Spirulina is a simple one-celled microscopic blue-green algae (Cyanobacteria). There are two species, Arthrospira platensis and Arthrospira maxima.
The Arthrospira platensis and Arthrospira maxima species were once classified in the genus Spirulina. There is now agreement that they are in fact Arthrospira; nevertheless, and somewhat confusingly, the older term Spirulina remains in use for historical reasons.
Under a microscope, spirulina appears as long, thin, blue-green spiral threads. The odor and taste of spirulina is similar to seaweed.