The atmosphere was made aerobic by cyanobacteria, a group of bacteria that includes most bacterial photosynthesizers. Originally they were called blue-green algae because of their ability to photosynthesize, but were reclassified when it was found that they are more similar to bacteria, existing as prokaryotic cells. They also can fix nitrogen into organic compounds that can be used in amino acids and nucleic acids. It is believed that the first chloroplasts were ancient cyanobacteria that were taken up by other cells. These bacteria have chlorophyll a and use two photosystems that spilt water and yield oxygen gas, like a plant's photosynthesis. They lack flagella and and can be single, colonial, or multicellular, with a division of labor similar to that which may have produced the first true multicellular species. They are usually present in water that has been polluted, often with phosphates and nitrates from runoff, because they thrive in water that has extra nutrients in it, and often explode in "algal blooms."