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. The biogeochemical impact of coccolithophores is defined not only by theiroverall abundance in the oceans but also by wide ranges in physiologicaltraits such as cell size, degree of calcification and carbon productionrates between different species. Species' sensitivity to environmentalforcing has been suggested to relate to their cellular PIC : POC (particulate inorganic carbon : particulate organic carbon) ratio andother physiological constraints. Understanding both the short-term andlonger-term adaptive strategies of different coccolithophore lineages, andhow these in turn shape the biogeochemical role of the group, is thereforecrucial for modeling the ongoing changes in the global carbon cycle. Here wepresent data on the phenotypic evolution of a large and heavily calcifiedgenus Helicosphaera (order Zygodiscales) over the past 15 million years (Myr), at twodeep-sea drill sites in the tropical Indian Ocean and temperate SouthAtlantic. The modern species Helicosphaera carteri, which displays ecophysiological adaptationsin modern strains, was used to benchmark the use of its coccolith morphologyas a physiological proxy in the fossil record. Our results show that, on the single-genotype level, coccolith morphologyhas no correlation with growth rates, cell size or PIC and POC productionrates in H. carteri. However, significant correlations of coccolith morphometricparameters with cell size and physiological rates do emerge once multiplegenotypes or closely related lineages are pooled together. Using thisinsight, we interpret the phenotypic evolution in Helicosphaera as a global, resource-limitation-driven selection for smaller cells, which appears to be a commonadaptive trait among different coccolithophore lineages, from the warm andhigh-CO2 world of the middle Miocene to the cooler and low-CO2conditions of the Pleistocene. However, despite a significant decrease inmean coccolith size and cell size, Helicosphaera kept a relatively stable PIC : POC ratio (as inferredfrom the coccolith aspect ratio) and thus highly conservativebiogeochemical output on the cellular level. We argue that this supports itsstatus as an obligate calcifier, like other large and heavily calcifiedgenera such as Calcidiscus and Coccolithus, and that other adaptive strategies, beyondsize adaptation, must support the persistent, albeit less abundant,occurrence of these taxa. This is in stark contrast with the ancestrallineage of Emiliania and Gephyrocapsa, which not only decreased in mean size but also displayedmuch higher phenotypic plasticity in their degree of calcification while becomingglobally more dominant in plankton communities.

biogeosciences

A 15-million-year-long record of phenotypic evolution in the heavily calcified coccolithophore &amp;lt;i&amp;gt;Helicosphaera&amp;lt;/i&amp;gt; and its biogeochemical implications