Most of the processes used in the marine aquarium hobby are not very scientific, or not implemented in a way that has been demonstrated scientifically to be effective. One example of this is lighting in the marine aquarium. Using a “scientific method” to light a reef aquarium is not currently easy to do, if possible. However, as caretakers of photosynthetic invertebrates, aquarists should have a basic knowledge of the science behind illumination and photosynthesis.
What is light?
There is an incredible amount to know about visible light, and this article will just be scratching the service by listing some very basic information about light and the measurement of light, that should be helpful to hobbyists.
Visible light is simply a small frequency range (frequency means how often a full wave propagates) in a very large range of occurring energy frequencies. Most of the visible light spectrum, which is energy with a wavelength between 400 and 700 nanometers (one billionth of a meter, abbreviated nm) will stimulate chloroplasts, resulting in photosynthesis. Wavelengths shorter than 400nm carry too much energy and can damage or destroy living tissues, while wavelengths longer than 700nm do not carry enough energy for photosynthesis to occur.
There are many ways to produce visible light, but all visible light, no matter what method used to produce it, is just that: visible light. Because of this, the typical lighting sources used by an aquarist (VHO, T-5, Metal Halide, LED, natural sunlight) are all equally sufficient options for lighting a reef aquarium when considering spectral production and PAR.
There are many ways to measure light energy and “brightness”, and a few of these methods that the hobbyist is likely to encounter are defined below.
Photosynthetically Active Radiation
PAR is the only useful way to measure light energy and quantity for the home aquarist, and is much simpler to define and measure than any other form of light measurement. PAR is the number of photons per meter squared per second of light that falls between 400nm and 700nm in wavelength. Although longer wavelengths (the short end of the infrared spectrum) are useful photosynthetically, the PAR definition doesn’t account for this. However, infrared energy is virtually useless to the home aquarist, as it doesn’t penetrate the water column to a significant degree.
When aquarists refer to light “intensity” in an aquarium, PAR is what they are actually referring to, as PAR is literally the amount of photosynthetically usable light per given area.
Contrary to the popular belief that “actinic” and/or blue lighting is not useful for photosynthesis, photosynthetic efficiency peaks at around 430 nanometers, which is approximately the major spectral output of a standard “actinic” bulb, and 680 nanometers, or approximately the major spectral output of a “daylight” bulb.
Intensity
Intensity is defined as “the measure of the time-averaged energy flux”, meaning the energy transferred from one medium to another (traditionally a waveform, such as light, to another object) per amount of time (in seconds). The human eye cannot accurately distinguish intensity, and there is no easy way to compute the intensity of a light source at a given depth and spectrum in an aquarium. Intensity as a measurement is essentially useless for the home aquarist.
Lumen
The lumen measures luminous flux, the perceived power, or brightness of light by the human eye. For example, it is possible for two light sources to have the same intensity but emit a different number of lumens. The source that emits a higher number of lumens will appear brighter to the human eye, but the intensity will be the same. This can also be somewhat useful for the aquarist, because higher lumen count can equate to a higher PAR count, depending upon the spectrum.
While lumens are a useful measurement for household light bulb comparison, this measurement usually only serves the energy conscious hobbyist by giving a lumen per watt count on the bulbs used (assuming that information is available from the manufacturer). Keep in mind that a bulb emitting 2000 lumens at a color temperature of 20,000 kelvin (K) won’t emit as much PAR as a bulb emitting 2000 lumens at a color temperature of 6,500K (explained below).
LUX
LUX is a unit of measurement of lumens per square meter, sometimes (and incorrectly) used synonymously with light intensity. Instead, LUX is the measurement of apparent intensity, as viewed by the human eye, per square meter. Because the human eye weighs certain parts of the spectrum (certain wavelengths) as brighter than others, two light sources can have the same intensity but a different LUX. LUX was somewhat useful to the aquarist before PAR meters became relatively affordable, and can still be useful if a PAR meter is unavailable. The author's S. haddoni in its species display
Kelvin Temperature
Kelvin temperature (K), is the scientific unit for temperature, and is often used to measure the color temperature of light, or more accurately, the measurement of the temperature of an object emitting black body radiation (also known as thermal radiation, or radiant heat) as visible light (to be useful to aquarists). All objects at a temperature greater than absolute zero emit some form of thermal radiation (although at room temperature the wavelength of this radiation is too long for the human eye to perceive). There is a great deal to know about kelvin temperature and thermal radiation (most of it mathematical) that exceeds the scope of this article, but kelvin temperature is a very useful way to measure the color spectrum of a bulb.
Kelvin temperature, in the hobby, is used virtually interchangeably with spectrum. Spectral analysis is more useful for determining PAR, but kelvin temperature has become the norm among bulbs produced for the hobby. Kelvin temperature is often associated with PAR: a light source with a given intensity from an object with a color temperature of 6500K has more PAR than light emitted from a source of the same intensity but having a color temperature of 14,000K. A light source with a color temperature of 6500k places more of the energy between the 400-700nm wavelengths.
Spectrum
The color spectrum of visible light is well known by most – Red, Orange, Yellow, Green, Blue, Indigo, and Violet. These color spectrums (also known as wavelength, measured in nanometers) are usually not a concern to the modern hobbyist, as broad spectrum bulbs are readily available. Photosynthetic animals require this broad spectrum lighting to fully utilize the energy from light. The spectral range useful for photosynthesis is cited as PAR. To be the most useful for photosynthesis, aquarist should use bulbs that peak at ~430nm and ~680nm spectrum/wavelength. Photosynthesis
According to the Oxford Dictionary of Biochemistry and Molecular Biology (1997), photosynthesis is the “synthesizing by organisms of organic chemical compounds, mainly carbohydrates, from carbon dioxide using energy obtained by light rather than the oxidation of chemical compounds”. More simply, this means that plant cells can use energy gathered from light to produce cellular chemical energy (ATP) and carbon products (carbohydrates) when combined with carbon dioxide.
In order for the photosynthetic process to take place, the chloroplast (the organelle of the cell where the light energy to chemical energy conversion occurs) must receive sufficient PAR. If the saturation or compensation point of the chloroplast isn’t met, the organelle will not produce the optimum amount of carbon bi-products (carbohydrates), and this excess energy will not be transferred to the host invertebrate. Obviously, the compensation point is something every aquarist will want to meet. In attempting to meet this compensation point, however, the aquarist must avoid photoinhibition. To the laymen, this means that an excess of light causes a cessation of photosynthesis altogether. Photosynthetic invertebrates have a host of light inhibiting pigments to protect themselves from tissue damage and photoinhibition, as photoinhibition occurs much more frequently than the compensation point not being met.
Some aquarists believe that some corals “only need light” to survive, but this is absolutely untrue. No known animal can survive solely on light, as there must always be a phosphate and nitrogen source for a living cell to create the compounds needed to function. Photosynthesis is the process used to gain the necessary energy to convert these phosphate and nitrogen sources into ATP from light, which can be used directly by the cell. Photosynthesis does not turn energy into matter!
Lighting Methods
Hobbyists often debate and discuss what method of lighting is “superior”, but these discussions are misplaced. To even attempt to compare different lighting sources in the way aquarists intend to, one must choose a specific basis for comparison. The only easily measurable units for comparison would be PAR at a given depth, LUX at a given depth, or lumens per watt (efficiency). Any other type of comparison is unlikely to yield any useable or accurate result(s). There is no “best lighting method”, excepting the sun, making this debate completely useless. Instead, the aquarist should be more concerned with achieving the proper PAR at a given depth for the animals he or she has chosen to keep, with a preference for the least energy usage.
Conclusion
The superiority of the sun cannot be replicated in captivity, although thankfully photosynthetic invertebrates are adaptable to a wide range of lighting conditions. Basic knowledge regarding lighting and the photosynthetic process are necessary to understand what is actually occurring in a reef aquarium every day, and very helpful in maintaining healthy animals in the long term.
While information in this short piece is somewhat optional for the home aquarist, it is essential knowledge for anyone attempting to propagate or otherwise make a profit growing or displaying photosynthetic marine invertebrates. The Author's Nano Reef Aquarium
References
All images by the author from the author's aquariums
Mendenhall, T. C. (1893). "Fundamental Standards of Length and Mass". Reprinted in Barbrow, Louis E. and Judson, Lewis V. (1976). Weights and measures standards of the United States: A brief history (NBS Special Publication 447). Washington D.C.: Superintendent of Documents. Viewed 23 August 2006 at http://physics.nist.gov/Pubs/SP447/ pp. 28–29.
Cecie Starr (2005). Biology: Concepts and Applications. Thomson Brooks/Cole. ISBN 053446226X. http://books.google.com/books?id=RtSpGV_Pl_0C&pg=PA94&dq=380+750+visible+wavelengths&as_brr=3&ei=g7x0R5erIISOsgOtsLGeBw&ie=ISO-8859-1&sig=wJ7g0EcU-QUF29vfvl36YNg-FtU.
http://www.life.uiuc.edu/govindjee/paper/gov.html, from "Concepts in Photobiology: Photosynthesis and Photomorphogenesis", Edited by GS Singhal, G Renger, SK Sopory, K-D Irrgang and Govindjee, Narosa Publishers/New Delhi; and Kluwer Academic/Dordrecht, pp. 11-51. From unpublished data.
Smith, A. L. (1997). Oxford dictionary of biochemistry and molecular biology. Oxford [Oxfordshire]: Oxford University Press. pp. 508. ISBN 0-19-854768-4. "Photosynthesis - the synthesis by organisms of organic chemical compounds, esp. carbohydrates, from carbon dioxide using energy obtained from light rather than the oxidation of chemical compounds."
D.A. Bryant & N.-U. Frigaard (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends Microbiol 14 (11): 488. doi:10.1016/j.tim.2006.09.001
http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html
Raszewski G, Diner BA, Schlodder E and Renger T (2008) Spectroscopic properties of reaction center pigments in photosystem II core complexes: Revision of the multimer model. Biophys. J. 95:105-119
Rappaport F, Guergova-Kuras M, Nixon PJ, Diner BA and Lavergne J (2002): Kinetics and pathways of charge recombination in photosystem II. Biochemistry 41, 8518-8527
NIST Guide to SI Units - 9 Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technolog
R. Bowling Barnes (24 May 1963). "Thermography of the Human Body Infrared-radiant energy provides new concepts and instrumentation for medical diagnosis". Science 140 (3569): 870 - 877. doi:10.1126/science.140.3569.870
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