Host-symbiont relationship between Symbiodinium sp. , E. pallida and B. stephanieae

Abstract

Marine ecosystems are constantly threatened by human activities. Beside water pollution, the constant increase of temperature has, among others, a sad impact on corals life. 20% of corals have already definitively disappeared and 50% are predicted to vanish for the next 30-50 years. Corals are Cnidarian that live thanks to their symbiosis, as many organisms, but with symbiotic algae in this case. Corals bleaching is a worldwide-spread phenomenon that results in a breakdown of the symbiosis, leading to the death of the Cnidarian. This poorly understood breakdown is caused by the expulsion of the algae, which becomes toxic for the host, or the algae leaves the host for a better place. This project will not answer this question, “who leaves who?”. Instead, this is a contribution to understand the behavior of the symbiotic alga Symbiodinium inside different hosts: (i) Exaiptasia pallida, a symbiotic anemone that has the particularity to be bleached without fast dying has been used. (ii) Berghia stephanieae, a sea slug that eats E. pallida and captures Symbiodinium inside external vesicles, has also been studied to determine if this is also a symbiosis or an opportunistic mechanism. Various parameters as photosynthetic activity, photoprotection and pigment composition have been analyzed. Results have shown that there are no major differences between the three living stages in term of photosynthesis and Symbiodinium is not affected by the digestion of E. pallida by B. stephanieae, they are even expelled in perfect condition. In contrast, photoprotection recorded by NPQ experiment and de-epoxydation rate, is positively impacted by the symbiosis with E. pallida, meaning that the host have a protective role by filtering the light and/or providing enough nutrient to Symbiodinium to prevent photosynthesis saturation. Light and heat stresses also have a different impact on Symbiodinium when it is in free living or in symbiosis. Algae is less affected by the light stress when they are in symbiosis but heat stress cause a lower photosynthesis efficiency if they are in symbiosis. Our results indicate that the host has his part of responsibility, even if algae are protected from high irradiance, other stresses inside E. pallida could drive Symbiodinium to leave the host for a better condition