Spirulina, a type of blue-green alga, was first used as a food source by ancient Aztecs. In modern times, it earned widespread popularity as a dietary supplement when NASA began to include it in the space diets of astronauts. Spirulina contains more than 50 percent protein, and is a rich source of essential fatty acids, vitamins, minerals and antioxidants. Recent research has revealed a number of health benefits and some risks of spirulina.