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Effect of ocean Acidification on Marine Invertebrates CALcification in sensitive ecosystems

Final Report Summary - AMICAL (Effect of ocean acidification on marine invertebrates calcification in sensitive ecosystems)

Objectives:

Ocean pH has already declined by 0.1 units since pre-industrial times and is predicted to further decline by another 0.3 units by the end of the century as a consequence of increasing anthropogenic carbon dioxide (CO2). Reef-building corals and calcifying cold-water organisms such as pteropods (molluscs) were identified as at higher risk, while these organisms carry critical ecological role: stony corals deposit the structural framework that supports and promotes the maintenance of biological diversity and complexity of coral reefs, and pteropods form the basis of the food chains that temperate marine fisheries rely on.

This project focuses on the effect of rising partial pressure of CO2 (pCO2) on two sensitive and complementary model organisms: tropical coral juveniles (Acropora millepora) and Mediterranean pteropods (Limacina inflata and Cavolinia inflexa). Both corals and pteropods construct aragonitic skeleton / shell, a crystalline calcium carbonate polymorph which is less stable than calcite and dissolves more easily under higher pCO2. Because of their ecological and economical roles, it is pivotal to understand and quantify their response to rising pCO2 in more detail. The first part of the project (outgoing phase) assesses the effect of rising pCO2 and temperature on coral juveniles. To date, studies on the effect of ocean acidification on corals have mainly focused on physiology. However, molecular tools provide a comprehensive and timely approach to better understand the coral calcification mechanisms, and evaluate environment-driven changes in the metabolic properties of corals, including their calcification. The second part of the project (return phase) focuses on the effect of rising pCO2 on two Mediterranean pteropods by complementary physiological and molecular tools. Together, those results increase our knowledge on how cnidarians and pteropods will cope with environmental change, and contribute to improve our knowledge on the essential processes of calcification in both organisms.

Performed work

Outgoing phase:

Experiments were performed during the 2009 and 2010 spawning seasons (October - December) at the Great Barrier Reef in Australia. We investigated the effect of elevated pCO2 (acute and chronic) as well as of the increased temperature combined with elevated pCO2 on coral juveniles. A. millepora larvae were kept in seawater that had been equilibrated with air containing 380 ppm (pH approximately 8.16) 750 ppm (pH approximately 7.96) or 1 000 ppm CO2 (pH 7.86) combined or not with two levels of temperature exposure (ambient temperature 27 and 2 degrees of Celsius). These environmental conditions reflect different atmospheric scenarios for the 21st century predicted by the Intergovernmental Panel on Climate Change (ICPC). The overall impact of elevated pCO2 on coral larvae was assessed by measuring survivorship, developmental rate, settlement success, and post-settlement growth rates, while the molecular responses were measured using high-throughput sequencing (Illumina RNA sequencing). This technique allows us to have an overview of the whole transcriptomic response of coral juveniles to climate change.

Return phase:

We focused on two Mediterranean pteropods, L. inflata and C. inflexa. Pteropod specimens were kept at low and control pH conditions (Ph approximately 7.90 and 8.10) for 3 days. Calcification and respiration rates were measured in order to assess the physiological impacts of elevated pCO2 on those species. As for corals, the molecular responses were measured using high-throughput sequencing (Illumina RNA sequencing).

Main results achieved so far:

Outgoing phase: Acropora millepora juveniles

The physiological approach revealed that elevated pCO2 does not have major impacts on the first larval development stages (before initiation of calcification), but negatively impact post-settlement growth rates. This approach led to three publications. The molecular approach revealed that ocean acidification strongly suppressed metabolism but enhanced extracellular organic matrix synthesis, whereas targeted analyses shown complex effects on genes implicated in calcification. In addition, this work led to the discovery of numerous novel candidate genes for a role in calcification, which provide a basis and direction for future studies on coral calcification. This study is the first exhaustive exploration of the transcriptomic response of a scleractinian coral to climate change, and provides an unbiased perspective on its effects on the early stages of calcification. This new approach also demonstrates how transcriptome sequencing of a non-model organism can be employed to gain initial insights into genetic regulation and interaction with the environment as well as to generate novel hypotheses to pave the way for more detailed functional studies. Those results are published in Molecular Ecology (Moya et al., 2012). Three other manuscripts are submitted or in preparation.

Return phase: Mediterranean pteropods (Limacina inflata and Cavolinia inflexa)

The physiological approach revealed that low pH does not have major impacts on the respiration rates of L. inflata, however, animals in low pH conditions showed a significant decrease in calcification rate (44.9 % lower than in control condition, p-value = 0.03). Absolutely no sequence data are available for those two species, meaning that de novo assembly is required for data analysis. This is presently being done in collaboration with Dr Sylvain Foret (ANU, Canberra, Australia). This part of the study will greatly increase the molecular data available for pteropods, and will allow us to have an overview of the whole transcriptomic response of those species to climate change.

Expected final results and their potential impact

This project focuses on a timely, scientifically challenging and unsolved problem, i.e. the response of marine invertebrates to global change by complementary physiological and molecular tools. These research objectives had not been addressed in this way before, and final results will be of broad interest to physiologists and molecular biologists interested both in molluscs and corals, but also to 'climate change' research communities. The molecular results will serve as a valuable resource to the entire marine research community, as well as to other research groups that are interested in comparative genomics. Therefore, this project significantly contributes to enhance European Union (EU) scientific excellence by providing the first assessments on the physiological and molecular response of two very sensitive marine invertebrates, corals and pteropods, to rising pCO2.

List of keywords: Climate change, Ocean acidification, Biological science, Calcification, Coral reefs, Coral juveniles, Molluscs, Pteropods, Large-scale functional genomics.

List of websites:
- http://onlinelibrary.wiley.com/doi/10.1111/j.1365-294X.2012.05554.x/abstract
- http://www.coralcoe.org.au/news/co2-is-messing-with-coral-skeletons

Scientist in charge:
Jean Pierre Gattuso (gattuso@obs-vlfr.fr via e-mail)

Marie-Curie fellow: Aurelie Moya (aurelie.moya@gmail.com via e-mail).