The green sulfur bacteria are a phylum, Chlorobiota, of obligately anaerobic photoautotrophic bacteria that metabolize sulfur.

Photoautotrophs are organisms that can utilize light energy from sunlight and elements from inorganic compounds to produce organic materials needed to sustain their own metabolism. This biological activity is known as photosynthesis, and examples of such photosynthetic organisms include plants, algae and cyanobacteria.