cyanobacteria or blue-green algae, photosynthetic bacteria that contain chlorophyll. For many years they were classified in the plant kingdom along with algae, but discoveries made possible by the electron microscope and new biochemical techniques have shown them to be prokaryotes more similar to bacteria than to plants, and they are now placed in the kingdom Monera. Cyanobacteria are familiar to many as a component of pond scum. Despite their name, different species can be red, brown, or yellow; blooms (dense masses on the surface of a body of water) of a red species are said to have given the Red Sea its name. Nitrogen-fixing cyanobacteria need only nitrogen and carbon dioxide to live.