-ah-zan-the-lae) are microscopic single-celled yellow-brown algae (Symbiodinium spp.) that are found living endosymbiotically within the tissues of many marine invertebrate animals, such as cnidarians, flatworms, giant clams, jellyfish, nudibranches, sea anemones, and sponges. These endosymbiotic algae have a mutualistic relationship with the host animal. The host animal provides a protected environment and the compounds the algae needs, and in return the algae produce oxygen and help the host organism eliminate its waste. Extremely small, about 10,000 of these algae cells would fit in the
area of the period at the end of this sentence. Closely related to the free-living planktonic dinoflagellates, zooxanthellae are critical to the overall health and productivity of corals. Possessing chlorophyll, the zooxanthellae are able to utilize sunlight, CO2 and water in order to conduct photosynthesis and produce O2 and energy rich glucose. Additionally they make glycerol and amino acids available to their host animals which are able to utilize these substances to produce calcium carbonate, carbohydrates, fats, and proteins. The relationship between the algae and its host animal creates a pathway for the recycling of nutrients in the nutrient poor waters around a coral reef. As much as 90 percent of the organic material the algae produce is transferred to their host animal.
Zooxanthellae are perhaps best known for their relationship with cnidarians, in particular the reef-building or stony corals. Corals which contain zooxanthellae are typically referred to as photosynthetic or Zooxanthellae corals (sometimes called symbiotic corals), while those that do not are typically referred to as aphotosynthetic, nonphotosynthetic, or Azooxanthellae corals. While it is commonly believed that photosynthetic corals do not need to be feed, since the zooxanthellae provide their host animals with sugars such as glucose, carbohydrates are carbon rich and nitrogen poor. The equivalent if you will of reef "junk food." Evidence suggests that even photosynthetic corals much supplement their diet with planktonic food in order to acquire the proteins they need in order to survive and thrive. Hence the items that has been shown repeatedly to most significantly increase the overall healthy and growth of corals is not trace elements or light (although light is certainly critically important), but rather feeding and water flow.
Yet another result of the symbiotic relationship of zooxanthellae with their host animals is color; these algae and the pigments they contain are what give many marine invertebrate animals their beautiful colors. The pigments produced by the zooxanthellae are visible through the clear tissue of the host animal and range in color from golden-yellow to brown. The color will vary or change depending upon the intensity of light to which the host animal and its algae are exposed. If light levels are low, the numbers of symbiotic algae may increase and they may also produce more pigments in order to increase photosynthesis. In higher lighting the corals may expel some of their zooxanthellae or the zooxanthellae may reduce the amount of chlorophyll within themselves. Coloration in these host animals however is far more complicated than just the amount of zooxanthellae which are present. It also has to do with algae and pigment placement within the tissue of the host, as well the pigments that the host animals itself contains. While the pigments in zooxanthellae typically reflect light in the red and yellow spectrum, pigments within the host organism, particularly in corals typically reflect the green and blue spectrum. Therefore bulb color or temperature has a great deal to do with color development within these host organisms. Bluer light not only highlights the blue pigments, but also activates the fluorescent proteins which are not affected by longer wavelength light.
Unfortunately when corals become stressed, a mass expulsion of zooxanthellae leaves corals appearing white. The corals appear white because their tissue is mostly transparent and their calcium carbonate skeletons are white. This process is known as "bleaching" and bleached corals typically struggle to survive without their zooxanthellae. Coral bleaching can be fatal since the corals main energy source is now gone, and the coral begins to starve. Additionally, bleached corals have a much greater susceptibility to disease. If conditions return to normal relatively quickly or extra care is taken to ensure the corals receive sufficient food energy and remain disease free, corals can regain their zooxanthellae and survive. A large variety of condition can cause bleaching ranging from bacterial infections to changes in temperature or lighting.
The information for this article was obtained and rewritten from a number of different articles and posts written by both hobbyists and professionals. Credit goes to the following hobbyists from various forums: Florida Joe, Greenbeen 191, and Spanko, and the following professionals Borneman, Hatcher, Perun, and Weis.
I'm not an expert, so if anybody has anything to add, please feel free to post it up.