16.3G: Sea Coral and Sea Anemone Zooxanthellae
- Outline the role Zooxanthellae play in animal sybiosis
Symbiodinium are colloquially called “zooxanthellae” (or “zoox”), 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. The genus Symbiodinium encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known to science. These unicellular algae commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where they translocate products of photosynthesis to the host and in turn receive inorganic nutrients (e.g. CO2, NH4+). They are also harbored by various species of sponges, flatworms, mollusks (e.g. 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 (note that in most mollusks, Symbiodinium are inter- rather than intra-cellular). 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 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, and only a single symbiont cell is found within 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.
Key Points
- Zooxanthellae species are members of the phylum Dinoflagellata. The most common genus is Symbiodinium.
- 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.
- Zooxanthellates mutualistic relationships with reef-building corals form the basis of a highly diverse and productive ecosystem.
Key Terms
- endosymbiont : An organism that lives within the body or cells of another organism.
- phagocytosis : the process by which a cell incorporates foreign particles intracellularly.
- benthic : Pertaining to the benthos; living on the seafloor, as opposed to floating in the ocean.