Explanation

Oscillatoria is a photosynthetic cyanobacterium. In this, photosynthesis water is electron donor and oxygen is a byproduct, i.e., oxygenic photosynthesis occurs. Rhodospirillum and Chlorobium are non-oxygenic photosynthetic, purple non-sulphur and green-sulphur bacteria. Chromatium is purple sulphur bacterium and also non-oxygenic photosynthetic.