Why do coccolithophores calcify? The potential of the calcium carbonate shell to serve as protection against grazers

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Coccolithophores are a group of globally distributed marine phytoplankton species which cover themselves with a shell (coccosphere) composed of tiny calcium carbonate platelets (coccoliths). Through their ability to calcify they contribute to the vertical gradient in ocean alkalinity, accelerate organic matter export to depth, and increase the Earth s albedo, making this functional group a key player in the climate system. Despite more than a century of intense research on coccolithophore calcification and its biogeochemical relevance, one of the most fundamental questions is still unresolved: Why do coccolithophores calcify? Answering this question is of outstanding importance because if we fail to unravel the link between calcification - the key trait - and their success in the natural habitat we remain unable to predict the fate of coccolithophores in the future ocean where calcification rates are thought to decline. In the project proposed herein we address this key question by investigating whether coccolithophores form the coccosphere to protect themselves against planktonic predators, infectious bacteria, and/or viruses. We therefore designed a novel experimental approach where calcified and de-calcified cells are exposed to different grazers or pathogens. Our approach will allow us to unravel: (1) whether calcified coccolithophore cells better withstand grazing and infection than cells without a coccosphere; (2) whether grazers and pathogens prefer cells without coccosphere when they are offered both, calcified and de-calcified cells: (3) whether the growth and reproduction of grazers and pathogens is impeded when they feed on or infect calcified cells.