Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-303 Manuscript under review for journal Biogeosciences Discussion started: 6 September 2017 c Author(s) 2017. CC BY 4.0 License. Overcalcified forms of the coccolithophore Emiliania huxleyi in high CO2 waters are not pre-adapted to ocean acidification. Peter von Dassow1,2,3*, Francisco Díaz-Rosas1,2, El Mahdi Bendif4, Juan-Diego Gaitán-Espitia5, Daniella Mella-Flores1, Sebastian Rokitta6, Uwe John6, and Rodrigo Torres7,8 5 1 Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile. 2 Instituto Milenio de Oceanografía de Chile. 3 UMI 3614, Evolutionary Biology and Ecology of Algae, CNRS-UPMC Sorbonne Universités, PUCCh, UACH. 4 Department of Plant Sciences, University of Oxford, OX1 3RB Oxford, UK. 5 CSIRO Oceans and Atmosphere, GPO Box 1538, Hobart 7001, TAS, Australia. 10 6 Marine Biogeosciences | PhytoChange Alfred Wegener Institute – Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany. 7 Centro de Investigación en Ecosistemas de la Patagonia (CIEP), Coyhaique, Chile. 8 Centro de Investigación: Dinámica de Ecosistemas marinos de Altas Latitudes (IDEAL), Punta Arenas, Chile. Correspondence to: Peter von Dassow (
[email protected]) 15 Abstract. Marine multicellular organisms inhabiting waters with natural high fluctuations in pH appear more tolerant to acidification than conspecifics occurring in nearby stable waters, suggesting that environments of fluctuating pH hold genetic reservoirs for adaptation of key groups to ocean acidification (OA). The abundant and cosmopolitan calcifying phytoplankton Emiliania huxleyi exhibits a range of morphotypes with varying degrees of coccolith mineralization. We show that E. huxleyi populations in the naturally acidified upwelling waters of the Eastern South Pacific, where pH drops below 20 7.8 as is predicted for the global surface ocean by the year 2100, are dominated by exceptionally overcalcified morphotypes whose distal coccolith shield can be almost solid calcite.