Microscopy insights into reef coral fluorescent proteins: Diversity, light energy transfer and biological function

Fluorescent proteins (FPs) and chromoproteins (CPs) of reef-building corals, which generate much of their vivid colouration, occupy a distinctive position at the interface of natural photonics, symbiosis biology and fluorescence microscopy. They include cyan, green, yellow-orange and red, photoconvertible, photoswitchable FPs and weakly emissive CP classes and are often expressed at unusually high local concentrations within patterned tissues. Coral tissues, therefore, provide a naturally evolved system in which fluorescence, scattering, re-absorption and proximity-dependent energy transfer can be examined in vivo. This review focuses on the light-driven functions of coral GFP-type proteins in symbiosis, treating them as host-controlled optical elements that shape the photon environment experienced by Symbiodiniaceae. Evidence is synthesised for two complementary outcomes: photoprotection under high irradiance and redistribution, wavelength transformation or spectral enhancement of light in low-light tissues and habitats. These functions are particularly relevant during thermal stress and bleaching, when loss of symbiont pigmentation alters the internal light field and can change host FP/CP expression; in several coral taxa, high host fluorescence has also been associated with reduced bleaching susceptibility. In corals, strong spectral overlap, oligomerisation, photoconversion, chromoprotein- dark states, tissue and skeletal scattering make interpretation based on steady-state fluorescence intensity alone unreliable. Microscopy was critical for distinguishing the spatial and photophysical mechanisms underlying these optical effects. Confocal microspectroscopy localised spectrally distinct GFP-type proteins within intact tissues and revealed the spatial organisation required for light-transfer phenomena. Fluorescence lifetime imaging microscopy (FLIM) provided a means of distinguishing radiative re-emission/reabsorption from proximity-dependent non-radiative transfer through changes in donor fluorescence lifetime. Building on this foundation, the review advances an expanded interpretation of coral FP function in which dense pigment ensembles may redistribute or dissipate excitation through a hierarchy of processes, from tissue-scale radiative transfer to nanometre-scale FRET and, potentially, ultrafast proton-coupled excited-state dynamics and excitation migration. This Festschrift review honors Associate Professor Guy Cox, whose contributions to quantitative fluorescence microscopy, fluorescence lifetime imaging and collaborative coral FP research were foundational to this field.