Symbiodinium

Symbiodinium is a genus of single-celled dinoflagellate algae, commonly known as zooxanthellae, that form the largest and most widespread group of photosynthetic symbionts living inside other marine animals. They typically inhabit the inner tissue layers of tropical corals, sea anemones and jellyfish, trading the sugars they produce through photosynthesis for inorganic nutrients supplied by their host, and similar partnerships extend to sponges, flatworms, mollusks such as giant clams, and some single-celled hosts. Thriving in warm, nutrient-poor tropical waters, they are among the most abundant microbes in coral reef ecosystems and are essential to reef health, since the loss of these symbionts under heat stress is what causes coral bleaching. A major taxonomic revision in 2018 split the genus, once broadly defined, into seven separate genera, with the name Symbiodinium now applied more narrowly.