The Role of Fluorescent Proteins on the Regulation of Coral-Symbiosis in Reef-Building Corals.
Supervisor: Loreto Mardones (SOTON, UK)
The health of reef-building corals is strongly influenced by light variability in their environment due to their reliance on algal symbionts (genus Symbiodinium) for many basic metabolic functions. When the irradiance levels exceed optimal conditions for the symbionts the host may expel their symbionts, resulting in bleaching. As an adaptation to light stress, the coral host produces and accumulates fluorescent proteins (FPs). FPs, naturally produced by many cnidarians, absorb high-energy light and re-emit it at longer wavelengths, potentially protecting algal symbionts from photodamage. This study investigated the interaction between host regulation of symbiont populations and the accumulation of FPs in five tropical Indo-Pacific coral species: Acropora millepora, Montipora digitata, Seriatopora hystrix, Porites lichen, and Montipora capricornis. The experimental study involved exposing coral fragments to two light intensity treatments: a control group receiving ~200 μmol photons m–2s–1 and a high irradiance group receiving ~390 μmol photons m–2s–1 for six weeks. The goal was to assess the effects of increased FP production on symbiont proliferation and the overall physiological response of the corals. Fluorescence intensity was measured using spectroscopic analysis,  while symbiont density and division rates were assessed through cell cycle quantification. After six weeks of exposure, the coral fragments of A. millepora, M. digitata, and P. lichen under high irradiance had higher fluorescence intensity than those in the control group with no significant change in their symbiont populations, suggesting a protective mechanism against photodamage. S. hystrix displayed a mild bleaching response attributed to chromoprotein expression and M. capricornis showed no significant changes, highlighting species-specific responses to light stress. The results emphasize that FPs serve as an effective photoprotective strategy in some coral species, potentially aiding in the prevention of bleaching and maintaining symbiont health under high irradiance.