In our last blog post, we talked briefly about what makes coral fluorescent and what purpose that fluorescence serves. With this post, we’d like to take a closer look at the benefits fluorescent pigments offer to marine coral. The mystery behind the glow has been debated in the scientific marine community for years, but the fact that so many species from widely varying habitats have evolved this capability indicates that it serves a valuable purpose. Recently, advances in measuring the functionality and mechanisms of this behavior have started to bear fruit, with evidence mounting that fluorescence serves more than one purpose. The two driving forces uncovered thus far for fluorescing pigments are protection from the sun’s rays and light generation for photosynthesis.
Protection Against Harmful Light
In order to deal with the high intensity sunlight in their native equatorial waters, corals have developed a photo inhibition, or protection from light, that while sounding intuitive, is actually quite complex. The widely accepted consensus is that bright, high-energy environments pushed coral to evolve fluorescing pigments to deal with harmful UV radiation from the sun. This protection extends not only to the coral itself, but also to zooxanthellae, a marine plankton that lives in a symbiotic relationship with the coral, providing it with glucose, glycerol and amino acids used to produce proteins, fats, and carbohydrates. The coral in turn provides the zooxanthellae a safe environment, free from predators.
A study conducted by researchers at the University of Sydney has provided a very strong link between the location of fluorescent pigment granules (FPGs) within the coral tissues and the tissues’ exposure to UV and visible solar radiation. Corals examined from the areas of a reef with the greatest exposure to sunlight revealed high concentrations of FPGs within the epidermal layer and the outer part of the endodermal layer. FPGs in these coral were also always found above, or closer to the surface, than the symbiotic dinoflagellates (the scientific term for zooxanthellae). The opposite was observed in deep water or shaded coral, with FPGs located below algal symbionts, and their concentration in the epidermal layer being much thinner.
It’s postulated that the thick “skin” of FPGs developed by shallow water coral act as a sunscreen, shielding the zooxanthellae from damaging UV rays. The study also found higher concentrations of FPGs on damaged areas of the coral, indicating that the sunscreen effect also benefits the coral itself.
The results of the study surrounding FPGs can be given further credibility by another study conducted at the University of Sydney, in conjunction with the University of Copenhagen. It found that 97 percent of coral sampled at shallow reef sites had a high fluorescent pigment concentration, with the concentration increasing in areas of direct sunlight. Non-fluorescent coral also showed a significant increase in photo inhibition during times of peak light irradiance when compared to their fluorescent counterparts. These results support the notion that coral exposed to intense light will stockpile fluorescent pigment near their surface in order to protect their symbiotic brethren and themselves.
Generation of Light for Photosynthesis
Before we start this section, we want to note that how fluorescence adds to photosynthesis was not a readily accepted theory when first proposed. However, recent evidence is mounting that it does frequently occur. As we mentioned in our last article, blue light penetrates water to a much higher degree than red light. We also said that coral has adapted to utilize blue light more efficiently in order to conduct photosynthesis, but what we didn’t mention is exactly what this adaptation is.
A study published by scientist D. Schlichter in the journal Oecologia found evidence of differentiation between pigments used to protect from light rays and those used to enhance photosynthesis. By exposing different light wavelengths to a variety of corals, Schlichter was able to find that short wavelength light elicits a strong reaction in high concentrations of fluorescent pigmentation, and that the resulting fluorescent glow is of a longer wavelength than the initially exposed light. Coincidentally, the preferred light spectrum used by symbiotic zooxanthellae is that of longer wavelengths. The coral’s adaptation is actually not to use the blue light, but to convert it into more photosynthetic-friendly long wavelength light.
Taking these results and applying them to the first Sydney study that found corals in deep water have pigment located below their algal symbionts, it can be said with confidence that their location has evolved as a way of enabling more efficient photosynthesis. In this case, the location of pigments does not shield the symbionts from the sun, but rather emits long wavelength light onto them. The resulting increase in photosynthesis is thought to give fluorescent corals a competitive advantage against other deep-water species.
While our understanding of fluorescent coral is still limited, ever advancing science is slowly but surely revealing more and more about our favorite marine invertebrates. If you share the same fascination with these marvelous creatures as we do, consider contacting us about setting up an at home or office habitat to grow your own reef. We specialize in reef lighting systems that ensure your coral is as healthy as it is stunning.
Stay tuned for our next segment on coral pigments, where we will discuss how scientists are using colorations to measure reef health via satellite.