Before discussing what kind of light corals need, let's get to know corals.
The first step should be to clarify whether corals are animals or plants. We often see corals living on the seabed with seagrasses and the like, so we mistakenly think that coral is a plant. In fact, corals are inferior animals in the ocean, coral belongs to the Coelenterata phylum (Coelenterata) (Anthozoa), the largest class in the coelenterate phylum, with more than 7,000 species, all of which are seafood. Polyps are further divided into eight-ample coral suborders and six-ample coral suborders. Depending on the skeletal texture and body size of the hydra, corals are generally divided into several types, such as large polyp scleractinian (LPS), small polyp scleractinian (SPS), soft coral, and anemone.
So what do corals feed on? One is to prey on plankton and organic debris through tentacles, cilia and mucus, and the other is that zooxanthellae, which live symbiotic with corals, can provide a source of nutrition for zooxanthellae, a type of seaweed, which is estimated to have 30,000 zooxanthellae per mm3 of coral tissue, and they coexist with polyps in reciprocity. Zooxanthellae are mostly autotrophies and provide the host with translocation reducing carbides, such as glucose, glycerol, amino acids and other photosynthetic products. Zooxanthellae can provide up to 90% of the energy needs of coral reefs. In return, corals provide zooxanthellae with protection, shelter, nutrition (mainly waste containing nitrogen and phosphorus) and a constant supply of carbon dioxide needed for photosynthesis. Corals living in deep waters have no symbiotic algae in their bodies because of the extremely weak light on the seabed, and they prey mainly in the first way. Corals living in shallow waters mostly have two feeding methods at the same time.
Now we know why corals need light, as the symbiotic zooxanthellae in their bodies need photosynthesis. Corals usually grow at a depth of no more than 20 m. In fact, the depth itself is not the decisive factor, but mainly determined by the radiation energy and light requirements of zooxanthellae; relative to the surface layer, the intensity of radiation energy is greatly reduced when the water depth exceeds 20m; in addition, the number of zooxanthellae coexisting with corals is usually the largest at a water depth of 4m-5m. Early researchers believed that zooxanthellae belonged to a single species, and the current discovery is that zooxanthellae are different species groups of many species, and it may have hundreds of species. At present, zooxanthellae are divided into A-G7 communities, of which the A and B groups are mainly present in shallow water corals (high radiation region), and the C group is mainly in deep-water corals (low radiation region).
Water surface (light blue), 5 m deep (blue), 15 m deep (dark blue)
The above figure is a plot showing the relationship between the spectral distribution and wavelength of different depths, from which we can see that there is still a certain proportion of light above 600 nm wavelength at a water depth of 5 meters, and when it reaches 15 meters, this part of the light almost disappears.
The following table is fiji's 3-month statistics, converted to the data, we can get the distribution of visible light power in different wavelength bands at different depths.
Above 5m water depth, 520 nm or more or higher radiation power, the above mentioned that the number of zooxanthellae symbiotic with corals is usually the most at a water depth of 4m-5m, indicating that the zooxanthellae photosynthesis of the worm part can use these lights, such as the A and B group of zooxanthellae; at 15m water depth at least 480nm wavelength light attenuation is not much, and above 520nm is sharply attenuated, I think part of the zooxanthellae can survive normally in the absence of light above the wavelength of 520nm, For example, C-series swarms of xanthophyllum.
The figure above is a light absorption map of a class of zooxanthellae, and we can see that the violet-blue light absorbs the most, and there are obvious peaks near the red light of 660-680 nm.
It can be seen that light in the 400-480 nm band is essential for photosynthesis of zooxanthellae, and in the natural environment of Fiji, corals receive 52-55W/m2 light in the 400-440 nm range and 60-64 W/m2 light in the 440-480 nm range.
The average total amount of solar radiation in Fiji is 413 W/m2, while the average value of China's Nansha Islands is 220W/m2. The total amount of solar radiation in different regions is very different, and the maximum amount of solar radiation that different corals can accept is also different. The authors of the article "Coral Reef Aquarium Light" pointed out that the reasonable maximum radiation intensity of the wavelength in the 400-440 nm range is about 45 W/m2, while the 440-480 nm range is 40 W/m2.
Through the previous analysis, I think that corals living in deep water can completely rely on the light in the 400-480 nm band to survive normally, and the total radiation of coral lamps is not more than 60W/m2; and for corals living in shallow waters, it is also necessary to have 400-480 nm of light, if the red light in the 660-680 nm band is increased, it is advisable to make corals live better, and the total radiation of coral lamps does not exceed 90W/m2.
My materials are mainly derived from "Introduction to Coral Reef Science" edited by Xu Kefu and "Coral Reef Aquarium Light" edited by Dmitry Karpenko, Vahe Ganapetyan.