Symbiodinium

Symbiodinium is a genus of unicellular dinoflagellates that live as endosymbionts inside the tissues of various marine invertebrates, most notably reef-building corals. Collectively known as zooxanthellae, these symbionts perform photosynthesis and translocate a portion of their photosynthetic products to their hosts, supplying a substantial part of the coral’s carbon needs and supporting reef productivity and calcification.

Taxonomy and diversity within the family Symbiodiniaceae have led to a complex naming framework. Symbiodinium is one of several closely related lineages, and genetic studies have grouped endosymbionts into clades labeled A through I (and sometimes J), with functional variation in traits such as light and thermal tolerance. The taxonomy of these organisms has undergone revisions, and many former species placed in Symbiodinium are now classified in other genera within Symbiodiniaceae, though the name Symbiodinium is still widely used in ecological literature.

Hosts include most reef-building corals and other marine invertebrates such as sea anemones; the symbionts are found in hospite (within host tissues) and can also exist freely in the environment. Transmission modes vary by host species, with some acquiring symbionts horizontally from the environment and others inheriting them vertically from parents.

Bleaching, a major threat to coral reefs, occurs when thermal or other stresses cause the breakdown of the symbiosis, leading to loss of Symbiodinium or pigment decline. Recovery can occur if symbiosis is re-established, sometimes involving shifts to more heat-tolerant clades.

Symbiodinium is central to coral reef ecosystems, influencing energy flow, resilience, and responses to climate change, and it remains a key focus of research on symbiosis and reef conservation.