Lighting, Photosynthesis, and Marine Invertebrates

Author: Mike Maddox

Understanding how light works and the basics of the type of light required is essential for anyone looking to set up a reef aquarium.

Many different organisms in the hobby require lighting to survive—corals, giant clams, and plants all have specific lighting needs. There is a lot of discussion about the best or ideal type of lighting for different plants and animals, although these opinions are not always based on sound science. In fact, taking a scientific approach to lighting is not easy to do for the average hobbyist. However, a basic understanding of light, how photosynthesis works, and how to apply that understanding to an aquarium can be a big help for aquarists interested in keeping photosynthetic organisms.

What Is Light?

There is an incredible amount to know about visible light, and I will just be scratching the surface of the subject. Every hobbyist who wishes to house photosynthetic invertebrates should have some very basic information about light and its measurement. After all, most of your corals and anemones need it to survive.

Quick Physics Lesson

Before diving into hobby applications of light, it is best to know some background physics. Visible light is a small part of the continuum known as electromagnetic radiation and includes such things as radio waves, X-rays, and microwave energy. These energies are measured by their wavelength, usually with a nanometer (billionth of a meter) scale, abbreviated “nm.”

Visible light is energy with a wavelength between 400 and 700 nm, and specific wavelengths within this range are used by photosynthetic organisms. Wavelengths shorter than 400 nm carry too much energy and can damage or even destroy living tissues (think of ultraviolet radiation, which leads to sunburn), while wavelengths longer than 700 nm do not carry enough energy for photosynthesis to occur.

Producing Visible Light

There are many technologies to produce visible light, and as long as you are producing visible light, you’re on the right track. The typical lighting sources used by reef aquarists are very high output (VHO), T-5 high output, and metal halide bulbs, as well as light-emitting diodes (LEDs) and natural sunlight.

Sounds simple, right? Wait, there’s a bit more. There are many ways to measure light energy and brightness, and a few of these methods and additional terms that the hobbyist is likely to encounter are defined below.

Photosynthetically Active Radiation

Photosynthetically active radiation (PAR) is, in my opinion, the only useful way to measure light energy and quantity for the home aquarist—I think it is much simpler to define and use than any other form of light measurement. To put it simply, it is the measurement of how much visible light energy reaches a square meter in one second. Aquarists often mistakenly use the term intensity instead of PAR, as PAR is literally the amount of usable light energy available for photosynthesis per given area.

Another common misconception aquarists have is that actinic blue lighting is not useful for photosynthesis. On the contrary, photosynthetic efficiency has two peaks, one at a wavelength of about 430 nm, which is approximately the main wavelength produced by a standard actinic bulb, and one at a wavelength of about 680 nm, which is approximately the major wavelength produced by the more commonly used daylight bulb. Both bulb types and wavelengths are useful in providing PAR to photosynthetic invertebrates, and a combination of blue and white bulbs is usually used for aesthetics.


Intensity refers to the amount of energy transferred from one medium to another per amount of time—as in the amount of light energy from and to another object per second. For the aquarium, it is a measurement of how much energy is transferred via light from the source (bulb) to an object (a coral) per second. This is all light, not just photosynthetically active energy.

The human eye cannot accurately determine intensity, and although there are meters available to measure light intensity, many of them actually measure PAR. It is difficult to determine the intensity of a light source at a given depth in the aquarium. That is why I say that intensity is a very misused term in the hobby and is not as applicable to aquariums as PAR.

Kelvin Temperature

Beware, kelvin temperature is another difficult term. The Kelvin scale is a standard scientific measurement for temperature. Absolute zero is zero kelvin degrees, abbreviated as 0K. (Note there is no degree symbol.) It is often used to measure the color temperature of light, or more accurately, to measure the temperature of an object emitting radiation; at certain temperatures this radiation will include visible light.

All objects at a temperature greater than absolute zero emit some form of radiation, although at room temperature the wavelength of this radiation is too large for the human eye to perceive. Think of it this way: At room temperature, an electric stove burner doesn’t emit any visible light. However, as the burner increases in temperature, the energy associated with the burner increases, and its radiation increases; thus the radiation the burner is emitting changes and it begins to emit that energy as visible light.

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 range is more useful for determining PAR, but kelvin temperature is the standard unit given on 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 with a color temperature of 14,000K. A light source with a color temperature of 6500K places more of the energy between the 400 to 700 nm wavelengths, thus there is more energy available for photosynthesis to occur.


The color spectrum of visible light is well known by most—red, orange, yellow, green, blue, indigo, and violet. A bulb’s color spectrum is usually not a concern to the modern hobbyist, as broad-spectrum bulbs are readily available, and virtually all bulbs made for reefs are broad spectrum. Photosynthetic animals require this broad-spectrum lighting to fully utilize the energy from light. The spectral range useful for photosynthesis is cited as PAR, as you’ll recall. To be the most useful for photosynthesis, aquarists should use bulbs that have a peak spectral output of about 430 nm and 680 nm.


The lumen measures luminous flux, or the perceived power or brightness of light by the human eye. This is purely a term used for our eyes, and not a measurement of light energy that is, say, shining on your coral.

For example, it is possible for two light sources to have the same intensity (same energy transfer) but emit a different number of lumens. The source that emits a higher number of lumens will appear brighter to our eyes, but the intensity will be the same. This can also be somewhat useful for the aquarist because a higher lumen count can equate to a higher PAR count, depending upon the spectrum.

While lumens are useful when, for example, comparing household light bulbs (as you will use the light to see), this measurement usually only serves the energy-conscious hobbyist. By comparing lumen-per-watt count on the bulbs (assuming that information is available from the manufacturer), you can determine how bright your tank will be with a given amount of energy. Keep in mind that a bulb emitting 2000 lumens at a color temperature of 20,000K won’t emit as much PAR as a bulb emitting 2000 lumens at a color temperature of 6500K.


Lux is a unit of measurement of lumens per square meter, sometimes incorrectly used synonymously with light intensity. Instead, lux measures apparent intensity (how bright it looks), as viewed by the human eye, per square meter. A light transmitting 100 lumens on a square meter area produces an illumination of 100 lux, but 100 lumens on 10 square meters gives a luminosity of 10 lux.

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, but now a PAR meter can be had for the same price and is much more useful.


Photosynthesis is how plants obtain sugar and other carbohydrates. Instead of consuming carbohydrates for energy as humans do, the process of photosynthesis allows plants and some invertebrates to make their own carbon products from carbon dioxide using energy obtained from light.

Converting Energy

In order for the photosynthetic process to take place, the organelle of the cell where the light energy is converted to chemical energy—the chloroplast—must receive sufficient PAR. All chloroplasts have something known as the compensation point, which is when the rate of photosynthesis equals the rate of metabolism. When only the compensation point is reached, the symbiotic algae (zooxanthellae) harbored by corals will survive and do well, but they will not transfer most of their carbohydrates to the host coral. The coral may get enough food to survive, but not enough to thrive.

Saturation Point

There is also the saturation point, the point at which photosynthesis is being performed at the maximum rate, so adding additional light will not increase the rate of photosynthesis any further. If the saturation point of the chloroplast isn’t met, the organelle will not produce the maximum amount of carbohydrates, and the excess energy will not be transferred to the host invertebrate.

Obviously, the saturation point is something every aquarist will want to reach at all depths in their aquarium. Remember, however, that different organisms have different saturation points, which is why corals are classified in terms such as low light and high light. Low-light corals, generally speaking, have a lower saturation point than high-light ones and therefore can be put in areas with a lower PAR, such as the bottom of the aquarium, farther from the light source.

Avoiding Photoinhibition

Knowing this information is important because in attempting to meet the saturation point, the aquarist must avoid photoinhibition. Photoinhibition is the result of an excess amount of light energy reaching the chloroplast—if too much energy is absorbed, photosynthesis can stop completely. As you may know, photosynthetic invertebrates have a host of light-inhibiting pigments that protect them from tissue damage and their zooxanthellae from photoinhibition. In fact, those pigments are the source of the pretty colors seen on corals and giant clams. Those pigments are present because, in the wild, too much light energy occurs much more frequently than insufficient light energy.

Phosphorous and Nitrogen

Some aquarists believe that some corals only need light to survive, but this is absolutely untrue. No animal known to science can survive solely on light energy. There must always be a source of phosphorous and nitrogen for a living cell to create the compounds needed to function, even if these organic chemicals are simply dissolved in the water column and taken up by the animal.

Photosynthesis Explained

Photosynthesis is a process in plants that converts the energy in sunlight into stored chemical energy. Animals that eat the plants (or that eat animals that ate the plants) can then use this energy source to utilize phosphate and nitrogen sources for their metabolic needs. Photosynthesis does not invent something from nothing; it is simply an energy conversion process using light energy to turn compounds in the water column into more advanced and usable compounds for the algae, and in turn, the host invertebrate.

Which Lighting Methods Are Best?

Knowing which lighting method is best is a hot topic among hobbyists, but a discussion about superior lighting methods is misplaced. Obviously, all artificial methods are inferior to sunlight, but it is a rare home that has enough ambient sunlight to grow corals.

Instead of concentrating on a misplaced and unscientific debate over which lighting method is best, we as hobbyists should focus our efforts on achieving a proper PAR range at a given depth for the animals we wish to keep. If sunlight can’t be used to achieve this, concentrate on achieving an ideal PAR range in the most efficient manner possible, such as using electrically efficient methods and long-life bulbs.

While the superiority of sunlight cannot be easily used except with skylights and/or windows, photosynthetic invertebrates are adaptable to a wide range of lighting conditions. Basic knowledge regarding lighting and the photosynthetic process is necessary to understand what is actually occurring in a reef aquarium, and it is very helpful in maintaining healthy animals for the long term and in alleviating debates over lighting methods that are almost always based purely on speculation. I hope this article has helped you make sense of your lighting options in the aquarium.

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