Blue light regulation of host pigment in reef-building corals

Reef-building corals harbor an astounding diversity of colorful GFP (green fluorescent protein)-like proteins. These pigments can easily be detected and thus may serve as intrinsic optical markers of physiological condition, provided that the determinants that control their expression are well understood. Here we have analyzed the effect of light on the regulation of major classes of GFP-like pigments in corals of the taxa Acroporidae, Merulinidae and Pocilloporidae. Pigment levels in the tissues of all studied species were observed to be tightly controlled by light. Two groups could be distinguished by their distinctly different light-dependent regulation. The low-threshold group contains mainly cyan fluorescent proteins; they are expressed in considerable amounts at very low light intensities, and their tissue content increases with light to a maximum at a photon flux of 400 umol m-2 s-1. The high-threshold group includes green and red fluorescent proteins as well as non-fluorescent chromoproteins. These pigments are essentially absent in corals grown under very low light, but their tissue content increases in proportion to photon flux densities >400 μmol m-2 s-1. The enhancement of coral pigmentation is primarily dependent on the blue component of the spectrum and regulated at the transcriptional level. The specific regulation patterns suggest complex functions of GFP-like proteins related to the photobiology of reef corals. Moreover, the distinct response of coral coloration to light climate promises that the pigment complement can also be predicted in natural habitats. Our results stress the potential of GFP-like proteins as intrinsic markers of physiological processes, as well as overall health, in corals.