Here's an interesting experiment using blue, green, yellow, and red LEDs shining on different parts of the same coral. The effect was that the red light bleached that part of the coral whereas the blue light made it turn brighter pink.
http://www.advancedaquarist.com/issu...03/feature.htm
Excerpts:
On Day 28, coral fragments were again examined under the light of the dive light and filters. The area illuminated by the blue LED had strong chlorophyll red fluorescence. Normal, but not elevated, chlorophyll fluorescence was noted on the fragment illuminated by the green LED. The bleached area illuminated by the red LED remained, as it did on Day 22, apparently free of zooxanthellae and no chlorophyll fluorescence was noted.
On Day 31, a spot of pink coloration (~5 mm in diameter) was noted at the area illuminated by the blue LED (see Figure 4). Since that day, the spot has intensified in coloration but not size.
Procedure - Round Two
It was decided that the second round of the experiment would utilize only the blue and red LEDs, plus an LED producing ultraviolet radiation.
On Day 71, it was found that the coral areas under the blue LED had gained pink coloration, and the area illuminated by the red LED had lost more zooxanthellae – it had bleached (See Figures 6 and 7). No visible changes were observed within the area irradiated by the UV LED.
Discussion
The results of this experiment suggest that narrow bandwidths of essentially ‘pure’ red and blue wavelengths have profoundly different effects on zooxanthellae health and host tissue pigmentation.
It appears that red light induced bleaching in the two experiments. It is also worthy to note that bleaching was noticed on Day 22 and Day 23 of the first and second set of experiments, respectively, even when red LED lamp intensity differed by ~20%.
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The reasons for corals’ production of a pink pigment under blue light are not as easily explained - a theory could be advanced that some corals (likely only those genetically predisposed – see Takabayashi and Hoegh-Guldberg, 1995) react to blue light by the manufacture of reflective/fluorescent pigments.