Green fluorescent protein-like pigments optimize the internal light environment in symbiotic reef building corals

Pigments homologous to the Green Fluorescent Protein (GFP) have been
proposed to fine-tune the internal light microclimate of corals,
facilitating photoacclimation of photosynthetic coral symbionts
(Symbiodiniaceae) to life in different reef habitats and environmental
conditions. However, direct measurements of the in vivo light conditions
inside the coral tissue supporting this conclusion are lacking. Here, we
quantified the intra-tissue spectral light environment of corals
expressing GFP-like proteins from widely different light regimes. We focus
on (1) photoconvertible red fluorescent proteins (pcRFPs), thought to
enhance photosynthesis in mesophotic habitats via wavelength conversion,
and (2) chromoproteins (CPs), which provide photoprotection to the
symbionts in shallow water via light absorption. Optical microsensor
measurements indicated that both pigment groups strongly alter the coral
tissue light environment. Estimates derived from light spectra measured in
pcRFP-containing corals showed that fluorescence emission can contribute
to >50% of orange-red light available to the photosynthetic
symbionts at mesophotic depths. We further show that upregulation of pink
CPs in shallow-water corals during bleaching leads to a reduction of
orange light by 10-20% compared to low-CP tissue. Thus, screening by CPs
has an important role in mitigating the light-enhancing effect of coral
tissue scattering during bleaching. Our results provide the first
experimental quantification of the importance of GFP-like proteins in
fine-tuning the light microclimate of corals during photoacclimation.