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When buying a light fixture for a marine fish tank, it is often sold with some technical data reguarding the spectral output of the light, with a peak in the blue region of visible light, somewhere around 430 to 460nm in air.

As light enters the water with a refractive index of approximately 1.33, the peak wavelength will reduce, but the frequency of light will remain constant. So, for example, a monochromatic light with wavelength 460nm in air would have a wavelength in water of about 345nm, but the frequency is the same. Physics so far, so here comes the biology question....chlorophyll a and b have peak responses between 400-500nm light, so is it the wavelength of light in water that is important to photosynthesis, or is it actually the frequency(therefore the photon energy) that is what counts?

If we were to discover a purely theoretical new plant that grew equally well in air or water, and discovered that photosynthesis only occurred at 650nm wavelength light in air, and not at all to any other wavelength. Then we place this plant in water indoors in the dark and shine a monochromatic 650nm light source at if from air, into the water, would photosynthesis still take place because the photon energy is the same in water or air (e = hf). Or would we need to shine a monochromatic light source of approx 865nm in air, which would reduce to 650nm in water (865/1.33)? I suspect the answer is that photsynthesis is dependent on the photon energy not wavelength, but I am far from sure about this!

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    $\begingroup$ Its is my hunch that it is the photon energy/frequency that counts not wavelength in much the same way our eyesight works with colours, but i would like to see it confirmed either way. I often wonder if artificial lighting for plant growth should be aimed more towards replicating natural sunlight (there is a Japanese company doing just this), especially regarding corals from shallow waters or propagated frags from tanks where some red light still makes it through the first few metres of water. Still I am interested to know what drives photosynthesis specifically underwater. $\endgroup$
    – Justanengineer
    Commented Apr 6, 2019 at 17:21

2 Answers 2

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Short answer: Coral doesn't need the light, it's the zooxanthellae that is symbiotic with some corals that relies on light to grow; it adapts to the frequency spectrum available at the depth the particular coral grows, full spectrum at shallow depths and at over ~30m the zooxanthellae relies on blue light because other frequencies are absorbed by water.

Some species of coral, such as Lophelia pertusa, grow in water so deep that there is too little light for them to rely on zooxanthellae, instead their polyps gather food directly with their tentacles, feeding heterotrophically.

There are even instances of coral feeding on jellyfish, see: "Opportunistic feeding by the fungiid coral Fungia scruposa on the moon jellyfish Aurelia aurita" (May 21 2009), by A. Alamaru, O. Bronstein, G. Dishon and Y. Loya

Longer answer:

The physics portion of your question is best answered on our Physics.SE site's question: "Why doesn't the frequency of light change during refraction?"

Light's wavelength and frequency are related to each other based on a mathematical formula, it's the intensity that is affected by passing through a medium; different materials will attenuate some frequencies more than others. Water doesn't transpose light's frequency or wavelength as much as it absorbs (filters) the frequencies of light affecting which color can penetrate deeper.

"What is a light wave?

Light is a disturbance of electric and magnetic fields that travels in the form of a wave. Imagine throwing a pebble into a still pond and watching the circular ripples moving outward. Like those ripples, each light wave has a series of high points known as crests, where the electric field is highest, and a series of low points known as troughs, where the electric field is lowest. The wavelength is the distance between two wave crests, which is the same as the distance between two troughs. The number of wave crests that pass through a given point in one second is called the frequency, measured in units of cycles per second called Hertz. The speed of the light wave equals the frequency times the wavelength.".

See this other Physics.SE question: "Does light color change when refracting?":

"The color will not change. What you're not taking into account is the speed of light in the medium. It's not the same $c$ en vacuo. The frequency stays the same. What changes is that speed of light in the refracting medium and as a result wavelength.".

Here is what sunlight (natural light) looks like underwater:

Blue light penetrates water better than other colors

Despite that fact in shallow water enough full spectrum light penetrates sufficiently, and the zooxanthellae adapts to the spectrum available, no longer requiring blue light for growth.

See: "The spectral quality of light is a key driver of photosynthesis and photoadaptation in Stylophora pistillata colonies from different depths in the Red Sea" by T. Mass, D. I. Kline, M. Roopin, C. J. Veal, S. Cohen, D. Iluz, O. Levy in the Journal of Experimental Biology 2010 213: 4084-4091; doi: 10.1242/jeb.039891

"Depth zonation on coral reefs is largely driven by the amount of downwelling, photosynthetically active radiation (PAR) that is absorbed by the symbiotic algae (zooxanthellae) of corals. The minimum light requirements of zooxanthellae are related to both the total intensity of downwelling PAR and the spectral quality of the light. Here we used Stylophora pistillata colonies collected from shallow (3 m) and deep (40 m) water; colonies were placed in a respirometer under both ambient PAR irradiance and a filter that only transmits blue light. We found that the colonies exhibited a clear difference in their photosynthetic rates when illuminated under PAR and filtered blue light, with higher photosynthetic performance when deep colonies were exposed to blue light compared with full-spectrum PAR for the same light intensity and duration. By contrast, colonies from shallow water showed the opposite trend, with higher photosynthetic performances under full-spectrum PAR than under filtered blue light. These findings are supported by the absorption spectra of corals, with deeper colonies absorbing higher energy wavelengths than the shallow colonies, with different spectral signatures.".

Deep vs. Shallow Adapted Coral

"Fig. 4. Percent surface irradiance received of different wavelengths at 3 m (grey) and 40 m (black) depth in the waters adjacent to the Interuniversity Institute for Marine Sciences, Eilat, Israel, on 3 April 2008.".

So if you deep dive for your own coral you should be aware that it will thrive under blue light. If you obtain shallow water coral, or buy from the local aquarium supply store, white light is suitable.

Also from: "The spectral quality of light is a key driver of photosynthesis and photoadaptation in Stylophora pistillata colonies from different depths in the Red Sea", (full info above):

"Studies in the Caribbean have found that other corals that are found along a broad depth gradient switch from hosting clade A or B zooxanthellae in shallow water to clade C in deeper water, possibly owing to the ability of the different clades to adapt to different light levels (Rowan and Knowlton, 1995; Baker et al., 1997).".

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    $\begingroup$ Hi Rob. Thanks for the detailed answer. I'm not sure it answers my actual question though. I have a physics degree, I understand electromagnetic radiation, so my question was not about physics. I understand that light will be scattered, reflected, absorbed in different ways in water. I also understand that it is the zooxanthellae that photosynthesize. I am not a biologist though, so, very specifically my question is this - is photosynthesis dependant on wavelength or frequency, baring in mind the wavelength changes in water, and the frequency doesn't (and therefore photon energy)? $\endgroup$
    – Justanengineer
    Commented Apr 6, 2019 at 7:17
  • $\begingroup$ @Justanengineer Is this answer more helpful, that is the frequency or wavelength in a vacuum, depending if you want to measure in THz or nm. Moving from space to air, glass or water affects the wavelength because the medium is changed, it affects the diffraction and filters which frequencies can pass more freely than others but it doesn't alter the frequency, the color, of the light by transposition. Changing the speed of light by passing through water only affects the wavelength. $\endgroup$
    – Rob
    Commented Apr 6, 2019 at 9:27
  • $\begingroup$ Hi Rob, all the information you have provided is great, although I feel i already have a good grasp on the physics of this question. Its the biology i need to learn about! I understand refractive index, downwelling irradiance from the sun, the filtering of different wavelengths of light dependent on water depth. Its the biology of photosynthesis i do not understand. It seems to me that it would be more helpful to measure solar irradiance in THz or electron volts, as this does not change in different media. I need to rephrase my question I think somehow.... $\endgroup$
    – Justanengineer
    Commented Apr 6, 2019 at 9:51
  • $\begingroup$ Maybe a better way to phrase this question is: "Is it accurate to say that photosynthesis occurs under light irrandiance of frequency 430-770THz, regardless of the refractive index of the medium it takes place in, and therefore regardless of the measured wavelength in that medium"? $\endgroup$
    – Justanengineer
    Commented Apr 6, 2019 at 11:07
  • $\begingroup$ Am I on the right track here...1.82ev is needed to activate the chlorophyll complex p680, which drives photosystem 2 at the start of the light dependent reactions and is needed to oxidize h20. So when we think of light for photosynthesis, we should think in terms of electron volts required for redox reactions? Can anyone point out my beginner mistakes or point me towards some further reading? $\endgroup$
    – Justanengineer
    Commented Apr 6, 2019 at 23:27
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Yes, coral photosynthesis and growth (actually, Symbiodinium symbiont photosynthesis) is dependent upon the spectra of light provided. Each photosynthetic organism has a characteristic 'action spectrum' which is the photosynthetic rate (Y axis) plotted vs. the wavelength of light (X axis). Or, we could plot an action spectrum for growth, which is a step removed from photosynthesis.

Either way the action spectrum is a product of: -absorbance of light at a given wavelength -yield of conversion of absorbed light to achieved photosynthesis (or growth), at a given wavelength. -second order effects related to the complex propagation and scattering of light through tissue or coral matrix

For any oxygenic photosynthesis the action spectrum is going to include absorbance by chlorophyll a (peaks at ~440 nm and at ~680 nm), but will be modified by other secondary pigments.

http://jeb.biologists.org/content/206/22/4041 Fig. 6, rough action spectra.

Some methodologies. https://www.advancedaquarist.com/2014/5/aafeature

The situation is complicated because: -some wavelengths (short blue, ~420 nm) can drive photosynthesis but also drive photoinactivation of photosynthesis more so than longer wavelengths. This can lead to cumulative light stress, depending upon the absolute level of light, the duration of illumination and the wavelength. -corals 'use' light for many things other than photosynthesis and these regulatory and signalling functions can be disrupted if the spectrum or duration of light is not appropriate. -different corals will have different requirements.

Long answer, tough question, still a research topic.

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