Coral Reef Health

Corals maintain an important lifelong partnership with dinoflagellates (also referred to as algae) or photoendosymbionts (photo = referring to photosynthesis used for energy; endo = referring to intracellular (within the coral host cell); symbiont = referring to the symbiosis). The partnership between coral and algae provides the coral animal with most of the energy it needs to grow and build reefs, which are home to thousands of different species on a coral reef.

Coral bleaching (when the coral turns from brown to white) occurs with the symbiosis when algae break down. The coral turns white because the algae are expelled and no longer kept within the coral cells. Once the algae is gone, all that is left is the transparent coral animal tissues, which are starved of nutrients. We then see the white skeleton of the coral.

Corals that have bleached will slowly starve if their symbiosis is not restored.

Bleaching occurs when water temperatures and light stress increase enough to damage the photosynthesizing ability of the algae. (Much like when you move a plant out into the hot sun; its leaves will bleach white as its ability to photosynthesize is damaged from too much heat and light).

Healthy corals

Healthy corals have dark, healthy tissue and often visible, extended polyps.

Partially bleached corals

When corals start to bleach, they expel their symbiotic algae. Without the algae, the coral polyps are mostly clear, and the white skeletons beneath become visible. This results in a patchy, partially bleached appearance.

Fully bleached corals

When corals fully bleach, they have expelled symbiotic algae from the coral tissues and the white skeleton beneath is clearly visible.