Abstract :
[en] Tropical Coral reefs are among the richest and most important ecosystem on Earth. This success would not be possible without the symbiosis established between corals and unicellular algae of the genus Symbiodinium that provide them with photosynthesis-derived carbon. Unfortunately, with the climatic upheaval that we witness today, the long-term survival of coral reefs could be in jeopardy. Massive loss of symbiotic algae, a phenomenon known as coral bleaching, becomes indeed more and more frequent throughout the globe and already urged scientists to study its mechanisms for more than a decade. Their research highlighted the central role of reactive oxygen species in the collapse of symbiosis. They also established that the expulsion of Symbiodinium from its host is mainly operated through the death of the host cell. The ensuing events, although determining the eventual survival of the energetically compromised coral, are however much less detailed. In this work, we decided to investigate these “post-bleaching” events and focused our efforts on the evaluation of cell proliferation and mucocyte number, for the role they may respectively play in regenerative processes and heterotrophic feeding. For this purpose, we worked with the sea anemone model A. pallida in which we analyzed the incorporation of a thymidine analogue (EdU). After preliminary experiments assessing the general repartition and the circadian variations of cellular proliferation in healthy specimens, we conducted a series of bleaching experiments using a variety of stresses. Every treatment, namely cold and darkness, heat and light or exposition to a photosynthesis inhibitor, drastically reduced the Symbiodinium density. This reduction was always accompanied by important histological modifications. In every case, we highlighted an increase in cellular proliferation in both the ectodermis and the gastrodermis as well as an increase in ectodermal mucocyte density. These values returned then to normal as algae that survived the stress progressively repopulated anemones. Further experiments showed that, following bleaching, a small fraction of the newly produced ectodermal cells migrate to the gastrodermis. Along with new gastrodermal cells, they most probably operate a regeneration of the wounded tissue, differentiating into host cells in order to harbor new algae. Another experiment also indicated that a small but significant part of ectodermal newly produced cells might differentiate into mucocytes, therefore explaining their increased density in bleached individuals. We hypothesize that the higher amount of mucus produced, in addition to providing protection against various aggravating stresses, would be a way to efficiently increase the feeding capacity of the bleached cnidarians. This heterotrophic shift would therefore allow a sufficient energy income until full restoration of the symbiosis. This work emphasizes the need to focus more attention on the post-bleaching period, a critical time in which some modifications might be decisive for coral and coral reef survival.