Spirulina is a source of protein, iron, carotenoids, and some vitamins. The protein in spirulina is like other plant proteins. Like other plant proteins, it's not complete. This means it doesn't contain all nine essential amino acids. Because of this, it shouldn't be used as a main source of protein.
Spirulina has been used as a feed extender for livestock. It can provide some of the total protein intake (up to 25%). But it must be supplemented with amino acids that aren't in spirulina. These often include lysine, methionine, and histidine. Spirulina protein isn't digested as well as other livestock foods.
Spirulina can also be used as a source of protein and some vitamins for humans. But it's important to get it from a tested source. Spirulina can be tainted with microbes and radioactive metal ions. It can also be tainted with heavy metals. These can include mercury, cadmium, lead, and arsenic.