Figure 4.

a) Data analysis procedures leading to the reconstructed representation of a C. braaudii coccolith dissolved in the opto‐electrochemical experiment. Further detail is provided in SI section 2.4. The temporal evolution of this coccolith during acid dissolution is shown in Figure 2 a) with an initial dissolution rate (v=dr _eff/dt) of 0.056 μm s⁻¹ as shown in Figure 2 b). The electrolyte is K/2 growth medium with 10 mM H₂BQ acid precursor. Scale bar=5 μm. b) A scatter plot showing the estimated mass of C. braaudii coccoliths in various electrolytes, all with 10 mM of H₂BQ acid precursor added prior to the experiment. The coccoliths within a distance 10–100 μm from the electrode were analyzed. The colour Scheme representing the electrolyte solution is unchanged from Figure 3 c. The different symbols with the same colour represent data from repeated experiments. The overlaid black dotted line is the estimation of coccolith mass using the recommended shape factor (k _s) of 0.06 and green shade is the range of k _s values (0.04–0.07) reported by Young et al. ^[13a]