Photosynthesis is the use of light as a source of energy for growth, more specifically the conversion of light energy into chemical energy in the form of ATP. Prokaryotes that can convert light energy into chemical energy include the photosynthetic cyanobacteria, the purple and green bacteria, and the "halobacteria" (actually archaea). The cyanobacteria conduct plant photosynthesis, called oxygenic photosynthesis; the purple and green bacteria conduct bacterial photosynthesis or anoxygenic photosynthesis; the extreme halophilic archaea use a type of nonphotosynthetic photophosphorylation mediated by a pigment, bacteriorhodopsin, to transform light energy into ATP.

Net equation:

6CO ₂ +12H₂O+LightEnergy → C₆H₁₂O₆+6O₂+6H₂0

 Photosynthetic reactions divided into two stages:

• •  Light reaction - light energy absorbed & converted to chemical energy (ATP, NADPH)

•  Dark reaction- carbohydrates made from CO₂ using energy stored in ATP & NADPH

Types of bacterial photosynthesis

Five photosynthetic groups within domain Bacteria (based on 16S rRNA):

1. Oxygenic Photosynthesis

• •  Occurs in cyanobacteria and prochlorophytes

•  Synthesis of carbohydrates results in release of molecular O₂ and removal of CO₂ from atmoshphere.

•  Occurs in lamellae which house thylakoids containing chlorophyll a/b and phycobilisomes pigments which gather light energy

•  Uses two photosystems (PS):

- PS II- generates a proton-motive force for making ATP.

- PS I- generates low potential electrons for reducing power.

2. Anoxygenic Photosynthesis

• •  Uses light energy to create organic compounds, and sulfur or fumarate compounds instead of O₂.

•  Occurs in purple bacteria, green sulfur bacteria, green gliding bacteria and heliobacteria.

•  Uses bacteriochlorophyll pigments instead of chlorophyll.

•  Uses one photosystem (PS I) to generate ATP in “cyclic” manner.