Autotrophic organisms can be divided into two categories: photoautotrophic and chemoautotrophic. Higher plants and cyanobacteria (primitive unicellular algae with photosynthetic pigments in their cells) can carry out photosynthesis, which belongs to photoautotrophic (photosynthesis). Nitrifying bacteria are chemoautotrophic, and they can use the chemical energy released when inorganic substances are oxidized.