Although they are sometimes called "blue-green algae," cyanobacteria are not plants and are prokaryotes, while green algae are plants and are eukaryotes. Both types of organisms obtain their energy from photosynthesis, but cyanobacteria do not have a membrane-surrounded nucleus, the lack of which is a structural characteristic of prokaryotes. Unlike green algae, cyanobacteria do not contain chloroplasts.
Green algae possess a nucleus which is enclosed in a membrane, a characteristic that classifies them as eukaryotes. Green algae cells are also significantly larger that prokaryote cells. Like land plants, green algae possess chloroplasts, but they do not have the differentiated tissues that would enable them to adapt to life on land and they must live and reproduce in water.
Although they are not plants, cyanobacteria contain chlorophyll, and many are oxygenic phototrophs that belong to a class of bacteria that can produce their own food through photosynthesis. Their cells have no interior compartments and are filled with photosynthetic pigments. Some species of cyanobacteria are able to grow in the dark using sucrose and glucose as a source of energy and carbon.
Some of the oldest fossils are of interconnected colonies of cyanobacteria and are generally acknowledged as having played a critical role in the evolution of oxygen-dependent terrestrial life. Known to be highly tolerant of extreme conditions, cyanobacteria were the first oxygen-producing phototrophs to evolve on Earth and helped to transform an anoxic environment into one supportive of organisms that require oxygen for respiration.