Zooxanthellae are microscopic algae that live inside the tissues of many marine invertebrates. This term is a colloquial name for single-celled organisms, primarily from the dinoflagellate genus Symbiodinium, that engage in photosynthesis. These tiny, golden-brown algae are a component of many marine ecosystems, converting sunlight into energy that sustains both themselves and their host organisms.
The Symbiotic Partnership with Corals
The relationship between zooxanthellae and corals is an example of mutualistic symbiosis. The algae live directly within the cells of the coral polyp’s tissues. Through photosynthesis, zooxanthellae produce energy-rich compounds like glucose, glycerol, and amino acids. This process supplies the coral with up to 90% of its energy needs and provides a steady supply of oxygen.
In return, the coral provides the algae with a protected environment away from predators. The coral’s metabolic waste products, including carbon dioxide, nitrogen, and phosphorus, are nutrients the zooxanthellae require for photosynthesis. This recycling of nutrients creates a highly productive system in the often nutrient-poor waters of the tropics.
This partnership has a direct visual impact on the reef. The vibrant colors of most shallow-water corals are not from the coral animal itself, but from the pigments within the billions of algal cells living in its tissue. Different species and genetic varieties of zooxanthellae contain different pigments, which contributes to the wide spectrum of colors seen across a healthy coral reef.
Role in Reef Construction and Ecosystem Health
The energy transferred from the zooxanthellae to the coral is a driving force behind reef formation. This energy fuels the process of calcification, where corals secrete calcium carbonate to build their hard skeletons. The rate of this skeletal growth is significantly enhanced by the photosynthetic activity of the symbiotic algae.
Over geological timescales, the slow accumulation of these calcium carbonate skeletons, layer upon layer, creates the massive, complex structures known as coral reefs. These structures can stretch for thousands of miles and are among the largest biological constructions on the planet.
These reefs become foundational habitats for countless other organisms. They provide shelter and breeding grounds for approximately 25% of all marine species, from small invertebrates to large fish. The energy produced by the zooxanthellae forms the base of the food web for the entire reef ecosystem.
The Phenomenon of Coral Bleaching
Coral bleaching is the visible sign of the breakdown of this symbiosis. When corals are subjected to environmental stress, they expel the zooxanthellae from their tissues. The primary trigger for widespread bleaching is an increase in sea surface temperatures. A sustained temperature increase of just 1-2°C (1.8-3.6°F) can be enough to disrupt the algae’s photosynthetic machinery, causing the coral to eject them. Other stressors like ocean acidification, pollution, and high solar irradiance also contribute to bleaching.
When the pigmented algae are expelled, the coral’s tissues become transparent, revealing the stark white of its calcium carbonate skeleton underneath. This is why the event is called “bleaching.” A bleached coral is not dead, but it is starving and becomes more vulnerable to disease without its primary food source.
If environmental conditions return to normal quickly, corals can reacquire zooxanthellae from the surrounding water and recover. If the stress is prolonged, the coral will not survive without its algal partners and will eventually die. This leads to the degradation of the reef structure and the loss of habitat for the species that depend on it.
Hosts Beyond Coral Reefs
While the partnership with corals is the most well-known, zooxanthellae are not exclusive to them. These versatile algae form symbiotic relationships with a variety of other marine invertebrates. Many species of sea anemones, for instance, host zooxanthellae within their tentacles and bodies, contributing to their coloration and nutrition.
Other hosts include giant clams of the genus Tridacna, which have specialized tissues to farm the algae, and certain types of soft corals. Sponges and some species of jellyfish, like the upside-down jellyfish (Cassiopea), also harbor these algal symbionts.