Bacterial nutrition divides bacteria into different classes, on the basis of carbon sources these are divided into two classes i.e. autotrophs and heterotrophs. Autotrophs use carbon dioxide as a sole source of carbon while heterotrophs utilize organic carbon.

Living organisms need a source of energy for survival, bacteria has been divided into two categories on the basis of the source of energy i.e. phototrophs which need light as their energy source and chemotrophs obtain energy from the oxidation of chemical compounds.

Bacteria which uses reduced inorganic substances as their electron sources are called lithotrophs while organotrophs are the bacterial class which utilizes organic compounds as their electron source.

Photolithoautotrophy is the class of the bacteria which utilize CO₂ as a carbon source, light energy as the energy source, and inorganic hydrogen as the electron source.

Purple and green non-sulfur bacteria use light energy as an energy source, organic carbon source, and organic hydrogen as the source of an electron.