Abstract :
[en] The symbiotic partnership between cnidarians and dinoflagellates from the family Symbiodiniaceae constitutes the basis of remarkably diverse ecosystems. This tight association between the cnidarian host and the intracellular photosymbionts displays a complex energetic metabolism, involving respiration from both partners and photosynthesis from the dinoflagellate symbionts. Despite the major importance of these two critical processes, their interplay and regulations remain poorly studied. Abiotic factors can unsettle the symbiotic balance, leading to the collapse of the partnership
and threatening the survival of entire ecosystems. Among them, the rise in sea water temperature is getting more and more concerning as global warming takes place.
To address this topic, our first approach consisted in subjecting Cassiopea, an established model organism for photosynthetic jellyfish, to a mild hyperthermic stress for 28 days. Despite an increase of 6°C of the water temperature, the stressed jellyfish kept on growing over the experiment, similarly to their control counterparts. In both groups, the symbiont density remained remarkably constant, as opposed to the bleaching that has already been observed in other photosynthetic cnidarians. Along this experiment, several photosynthetic parameters were measured, among which the relative electron transport rates through both photosystems. No differences could be observed between the two jellyfish populations, and the relationship between the activity of PSII and PSI, measured
concurrently, remained linear, indicating the absence of adjustment related to a circular electron flow in the dinoflagellate. The relative electron transport rate through PSII was also measured simultaneously with the production of dioxygen. Our results also show a linear relationship between these photosynthetic parameters, suggesting the absence of alternative electron flow involving oxygen, such as the Mehler reaction, in stressed jellyfish. The measurement of the level of pigments in Symbiodinium cells hints at an adaptation of the photosynthetic apparatus to heat stress, with the slight increase of the amount of total pigments per cell and the transient rise in the deepoxidation of xanthophylls. A re-organization of the light-harvesting complexes of the dinoflagellate is also suggested by the variation of the relative importance of peaks corresponding to the main antennae on fluorescence spectra measured at 77K. Despite the minor impact of heat stress on the photosynthesis of symbiotic Cassiopea, an increased respiration along with an increase in bell pulsation rate were observed in the heat-stressed population. To dig more into the cellular scale, we are repeating this long-lasting heat stress experiment, collecting samples to carry out bottom-up proteomics analysis. The production of reactive oxygen species is also investigated, along with the fatty acid composition of both symbiotic partners.
All in all, our work aims at taking advantage of the biophysical and spectroscopic techniques in the context of the mutualistic partnership between cnidarians and dinoflagellates, as well as its disruption. Our results show the tolerance of Cassiopea andromeda jellyfish and their dinoflagellate symbionts to a long-lasting hyperthermic stress.
