Re-examining extreme carbon isotope fractionation in the coccolithophore Ochrosphaera neapolitana

Reply to: Re-examining extreme carbon isotope fractionation in the coccolithophore Ochrosphaera neapolitana

“…In the accompanying Matters Arising, Zhang et al 1 report that they did not generate the same fractionation in δ 13 C between seawater dissolved inorganic carbon (DIC) and both inorganic and organic carbon of the coccolithophore Ochrosphaera neapolitana that were observed in our study 2 . These differences in findings are an opportunity to discuss the uncertainties and limitations of different designs of ocean acidification experiments.…”

“…In our original study 2 , we clearly state our assumptions in modeling the δ 13 C of DIC in the experiment and provide experimental data on the δ 13 C of DIC from a similar experiment using the same reagents. The theoretical analyses presented by Zhang et al 1 on timescales of equilibration in hypothetical experiments don't invalidate our assumptions because they show that we pre-bubbled our experimental treatments long enough for pCO 2 of the air and mixed gases to achieve equilibrium.…”

“…To further clarify this matter, the treatments in our experiment were continuously bubbled at the rate of ~1.5 L/min with the mixed gases using a microporous bubbler for 2 weeks before the experiment and throughout its 2-week duration. Owing to this rapid bubbling rate (~1.5 L/min), the blue curves (corresponding to "slow-bubbling" rates) in Figure 2 and Supplementary Figure 2 of Zhang et al 1 are not applicable to our experiment. Moreover, under the "fast-bubbling" scenario (red curves in Figure 2 and Supplementary Figure 2 of ref.…”

“…These differences in findings are an opportunity to discuss the uncertainties and limitations of different designs of ocean acidification experiments. Here, we note that Zhang et al 1 did not closely replicate our experimental conditions, meaning that their alternative hypotheses about what could be occurring in our experiment have ultimately not been tested. Additionally, we highlight elements of their analyses that are under-constrained and how it is difficult to fully resolve these issues without more experimental work.…”

“…), isotopic equilibrium was achieved even for the lowest pCO 2 treatment by day 14 of pre-bubbling (with ~66% equilibrium achieved by day 5). Thus, according to the model of Zhang et al 1 , it seems reasonable to assume that pCO 2 and δ 13 C of the mixed gases and seawater treatments should have been close to equilibrium throughout the duration of the experiment.…”