Optimal Lighting for a Reef Aquarium

The proper light for the reef aquarium is important not only for coral growth and health, but for the overall functioning of the food web. A large number of organisms involved in nutrient recycling depend on light for survival. Photosynthetic bacteria, photo-autotrophic protists, cyanobacteria, diatoms, phytoplankton, zooxanthellae and algae perform photosynthesis to feed and build their tissues. To understand how corals take advantage of light, it is important to know that, for biological purposes, there is no difference between the light emitted by a HQI bulb, a T5 fluorescent tube or a LED fixture. In all cases it is a photon flux, which is used by zooxanthellae to perform photosynthesis, providing food to the coral.

Figure 1 shows the irradiance pattern according to time of day and sun elevation on an Australian coral reef. Phases of dawn, day and dusk are depicted. To follow this pattern in an aquarium setting, dimmable lights are required.​

Depending on its location on the coral reef or in the aquaculture farm, a coral will be adapted to certain lighting conditions and water movement. In the ocean, depth determines the amount and type of light that strikes the zooxanthellae. This light is subject to very small variations. Variations are determined by the time of day, movement of clouds, water turbidity or sea swell intensity. Corals, anemones and giant clams respond very poorly to sudden increases in light intensity, so light acclimation is necessary before placing them in their final spot within the aquarium.

Corals located near the water surface receive a large amount of solar radiation and are forced to reduce their zooxanthellae population to reduce oxidative stress. This reduction results in a much lighter coloration. However, corals in deep zones, in order to maintain the same photosynthesis rate, must develop a higher density of zooxanthellae, presenting much darker shades. Another factor that significantly affects the coloration of corals, anemones and giant clams are the protective pigments they develop in response to UV and intense light radiation. These pigments provide them with very attractive colors, in shades of blue, mauve, yellow or pink, and their function is to reduce oxidative stress. In the reef aquarium, intense lighting, within reasonable limits, favors the formation of these pigments and the corresponding associated colors. These colors are a natural response and are consolidated when environmental conditions are extraordinarily stable. Frequent changes in the location of corals into a reef tank greatly limit the development of these pigments.

Light is an electromagnetic radiation that has the same nature as radio or cell phone signals. The energy carried by a light source is inversely proportional to its wavelength, i.e., the longer the wavelength the lower the energy. If an electromagnetic radiation has a wavelength between 400 nm and 700 nm (1 nm being one millionth of a millimeter), it is perceived by the human eye in the form of light and colors. Each color corresponds to exactly one wavelength. This electromagnetic spectral range is known as the “visible spectrum”. Light from the sun carries all the wavelengths of the visible spectrum and additionally those corresponding to infrared and ultraviolet. Ultraviolet radiation is located before 400 nm and infrared radiation after 700 nm, i.e., at the borders of the visible spectrum above and below. The approximate equivalences between wavelengths and colors are described in Table 1.​

Figure 2 shows a plot of the spectrum of the average solar radiation incident on the ocean surface of a coral reef at the brightest time of the day.

 

As light penetrates the ocean it interacts with water and organisms. Shorter wavelength colors (blue) with higher energy are able to penetrate deeper than longer wavelength colors such as red. As a result, when depth increases the available light becomes bluer and less red. The spectrum of sunlight that reaches fish, corals and invertebrates submerged in the coral reef would be the result of combining the data from Figure 2 and Figure 3. This spectrum is represented in the Figure 4 and corresponds to the existing light at about 5 meters depth. This would be the type of lighting pattern you would use if you were trying to replicate nature in your reef tank. 

 

However, there are other important considerations that make this spectrum less than optimal for aquarium lighting.​

 

Zooxanthellae do not use all colors equally for photosynthesis. For solar radiation to be transformed into available energy for photosynthesis, it is necessary for organisms to possess certain absorption pigments. These pigments would be a kind of “organic photoelectric cells”, i.e., molecular structures that absorb certain wavelengths, converting light energy into chemical energy. The most abundant absorption pigment among photosynthetic organisms is chlorophyll, present within zooxanthellae in the A, B and C forms. Zooxanthellae also possess carotenoid absorption pigments, such as beta-carotene, peridinin and diadinoxanthin. The absorption response for all these pigments is very high in the violet/blue (400-500nm) and red (650-700 nm) range, ensuring that all available light on the reef is utilized. In figure 5 we can verify that the range between 550 nm and 650 nm (green and yellow colors) is of rather little importance for the metabolism of these symbiont algae. Therefore, in our aquarium lighting system, we can greatly reduce the green and yellow colors without significant impact on the biology of corals, clams and invertebrates harboring zooxanthellae.
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Secondly, there is considerable scientific evidence of the detrimental effects of intense red-light radiation on corals (R.A. Kinzie & T. Hunter 1987). Intense red light frequently produces photosynthetic photoinhibition reactions in zooxanthellae, triggering their expulsion by the coral host and subsequent death. It has also been verified in laboratory studies with corals that blue light produces higher photosynthesis rates, zooxanthellae density, chlorophyll content and overall growth, than red light.

The type of light depicted in Figure 4 has everything biologically necessary for the health of fish, corals, invertebrates and the associated food web of a reef aquarium. However we can optimize this spectrum by reducing greens, yellows, oranges and reds, without any detrimental effect on fish, corals and invertebrate’s health. The reduction of wavelengths near to red, not only protects our corals from oxidative stress, but also minimizes the probability of occurrence of unwanted algae. In tide pools, close to the reef, the light received has a large amount of red color, which is used by hair and macro algae for colonization and growth. The same thing can happen in the aquarium if a lot of red-light radiation is used.

Figure 6 shows what the reference light spectrum would look like for reef tank application. HQI bulbs, fluorescent tubes and commercially available LEDs fixtures are designed to provide this spectrum to a greater or lesser extent, with slight modifications. Taking this spectrum as a baseline, if we wish to recreate shallow areas or even intertidal pools, it is recommended to increase the content of green, yellow and red colors. In the case of recreating deeper areas, we can reduce the greens, yellows and reds as much as necessary, without limiting the health or growth of the corals, giant clams and invertebrates.​

As an example, Figures 7 to 10 show the spectrum provided by the manufacturer Giesemann in four HQI bulb models, together with their corresponding color temperature. We verify that the color temperature increases with blue light content, decreasing when there is a greater amount of red and yellow components.​