Photosynthetic organisms use light energy to fuel their biosynthetic processes. Oxygen is generated in oxygenic photosynthesis where water is used as the electron donor. In anoxygenic photosynthesis, organic or sulfur compounds are used as electron donors. Plants, algae and cyanobacteria carry out oxygenic photosynthesis, whereas the photosynthetic bacteria obtain energy from anoxygenic photosynthesis. Aerobic anoxygenic phototrophic bacteria use light energy in a similar way as the purple bacteria, and are a group of photosynthetic bacteria that grow under aerobic conditions.

Phototrophic organisms have a photosynthetic apparatus consisting of a reaction centre intimately associated with antenna molecules (or a light-harvesting complex). The antenna molecules and the reaction centre absorb light energy. The energy is concentrated at the reaction centre that is activated and initiates light-driven electron transport. Halophilic archaea convert light energy through a photophosphorylation process.

Photosynthetic microorganisms

Microorganisms utilizing light energy include eukaryotic algae, and cyanobacteria, photosynthetic bacteria and aerobic anoxygenic phototrophic bacteria among the prokaryotes. The halophilic archaea synthesize ATP through photophosphorylation, but they are not considered to be photosynthetic organisms since they lack photosynthetic pigments.

Algae and cyanobacteria have similar photosynthetic processes, using chlorophyll, as plants. However, cyanobacteria are members of the proteobacteria according to their cell structure and ribosomal RNA sequences. Photosynthetic bacteria are different from other photosynthetic organisms. They have different photosynthetic pigments and do not use water as their electron donor. Some of them can grow chemoorganotrophically in the dark.