WHAT YOU NEED TO KNOW ABOUT THE COLORATION OF STONY CORALS

Pigments – the colours of nature

Molecules or substances that are responsible for color in biological systems are generally referred to as pigments. In stony corals, these pigments occur both in the chloroplasts of the zooxanthellae and in the tissues of the corals. When light hits the pigments, it is either absorbed (= taken in) or reflected (= thrown back).

Chromophores – center of power

While reflection does not cause any change in the pigment molecules or the reflected light, the light energy absorbed during absorption is transferred to a special chemical group called a chromophore.

The chromophore is the part of the pigment that is responsible for its color. Within the chromophore, individual electrons are raised to a higher energy level by the absorbed light energy.

However, this higher energy state is extremely unstable and after only a short time the excited electrons return to their original energy level. The excess energy is released into the environment in the form of heat or light.

The release of energy in the form of light is called fluorescence. The fluorescent light emitted by the pigment molecules has a lower energy content than the previously absorbed light and therefore usually has a shorter wavelength. Each pigment absorbs (=absorbs) and emits (=releases) light of a specific wavelength.

The different types of pigments in stony corals

Pigments that occur in the tissues of stony corals are divided into fluorescent, non-fluorescent, kindling and photosynthetic pigments. Non-fluorescent pigments are also called chromoproteins.

Very well studied: fluorescent proteins

The group of fluorescent proteins is very well described scientifically because they are used as marker proteins in medicine, among other things. Marker proteins can be used to study the synthesis and dynamics of proteins in the human body.

The first fluorescent protein to attract attention in medical research was the so-called green fluorescent protein (GFP), which was isolated from the jellyfish Aequorea victoria . A large number of differently colored fluorescent proteins are now known, but they all have a common basic spatial structure. This structure consists of a barrel-like amino acid chain (=peptide) in the center of which three amino acids (=tripeptide) form the chromophore. If the composition of this tripeptide is changed, the wavelength of the absorbed and emitted light changes, and a fluorescent protein with new properties is obtained.

This is how fluorescent proteins are formed and activated in the organism

The formation of proteins is identical in all organisms. The information required for this is stored in the DNA. In the first step, this information is read and copied in the nucleus of the cells (=transcription). The copies created (=RNA; ribonucleic acids) are then transported from the cell nucleus into the plasma and translated into an amino acid sequence in the second step (=translation). In the last step, a spatial structure is formed from the synthesized sequence (=post-translational modification), which creates a functional protein.

In the case of fluorescent proteins, the post-translational activation or modification of the chromophoric groups occurs automatically. This process comprises three steps: (1) cyclization, (2) dehydration and (3) oxidation. Only when these three steps have been completed can the protein absorb and emit light of specific wavelengths.

Depending on the emitted light, a total of five different groups of fluorescent proteins are distinguished: (1) turquoise (CFP), (2) green (GFP), (3) yellow (YFP), (4) orange (OFP) and (5) red (RFP) fluorescent proteins.

The role of fluorescent proteins in stony corals

So far, the biological function of fluorescent proteins in tropical stony corals has not yet been clearly clarified and various theories are currently being discussed.

One of the best-known explanations assumes a photoprotective function of the fluorescent proteins. Natural sunlight, especially the UV light it contains, is very energetic and can, under certain circumstances, lead to damage to the photosystems in the zooxanthellae. In the tissues of the stony corals, the FPs are sometimes located above the zooxanthellae and thus shade them from the harmful radiation.

Furthermore, the fluorescent proteins could act as so-called antioxidants. Dangerous oxygen radicals are released as unwanted byproducts during photosynthesis, which can lead to serious cell damage. Fluorescent proteins could bind such molecules in stony corals and thereby protect the zooxanthellae or coral cells from cell damage.

Zooxanthellate stony corals can also be found in the deeper areas (= twilight zones) of tropical coral reefs. In the Red Sea, for example, the large-polyp stony coral Leptoseris fragilis can be found at depths of up to 160 meters. However, the amount of available light is very low there. In addition, certain areas of the light spectrum are completely missing. So how do the zooxanthellae of L. fragilis manage to carry out photosynthesis in this extreme habitat? Another possible function of the fluorescent proteins could provide an answer to this question. The fluorescent proteins contained in the tissues of L. fragilis can absorb the strongly blue-colored light at depth and then emit fluorescent light that has a wavelength that can be used by the symbiotic algae for photosynthesis.

New scientific territory: Chromoproteins

In contrast to fluorescent proteins, non-fluorescent or chromoproteins have received little scientific attention in the past.

An exception are the pink and blue pocilloporins, which were first detected in corals of the taxon Pocilloporidae.
Pocilloporins have chromophoric groups that do not contain amino acids, but rather non-proteinogenic molecules or metal ions. The absorption spectrum of pocilloporins lies mainly in the wavelength range of 570 to 580 nm. This corresponds to a green to orange-yellowish light.
In the natural habitat, the concentration of pocilloporins in coral tissues decreases with increasing depth. In addition, strong light with a high UV content is required for the initiation of pocilloporin synthesis. Therefore, pocilloporins could possibly also fulfill a photoprotective function.

Neither one thing nor the other: Kindling proteins

Basically, kindling proteins are an intermediate form of chromoproteins and fluorescent proteins. They have the ability to either reflect or absorb radiation depending on the wavelength of the incident light. Light in the green spectral range in particular stimulates the transition of kindling proteins from chromoproteins to fluorescent proteins. Due to the very sparse data available, the importance of kindling proteins for the coloration of stony corals cannot currently be precisely assessed.

Photosynthetic pigments

The last large group of pigments that can be found in stony corals are the pigments of endosymbiotic algae (=zooxanthellae). These are located in the antenna complexes of the photosystems and serve primarily to convert light energy into chemical energy. Although the main function of the photosynthesis pigments is not in the area of ​​coloring, they still contribute to it to a large extent. Since these pigments absorb mainly light in the blue and red spectral range, mainly light in the orange-yellow spectral range (575 to 650 nm) is reflected and an overall brownish color impression is created. This is why stony corals that have a high concentration of zooxanthellae in their tissues appear more brownish in color.

It's all about the mix!

The color of stony corals is ultimately the result of the reflection and/or absorption of light by all the pigments listed here. However, their concentration is influenced by many other factors (light intensity, light spectrum, nutrient conditions, etc.) in addition to genetic predisposition, which is why it is generally very difficult to predict how the color of a stony coral will behave or change under certain conditions. The color formation of stony corals is an extremely complex topic and so far only a few scientific papers have been published on the subject. However, some general assumptions and relationships can be formulated.

Conclusion: Fluorescence and chromoproteins vs. photosynthetic pigments

Put simply, the coloration of stony corals results from the interaction between fluorescence and/or chromoproteins and the density of zooxanthellae. If the density of zooxanthellae and thus the concentration of photosynthetic pigments in the coral tissues is high, these overlay the fluorescence and chromoproteins, resulting in an overall brownish appearance of the coral. This effect is intensified when the concentration of fluorescence and chromoproteins is low.

In order to achieve an intensive coloration of stony corals, the zooxanthellae density should be as low as possible, while the concentration of fluorescence and chromoproteins should be as high as possible.

Reduce zooxanthellae density

The first approach to reducing the zooxanthellae density is to lower and stabilize the nutrient concentrations. Dissolved nutrients (NH4+, NO2-, NO3-, PO43-) can be absorbed by the zooxanthellae from the environment and converted into biomass. Ideally, the concentrations of ammonium and nitrite should be below the detection limit, the nitrate concentration should be a maximum of 3 - 5 mg/l and the phosphate content a maximum of 0.03 - 0.05 mg/l. Different methods can be used to reduce the nutrient concentrations. Operating a pellet or zeolite filter is particularly suitable for this. In both cases, a powerful (but not oversized) skimmer should also be used.

The use of very strong lighting also has a positive effect on the density of zooxanthellae. Since more light energy is available, fewer zooxanthellae are needed to cover the energy requirements of the corals. To achieve optimal results, the output of the selected lighting should be between 1.0 and 1.2 watts per liter of tank volume or 450 to 550 watts per square meter of tank area. To achieve the best results in color formation, a PAR value (=Photosynthetically Active Radiation) of 450 to 650 µmol*m-2*s-1 is recommended when caring for SPS corals.

The corals should also be fed regularly. If they have no additional source of energy and nutrients, they have no choice but to rely on the photosynthesis products of the zooxanthellae. However, this inevitably leads to the density of the zooxanthellae increasing. Feeding with dissolved organic substances, such as amino acids, live food or special dry food mixtures, has proven particularly effective in this regard.

Increase fluorescence and chromoprotein concentrations

Since the fluorescence and chromoproteins are suspected of acting as photoprotective substances, their concentration can be effectively increased by using strong lighting (see above). In addition, the spectrum used plays an important role. While a spectrum of 380 to 720 nm is required for the synthesis of chromoproteins (pink & blue shades), the spectrum for the formation and activation of fluorescent proteins should cover a wavelength range of 380 to 500 nm.

Another way to increase the concentration and formation of fluorescence and chromoproteins in coral tissue is to add specific (trace) elements. However, this is not a scientifically proven finding, but rather the experience and observations of aquarists.

Since the molecular structure of the color-producing proteins themselves does not contain any (trace) elements, the elements could, for example, influence the spatial structure of the proteins by accumulation and thereby enhance the fluorescence effect or, due to their photoprotective and antioxidative function, help physiological processes to take place under strong light conditions.

In practice, it has been observed that the addition of iron supports and promotes the formation of the colour green, the addition of potassium and manganese supports and promotes the formation of the colours red and pink, and the addition of iodine supports and promotes the formation of the colours blue and violet.

Ready-made products should be used to dose these elements. However, it is recommended to start with only half the dosage specified by the manufacturer and observe how the corals behave or the aquarium develops overall. The elements should only be added until the existing color deficiency has been eliminated. Dosing should only be started again when color loss occurs again. In addition, the concentration of the added elements should be checked at regular intervals. Commercially available test kits (iron, iodine, potassium) or an ICP analysis are suitable for this.

An overdose of the elements is reflected in a brown coloration of the corals and/or in an increased growth of algae. In addition, an overdose of potassium can lead to so-called "tip burning", whereby the tissue at the growth tips of SPS corals detaches.

When trying to increase the colors of corals, all changes should be made slowly. The corals need some time to adapt to the new conditions. This applies in particular to increasing the lighting intensity or changing the spectrum used. The first effects of an additional element dosage can be seen after at least three days.

Under what conditions do you achieve the best coloration of your corals? What do you do to get more color out of the corals and what would you advise against?