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See detailHISTOLOGICAL RESPONSES OF THE SEA ANEMONE A. PALLIDA TO BLEACHING INDUCING STRESSES
Fransolet, David ULg

Doctoral thesis (2014)

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 ... [more ▼]

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. [less ▲]

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See detailImpairment of symbiont photosynthesis increases host cell proliferation in the epidermis of the sea anemone Aiptasia pallida
Fransolet, David ULg; Roberty, Stéphane ULg; Plumier, Jean-Christophe ULg

in Marine Biology (2014)

Corals exposed to environmental stresses need to engage appropriate physiological strategies to survive. Here we examined tissue modifications following algal dysfunction. Aiptasia pallida was exposed ... [more ▼]

Corals exposed to environmental stresses need to engage appropriate physiological strategies to survive. Here we examined tissue modifications following algal dysfunction. Aiptasia pallida was exposed during one week to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the herbicide called Diuron. DCMU treatment produced a drastic loss in photosynthetic efficiency and a subsequent diminution of algae density over the following days. Cell proliferation evaluated by measuring the number of cells labeled with a thymidine analogue (EdU) revealed a significant increase in EdU+ cells in the epidermis after one week of DCMU incubation and in the gastrodermis at four weeks. TUNEL histology showed that the extent of cell death was however similar in the epidermis of control and treated specimens. In addition we noticed a significant effect of DCMU treatment on the density of epidermal mucocytes after one, two and four weeks. These results show that inhibition of Symbiodinium photosynthesis in the absence of any known direct effect of DCMU on host cells can induce an increase of epidermal host cell proliferation in both the epidermis and the gastrodermis. While new host gastrodermal cells are likely to promote tissue regeneration in order to recruit new algae, the new host epidermal cells may contribute to tissue adaptation following a decrease in energy income. Some of these new epidermal cells, such as mucocytes, may contribute to an eventual increase of the host heterotrophic ability until restoration of algal autotrophic contribution. [less ▲]

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See detailIncreased Cell Proliferation and Mucocyte Density in the Sea Anemone Aiptasia pallida Recovering from Bleaching
Fransolet, David ULg; Roberty, Stéphane ULg; Herman, Anne-Catherine et al

in PLoS ONE (2013), 8(5), 65015

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See detailIncreased cell proliferation in Seriatopora hystrix following heat-induced bleaching
Fransolet, David ULg; Ugille, Aurélie; Leblud, Julien et al

Poster (2012, July)

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See detailEstablishment of endosymbiosis: The case of cnidarians and Symbiodinium
Fransolet, David ULg; Roberty, Stéphane ULg; Plumier, Jean-Christophe ULg

in Journal of Experimental Marine Biology and Ecology (2012), 420–421

The symbiosis between cnidarians and Symbiodinium algae (dinoflagellates) is the keystone responsible for the formation of the huge and important structures that are coral reefs. Today many environmental ... [more ▼]

The symbiosis between cnidarians and Symbiodinium algae (dinoflagellates) is the keystone responsible for the formation of the huge and important structures that are coral reefs. Today many environmental and/or anthropogenic threats compromise this tight relationship and lead to more frequent events of drastic loss of Symbiodinium pigments and eventually of algae themselves from cnidarians, better known as cnidarian bleaching. While the mechanisms underlying the collapse of the algae–coral symbiosis are progressively getting unraveled, the understanding of the mechanisms involved in the de novo infection of bleached cnidarians by Symbiodinium remains elusive. In this review, we describe the various steps needed to establish a stable symbiotic relationship between Symbiodinium and cnidarians. We review the mechanisms implicated in host–symbiont recognition and in symbiosome formation and persistence, with a special emphasis on the role played by lectins and Rab proteins. A better understanding of these molecular mechanisms may contribute to the development of strategies to promote post-bleaching recovery of corals. [less ▲]

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See detailIncreased number of mucocytes in Aiptasia pallida following heat-induced bleaching
Fransolet, David ULg; Herman, Anne-Catherine; Roberty, Stéphane ULg et al

Poster (2012)

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See detailIncreased number of mucocytes in Aiptasia pallida following bleaching
Fransolet, David ULg; Herman, Anne-Catherine; Roberty, Stéphane ULg et al

in Yellowlees, D; Hughes, T P (Eds.) proceedings of the 12th International Coral Reef Symposium (2012)

While many studies have investigated histological changes occurring in cnidarians during bleaching, only a few have focused on continuing changes in tissues during the recovery period. Here, we examine ... [more ▼]

While many studies have investigated histological changes occurring in cnidarians during bleaching, only a few have focused on continuing changes in tissues during the recovery period. Here, we examine the response of the sea anemone Aiptasia pallida to a transient elevation of water temperature combined with high illumination. Following 30h of exposure to stress conditions (33°C and 1900μE/m2.s), anemones show a significant reduction in their Symbiodinium concentration followed by a progressive recovery over 8 weeks. Histological analyses show an increase in cell proliferation in both ectoderm and gastroderm tissues one day following the stress. This increased proliferation seems to be sustained after 3 weeks before returning to normal after 8 weeks. Moreover, our results show a progressive increase in the number of ectodermal mucocytes over 3 weeks before returning to a normal level after 8 weeks. While the new cells formed in the gastroderm would most likely host new Symbiodinium, the fate of new cells in the ectoderm is still not completely understood. These new cells may contribute to the increased number of mucocytes which could eventually help shift the feeding mode temporarily to a heterotrophic state until restoration of the symbiosis. [less ▲]

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See detailPhotoacclimation responses of a symbiotic sea anemone reveal an important host cellular plasticity
Roberty, Stéphane ULg; Fransolet, David ULg; Ladrière, Ophélie ULg et al

Poster (2010)

The high productivity of coral reef ecosystems is largely attributed to the mutualistic symbiosis between reef-building corals and their intracellular dinoflagellate in the genus Symbiodinium commonly ... [more ▼]

The high productivity of coral reef ecosystems is largely attributed to the mutualistic symbiosis between reef-building corals and their intracellular dinoflagellate in the genus Symbiodinium commonly referred to as zooxanthellae. These photosynthetic algae translocate a majority of their photosynthetically fixed carbon to the host and contribute to their metabolic needs and the calcification process. <i>Symbiodinium</i> must maintain a balance between the energy derived from the light reactions in the chloroplast and the amount of energy used during dark reactions and other metabolic processes. Nevertheless, in the natural environment the holobiont have to cope with daily and seasonal changes in light intensity, upsetting that balance and creating a stress that induces a physiological response (photoacclimation) to optimize growth rates. After a ten day exposition to high and very low light intensity, morphological and photophysiological analysis conducted on the symbiotic sea anemone, Anemonia manjano, reveal significant modifications of the host tissues ultrastructure and the Symbiodinium metabolic processes (photosynthesis, respiration). Those results highlight particularly important gastrodermal and ectodermal plasticity in which symbiotic cnidarians acclimate to the Symbiodinium physiological status (mainly photosynthesis) by varying the density of particular cellular types (e.g.: cnidocytes, gastrodermal cells) contained in their tissues. [less ▲]

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See detailIncreased mitosis in the bleached gastrodermis of the sea anemone A. pallida
Fransolet, David ULg; Roberty, Stéphane ULg; Plumier, Jean-Christophe ULg

Poster (2010)

Today, coral bleaching represents a major concern for marine biologists, especially considering the upsurge of this phenomenon possibly linked to climate change. Bleached corals, deprived of most of their ... [more ▼]

Today, coral bleaching represents a major concern for marine biologists, especially considering the upsurge of this phenomenon possibly linked to climate change. Bleached corals, deprived of most of their energy incomes, may show a partial or total mortality, which ultimately lead to shifts in reef communities. Studies focusing on cellular bleaching mechanisms have shown different ways by which algae may be expelled from gastrodermal host cells. Among those mechanisms, major emphasis has been put on host cell death, most probably due to both apoptosis and necrosis. Recovering gastrodermis is then expected to undergo regeneration process in order to be reinfected by new algae. We describe here this regeneration process in the bleached sea anemone model A. pallida. [less ▲]

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