Be careful with graphs like this. Looking at this graph leads one to believe spectrum based on these curves shown is somehow best. I'm not saying success can't be achieved making decisions on this info, but there's a lot more to the story. Look at the line showing absorption for carotenes - this includes beta-carotene that can actually act as a photo-protectant (beta-carotene is found in carrots (! -carrot-tene) and makes them appear orange because they absorb violet/blue light.) Hence, they compete with chlorophylls for blue light. The same can be said for xanthophylls - oxygenated carotenes - they absorb blue light as well. In short, merely looking at the graph leads one to think blue light is most strongly used in photosynthesis. Based on PAM fluorometry experiments' results, red light is most efficient while graphs like this suggest otherwise. It is no accident that fluorescent pigments that absorb UV-A, blue and violet light and hence subtract from light available to photosynthesis if blue light is of high intensity (relative to what is seen in nature.) Coral fluorescence is another tool in the arsenal of zooxanthellae photoprotection.