Layers of chemolithotrophic purple photosynthetic bacteria are often found in anoxic conditions in lakes or on the mud surface of tide flats. Unlike algae, photosynthetic bacteria cannot tolerate oxygen.These bacteria are known for their ability to exist in platelike layers at very low light levels in or just below the metalimnion. The plate is usually found below the zone where algea are likely to grow. Photosynthetic bacteria occur in many productive lakes,particularly saline ones,and,because of their narrow vertical distribution, may go unnoticed during routine sampling. Cyanobacteria,procaryotic cells that produce oxygen in photosynthesis, are also called blue-green algea.