δ44/40Ca From Coccolithophores May Reveal a Link Between the Ca and C Systems Upon Regulating the Physiological Adaptations of Calcification and Photosynthesis to Varying CO2 Concentrations

The sedimentation of calcium carbonate (CaCO₃) is the largest carbon (C) sink in the combined biosphere, atmosphere and ocean systems, and therefore influences the global C cycle. Coccolithophores are important contributors to CaCO₃ sediment production, with contributions varying from 95% of the total marine CaCO₃in the Cenozoic to 50% in the modern ocean. Consequently, physiological adaptations of coccolithophores′ calcification and photosynthesis to varying ambient conditions have implications for the C cycle.

It has been recently shown that under low CO₂ concentrations (CO₂ threshold of ~20 μM), coccolithophores reallocate HCO₃^- from the calcification vesicle to the chloroplast to cope with the decrease in CO₂ available for photosynthesis. This adaptation was first observed in the late Miocene, as δ¹³C of the more sensitive larger coccoliths (with lower surface to volume ratios) became lighter due to the diminished use of HCO₃^- and increased use of CO₂ as C source for calcification (Bolton and Stoll, 2013). Without further physiological adaptations to maintain calcification, reduced HCO₃^-availability for calcification may result in less calcified coccoliths (e.g. thinner and lighter).

Here we report δ^44/40Ca and δ¹³C measurements of cultured Emiliana huxleyi, Calcidiscus leptoporus and Gephyrocapsa oceanica grown under varying CO₂ concentrations. We test the hypothesis whether Ca transport is influenced by coccolithophores to maintain calcification at low CO₂ concentrations. It is possible that as the reallocation of HCO₃^- from the calcification vesicle to the chloroplast is increasing in response to low CO₂ availability for photosynthesis, Ca transport and concentration would also increase to maintain high saturation at the site of calcification. We also present δ^44/40Ca and δ¹³C from two coccolith size fractions from site 925 in the Western Equatorial Atlantic from the last ~11 Myr, to access how the Ca system of coccolithophores of different sizes may have responded to the decrease of atmospheric CO₂ concentrations over this time interval.