Cyanobacteria and purple sulfur bacteria both obtain energy from light. however, only the are capable of releasing

Cyanobacteria, oxygen

Cyanobacteria also known as Cynophyta and blue green algae. It is a phyllum of bacteria that produces it's own food like plants by the process of photosynthesis. It is found in freshwater lakes, damp soils, oceans, moistened rocks and even in animal fur. The purple sulfur bacteria belongs to the group of Proteobacteria. It is an anaerobic bacteria. It is found in hot springs, and stagnant water. It is a photosynthesizing bacterial strain like Cyanobacteria but it does not utilize water to be reduced in the photosynthesis process like in Cynobacteria, to produce oxygen.

Hence, only the Cyanobacteria are capable of releasing oxygen.  

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