How Do Corals Fluoresce
How do corals glow in the dark? Read on as Nicole Helgason peels away the secrets behind its mysterious glow.
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.
White Light
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.
Daylight
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.
Fluoro Light
Though UV lights were often used in the early days of fluoro diving, they have been found to be less effective at making corals fluoresce and may even be harmful to the organisms. We’ve been told that staring into the sun isn’t good for our eyes due to the UV light it emits. The same concept applies to marine animals. As such, today UV light has been replaced by precision blue LEDs emitting light in the range from 450 to 470 nanometres.
To see the psychedelic colours of fluorescent light, it is highly recommended to use a blue excitation light, which are commonly called fluoro lights, or fluoro torches. It’s important to purchase a blue light made with blue LEDs for fluoro diving, and not a purple hue UV light or a white light with a blue filter.
Fluoro torches feature a dichroic, interference or excitation filter in front of the lens. The filter is there to optimise the range of fluoro light and only lets a certain wavelengths pass through, while reflecting all other colours. Any new LED light that markets itself as a fluoro light should have these features.
When using a blue fluoro light, corals will glow in psychedelic shades of blue, pink, and purple. If the thought of night diving makes you feel uncomfortable, you can still see these colours during the day, although not as bright. I personally love using blue fluoro light to search for little zoanthids in the sand. They look like colourful flowers, each around three millimetres wide. The tiny polyps close when disturbed and unfurl when at rest. A fluoro light is perfect for spotting these little treasures as they light up amongst the muck.
Read the rest of this article on the fluorescent world of corals in Scuba Diver Australasia + Ocean Planet No. 113 Issue 3/2018 by downloading a digital copy here!