JCJC SVSE 7 - JCJC - SVSE 7 - Biodiversité, évolution, écologie et agronomie

Symbiotic Interface of a cnidarian-Dinoflagellate Endosymbiosis – inSIDE

Host-symbiont signalling: what drives coral endosymbioses?

In this project, we focus on the metabolic exchanges and on the molecular dialogue that is established between cnidarians (sea anemones, corals) and photosynthetic dinoflagellates (unicellular algae). The aim is to address the physiological mechanisms that underlie this symbiosis, as well as events that ultimately lead to its breakdown (coral bleaching).

Ups and downs of the cnidarian-dinoflagellate association

The symbiotic interaction between cnidarians and dinoflagellates form both the trophic and the structural foundation of the coral reef ecosystem, and this partnership is centered on metabolic exchange. The aim of this multidisciplinary project is to better characterize these exchanges (identity of the metabolic compounds), as well as the mechanisms that control their exchange across the host-symbiont interface. These investigations are essential to understand what mechanisms drive the symbiosis breakdown (bleaching) and which partner causes the divorce.

The originality of this project relies on a multidisciplinary approach, which is developed by biologists, biochemists and chemists. We plan to use either targeted approaches (biochemistry, cell biology, analytical chemistry), or high-throughput techniques (proteomics, metabolomics) to characterize the mediators involved in the symbiotic interaction established between cnidarians and dinoflagellates. Furthermore, we will develop mass spectrometry imaging to localize these mediators (metabolites, lipids, proteins) inside cnidarian tissue samples.

We have demonstrated the specific recruitment around symbionts of a cnidarian protein involved in sterol transport in vertebrates. We have also determined the ultrastructure of the symbiotic interface in this sea anemone using transmission electron microscopy. Moreover, mass spectrometry imaging techniques have been adapted to study sea anemone tissue samples. Finally, using a metabolomic approach, we have characterized secondary metabolites and lipids, which may be involved in the symbiotic interaction.

Coral reefs harbour more than one quarter of the ocean's biodiversity. But this ecosystem is threatened by numerous environmental perturbations that disturb the symbiotic association and may ultimately lead to the symbiont expulsion (bleaching). It is therefore essential to better understand the physiological mechanisms of the symbiotic interaction, as well as the mechanisms by which cnidarians are impacted by stress, to predict whether or how corals and other symbiotic cnidarians might survive climate change and other environmental perturbations.

Ten communications have been presented at both national and international conferences, and two publications have been submitted. The first one describes a cnidarian protein which is specifically localized around the symbionts (Figure B) and that seems involved in the symbiotic interaction. The second, which is written by all participants, describes the adaptation of mass spectrometry imaging techniques to study cnidarian-dinoflagellate symbiosis (Figure C).

Coral reefs are among the most beautiful and the most biodiverse marine ecosystems, but they are suffering massive declines due to environmental Coral reefs are among the most beautiful and the most biodiverse marine ecosystems, but they are suffering massive declines due to environmental perturbations, partly related to global climate change. Reefs are primarily built by calcifying corals belonging to Anthozoans (Cnidaria), such as sea anemones. Many Anthozoans rely on photosynthetic endosymbionts, dinoflagellates of the genus Symbiodinium, which are broadly referred to as zooxanthellae. The anthozoan-dinoflagellate partnership is centered on nutritional exchange: the dinoflagellate symbionts translocate a majority of their photosynthetically fixed carbon to the host in exchange for inorganic nitrogen, phosphorus and carbon from the host, in addition to a high light environment and refuge from herbivory. Various environmental stressors, such as elevated temperatures associated with global climate change, induce several cellular events that ultimately lead to loss of symbionts from host tissue, a phenomenon called “bleaching”. Bleaching can have a large variety of negative consequences specific to corals (mass mortality) as well as many that impact the reef ecosystem as a whole. Symbiosis maintenance is therefore central to coral health. Insights into the physiological mechanisms that underlie healthy as well as stressed (or bleached) anthozoans are thus critical to predict whether corals will be able to adapt to and survive climate change. Anthozoans harbour their unicellular symbionts intracellularly in vacuoles (symbiosomes) within cells in the inner or gastrodermal tissue layer. The establishment and maintenance of the symbiotic partnership must therefore be dependent on intimate molecular communications between the partners, including recognition and tolerance of symbionts, as well as adaptations for mutual transport and exchange of nutritional resources.
Several genomic and cellular studies strongly suggest the putative role of host membrane as signalling and interacting/entry platforms for zooxanthellae. Indeed, the symbiosome membrane is originally derived from host plasma membranes during phagocytosis of zooxanthellae. However, its molecular components and functions are not well established. Previous results identified several proteins (Sym32, NPC1 and NPC2-D) that putatively maintain the interaction in the sea anemone Anemonia viridis. The aim of the inSIDE project is therefore to contribute to better understand the biology of symbiotic Anthozoans by focusing on the characterization of the symbiotic interface to appreciate its role in the symbiotic interaction. Our main objectives are to i) demonstrate the involvement of Sym32, NPC1 and NPC2-D in the interaction (localization and regulation), ii) fully characterize the host symbiotic membrane (symbiosome) proteome, and ii) identify new mediators of this interaction (lipids and secondary metabolites).

Project coordinator

Madame Cecile SABOURAULT (UMR7138 Systématique Adaptation Evolution) – Cecile.Sabourault@unice.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UPMC-SAE UMR7138 Systématique Adaptation Evolution

Help of the ANR 240,000 euros
Beginning and duration of the scientific project: December 2012 - 36 Months

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