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