What is zooxanthella?

• A group of unicellular brown algae known as dinoflagellates

• Can carry out photosynthesis (i.e. using sun energy, carbon dioxide, water and inorganic materials to produce organic food and oxygen)

• Benefit hermatypic corals in two ways

  1. leaks out 94-98% of the photosynthetic product to the coral polyp as food source

  2. enhances coral depositing the limestone skeleton by 2-3 times so that it enables corals to grow faster than they are eroded by the action of the sea and eroding organisms

Bleached Coral Colonies in Clear Water Bay

Skeleton of most corals is white and the soft living tissues are transparent.

Many of the splendid colours of corals come from the symbiotic zooxanthella and therefore breakdown of the association will result in coral bleaching.

When subject to adverse changes, such as temperature increase, corals release increased numbers of algae and hence impair the symbiotic relationship and cause coral bleaching.