Coral reefs aren’t just known for the amazingly colorful fish that inhabit them, but also for the beautiful colors of the corals themselves. The colors of the coral don’t just come from the corals alone, but from the combination of the colors of the corals and the little microalgae, or zooxanthellae, that inhabit them. The molecules responsible for color are known as pigments, and aside from making the corals very attractive, they perform a number of other important functions as well.
So, What Are The Roles Of Pigments?
The primary function of pigments is to capture light. Photosynthetic organisms, such as the zooxanthella, which inhabit the coral, are able to capture light energy and transform it into chemical energy. This chemical energy is stored in the form of carbon-based organic compounds, such as glucose. The glucose produced by the microalgae is shared with the coral and provides up to 80% of its nutritional requirements, in return for safe housing and protection. The co-operative nature of the microalgae and coral is known as symbiosis.
A Bit More On Light…
Different pigments absorb different types of light depending on their chemical structure. Light is electromagnetic radiation that travels in the form of waves. The visible light spectrum includes the portion of the electromagnetic spectrum which can be perceived by the human eye, specifically defined as light with wavelengths between 400-700 nm, or what you and I would identify as the colors of the rainbow.
The 400 nm end of the spectrum corresponds to the blue end of the light spectrum, with ultraviolet light occupying the below 400 nm range of the spectrum. Red light has wavelengths which occupy the 700 nm end of the spectrum. Blue spectrum light, which has a shorter wavelength and therefore more energy than red spectrum light, is able to penetrate deeper through salt water than red light, which has a longer wavelength and is therefore less energetic. Theoretically, that is why corals nearer to the surface may seem redder, because there is more red spectrum light available to be reflected by the corals than in deeper water. Anyone with experience scuba diving will be able to tell you that red is the first color you lose as you dive deeper, and if you go deep enough, everything ends up looking blue.
Pocilloporin And The Acroporidae Family
One of the color pigments identified in corals is known as pocilloporid, which is a protein that works in the blue light spectrum. This pigment was identified by a group of scientists working in the Acroporidae coral family. The Acroporidae family is a group of small polyped stony corals, commonly known as staghorn corals.
Pigment Functions In Coral:
This is similar to how melanin, or the pigment that gives human skin its color, protects our skin from UV light damage from the sun.
Increase Photosynthetic Light Availability
Although the zooxanthellae rely on light to produce energy in the form of carbohydrates, too much light or strong solar radiation (such as ultraviolet light) can cause corals to become stressed and induce a process known as coral bleaching. Coral bleaching is when corals expel the zooxanthellae that live within their bodies. This causes a loss of color in the corals, which often become bone white, which is why the process is known as bleaching. This is very dangerous to the corals, as they cannot survive long without their zooxanthellae.
One of the main roles thought to be performed by the blue pigment pocilloporid is that of photo-protection or protecting the micro-algae against damage from too much solar radiation. In environments where there is too much light, such as in shallower water, the pocilloporids act as a shield, preventing sun damage to the micro-algae by dissipating excess energy in non-photosynthetic wavelengths. Too much light can be detrimental to coral health, as if the quantity of light exceeds that needed for photosynthesis, the excess light can generate reactive oxygen species (ROS), which can trigger coral bleaching. These pigments are located in the upper part of the micro-algae and have been shown to enhance resistance to coral bleaching.
In addition to reducing the light levels and therefore damage from solar radiation in high light environments, some pocilloporids can convert harmful ultraviolet light to light in the blue spectrum (440-445 nm) or blue-green spectrum (480-490 nm and 500-505 nm), which can then be absorbed by the micro-algae for photosynthesis. Because the rate of photosynthesis and therefore the energy produced in the process varies depending on the wavelength of light captured by the color pigments, it is very important that the micro-algae capture the right spectrum light.
Using Coral Colour Pigmentation As An Indicator Of Environmental Conditions
Because the regulation of photo-pigments in coral reefs is closely linked to the intensity and spectral composition of irradiating light conditions, scientists have suggested that coral pigmentation may be a sensitive indicator of light conditions within a habitat. Typically, corals have a higher concentration of pigments in areas closer to the surface or in direct sunlight.
Going Bright May Help Corals Recover From Bleaching
Although corals typically turn white when bleaching occurs, they occasionally become very colorful – as was observed in Acropora corals in the Philippines after the water temperature was slightly higher than the bleaching threshold for a couple of weeks. Scientists demonstrate that this colorful display is correlated with mild heat stress and linked to increased light fluxes in bleached coral tissue. Scientists also suggest that this could facilitate the recolonization of coral tissue by symbiotic micro-algae as the display indicates an offer of mitigated light stress to the micro-algae.
In summary, pigments perform important functions in the relationship between corals and their symbiotic micro-algae. Specific blue pigments, known as pocilloporins, act both to protect corals from sun damage in high light conditions and also to improve light availability for photosynthesis in environments with low light conditions. Therefore, having pigments allows corals to adapt to the lighting conditions present in their environment and to provide the optimal conditions for their symbiotic microalgae to photosynthesize.