Even though the Cyanobacteria are classified as bacteria (lacking a membrane-bounded nucleus). However, because they are photosynthetic and aquatic, cyanobacteria are often called "blue-green algae" and included in algal collections and taxonomic hierarchies. This name is convenient for talking about organisms in the water that make their own food, but does not reflect any relationship between the cyanobacteria and other organisms called algae. Cyanobacteria are relatives of the bacteria, not eukaryotes, and it is only the chloroplast in eukaryotic algae to which the cyanobacteria are related.
Blue-green algae are common in soil, in both salt and fresh water, and can grow over a wide range of temperatures. They frequently form mats. Cyanobacteria can also occur as symbionts of protozoans, diatoms and lichen-forming fungi, and vascular plants.