More Than Just Pretty Colors: Photoprotection by means of Fluorescence

Written by Ayla Sage

One thing that relates all reef-forming corals is the presence of their algal symbionts! Algal symbionts are corals’ mutualistic plant partners that live inside them and produce energy. Since these symbionts use photosynthesis as a means of providing food to their coral host, these corals must remain in the photic zone (sunlit zone). In this zone, light intensity ranges from an extreme low to an extreme high. On the high end, light intensity includes damaging levels of Ultraviolet (UV) radiation [1]. UV radiation is the form of light that causes damage to human skin as well. You can blame the UV next time you get a bad sunburn.

The light spectrum is quite large and increases in energy and wavelength as you move up along it. Humans are actually only able to see a small portion in a section that we call the ‘visible light spectrum’. The visible light spectrum ranges from 400 nm (purple light) to 700 nm (red light). Just below this visible light spectrum is where the UV light/radiation lies on the scale.

When exposed to these high UV levels for too long, some corals undergo bleaching. Coral bleaching is a major threat to reef survival and occurs when the coral host begins to expel its brownish-colored algal symbionts from its tissue [2]. In 2015-2017, coral reefs around the world experienced the most widespread and destructive mass coral bleaching event ever seen [3]! The increasing frequency of these events is most likely linked to human induced climate change, meaning that these events are likely to continue unless we drastically change our ways.

Reducing the effects of climate change will be no easy feat. It will take years or even decades to bring Earth back to past baseline levels, if that is even possible. With the human population growing at an exponential rate, we must become more sustainable and turn to using more renewable resources. Even small contributions from individuals like not using single use plastics (i.e. cutlery, water bottles, plastic bags) is an act that can help. Humans have drastically manipulated the planet’s natural patterns and it is time to stop. Earth and its organisms deserve a fighting chance against the damage that we have already caused.

This raises a question: how will underwater and mostly sessile animals protect themselves? In healthy corals, the photosymbiont pigments are able to absorb the incident light [2] and use the energy for other functions. However, when the corals become bleached, the light that hits the corals is reflected off the skeleton instead of the symbionts and causes the animal to appear a bright white bleached color [2] (Figure 1). If a coral host loses its algal symbionts for too long, it will die. Seeing that corals have the ability to tolerate this wide range of light implies that corals must have other effective mechanisms for light acclimation and adaptation.

In some circumstances, rather than bleaching white, corals have been known to appear vibrantly green, yellow, and even purple-blue [2] (Figure 2). These colors are derived from green fluorescent protein (GFP)-like pigments which reside in many scleractinian (stony) corals [2]. Included in these pigments are fluorescent proteins (FP) that contain a light absorbing chromophore (atoms that allow for coloration) and chromoproteins (proteins that allow for coloration) (CPs). The chromophore emits transmitted red wavelengths, and the CPs strongly absorb the light present in the visible range but emit low levels of light [2]. The combinations of these absorptions and transmissions give the corals their different colors.

Since these proteins are found in the ectoderm (outer layer) of the host coral tissue, they are therefore regulated by light intensity [2]. Regulated means that the amount of proteins present in the outer layer will change depending on how much light is present. The two categories of light regulation response are: low threshold and high threshold. In the high threshold group, found in shallow waters, corals show green and red FPs and pink to purple-blue CPs [2]. In the low threshold group, found in deeper waters, FPs are biologically and functionally distinct. In the deep, these corals appear even more colorful [2].

Also of significance is the internal location of these proteins in the shade-adapted and high light-adapted corals. Location of proteins is a major factor in how corals are adapted to handle light. Corals exposed to significant light have proteins closer to their outer layers compared to deeper or less exposed corals. The deeper water corals hold their FPs in the same sections that their endosymbionts are located in.  This is important because it changes the amount of light energy that they use for photosynthesis [1].

It has since been hypothesized that FPs are likely to prevent bleaching in shallow water corals exposed to high solar radiation. [1]. After the severe 1998 bleaching event in the Great Barrier Reef, researchers sampled 21 common coral species affected by bleaching and found that there was a significant correlation between the amount of FPs present within the tissue and the likelihood of becoming bleached [1]. In general, the more FPs the coral tissue contained, the less likely it was to become bleached.

    These more colorful corals were healthier than the ones that were turning white aka bleaching. It became evident that there was a distinct difference between the two. The observation of more FPs became the first step in an experiment that showed that more colorful corals were better at handling higher light intensities. What they found is what we’ve been hinting at all along.

The brighter the colors, the more protection against the sun the corals have.  While functioning to combat light, corals will actively change the amount of proteins at the surface of their tissues. You can think of this as the difference between low and high SPF sunscreens. They do this by contracting their polyps [1]. This contraction leads to a denser concentration of tissue GFPs and FPs, allowing no light to come through [1]. Essentially, this layer of proteins acts as a layer of sunscreen by scattering light and transforming shorter, more damaging wavelengths to longer, less destructive wavelengths!

      While this all sounds great, one might ask how the experiment itself was tested. To show that fluorescence is not an automatic response to bleaching but rather a function of changes in light intensity, researchers conducted a study where they exposed different sections of a large bleached coral to different colors of light[2]. Initially, sections of the coral were bleached through exposure of focused red light – red light causes stress but does not cause GFPs to come to the surface of the tissue[2]. The color of visible light that the corals are exposed to is therefore important. It is exposure to blue light (not red) that causes a large amount of GFPs to move towards the surface. Afterwards in the experiment, the bleached areas were exposed to either blue or green light. It was only the areas exposed to blue light that showed an increase in green tissue fluorescence, a change that can be related to exposure of high amounts of light [2]. There was no increase in tissue fluorescence in the sections of the coral exposed to green light. This experiment confirms that CFP/GFP amounts are driven solely by light, and not by the bleaching process [2].

            So, there you have it. Corals may not be as defenseless as we think! You now know that when you see a brightly colored coral on a reef that it’s not fluorescing just to show off — it’s doing it as a way to protect itself from the sun’s harsh UV rays just as humans do when we apply a layer of sunscreen. Except, the good news for them is they don’t have to continuously reapply!

Resources

[1] Salih, Anya & Larkum, Anthony & Cox, Guy & Kühl, Michael & Hoegh-Guldberg, Ove. (2001). Fluorescent Pigments in Corals are Photoprotective. Nature. 408. 850-3. 10.1038/35048564.

[2] Bollati et al., 2020, Current Biology 30, 2433–2445. July 6, 2020 a 2020 The Authors. Published by Elsevier Inc. https://doi.org/10.1016/j.cub.2020.04.055

[3] Hughes, T.P., Kerry, J.T., Alvarez-Noriega, M., Alvarez-Romero, J.G., Anderson, K.D., Baird, A.H., Babcock, R.C., Beger, M., Bellwood, D.R., Berkelmans, R., et al. (2017). Global warming and recurrent mass bleach- ing of corals. Nature 543, 373–377.

[4] Featured Image: https://www.sciencenews.org/article/corals-algae-neon-colors-bleaching-recovery-oceans