Zooxanthellae
Symbiotic microalgae that live in coral tissues. They provide energy through photosynthesis in exchange for protection and nutrients.
Detailed explanation
Zooxanthellae (genus Symbiodinium, now reclassified in Symbiodiniaceae) are unicellular algae that live inside coral cells. This symbiosis is the basis of the algae reef ecosystem.
The coral offers the zooxanthellae a protected environment and the nitrogen and phosphorus compounds produced by its metabolism. The zooxanthellae in return photosynthesize and give the coral up to 90% of the energy produced in the form of sugars, amino acids and fatty acids.
It is the zooxanthellae that give colour to corals. A brown coral has many zooxanthellae. A brightly coloured coral (purple, green, red) has fewer zooxanthellae and the coral's protective pigments become visible. A white coral has lost all its zooxanthellae: this is bleaching.
Bleaching occurs when the coral expels zooxanthellae due to stress. The most common cause is too high a temperature (above 28-30°C for prolonged periods). Excessive light, disease and pollutants can also cause bleaching.
A bleached coral doesn't die immediately. If conditions become favorable again, zooxanthellae can recolonize within weeks or months. If the stress persists for too long the coral dies due to lack of energy.
In the aquarium, managing zooxanthellae translates into managing light and nutrients. Too much light with too many nutrients produces brown corals. The right light with moderate nutrients produces the best colours. Few nutrients with a lot of light produces bleaching.
Different Symbiodiniaceae species have different temperature tolerances. C clades are the most common but D clades are more thermotolerant. This diversity is being studied for the resilience of coral reefs to climate change.