Spirulina is a type of blue-green algae that is rich in protein, vitamins, minerals, carotenoids, and antioxidants that can help protect cells from damage. It contains nutrients, including B complex vitamins, beta-carotene, vitamin E, manganese, zinc, copper, iron, selenium, and gamma linolenic acid (an essential fatty acid). Spirulina -- like any blue-green algae -- can be contaminated with toxic substances called microcystins. It can also absorb heavy metals from the water where it is grown. For these reasons, it is important to buy spirulina from a trusted brand.