How many of us have bought a brightly coloured coral from an aquarium shop only for it to slowly loose its stunning colours and turn brown over the following weeks in our home aquarium? Occasionally the reverse might be true, we buy a dull brown colony and much to our delight it slowly develops in to a bright green or pink coral under high intensity lighting.
The reasons that certain corals appear certain colours and the factors that affect coral colouration are diverse and rather complicated. In this article I will look at some of the basic principles that govern colouration in corals and what we can do to obtain the most vivid colours from our corals.
The majority of corals that we keep in aquaria have a symbiotic relationship with algal cells known as zooxanthellae, it is these algal cells that photosynthesise and provide the coral with nutritional compounds such as glucose, glycerine and amino acids that are passed from the algal cells to the corals own tissue cells. The zooxanthellae themselves gain nutrients that they need to photosynthesise directly from the sea water such as ammonia, nitrite, nitrate, carbon dioxide and phosphate as well as metabolic waste products from the corals own cells. These are used in the formation of the previously mentioned sugars and amino acids that are then passed on to the coral. The coral uses these nutrients and produces waste products that are recycled back to the zooxanthellae to be used again in photosynthesis. There is a balance that needs to be maintained between the coral and the zooxanthellae in terms of the amount of photosynthesis that occurs within the corals tissues. Too many zooxanthellae and too high a rate of photosynthesis can be just as damaging to the coral at too little. It is this adjusting of the concentration of zooxanthellae by the coral that is a major factor in the colouration of corals. Zooxanthellae are all brown or golden-brown, they do not exist in any other colour and this is why most corals are brown.
Fluorescence is the term used to describe the behaviour of an object that absorbs light of one particular wavelength (colour) and emits it at a different wavelength. This change in emitted light is due to changing energy levels of photons, the energy levels of photons is related to it’s wavelength which we perceive as colour. Fluorescent pigments absorb light of a specific wavelength and then emit light in a wavelength that is longer than the original one. This fluorescence has a large effect on the perceived colour of a coral particularly when the coral is illuminated by light that is inclined to produce fluorescence such as actinics or high Kelvin rated metal halide lamps.
Non-fluorescent colour pigment
As the name suggests these pigments do not visibly fluoresce, these include the non-fluorescent pink and blue coral pigments known as pocilloporans and purple/red pigments found in some corals. The overall colour that your corals will appear is a direct relationship between the abundance of zooxanthellae and fluorescent and non-fluorescent pigments that may or may not be present.
Lighting is probably the single biggest factor in terms of coral colouration. It is not only the intensity of light that is important but also the colour or concentration of certain wavelengths of light.
Light is composed of various different wavelengths and each wavelength corresponds to a particular colour. The human eye cannot detect all of these wavelengths and those that fall below the levels detectable by the human eye are known as ultraviolet light and those that are above the upper detectable levels are known as infrared light. The wavelengths that we can see range from 380 nm (nm stands for nano meters which is the unit of measurement for wavelength) to 780nm.
It is important for marine aquarists to have an understanding of ultraviolet light as well as visible light. Ultraviolet light from 100 to 400 nm is divided into three different groups:
UV-A: This is light that falls in the wavelengths between 315 – 400nm, UV-A will pass through normal silicate glass.
UV-B: This is light that falls in the wavelengths between 280-315nm. It is UV-B that will cause sunburn. UV-B will not pass through normal silicate glass and is absorbed quickly in water, however in crystal clear waters of coral reefs both AV-A and UV-B can penetrate over 30m deep.
UV-C: This is light that falls in the wavelengths between 100 – 280nm. UV-C is very harmful to living tissue and is the type of light that is used in UV sterilizers. UV-C is rapidly absorbed by the earths atmosphere. It should be noted that UV-C is given off by single-ended metal halide lamps but it is absorbed by the filters used on the bulbs and cover glass. Most metal halide lamps also produce some UV-A and UV-B radiation whereas fluorescent tubes usually do not.
As we know that coral reefs are exposed to high levels of UV-A and UV-B particularly in shallow water it follows that the corals have come up with ways to protect themselves from being burnt. It has been known for a long time that corals can produce a sunscreen to protect themselves from UV radiation, the most important of these that have been found are known as S-320 or mycosporine-like amino acids. It was thought that it is these sunscreens that are produced in corals in shallow waters that give corals in these areas their bright colours, however it is known that these pigments are transparent to UV-A and rather are used to block UV-B radiation and these wavelengths are below those that are known to induce colouration. However there does seem to be some disagreement on this point as Dunlap and Chalker (1986) found three S-320 compounds in Acropora formosa which absorb light between 310 and 340nm and claim that these compounds are seen in corals as violet or fluorescent pigments.
Metal halide lamps produce peaks of light in certain UV wavelengths, for example mercury used in metal halide bulbs produces a peak at 365nm, while thallium and scandium produce peaks at 378nm, 391nm and 393nm. It is known that these wavelengths induce colouration in in certain corals, so you might think to yourself that it is a good idea not to shield your metal halide lamps and expose your corals to as much UV light as you can to help your corals colour up. Unfortunately this is not the case, it has been shown that UV radiation in the wavelengths also known to induce colouration in corals can result in photoinhibition when exposed at the levels given off by metal halide bulbs. Photoinhibition is the process when photosynthesis in the coral ceases and the coral can end up being starved of nutrients from the zooxanthellae.
Light of 400nm is the lower limit at which photosynthesis can occur, light at wavelengths lower than this is known to cause chloroplast damage. It is thought that the UV-A light that corals receive is channelled through photosynthetic pathways that re-emit the light at longer wavelengths which can then be used by the zooxanthellae for photosynthesis and as mentioned previously causes the corals to fluoresce. By using this system as well as sunscreen pigments and UV resistant strains of zooxanthellae the coral is able to live and photosynthesis under exposure to UV radiation. However, there are limits to what a coral can tolerate and as previously mentioned photoinhibition should be of concern to hobbyists. Photoinhibition can occur from overexposure across a range of wavelengths and not just from UV light.
When zooxanthellae photosynthesise they produce hydrogen peroxide which is harmful to living tissues, to counteract this the coral itself produces an enzyme which neutralises the hydrogen peroxide. If the rate of photosynthesis by the zooxanthellae is too high, there is a sudden increase in the number of zooxanthellae or the coral is not able to produce enough of the neutralising enzymes to combat the hydrogen peroxide then the coral will react by expelling some or all of it’s zooxanthellae. The expulsion of some of the algal cells may seem like a benefit if we are trying to obtain bright colours in our corals but usually in this situation the coral is weakened, growth slows and the production of colour pigments fades as the coral becomes stressed. Conversely, if a coral has the light intensity that it is exposed to reduced then the coral will respond by producing more zooxanthellae to meet the demands of the hosts tissues. So if we take a coral from a high light intensity environment like a coral reef and then place it in an aquarium with much lower light intensities then the net rate of photosynthesis by the zooxanthellae will decrease. In response to the lower levels of nutrients that the coral is receiving from the zooxanthellae, the coral encourages more zooxanthellae to be produced by releasing more metabolic waste products resulting in more photosynthesis and the coral recovering it’s required nutrients from the zooxanthellae and causing the coral to turn brown.
There is another aspect to this process as well. On coral reefs there is a huge abundance of zooplankton and phytoplankton that a coral can capture and utilise as an energy source. If this coral is placed in an aquarium where the levels of prey abundance are much lower then the coral needs to get it’s nutrition from elsewhere, as a result the coral increases the numbers of zooxanthellae that it holds in it’s tissues, regains it’s nutritional requirements and has turned brown.
Another factor effecting the abundance of zooxanthellae in a corals tissues is the amount of dissolved organic matter that occurs in the water. Naturally the levels of these nutrients in the sea is very low and the coral subsidises what the zooxanthellae can receive from the surrounding water by providing them with waste products from it’s metabolic processes. If the concentration of dissolved organic matter in the water is greatly increased as in the case of our aquarium water then the zooxanthellae have ample nutrients to grow and multiply inside the coral and hence mask colours. Take this abundance of nutrients too high and the coral cannot deal with all the photosynthetic processes going on in it’s tissues and it will begin to expel the zooxanthellae and bleach.
Zooxanthellae have a lower light limit known as the compensation point. This is the point at which photosynthesis by the zooxanthellae equals respiration by the zooxanthellae. If light intensities are too low then the zooxanthellae will not be able to pass on enough of the photosynthetic products or consume enough of the corals metabolic waste products to enable the coral to grow and thrive. At the other end of the scale zooxanthellae also have what is known as the saturation point, this is the point at which the rate of photosynthesis is at it’s maximum and any increase in light intensity will not increase the rate of photosynthesis. The saturation point of corals will vary from species to species and also the environment that they have adapted to, generally the saturation point of many species of coral will not exceed 300 µmol·m²·second. This is the level of light that is typically achieved 6” under the water surface directly under a 250w 10K metal halide bulb that is mounted 8” above the surface of the water. However, the saturation point of some corals is often reached well below this light intensity and exposure to intensities above the saturation point will lead to photoinhibition, the expulsion of zooxanthellae and the weakening and paleing of a corals colours, often resulting in death.
As you can see the key to good colouration in corals is very much a sliding scale dependent on the light intensity, light spectrum and nutrient load of the water. The key is to get the correct balance of all of these things and maintain it there. However the story does not end here and there are a few more things to consider.
Trace elements are widely proclaimed to be able to greatly enhance the colours of your corals by many aquarium additive manufacturers, however the role of many trace elements in corals is poorly understood but there is a little information that is starting to make it’s way forward. Although it is by no means conclusive there is some evidence pointing towards iodide playing a role in transitioning apparent colouration in corals. There are certain genetically engineered variants of yellow fluorescent proteins that have shown to be affected by certain halides such as fluoride, iodide, chloride and bromine. Conversely certain pocilloporans (non-fluorescing colour pigments) extracted from montipora species have been shown to be insensitive to iodide additions. It is also known that certain trace elements facilitate enzymatic and photosynthetic reactions and so could end up playing an important role in coral colouration, however much more research needs to be done in this field. But before you go tipping loads of trace elements into your aquarium in the hope that the colours of your corals will improve, don’t forget that for the majority of aquariums that have a low to moderate coral density, the demand for most trace elements is met by regular water changes.
Interestingly, some research by Dana Riddle and Andy Amussen in the late 1990’s seemed to show that lower alkalinity levels can lead to loss of colour pigmentation in some corals. Of the ten systems that they were experimenting on, they allowed the alkalinity in four of the systems to drop to levels of 7dKH and noticed that many of the sps colours faded. They then used alkalinity buffers in these systems to quickly raise the dKH back up to 10. They found that some corals intensified their pigmentation over night while in others colour returned over a few days.
Ultra Low Nutrient Systems
In recent years there has been a move towards reef systems that have very low nutrient levels in the water, Fauna Marin and Prodibio are two brands that have been quite popular here in the UK. These systems work by providing the filtration bacteria with an extra carbon source to increase their population size and efficiency at removing nitrates and phosphates from the water. This technique is combined with regular additions of trace elements and amino acids. The system clearly works in terms of reducing nitrates and phosphates and as a result of the lower nutrient concentrations in the water there are less ‘fertilisers’ available for use by the zooxanthellae and their concentrations drop and allow the fluorescent and non-fluorescent pigments to show through. The addition of trace elements ensures a good supply of those elements needed for photosynthetic and metabolic processes and the addition of amino acid provides the building blocks that the coral may use for various protein, enzyme and pigment synthesis. These additives should however be used carefully, although nitrates and phosphates in excess may cause browning of corals they are essential in low levels.
The levels of nitrate in natural reef waters are too low to maintain the biodiversity of corals that occur on reefs, the zooxanthellae must be subsidised with metabolic waste products from the coral to make up the deficit. If the levels of metabolic waste products that a coral is passing to zooxanthellae is too low then in aquariums incorporating ultra low nutrient systems the zoxanthellae may starve leading to the death of the coral. It is likely that the amino acid supplements incorporated in these additives are taken up directly by the coral, used in various metabolic activities and enable the coral to provide the zooxanthellae with necessary waste products in an environment that is otherwise devoid of nutrients.
There is no short answer to obtaining the best colours in your corals, as we have seen lighting is a major factor. It is known that certain wavelengths particularly in the blue part of the spectrum stimulate some fluorescent pigments. Increasing light intensity will cause the reduction in zooxanthellae abundance and hence colour pigments will become more apparent, however under light intensities that exceed a corals saturation point metabolic activity by the coral will decrease, colours will pale and in extreme cases the coral will bleach. High levels of nutrients such as nitrates will cause an increase in the concentration of zooxanthellae which will mask colours. Maintaining low levels of nutrients will reduce the number of zooxanthellae that occur but under these circumstances if a coral cannot supply the remaining zooxanthellae with enough metabolic waste products derived from prey capture or amino acid uptake then the zooxanthellae will not be able to support the coral and it will starve as well.
There is some evidence that trace elements are necessary for pigment production, they are certainly required for many metabolic and photosynthetic processes and hence will have an impact on perceived colour, similarly there is some evidence pointing to alkalinity effecting colouration.
The correct level of lighting combined with good maintenance in the form of water changes and filtration will usually sustain good colouration in most aquariums. Regular water changes will provide the required trace elements and corals will be able to gain necessary amino acids and other nutrients from your fish waste and other foods offered. In aquariums that are heavily stocked with corals then trace element and amino acid supplementation may be required. The new ultra low nutrient systems are showing some impressive results in coral colour, however we have seen that the health of corals is a delicate balance resulting from many factors and as such any changes that you make to your aquarium in an attempt to improve colour should be made very gradually.
References Fossa and Nilsen: The Modern Coral Reef Aquarium Veterinary & Aquatic Services Department, Drs. Foster & Smith, Inc.: Colors of Corals: Light, Chlorophyll, and Other Pigments Benoît Finet, Florian Lesage: Colors by the Thousands – Light, Colors and Corals, Part I Dana Riddle: Coral Coloration: Fluorescence: Part 1 Dana Riddle: Coral Coloration, Part 2: Fluorescence: Pigments 510 – 565 and Notes on Green Fluorescent Proteins Dana Riddle: Coral Coloration, Part 3: Pigments Responsible for Yellow and Orange Coloration, With Notes on Photoconversion Dana Riddle: Coral Coloration, Part 4: Red Fluorescent Pigments, a Preliminary Report of Effects of Various Environmental Factors and Color Mixing Anya SALIH1, Guy COX1, Ron SZYMCZAK2, Steve L. COLES3, Andrew H. BAIRD4, Andrew DUNSTAN5, Giordana COCCO6, Jacqui MILLS7, Anthony LARKUM6: The role of host-based color and fluorescent pigments in photoprotection and in reducing bleaching stress in corals. Jake Adams: Water flow is more important for corals than light. Part II: The science of corals and water flow