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Nathan has it right.
We would have to look at how algae absorbs light. That information is in the form of an absorption spectrum, i.e. the percentage of light that is absorbed as a function of wavelength. Green plants are green because chlorophyll absorbs the spectral components of light on the red end AND the blue end (thus reflecting green which is in the middle). All plants, from microalgae to trees, will have a different absorption spectrum depending on the environment of the chlorophyl molecules. Pigments that are found in coral will shift the wavelengths that the xooxanthallae absorb.
After we have the absorption spectrum of the coral or algae in question we next have to look at the emission spectra of the light source(s) which shows the intensity of light given off as a function of wavelength. If we lay the emission spectrum of a given bulb on top of the absoprtion spectrum for the species in question we can which wavelengths of the bulb are absorbed most by the plant or animal. This can all be done quantitatively and forms the basis of Sanjay's newly defined term, PUR, or photosynthetically useable radiation. So if, for example, an algae abosrbs strongly at the red wavelengths then we compare the output of two lamps (old vs. new or 6500 K vs. 10,000 K) and see which bulb's emission spectrum has more intensity at the red wavelengths.
Give us the spectra with real numbers and we can answer anything!
Mark
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