Zooxanthella
Zooxanthellae (Template:IPAEng) are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans.  They are members of the phylum Dinoflagellata and are typically dinoflagellate algae, although algae such as diatoms can also be zooxanthellae. They may be acquired by direct ingestion, and subsequently reproduce by splitting apart; a process known as budding. In other cases, zooxanthellae may be transmitted by the coral eggs and planulae. Most are autotrophs and provide the host with energy in the form of translocated reduced carbon compounds derived from photosynthesis. Zooxanthellae can provide up to 90% of a corals energy requirements. In return, the coral provides the zooxanthellae with protection, shelter, and a constant supply of the carbon dioxide required for photosynthesis. Their population in the host tissue is limited by available nutrients and incident light, and by expulsion of excess cells.  However, zooxanthellae do not appear to be digested by their hosts.
Hermatypic (reef-building) corals have zooxanthellae and are largely dependent on them, limiting their growth to the photic zone.  The symbiotic relationship is probably responsible for the phenomenal success of corals as reef-building organisms in tropical waters. However, when corals are subjected to high environmental stress, they can lose their zooxanthellae by either expulsion or digestion and die. The process known as coral bleaching occurs when the zooxanthellae densities within the coral tissue become low or the concentration of photosynthetic pigments within each zooxanthella decline. Color loss is also attributed to the loss or lowering of concentrations of Green Fluorescent Proteins (GFP) from the cellular pigments of the cnidarian itself. The result is a ghostly white calcareous skeleton, absent of zooxanthellae, with the inevitable death of the coral unless conditions improve, allowing for the zooxanthellae to return.
Coral are under constant disturbance, which is ultimately  felt by the zooxanthellae living within their tissues. Exposure to air during extremely low tides or damage from intensifying solar radiation in shallow water environments are some of the ecological stressors zooxanthellae face. Temperature changes have provided the most stress to the zooxanthellae-coral relationship. A rise in temperature of 1-2 degrees Celsius for 5-10 weeks or a decline in temperature of 3-5 degrees Celsius for 5-10 days has resulted in a coral bleaching event. Strong temperature changes shock the zooxanthellae and cause them to suffer cell adhesion dysfunction which sees the detachment of the cnidarian endodermal cells from the zooxanthellae.
Other organisms which may have zooxanthellae include jellyfish, clams, sea slugs, and radiolaria.  There are several different species of zooxanthellae, typically grouped together as the genus Symbiodinium, which appears to be monophyletic.
The genus, Symbiodinium, was created by Hugo Freudenthal in 1959, after his identification of the life cycle of zooxanthella from Cassiopea.  At that time he proved that they had a motile stage which resembled a "gymnodinioid" dinoflagellate. Being both symbiotic and a dinoflagellate, he named the Genus Symbiodinium, and the species "microadriaticium," after its resemblance to a similar free-living species.  There is considerable disagreement as to whether there are a single or many species of Symbiodinium.  DNA testing shows differences between the symbionts from different corals, but the issue is whether or not these are significant enough to represent different species.  Dr. Freudenthal demonstrated that the zooxanthellae go through a vegetative stage, a cyst stage, and a motile stage as part of their life cycle.