The use of blue light in the hobby is a topic often debated, sometimes contentiously. I know. As a seller of corals, I often face skeptics of the blue cast in my photos and videos. But here’s the thing, my goal has always been to portray the corals as they appear to my eyes. My focus is always on the corals. So when the public scrutinizes the blue cast over the sandbed or my hand, I don’t engage it. It doesn’t tell the full story. But the question still warrants an answer: beyond just look, what does blue light do to the corals? As I have come to understand, blue light plays a major biological role for many corals in the hobby, especially those adapted to clear water environments where the spectrum becomes increasingly blue-heavy with depth. The beauty of the colors happens to be a byproduct that we enjoy. In this article, we will get a deeper understanding, based on current scientific research, of the role that blue spectrum lighting plays in delivering biological benefits to our corals, while also allowing us to appreciate the wide range of colors they display.
I want to preface this by saying that I am not a scientist. This is a hobby-grade interpretation of current research, intended to provide science-driven information without turning a complex subject into a scientific review. Results can vary by species, spectrum, intensity, and acclimation, and new research may refine our understanding. But the evidence is strong enough to support practical conclusions about the role of blue light in reef keeping.
What blue means
Throughout this article, I use “blue” as hobby shorthand for the violet-to-blue part of the spectrum, not one exact wavelength or LED channel. In clear ocean water, depth removes longer wavelengths first, such as red and yellow, leaving a broad range dominated by blue light. Aquarium lights distribute their output differently within this range. Some peak in royal blue, while others peak further into violet. Which mix is best is beyond the scope of this article. So when I say blue, I mean this broader short-wavelength range, while recognizing that individual wavelengths can have different effects.
Two separate processes are also important. Expression is the long-term response in which light causes a coral to produce more pigment. Excitation happens instantly when an existing pigment absorbs light and glows. Different fluorescent proteins respond to different wavelengths. Violet can excite some cyan and blue proteins, blue excites many green proteins, and some red proteins respond best to blue-green or green-yellow light. Other proteins require near-UV light to glow a different color. These are well documented, but this article focuses on the broader blue region and its role in coral biology.
Blue light dominates in the ocean
It’s well known that light gets filtered out by seawater at different depths based on wavelength. For example, red is easily absorbed on the surface. By about 5 meters or 15 feet most of the red no longer reaches. Blue light penetrates well past 50 meters or 160 feet in clear water. So a coral living deeper than that must make use of what is available. This is probably why so many deep reef fish are bright red when you bring them up, yet look almost black down where they live. With no red light to reflect, red is the best camouflage there is in deeper water.
When I dove in Tahiti in 2013, I saw SPS corals all the way down to 100 feet of water. And everything was a blue wash. These are pictures I took on that trip. Note the depths we were in.
Of course, plenty of corals do live in the shallows, under the full blast of the sun. But only a few meters down, the spectrum becomes increasingly blue-heavy. So when someone tells you blue doesn’t look natural, it really comes down to which part of the reef they prefer.
Blue dominates our tanks too
Let’s take metal halide for example, which has been known to give off that full spectrum look. But have you ever seen a photo of a tank under metal halide and it still comes out bluer than it appears to your eyes? That’s because halide throws plenty of blue too. The most intensity comes from the blue/violet wavelengths even in a 10,000k bulb. Since our brain is not built to see white as a color, but rather just blends colors together, our eyes don’t register that peak. But a camera’s sensor is designed to process a broad spectrum of raw data. When your most intense spectrum is blue and violet, you can bet the photo will be blue-heavy.
A 12,000K bulb peaks at 450nm, commonly referred to as royal blue. This means this bulb is most intense in the blue spectrum, even though more of its PAR is from the other colors.
One thing to note: PAR meters count photons. Blue photons carry more energy individually than red photons, but that does not make a lower PAR reading of blue equivalent to a higher full-spectrum reading. How useful that light still depends on the exact wavelengths, how the coral and its symbionts absorb them, and how they have adapted or acclimated.
Here’s an old picture from an article on metal halide by Marc Leveson. Notice the blue tint on even the 10,000k bulb:
Zooxanthellae are brown, what gives coral colors?
The algae living in coral tissue, the zooxanthellae, use chlorophyll a and c and peridinin, pigments that evolved to absorb blue and violet efficiently, as well as red if available. What they don’t absorb, the combined reflected light, is the golden brown you see in zooxanthellae. It’s why a coral carrying a lot of them looks brown.
You probably already know that. But you may ask if the algae make the coral look brown, what gives the coral colors? Researchers found the answer. They took Acropora millepora polyps that had only just settled, before they had picked up any zooxanthellae, so there are no algae in them at all. Some went under red light, some under blue, and they left them for five days.
The one under blue lit up significantly brighter. Green fluorescence in the tentacles came out significantly higher. Remember there were no algae in there to give off any color.
The same baby Acropora millepora polyps after five days under red light and under blue. No zooxanthellae in either. The chart is the measured difference across 28 polyps. From D’Angelo et al. 2008, Fig. 4.
Corals have been well documented to utilize blue light independent of zooxanthellae. And cryptochromes, specialized light-sensitive proteins found in plants and animals that absorb blue and ultraviolet light to regulate biological process, are found in Acropora millepora itself.
The algae need light to power photosynthesis to feed the coral, but they can also get too much of it. Past a certain point the extra energy stops being useful and starts doing harm, and that leads to bleaching. Next we’ll look at what fluorescent proteins can do to help.
How Blue Light Impacts Coral Colors - Fluorescent Protein vs Chromoprotein
These are the proteins responsible for the pigments of our corals. And they behave differently. Understanding how they work helps us understand the role of blue light.
Two kinds of pigment
Blue and violet light have short wavelength, which carry higher energy than longer wavelength light such as green, orange and red. Corals make fluorescent proteins that make light, not reflect them. So when a high energy blue light hits the protein, it’s absorbed and used, then converts to a fresh photon carrying slightly less energy, giving us the green, orange or red.
Chromoproteins on the other hand reflect light. They absorb some wavelengths and reflect the rest, and the part they reflect is the color you see. Chromoproteins play a smaller part in coloring your corals. The deep purples and blues on acro tips are usually chromoproteins, so they reflect instead of glow.
Research has shown that blue light builds both of these proteins. The difference is in the way colors are presented, reflected or made. This is where corals are different from plants. A leaf’s color is the light it reflects, so if a leaf absorbed every wavelength perfectly, it would look black to us. On a coral, we mostly see the color that the algae reflects, which is brown. The other colors on a coral are made by the fluorescent proteins.
Reflecting vs glowing
Say you’ve got two corals that both look colorful in your tank. One is an acro with purple tip, purple because of a chromoprotein, so it reflects light. The other is a red that glows from fluorescent proteins, so it makes light.
Under white light, the purple tip looks purple, bouncing purple back at you. White light contains every color, so there’s plenty for it to reflect. The red one looks dull by comparison, but it doesn’t mean it’s not glowing. Its glow simply looks subdued next to all the white light bouncing off every surface of the tank. Same reason you can’t see stars during the day. They’re still up there. The sky is just brighter.
Now take the white away and leave only blue light. The red one now pops, because there’s no longer the overpowering white light competing with it. The purple tip changes too, now in a subdued way. Purple needs some red in the mix to make purple, and blue light doesn’t have that, so the tip turns to blue or simply looks darker. It’s still reflecting light, it just has less to reflect.
The same Acropora millepora colonies, photographed twice. Top under white light, bottom under blue light. From D’Angelo et al. 2008, Fig. 1E.
Corals are built for blue light. Color is part of the response
Blue light is the trigger for the glow that researchers believe to act as a light-protective response. Here are three things that happen when blue light hits a coral.
- The zooxanthellae absorb blue to power photosynthesis, and the sugars they make feed the coral. This is the process that keeps the coral alive.
- The coral’s own photoreceptors pick up blue as a signal and switch on the genes that build fluorescent proteins. The light acts as a signal, which must be persistent. That’s why coloring up corals can take time.
- The fluorescent proteins act to take some of the light off the algae. They absorb blue photons that would otherwise land on the algae. A little of its energy is shed as heat, and the rest is converted to a weaker photon at a longer wavelength. The energy leaves as light instead of piling onto a photosynthetic system that already has plenty. This is the leading explanation for why corals fluoresce, based on Salih and colleagues, Nature, 2000, linked below.
This is why blue light plays a big role here. It’s a signal that triggers the production of fluorescent proteins, which take on the excess light that would otherwise overwhelm the zooxanthellae. Without this signal corals can ramp down the production of these proteins. I should note that I am oversimplifying it here. Not all proteins act this way. Certain photoconvertible proteins such as the red fluorescent proteins actually pass light to the algae in light-scarce environment. Overall, the impact seems to be more pigment regulation. The effect depends on the particular pigment, where it sits in the coral and the environment in which the coral lives.
What the research shows
I want to give credit to and highlight the research behind this article.
D’Angelo and colleagues, Marine Ecology Progress Series, 2008. In this paper, researchers ran two experiments on five species, two of them acros, six weeks each.
The first was about brightness. Colonies went under white metal halide at four intensities, from 80 up to 700 µmol, a unit used to count photons. The second was about color. Colonies went under red, green or blue, every group at the same 200 µmol, so the only thing separating them was wavelength.
On color, the corals under blue made the most pigment. The stronger pigments were essentially undetectable under red, and under green they never reached half of what blue produced. The authors note their green filter still leaked about a quarter of its photons as blue, so even the green result was likely blue doing the work.
The brightness experiment arrives at the same conclusion. Their halides put out about half their total photons in the blue band, which they measured. So the 100 µmol group was living on roughly 50 of blue and the 400 group on roughly 200. The authors say the pigment difference between those two groups resembles the difference they got between green and blue light. The more blue light given, the more colorful the corals.
Two Seriatopora hystrix colonies after six weeks. M was grown at 400 µmol, L at 100. Under the metal halides in the study, that works out to roughly 200 versus 50 µmol of blue. Both photographed under plain white light, so the color is chromoproteins at work. From D’Angelo et al. 2008, Fig. 1B.
In addition, the genes for those pigments were barely switched on under red, busier under green, and highest under blue. The coral regulates pigment production based on wavelength. It senses the light and turns color production up or down.
Brightness matters just as much as color. Some pigments barely appear until light reaches a certain level, then keep climbing with it. Others show up in dim light but fade away when it gets brighter, as in the case of cyan. That can explain why the same coral can have color discrepancy under different lighting. This is a gradual process. Eight hours under blue light was enough to see the genes respond, but nowhere near the levels in corals that had been under blue for four weeks.
Ecological Indicators, 2023. Blue light increased thermal bleaching tolerance by remodeling the relationship between the coral and its Symbiodiniaceae. Researchers exposed Acropora tenuis to blue-only light vs a broader spectrum, then raised the water temperature to 32°C, or 89.6°F for nine days. More than 90% of the corals under the broader spectrum bleached by day five, while the blue-light group showed little bleaching. Blue light slowed the algae’s photosynthesis and metabolism, while reducing oxidative stress in both the algae and the coral. This study shows that light spectrum can affect heat tolerance, but it tested only one species. I hope to see more studies around this area as coral bleaching is a more mainstream concern.
Salih and colleagues, Nature, 2000. This one studies the benefits of the fluorescent proteins to the corals. Their research points to the proteins acting as sunscreen. In strong light the fluorescent proteins take in energy and hand it back at wavelengths photosynthesis barely uses, so it leaves the coral as light instead of piling onto the algae. They also found corals carrying more of these proteins bleached less under heat stress, which is a call back to the 2023 paper above. Interestingly, different proteins in the same family do the opposite in deep water. Below about 20 meters, red fluorescent proteins re-emit the blue that reaches them as orange and red, which carries further into the coral’s tissue and reaches algae sitting in shade. That’s spreading light around rather than getting rid of it, to enhance light availability to the algae when light is scarce.
Effects of blue light and feeding on the physiological performance of reef corals. In a 75-day blue-only experiment, Stylophora pistillata showed the greatest branch extension when stronger blue light was combined with higher feeding, significantly outperforming the other in that growth measure. Pocillopora damicornis grew under all conditions but responded less strongly, showing that light and feeding affect species differently. Both corals also became lighter under stronger blue light while remaining healthy. The exact cause was not measured, but the change is consistent with fewer symbiotic algae.
How orange filters work
Point a camera at a blue tank with no filter and you get a blue wash. This is not what your eyes see. Your eyes adjust to colors and light intensity, the same way a white shirt still looks white indoors and out. Standing in front of the tank, you are seeing coral colors, not seeing blue.
A camera sensor doesn’t do that. Modern LED blue is narrow and intense, and the blue channel on the camera can only process so much light data before it maxes out. When each pixel maxes out it stops recording differences. Photographers call this clipping. Bright blue and brighter blue come back identical, so that detail is not recorded. The camera then pulls the exposure down to cope, which leaves other bands such as the greens and reds dim. What you get is a flat blue wash.
An orange filter, also called a long pass filter, fixes that by transmitting wavelengths longer than a certain cutoff while blocking shorter ones. This effectively blocks the blue before it ever reaches the sensor, so the channel stops clipping and the rest of the information survives. What’s left is what the corals are actually giving you, the fluorescence they emit and the wavelengths they reflect.
That’s why I use orange filters. It isn’t a gimmick or a trick shot. It’s not altering the photo to make it into something it’s not. Rather, it’s the closest I can get a camera to record what my eyes see under the blue light. A filtered photograph shows the coral’s appearance under that blue-excited fluorescence state. It is not how the coral will appear under full-spectrum lighting. But it is definitely how I prefer to view the corals in.
There are exceptions. Blues and purples are the hardest to photograph, because they are often the result of the coral reflecting blue and violet, and blue and violet are exactly what the filter removes. If a coral’s color is primarily blue, a photo taken with a filter will make it look dull. In this case, you want to turn on full spectrum and ditch the filter to get those colors right.
Will it still look like that in your tank?
We are not talking about intentionally altering photos to make corals look better than what their conditions render them. But there is an explanation in some benign cases where if a coral looked incredible on someone’s rack and now duller in your tank, that doesn’t mean the photo lied.
Fluorescent proteins are made and their abundance fluctuates in corals. The coral makes them in response to the light it’s given, and it winds production back down when that signal fades. Take a coral that colored up under heavy blue, put it under another light with much less blue, and over the next few weeks it will make less of the thing you bought it for. The color needs to be paid for by energy.
Of course, light isn’t the only thing that impacts color, nutrients and flow play a role too, but this article is about light, and it’s the one we tinker with the most.
A blue hand or blue sand bed appear blue because they mostly reflect the blue light rather than fluoresce. That alone does not prove the coral’s color is inaccurate. The orange filter reduces the reflected blue light reaching the camera, allowing the coral’s genuine fluorescence to be shown more clearly. If anything, the blue cast tells you the lighting condition under which the photo was taken in. The next question is whether your tank can reproduce that condition and whether the coral will maintain the same pigmentation after it acclimates.
And honestly, blue light makes the hobby fun
Finally, the hobby is supposed to be fun. Blue light gives our corals the ability to unveil colors that are natural but invisible to you without the right light, and that’s a big part of why people get hooked on the hobby.
Hobbyists ran heavy blue long before LEDs showed up. I met a guy in Long Island once when I first started in this hobby, he grew Oregon Tort out the water running only blue and violet-heavy power compact bulbs. Mike Paletta, who has his own corner of this community now, has been saying for years that the hobby is supposed to be fun and enjoyable. If blue light makes the colors pop more than non-blue, that’s part of the fun. But if you like a full spectrum tank, run that too. There is appeal in that as well. One benefit is you’re able to see the colors of the fish better since their colors come from reflecting light. I’m not telling anyone what to hang over their tank. But the idea that displaying corals under blue is somehow wrong deserves a second look.
