• Zooxanthellae are single-celled plants that live in the tissues of animals. These organisms are part of a group of dinoflagellates that are most often found as plankton. (phytoplankton)
• Like most plants, phytoplankton are able to convert the suns energy into food through a process called photosynthesis, so to survive they are only found in the upper layers of the sea and lakes where sunlight can penetrate.
Zooxanthellae in Gastrodermal Cells of Corals"Zooxanthellae are very small that they can only be seen through electron microscope"
Zooxanthellae is best known for a mutualistic relationship they have with reef building corals
The coral reefs have formed as the result of thespecial symbiotic relationship which exists between polyps andthe zooxanthellae inhabiting their cells.
Corals provide a protected environment andcompounds needed for photosynthesis tozooxanthellae.In turn, zooxanthellae provide food as productsof photosynthesis to coral.This gives corals a boost of nutrients, so they cansecrete the calcium carbonate skeleton that servesas the foundation for coral reef.This important relationship represents a highlyefficient exchange of nutrients in a nutrient-poorenvironment.
Benefits to the Coral Host• Higher rates of calcification (skeleton deposition).• Energy and materials for growth, repair, and reproduction.• Removal of chemical wastes from animal metabolism (nutrient recycling).• Receive chemicals that absorb damaging ultraviolet light.
Benefits to the Algal Symbiont • Chemical wastes from animal metabolism are important inorganic nutrients for plants (nutrient recycling). • Surrounding animal tissues can concentration substantial amounts of ultraviolet light absorbing compounds. • Zooxanthellae, nonmotile single cells, get a place to live in the crowded reef environment.
Corals are not the only hosts of zooxanthellae. Some other reef hosts include:• Anemones• Giant clams• Gorgonian corals or sea fans• Soft corals• Sea whips• Nudibranchs• Jellyfish
Zooxanthellae live in other protozoa (foraminiferans and radiolarians)and in some invertebrates.
Transmission of Zooxanthellae into Host Tissues• Maternal (direct) transmission.• Environmental (indirect) transmission
•Polyps can acquire Zooxanthellae by direct ingestion. However, their hosts donot digest them. In other cases, zooxanthellae may be transmitted by coral eggsand planulae. Zooxanthellae reproduce asexually by budding (one individualsplitting into multiple descendants.
Healthy Bleached coral coral Corals bleaching occurs when environmental stressadversely affects the mutually beneficial relationshipbetween the host organism and its zooxanthellae.
• Coral bleaching occurs when zooxanthellae densities within coral tissue become low or the concentration of photosynthetic pigments within each zooxanthella decline.• Color loss also comes from reduced concentrations of pigments produced by the cnidarian itself.
What can stress a coral?- High light or UV levels- Cold temperatures- Low salinity and high turbidity from coastal runoff events or heavy rain- Exposure to air during very low tides- Major: high water temperatures
Thermal stress- Corals live close to their thermal maximum limit- If water gets 1 or 2 C higher than the summer average, corals get stressed and bleach
Healthy Bleached coral with coral with algae no algae• - As a stress response, corals expel the symbiotic zooxanthellae from their tissues• - The coral tissue is clear, so you see the white limestone skeleton underneath
Can corals recover? - Yes, if the stress doesn’t last too long - Some corals can eat more zooplankton to help survive the lack of zooxanthellae - Some species are more resistant to bleaching, and more able to recover - Corals may eventually regain color by repopulating their zooxanthellae