Minerals: Besides the above mentioned high protein content and digestibility, Spirulina come from waters with minerals deposited from ancient soils and mountains.No other plants can live in these areas due to the mineral content.
An animal (fish for our purposes) produces unconjugated biliverdin, a yellow colored breakdown product of normal heme catabolism, formed by failing red blood cells.
Heme (also called Haem in the UK) is composed of iron plus amino acids from globin of hemoglobin.
Fish can ingest high amounts of 'artificially' added inorganic minerals without benefit to health because the fish body (or other aquatic organism such as shrimp) does not know what to do with these incompatible forms.
In fact, evidence is accumulating that the inorganic minerals can block absorption of the organic forms, leading ultimately to mineral deficiency diseases (most fish foods are low in natural calcium and need added calcium to meet requirements).
Aphanizomenon Flos-Aquae is a related algae found in Klamath Lake Oregon (see References).