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
Abstract: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 δ44/40Ca 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 δ44/40Ca 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.