How Do Corals Fluorescence?
Corals get their colours in two ways. They have photosynthetic marine algae living inside their cells which convert sunlight into energy. The brownish green colour you see in corals under normal daylight is from these algae, called zooxanthellae. When corals are bleached, they turn white because they expel the zooxanthellae.
But then there are the blues, the greens, purples, and reds which come from a family of Nobel prize-winning fluorescent proteins. The short answer is there are protein pigment in the tissue of corals which absorbs light in one colour, and re-emits the light in a different colour. These pigments look different under white light, daylight, and blue light.
In 2008, the Nobel Committee awarded Osamu Shimomura, Marty Chalfie and Roger Tsien with the Chemistry Nobel Prize for the discovery and development of the green fluorescent protein (GFP). Using GFP, we can see when proteins are made, and where they can go. This is done by joining the GFP gene to the gene of the protein of interest so that when the protein is made, it will have GFP hanging off it. Since GFP fluoresces, one can shine light at the cell and wait for the distinctive green fluorescence associated with GFP to appear.
Using only a white light, brighter reds, orange and yellow can be seen. Some of the light is absorbed into the protein while the colour we see is what is being reflected or re-emitted. That is also why companies sell red filters to go over cameras to bring the red back to life.
In regular sunlight, the deeper you dive, the bluer corals will appear. Any deeper than a few metres and longer wavelengths of light such as red, orange, and yellow are quickly absorbed in the water column. Once you reach 10 metres, everything looks blue. A flashlight or external LED or strobe light will bring these colours back into the environment.
Blue and Ultraviolet (UV) Light
To see fluorescent colour, a long wavelength of light must be used, such as blue and UV light. When shined on certain corals, the protein re-emits a fluorescent pigment as biofluorescence, glowing a bright neon colour. This colour is invisible under normal light conditions and they can only be seen when pigments are excited.
Read the rest of this article on the fluorescent world of corals in No.113 Issue 3/2018 of Scuba Diver magazine.