Symbiodinium is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known and have photosymbiotic relationships with many species. These unicellular microalgae commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where the products of their photosynthetic processing are exchanged in the host for inorganic molecules. They are also harbored by various species of demosponges, flatworms, mollusks such as the giant clams, foraminifera (soritids), and some ciliates. Generally, these dinoflagellates enter the host cell through phagocytosis, persist as intracellular symbionts, reproduce, and disperse to the environment. The exception is in most mollusks, where these symbionts are intercellular (between the cells). Cnidarians that are associated with Symbiodinium occur mostly in warm oligotrophic (nutrient-poor), marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems. Symbiodinium are colloquially called zooxanthellae, and animals symbiotic with algae in this genus are said to be "zooxanthellate". The term was loosely used to refer to any golden-brown endosymbionts, including diatoms and other dinoflagellates. Continued use of the term in the scientific literature is discouraged because of the confusion caused by overly generalizing taxonomically diverse symbiotic relationships. In 2018, the systematics of Symbiodiniaceae was revised, and the distinct clades have been reassigned into seven genera. Following this revision, the name Symbiodinium is now sensu stricto a genus name for only species that were previously classified as Clade A. The other clades were reclassified as distinct genera (see Molecular Systematics below).
Intracellular symbionts
Many Symbiodinium species are known primarily for their role as mutualistic endosymbionts. In hosts, they usually occur in high densities, ranging from hundreds of thousands to millions per square centimeter. The successful culturing of swimming gymnodinioid cells from coral led to the discovery that "zooxanthellae" were actually dinoflagellates. Each Symbiodinium cell is coccoid in hospite (living in a host cell) and surrounded by a membrane that originates from the host cell plasmalemma during phagocytosis. This membrane probably undergoes some modification to its protein content, which functions to limit or prevent phago-lysosome fusion. The vacuole structure containing the symbiont is therefore termed the symbiosome. A single symbiont cell occupies each symbiosome. It is unclear how this membrane expands to accommodate a dividing symbiont cell. Under normal conditions, symbiont and host cells exchange organic and inorganic molecules that enable the growth and proliferation of both partners.
Natural services and economic value Symbiodinium is one of the most studied symbionts. Their mutualistic relationships with reef-building corals form the basis of a highly diverse and productive ecosystem. Coral reefs have economic benefits – valued at hundreds of billions of dollars each year – in the form of ornamental, subsistence and commercial fisheries, tourism and recreation, coastal protection from storms, a source of bioactive compounds for pharmaceutical development, and more.
Coral bleaching
The study of Symbiodinium biology is driven largely by a desire to understand global coral reef decline. A chief mechanism for widespread reef degradation has been stress-induced coral bleaching caused by unusually high seawater temperature. Bleaching is the disassociation of the coral and the symbiont and/or loss of chlorophyll within the alga, resulting in a precipitous loss in the animal's pigmentation. Many Symbiodinium-cnidarian associations are affected by sustained elevation of sea surface temperatures, but may also result from exposure to high irradiance levels (including UVR), extreme low temperatures, low salinity and other factors. The bleached state is associated with decreased host calcification, increased disease susceptibility and, if prolonged, partial or total mortality. The magnitude of mortality from a single bleaching event can be global in scale as it was in 2015. These episodes are predicted to become more common and severe as temperatures worldwide continue to rise. The physiology of a resident Symbiodinium species often regulates the bleaching susceptibility of a coral. Therefore, a significant amount of research has focused on characterizing the physiological basis of thermal tolerance and in identifying the ecology and distribution of thermally tolerant symbiont species. The symbiosis Symbodinium-coral could provide higher resistance to multiple stress (desiccation, high UVR) to the coral holobiont through its mycosporine-like amino acids (MAAs). The concentration of MAAs increases with stress and ROS in Symbodinium. These UV-absorbing MAAs may also support light-harvesting pigments during photosynthesis, be source of nitrogen storage and for reproduction. More than half of the Symbodinium taxa contain MAAs. Symbiodinium trenchii is a stress-tolerant species and is able to form mutualistic relationships with many species of coral. It is present in small numbers in coral globally and is common in the Andaman Sea, where the water is about 4 °C (7 °F) warmer than in other parts of the Indian Ocean. In the Caribbean Sea in late 2005, water temperature was elevated for several months and it was found that S. trenchii, a symbiont not normally abundant, took up residence in many corals in which it had not previously been observed. Those corals did not bleach. Two years later, it had largely been replaced as a symbiont by the species normally found in the Caribbean. Symbiodinium thermophilum was recently found to make up the bulk of the algal population inside the corals of the Persian Gulf. It is also present in the Gulf of Oman and the Red Sea, at a much lower concentration. Coral that hosted this species was able to tolerate the 35 °C (95 °F) waters of the Persian Gulf, much hotter than the 31 °C (88 °F) of coral reefs globally.
Molecular systematics
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