Mushroom Coral as a Sunscreen

In an episode of Man vs. Wild that I was watching, a Mushroom Coral's mucus was used as a sunscreen, so me as a chemist interested in cosmetics, I wanted to know what photoprotecting chemicals were in this mucus.  The coral Fungia fungites and a couple other species gained this ability to protect themselves because they would grow in shallow water of the Adaman Sea where a lot of UV radiation penetrates through the clear water.  First looking at this ability one would expect the coral to synthesize these molecules, but it turns out that the culprits are tiny symbiotic dinoflagellates. In a paper from 1999 by Brown¹ studying the makeup of the mucus required for photoprotection, they look at the concentration of xanthophylls in the mucus, the primary photoprotector. Their study looked at how the concentrations of xanthophylls varried throughout the day, and showed that it is best to harvest the mucus at noon when the xanthophylls are at their peak concentration.

So finally to the chemistry of all of this. The molecule shown above is called diatoxanthin, which is a type of xanthophylls, and this is the molecule mostly responsible for filtering out the harmful rays.  So how does this happen? Well molecules and atoms absorb light one way or another through absorbing the photons, resulting in electron excitation, and in this case, the molecule absorbs in the UV portion of the electromagnetic spectrum. So what happens after the diatoxanthin molecules become excited? Well biology has found a solution for that, which has been named the Xanthophyll Cycle. It's job is to convert all the used up diatoxanthin back into diatoxanthin.

References:
1. Fitt, W. K., R. P. Dunne, S. W. Gibb, D. G. Cummings, I. Ambarsari, B. E. Brown, and M. E. Warner. "Diurnal Changes in Photochemical Efficiency and Xanthophyll Concentrations in Shallow Water Reef Corals : Evidence for Photoinhibition and Photoprotection." Coral Reefs (1999): 99-105.