Lighting the Reef Aquarium - Spectrum or Intensity?
By: Dana Riddle and Miguel Olaizola
It seems that avid reef aquaria hobbyists are constantly in search of a better lighting system. Perhaps one is motivated by a desire for more rapid coral growth, or simply an aquarium that is more pleasing to the eye. The question "What is the best lamp?" is often asked. Although the question is valid, the "best lamp" means different things among hobbyists. Is an aesthetically pleasing aquarium the goal, or is the promotion of photosynthesis the ultimate objective? The former is purely subjective. Finding an answer to the latter is quantifiable, but requires some rather sophisticated equipment. This article suggests an answer to the photosynthesis issue based on results of experiments conducted with a newly available research instrument.
Those factors promoting photosynthesis must be given serious attention since most tropical corals of interest to hobbyists contain symbiotic zooxanthellae algal cells. Of these factors, lighting is of primary importance. Debates have raged over which parameter, light intensity or spectral quality is more important. Both, of course, play a part in promoting photosynthesis in zooxanthellae. Light intensity or Photosynthetic Photon Flux Density (or PPFD, simply the number of light particles - photons - falling upon a given surface) must meet zooxanthellaes’ minimal requirements or the algal cells eventually die. If spectral quality is not correct, photosynthesis is not promoted and zooxanthellae become "starved" for proper light and will soon perish.
Estimating the spectral requirements of zooxanthellae is not particularly easy. Zooxanthellae contain various photosynthetic pigments, including chlorophyll A, chlorophyll C2, the carotenoid peridinin and perhaps others (all of which may be in varying proportions due to the photoadaptive capabilities of zooxanthellae). However, researchers have established the quality of light absorbed by these pigments and we can safely assume certain wavelengths are required.
Figure 1 demonstrates the light energy harvested by zooxanthellae isolated from the stony coral Favia, in a chart called an "action spectrum." An action spectrum describes the relative effectiveness of energy at different wavelengths in producing particular biochemical or biological responses (such as oxygen evolution, carbon uptake, electron transport rate, etc., during photosynthesis).
(FIGURE 1 NOT PROVIDED AS PART OF THIS EXCERT)
Figure 1. An "Action Spectrum" for zooxanthellae. See text. After Muscatine, 1980.
To believe that blue (430-480 nm) and red (600- 700 nm) wavelengths are required is only partially true. A wide range of wavelengths are absorbed by chlorophylls A and C2; however, peridinin and perhaps other photopigments, effectively harvest light energy outside of the range normally associated with photosynthesis.
Researchers have addressed light quality and its effects on zooxanthellae and coral growth. Perhaps the most interesting is a paper by Kinzie et al. (1984); they presented evidence that corals grown more rapidly under blue and white light of the same intensities (~12% of solar Photosynthetically Active Radiation - PAR, ~250 µMols·m2·sec, or 10,000 lux) than under "green" or "red" light of equal intensities. These scientists used clear or colored acrylic filters and natural sunlight. The blue filter transmitted wavelengths of ~ 400 to 500 nm and the clear filter (transmission quality not shown in the paper) likely was a fair representation of sunlight (although most acrylics attenuate all wavelengths but tend to decrease violet and blue disproportionately). "Blue" light is suggested to have some rather "magical" properties - it has been noted to increase rates of protein synthesis in some algae, as well as cause shifts in photosynthetic pigment concentrations in zooxanthellae. Blue light has also been reported to increase rates of photosynthesis (Kinzie and Hunter, 1987). Are spectral characteristics of "blue" metal halide lamps sufficient to promote photosynthesis more efficiently in zooxanthellae of captive corals?
Unfortunately, the spectral qualities of light transmitted by these researchers’ filters only faintly resemble those of lights used over aquaria. It is a leap of faith to apply the results obtained under filtered sunlight to artificial light sources, which have spectral spikes. However, this has not stopped many from interpreting that higher Kelvin lamps are best for promoting photosynthesis in corals.
In an excellent series of articles, Joshi and Morgan (1998; 1999) presented spectral qualities of many metal halide lamps commonly sold in the pet industry, but stopped short of making recommendations to hobbyists. So, the question remains - are there major advantages to zooxanthellae/corals when using certain lamps, or is there only aesthetic appeal? Do common lamps with output weighted in the violet/blue regions of the spectrum and readily available to hobbyists actually increase the rates of photosynthesis?
Two lamps were chosen for use in an experiment designed to determine if differing spectral qualities do indeed make a difference in photosynthesis rates. The first lamp is a Philips 175-watt 4,000° K metal halide lamp (usually available for less than $20 in major home improvement centers). The second lamp is an Aquarium Lighting Systems 175-watt 12,000°K "Sunburst" metal halide lamp. Spectral signatures of these lamps were determined with an Ocean Optics spectrometer. Spectral compositions were estimated by use of a LiCor quantum meter and glass cut-off filters. Use of these filters provides reasonable estimates of violet and blue wavelengths (400-465 nm) and red wavelengths (600-700 nm). These filters transmit few wavelengths in the yellow and orange portion of the spectrum. Considering that metal halide lamps have spectral spikes at 575 and 577 nm (due to the element mercury contained within the arc tubes), the percentages of blue radiation shown in the pie charts are slightly overstated (See Figures 2 - 5). However, the Sunburst 12,000°K lamp is the "bluest;" the Philips lamp less so.
(CONT)
By: Dana Riddle and Miguel Olaizola
It seems that avid reef aquaria hobbyists are constantly in search of a better lighting system. Perhaps one is motivated by a desire for more rapid coral growth, or simply an aquarium that is more pleasing to the eye. The question "What is the best lamp?" is often asked. Although the question is valid, the "best lamp" means different things among hobbyists. Is an aesthetically pleasing aquarium the goal, or is the promotion of photosynthesis the ultimate objective? The former is purely subjective. Finding an answer to the latter is quantifiable, but requires some rather sophisticated equipment. This article suggests an answer to the photosynthesis issue based on results of experiments conducted with a newly available research instrument.
Those factors promoting photosynthesis must be given serious attention since most tropical corals of interest to hobbyists contain symbiotic zooxanthellae algal cells. Of these factors, lighting is of primary importance. Debates have raged over which parameter, light intensity or spectral quality is more important. Both, of course, play a part in promoting photosynthesis in zooxanthellae. Light intensity or Photosynthetic Photon Flux Density (or PPFD, simply the number of light particles - photons - falling upon a given surface) must meet zooxanthellaes’ minimal requirements or the algal cells eventually die. If spectral quality is not correct, photosynthesis is not promoted and zooxanthellae become "starved" for proper light and will soon perish.
Estimating the spectral requirements of zooxanthellae is not particularly easy. Zooxanthellae contain various photosynthetic pigments, including chlorophyll A, chlorophyll C2, the carotenoid peridinin and perhaps others (all of which may be in varying proportions due to the photoadaptive capabilities of zooxanthellae). However, researchers have established the quality of light absorbed by these pigments and we can safely assume certain wavelengths are required.
Figure 1 demonstrates the light energy harvested by zooxanthellae isolated from the stony coral Favia, in a chart called an "action spectrum." An action spectrum describes the relative effectiveness of energy at different wavelengths in producing particular biochemical or biological responses (such as oxygen evolution, carbon uptake, electron transport rate, etc., during photosynthesis).
(FIGURE 1 NOT PROVIDED AS PART OF THIS EXCERT)
Figure 1. An "Action Spectrum" for zooxanthellae. See text. After Muscatine, 1980.
To believe that blue (430-480 nm) and red (600- 700 nm) wavelengths are required is only partially true. A wide range of wavelengths are absorbed by chlorophylls A and C2; however, peridinin and perhaps other photopigments, effectively harvest light energy outside of the range normally associated with photosynthesis.
Researchers have addressed light quality and its effects on zooxanthellae and coral growth. Perhaps the most interesting is a paper by Kinzie et al. (1984); they presented evidence that corals grown more rapidly under blue and white light of the same intensities (~12% of solar Photosynthetically Active Radiation - PAR, ~250 µMols·m2·sec, or 10,000 lux) than under "green" or "red" light of equal intensities. These scientists used clear or colored acrylic filters and natural sunlight. The blue filter transmitted wavelengths of ~ 400 to 500 nm and the clear filter (transmission quality not shown in the paper) likely was a fair representation of sunlight (although most acrylics attenuate all wavelengths but tend to decrease violet and blue disproportionately). "Blue" light is suggested to have some rather "magical" properties - it has been noted to increase rates of protein synthesis in some algae, as well as cause shifts in photosynthetic pigment concentrations in zooxanthellae. Blue light has also been reported to increase rates of photosynthesis (Kinzie and Hunter, 1987). Are spectral characteristics of "blue" metal halide lamps sufficient to promote photosynthesis more efficiently in zooxanthellae of captive corals?
Unfortunately, the spectral qualities of light transmitted by these researchers’ filters only faintly resemble those of lights used over aquaria. It is a leap of faith to apply the results obtained under filtered sunlight to artificial light sources, which have spectral spikes. However, this has not stopped many from interpreting that higher Kelvin lamps are best for promoting photosynthesis in corals.
In an excellent series of articles, Joshi and Morgan (1998; 1999) presented spectral qualities of many metal halide lamps commonly sold in the pet industry, but stopped short of making recommendations to hobbyists. So, the question remains - are there major advantages to zooxanthellae/corals when using certain lamps, or is there only aesthetic appeal? Do common lamps with output weighted in the violet/blue regions of the spectrum and readily available to hobbyists actually increase the rates of photosynthesis?
Two lamps were chosen for use in an experiment designed to determine if differing spectral qualities do indeed make a difference in photosynthesis rates. The first lamp is a Philips 175-watt 4,000° K metal halide lamp (usually available for less than $20 in major home improvement centers). The second lamp is an Aquarium Lighting Systems 175-watt 12,000°K "Sunburst" metal halide lamp. Spectral signatures of these lamps were determined with an Ocean Optics spectrometer. Spectral compositions were estimated by use of a LiCor quantum meter and glass cut-off filters. Use of these filters provides reasonable estimates of violet and blue wavelengths (400-465 nm) and red wavelengths (600-700 nm). These filters transmit few wavelengths in the yellow and orange portion of the spectrum. Considering that metal halide lamps have spectral spikes at 575 and 577 nm (due to the element mercury contained within the arc tubes), the percentages of blue radiation shown in the pie charts are slightly overstated (See Figures 2 - 5). However, the Sunburst 12,000°K lamp is the "bluest;" the Philips lamp less so.
(CONT)
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