The cost of (re-) calcification in a high CO2 world

To understand the present distribution and potential resilience of a given species in response to today’s and to future environmental variability, it is crucial to understand its physiological ability to buffer and/or regulate when exposed to these changes. For example, it is believed that shells of marine calcifiers will be more exposed to dissolution under elevated pCO2 environment but very little information is available on mechanisms, energy costs and fitness consequences of perturbation in the balance between calcification/dissolution. Combining internal pH mapping techniques and a newly developed physiological assay to manipulate calcification/dissolution/re-calcification, we will show that sea urchin larvae possess an unexpectedly high resilience to low pH conditions. Under moderate ocean acidification conditions (e.g. ΔpH=0.4), sea urchin larvae are able to quickly compensate for environmental pH changes by using ion channels (e.g. H+/K+-ATP) and keep their cellular pHi constant (e.g. PMCs involved in calcification). Following the re-calcification process after a major dissolution, we were able to estimate the cost of calcification as less than 10% of the larval energy budget in control conditions. Under high pCO2, an additional 10% energy cost is needed to regulate pHi. Impact of calcification/dissolution on sea urchin fitness was measured through larval performance (growth, calcification, settlement success, gene expression, swimming) and energy budget (respiration, feeding). We demonstrated that sea urchin larvae appeared to be much more resistant to elevated pCO2 (e.g. normal development till pH>7.3) than previously expected and are able to quickly recover from major episodes of decalcification (e.g. 24h exposure to pH 5.3).