Cyanobacteria, also known as blue-green algae, differ most prominently from other bacteria in that cyanobacteria possess chlorophyll A, while most bacteria do not contain chlorophyll. This gives them their characteristic blue-green color and causes them to give off oxygen during photosynthesis.
Other differences exist between bacteria and cyanobacteria. While bacteria may or may not possess small tails, or flagella, to propel them forward, cyanobacteria do not. Cyanobacteria are also typically larger than other bacteria. While most bacteria are heterotrophic, meaning that they consume other organic matter for food, cyanobacteria are autotrophs, able to produce their own food out of organic compounds via photosynthesis.
Cyanobacteria also differ from bacteria in terms of how helpful or harmful they may be. The ability of cyanobacteria to give off oxygen during photosynthesis is directly responsible for the presence of oxygen in early Earth's atmosphere, creating conditions that allowed life to develop. Cyanobacteria also possess the ability to fix nitrogen, which can be both helpful and harmful to humans. Nitrogen fixation makes cyanobacteria useful as an agricultural fertilizer, but nitrogen-based runoff from fertilizer or other waste can pollute waters and destroy river and marine life. Like bacteria, however, cyanobacteria may be useful in preventing or treating diseases.