Physiological and genetic properties of two fluorescent colour morphs of the coral Montipora digitata

Abstract

A comparison of strongly, weakly or non-fluorescent colour morphs has the potential to provide an ideal model for investigating the possible ecological function of GFP-homologs in reef-building corals. It was the aim of this study to identify the physiological and genetic characteristics of two fluorescent colour morphs of Montipora digitata, both of which are abundant in the shallow waters of Heron Island Reef, GBR. Morphological and physiological differences among corals in response to photo-acclimation from the algal perspective have been the subject of numerous studies. However, corals themselves may also respond by varying the intensity and type of host pigmentation. Both colour morphs can be assigned to our current taxonomic understanding of M. digitata. The symbionts of both morphs belonged to the same genotype of Symbiodinium (clade C15) and as such is unlikely to be accountable for the ecological differences observed. Spectral characterisation of the purified fraction of GFP-like proteins revealed the presence of three different GFP-like proteins that were distinguishable by their excitation and emission spectra. Moreover, fluorescence intensities and the content of GFP-like proteins were found to be significantly higher in the strongly fluorescent colour morph. High concentrations of pigments in shallow water corals have been postulated to simulate a low-light environment for symbiotic dinoflagellates, which live beneath the pigment layer. In the strongly fluorescent colour morph, symbiotic dinoflagellates occurred in greater abundance, providing support for this hypothesis, which is further sustained by the occurrence of higher concentrations of chlorophyll a. While the differences in the parameters observed cannot offer sufficient explanation for underlying physiological mechanisms, the approach of this study can be further extended to include all colours associated with the expression of GFP-like proteins. The observed physiological flexibility of the coral host observed in this study further emphasizes the fundamental problem of focusing solely on the dinoflagellate symbiont to explain the variability in reef-building corals with respect to their tolerance to heat and light.