published in life
Each polyp hosts unicellular algae called “zooxanthellae”, which give them a brown-greenish colour. This particular combination is called “”mutualism”, i.e. both species benefit from living together. The algae, through the photosynthesis of the chlorophyll, supply the polyp with energy in the form of sugars, produce oxygen and remove carbon dioxide (which could form carbonic acid and damage the calcareous skeleton of the polyps). In return, the polyp offers protection to the microscopic and numberless algae. Every square centimetres of the madrepore can contain up to one million zooxanthella algae.