Evidence of Photoenhancement by Green Fluorescent Proteins in Low Light Conditions: A Potential Factor of Habitat Distribution for the Sea Anemone Anthopleura sola

Ambient light intensity is a critical factor affecting species distribution, including the sea anemone, Anthopleura sola. In coastal waters, ambient light varies greatly with geographical span and within one location, for instance with increasing depth of the intertidal zone or within crevices of rocks in tidepools. A. sola inhabits diverse microhabitats within the subtidal and intertidal zones that are subject to differing light intensities, making it important to understand the effects of solar radiation on the sea anemone distribution in relation to its endosymbiotic zooxanthellae. A particularity of A. sola is it’s brightly fluorescent, which could be related to the presence of the species in the supratidal, well exposed to intense sunlight, in which case the fluorescence could be acting as a photoprotective mechanism. In this case, however, the species is also reported to cover itself with pieces of debris, thus mimicking exposure to shade rather than sunlight. Here, we postulated that the green fluorescent proteins (GFPs) of A. sola act as photoenhancement to redirect light to fuel photosynthesis for their endosymbiotic zooxanthellae. This was tested by investigating the native auto-fluorescence of A. sola in response to changes in low intensity of ambient light (to mimic shade), and to simultaneously monitor the photochemical efficiency of photosystem II of the zooxanthellae during a light controlled and flow through 106-day laboratory aquarium experiment. The experiment included an 82-day exposure period with three levels of ambient light intensities: 100% (control), 53%, and 22%, and a 24-day recovery period without light shading (100% of ambient light intensity). This research combines measurements of light intensity in the field and an experimental aquarium setting. Image analysis and an underwater fluorometer demonstrated changes in fluorescence intensity of sea anemones and the photosynthetic efficiency of zooxanthellae in response to light changes. Only the lowest light condition (22% of ambient light intensity) caused an increase in green fluorescence intensity, which suggests photoenhancement of GFPs. The endosymbiotic zooxanthellae chlorophyll maintained high efficiency of photosystem II in all light conditions and the zooxanthellae density remained unchanged; therefore, they seemed to not be affected by different light shading. The photoenhancement of A. sola’s GFPs in this experiment could explain A. sola’s ability to live and adapt to low light conditions in the intertidal and particularly the subtidal zone, explaining one factor affecting their geographical and local distribution