Fit procedure with experimental data

Maximum measured calcite to organic carbon (PIC:POC) ratio and maximum population mean coccosphere diameter of the four coccolithophore investigated here. (Values were extracted from data by Bach et al., 2013, Bach et al., 2012, Bach et al., 2011, Langer et al., 2006 and Sett et al., 2014, and this study). Maximum values for PIC:POC and coccosphere diameter are a good indicator for the species' calcification and cell size potential. Note, however, that each species can have lower PIC:POC ratios and be smaller under non-optimal carbonate chemistry conditions. Sensitivity parameters (a, b, c, d) of calcification rates were obtained by fitting measured calcification rates of 4 coccolithophore species (given in table S1: hdl:10013/epic.47926.d001) with Eq. (5). Note that their units need to be a = dimensionless, b = mol/kg, c and d = kg/mol and concentrations of CO2, HCO3- and H+ in mol/kg in order to get a dimensionless factor for calcification rates from Eq. (5). Carbonate chemistry conditions where calcification rates either reach 50% of their maxima (OPT1/2 - i.e. left of the optimum; Fig. 3) or decrease down to 50% after having reached maximum values (SubOPT1/2 - i.e. right of the optimum; Fig. 3) were calculated from pCO2 and TA determined in Fig. 3. R2 values determine to what extent a fit with Eq. (5) is able to reproduce the data. For further analysis of the fit quality see Fig. 1.