Investigating the change of pH in the internal vesicle of coccolith formation as a potential mechanism of adaptation of coccolithophores in the face of acidification

Chair: Sue-Ann Watson

N. Delebecque (1)*, M.-A. Sicre (2), D. De La Broise (1), E. Ponzevera (3), I. Probert (4), L. Beaufort (5), M. Boye (1)

1 Laboratoire des Sciences de l’Environnement Marin, Institut Universitaire Européen de la Mer (IUEM), Technopole Brest Iroise, Plouzané, France
2 Laboratoire d’océanographie et du climat: expérimentations et approches numériques, Institut Pierre-Simon Laplace / Université Pierre-et-Marie-Curie / CNRS / IRD, Paris
3 IFREMER, Centre de Brest, Département REM/GM/Laboratoire Cycles Géochimiques et Ressources, CS 10070, Plouzané, France
4 CNRS–UMPC, FR2424, Roscoff Culture Collection, Station Biologique de Roscoff, Roscoff, France
5
Environmental Geosciences, CEREGE (CNRS-Université Aix Marseille), Aix en Provence 13545, France

Carbon dioxide produced by anthropogenic activities is absorbed in surface oceans, decreasing seawater pH and the saturation state of calcium carbonate (CaCO3). This process, called ocean acidification (OA), will likely lead to profound changes in marine ecosystems, but it’s future impact on pelagic carbonate production is difficult to constrain. Coccolithophores (unicellular haptophyte algae that produce minute calcite scales to cover the cell) together with Foraminifera (unicellular amoeboid protists that produce calcitic shells) produce more than 90% of pelagic carbonate in the modern ocean. In culture experiments the response of coccolithophore calcification to acidification varies between and within species, complicating the prediction of overall community responses. The underlying processes leading to different sensitivities in calcification among coccolithophores are not known. Coccoliths are formed inside the cell in an internal vesicle where pH can vary (~7.6 to ~8.3). The capacity to regulate pH in the vesicle relative to the decreasing seawater pH would favour calcite precipitation, potentially explaining the adaptation of certain coccolithophores. To address this question, low-calcifying and super-calcifying E. huxleyi morphotypes were grown in turbidostats under different pH conditions to simulate OA. Physiological parameters (growth, photosynthesis and calcification) were examined and changes in vesicle pH were assessed by the use of emerging proxies of carbonate ion concentration (B/Ca) and pH (δ11B) analysed in coccolith calcite. Several other parameters were recorded including full characterisation of the carbonate system and of coccolith shape and thickness. These results will be presented and discussed. This study provides insights into the biomineralization process of coccolithophores and opens new perspectives for estimating the evolution of pelagic calcification under OA.