, which cannot be synthesized by itself, must be obtained from the outside. The most suitable carbon sources for autotrophic microorganisms are carbonate and bicarbonate, because they can survive by converting inorganic carbon into organic matter.

In the process of assimilation, organisms cannot directly use inorganic substances to make organic substances, but can only convert ready-made organic substances absorbed from the outside into their own components and store energy. This metabolic type is called heterotrophy.