Cyanidium caldarium belongs to the Cyanidiaceae family. It is a microscopic red algae that is found in acidic hot streams, bogs, and moist acidic soils all over the world. It is the only species in the genus Cyanidium. This algae use photosynthesis to acquire its nutrient. It contains the photosynthetic pigment, phycoerythrin, in addition to chlorophyll.