ESCOR · Environmnental stresses in a scleractinian coral-dinoflagellate symbiosis: a genomics approach

Environmnental stresses in a scleractinian coral-dinoflagellate symbiosis: a genomics approach

Coral reefs are among the most productive and biologically diverse marine ecosystems on Earth. At the heart of the success of corals as architects of reefs is their mutualistic symbiosis with dinoflagellate algae (Symbiodinium sp.), which provides enormous amounts of energy to the animal host. During recent decades, mass coral bleaching, which involves the breakdown of the coral-dinoflagellate symbiosis, has increased in scale and frequency across the world's coral reefs, being driven primarily by environmental changes including increased levels of thermal stress, ocean acidification, flooding and elevated Nitrogen loads. This project aimed to provide new insights into corals response to environmental stress at both a molecular and functional level by deploying cutting edge technologies including quantitative real-time PCR and a new imaging technique referred as Secondary ion mass spectrometry (NanoSIMS). In this context, the main scientific objectives of this project, including an outgoing phase in the Centre for Marine Studies (Australia), were: i) to assess the variation in the level of expression of several key genes involved in the regulation of cnidaria-dinoflagellate symbioses in response to different climate change scenarios; ii) to quantify the change in uptake and translocation dynamics in cnidaria-dinoflagellate symbioses in response to anthropogenic stress such as high level of dissolved inorganic nitrogen. Five campaigns of collection and experiment in the field were realised at Heron Island Research Station (HIRS, Great Barrier Reef, Australia) in September 2008, April 2009, and March 2010 and at Lizard island research station (Great Barrier Reef, Australia) in February 2010 and June 2010. The first results suggest important molecular regulations in both symbiont and coral host compartment in response to increased temperature with significant changes in the expression level of key genes such as cytochrome P450 and Heat shock proteins for the symbiont (Rosic et al, 2010; Rosic et al, 2011) and Bcl-2 family members and cytochrome C for the coral host (Pernice et al, 2011; Dunn et al, submitted). Concerning the response of corals to Nitrogen loads, we developed the use of NanoSIMS analysis to bring new mechanistic insights into the incorporation of ammonium within the coral-dinoflagellate symbiosis. Further, by using this methodology, we quantify for the first time the relative strengths of the capability to assimilate nitrogen for each cell compartment and infer that dinoflagellate symbionts can fix 14 to 23 times more nitrogen than their coral host cells in response to a sudden pulse of ammonium-enriched seawater (Pernice et al, 2011b). Given that the economic value of coral reefs has been estimated around USD 375 billion per year and that coral reefs support over 500 million people through the services and food that they provide, losing corals from reefs systems would have substantial impacts on coastal people all over the world. In this context, there is an urgent need to predict and respond to the impacts of climate change and anthropogenic stresses on coral reefs. By yielding new data on the expression of key genes and the metabolic changes occurring in corals during stress conditions, this research program provides new insight into the complex mechanisms that drive the coral sensitivity to environmental changes. Further, the methodology developed may help identify the effects of stressors at the sub lethal level, i.e. when mitigation responses have the most potential for positive outcomes. We propose, for instance, that the differential expression of Bcl-2:Bax ratio could be used as a potential marker for the early detection of physiological stress among coral populations (Pernice et al, 2011), and could be combined with other traditional techniques to address the substantial loses being experienced by coral reefs worldwide. These molecular markers, when complemented with environmental analyses (water quality, temperature and light) and coral population assessments can provide vital information for identifying the nature and source of coral-reef degradation. The results can allow the development of more effective conservation and management programs as well as be used to evaluate the effectiveness of mitigation measures in a timely and more cost-effective manner. Websites where additional information may be found: - http://www.coralreefecosystems.org/people/Mathieu - http://www.upmc.fr/fr/recherche/europe/7e_pcrd/mathieu_pernice.html - http://mathieu-pernice.com

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