Article: Gittens JR, D’Angelo C, Oswald F, Edwards RJ, and Wiedenmann J (2015) Fluorescent protein-mediated colour polymorphism in reef corals: multicopy genes extend the adaptation/acclimatization potential to variable light environments. Molecular Ecology, 24, 453-465. Doi: 10.1111/mec.13041
Cover Photo Credit: Steene Roger. “Acropora Mellopora”. Photograph. AIMS. <http://coral.aims.gov.au/factsheet.jsp?speciesCode=0047>.
Coral bleaching has posed a huge threat to many coral reefs across the globe. Bleaching occurs when the heat and light intensity of the sun irritates the zooxanthellae (type of algae) cells living within corals. Zooxanthellae and coral are great friends, having a mutual beneficial relationship with one another (a symbionic relationship if you will). The coral provides a protected environment for the algae and the corals waste provides the essential compounds needed for photosynthesis. In return, the algae help the coral get rid of its waste as well as produces oxygen for the coral’s survival. The heat and light of coral reefs can get pretty intense, especially for those shallow living corals. Therefore, corals and their symbionts (zooxanthellae), rely on multiple strategies to protect themselves.
One strategy thought to help protect the corals from sun exposure is the expression of a certain protein in corals, allowing them to regulate light intensity by color morphing. When it is a blazing hot day, you are more likely to get hot if you are wearing black as opposed to white, right? This is the same with corals, thus you often see lighter/less sun absorbing colors in corals as opposed to darker corals. How exactly are these corals changing colors and why do different corals of the same species display such an array of colors? Do they do it to show off to one another, or is there a biological advantage to being a certain color? Scientists from the UK set out to understand more about color morphing in corals. Variation in gene copy number can result in several different gene expressions, so does this mean that the number of gene copies a coral has relates to its color?
To understand how corals change color, researchers took three colonies of the branching stony coral Acropora millepora and studied its response to different environmental stimuli. The red color of these corals differs considerably under the same light in the shallow waters of the Great Barrier Reef (Fig. 1), so researchers selected different representatives of these corals that differ in their degree of redness. Under identical light conditions, corals were cultured for over six months side by side, regulating the red tissue fluorescence. Genetic coding and sequencing was then performed on each coral tissue to better understand the molecular differences in color morphing.
Results and Implications
As it turns out, changing color in response to light is a pretty complicated business! Looking at just the red fluorescent transformation of Acropora millepora, it took six weeks for the coral to fully adjust to a changing light level (and even then, the corals remained different from one another). Once fully color morphed, it was evident that the upper branches of the coral displayed a much brighter shade of red than the lower branches of the coral. Digesting this further, it was found that a specific gene directly affects the red coloration in the coral. The more copies of this gene, the greater strength of color. Producing multiple copies of a gene takes a lot of energy, thus maintaining a high pigment concentration is a significant energy investment.
So why would a coral bother to invest it’s energy into being bright? This comes back to the coral/algae symbiotic relationship. Algae requires some sunlight to survive, but too much light can harm or even kill the algae. In order for corals to protect the algae (again, algae helps provide the essential nutrients corals need to survive), the corals invest their energy into producing more pigment (more color) when exposed to high levels of sunlight, shading the algae from too much sun exposure. The upper coral branches receive the most light, meaning they need to be the brightest, while the lower coral branches are more shaded, so are able to stay darker. The corals are essentially providing a bright red umbrella for their zooxanthellae friends to hide under, keeping them safe from the sun, and in turn the keeping themselves safe from bleaching out.
These results provide a great roadmap to further understand coral relationship and its genetic code. This can help predict how corals will react to a changing environment and additional stressors that may be placed on them (warmer water temperatures, more sun exposure, etc…).