Coral and Algae, the colorful symbiotic relationship

Corals are animals with a range of colors like red, green, purple, etc. However, do corals themselves produce these colors, or something else makes them stand out from the blue of the ocean?
Introduce

Corals belong to the class Anthozoa and are categorized as hard corals and soft corals. The colorful coral reefs around the world are mainly hard coral, specifically stony corals (Scleractinians).

Stony corals are colonial organisms made up of hundreds to hundreds of thousands of individuals, called polyps. Each coral polyp was formed from 3 basic layers including an outer epidermis, an inner layer and interspersed between 2 layers is mesoglea. And almost all stony corals, gastrodermal cells (the inner layer) of polyps contain algae with a symbiotic relationship that not only provides food for corals but also creates their color.

The factors determine the color of corals

There are three factors that determine the color of corals – photosynthetic pigments, non-fluorescent chromoproteins and fluorescent proteins. The algae symbiotic with corals are dinoflagellates belonging to the genus Symbiodinium, and the common name is Zooxanthellae. Like other algae, Zooxanthellae contain chlorophyll so they have photosynthesis to produce the nutrients needed for corals. So light intensity is one of the important factors for the “health” of this symbiotic relationship. Zooxanthellae are very sensitive to high-intensity light, which can affect the “health” of corals. On the other hand, corals also have their own mechanisms for adjusting Zooxanthellae density to suit the environment. If the light intensity is elevated, corals will tend to “expel” Zooxanthellae out of their body, and when the light intensity is low, Zooxanthellae density will increase.

The colors of Zooxanthellae range from golden-yellow to brown, but they still can make deep red or bright orange in some cases. For example, Phycoerythrin is a photosynthetic pigment present in Zooxanthellae cells that can fluoresce a bright orange.  Similar to Phycoerythrin, corals produce some proteins that can absorb light of one wavelength and fluoresce a different wavelength which has lower energy to make red, green, cyan and an estimated 85 fluorescent pigments were found. In addition, corals also manufacture another pigment protein that they are incapable of fluorescence, although they can still produce colors such as purple, red, blue.

However, today more and more coral reefs in the world are losing their color, instead of the white of dead corals, it is called “coral bleaching”. The main reason for this situation is climate change, the increase of the sea surface temperature, corals expelling Zooxanthellae from their bodies, this not only affects the life of corals but also loses photosynthetic pigments that play an important role in the formation of coral colors.

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