Zooxanthellae (Symbiodiniaceae)

node · earth · Global tropical and subtropical shallow seas, inside host tissue

Single-celled photosynthetic dinoflagellates living inside the gastrodermal cells of the coral host, at densities on the order of a million cells per square centimetre of tissue. They fix carbon and pass a large majority of it to the host as small organic compounds — glycerol, glucose, amino acids — which is what allows a large animal to live in tropical water so nutrient-poor that it is sometimes described as a marine desert. The commonly cited figure is that the symbionts can supply up to about ninety per cent of the host colony's energy requirement. They also supply the coral's colour: bleached coral is white because the host tissue is nearly transparent and the aragonite beneath it is not. Until 2018 the whole group was placed in one genus, Symbiodinium, and referred to by clade letters A to I; a systematic revision that year split it into a family with several genera, of which Cladocopium and the heat-tolerant Durusdinium are the ones that matter most on modern reefs.