A 15 million-year long record of phenotypic evolution in the heavily calcified coccolithophore Helicosphaera and its biogeochemical implications 5 Luka Šupraha1, a, Jorijntje Henderiks1, 2 1 Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden. 2 Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biosciences, University of Oslo, P.O. Box 1066 Blindern, 0316 Oslo, Norway. a Present address: Section for Aquatic Biology and Toxicology (AQUA), Department of Biosciences, University of Oslo, P.O. 10 Box 1066 Blindern, 0316 Oslo, Norway Correspondence to: Luka Šupraha (
[email protected]) Abstract. The biogeochemical impact of coccolithophores is defined by their overall abundance in the oceans, but also by a wide range in physiological traits such as cell size, degree of calcification and carbon production rates between different species. Species’ “sensitivity” to environmental forcing has been suggested to relate to their cellular PIC:POC ratio and other 15 physiological constraints. Understanding both the short and longer-term adaptive strategies of different coccolithophore lineages, and how these in turn shape the biogeochemical role of the group, is therefore crucial for modeling the ongoing changes in the global carbon cycle. Here we present data on the phenotypic evolution of a large and heavily-calcified genus Helicosphaera (order Zygodiscales) over the past 15 million years (Ma), at two deep-sea drill sites from the tropical Indian Ocean and temperate South Atlantic. The modern species Helicosphaera carteri, which displays eco-physiological adaptations 20 in modern strains, was used to benchmark the use of its coccolith morphology as a physiological proxy in the fossil record.