Ocean Acidification

Effects of ocean acidification on Emiliania huxleyi strain RCC 1216 (calcifying, diploid life-cycle stage) and RCC 1217 (non-calcifying, haploid life-cycle stage) were investigated by measuring growth, elemental composition, and production rates under different pCO₂ levels (380 and 950 μatm). In these differently acclimated cells, the photosynthetic carbon source was assessed by a ¹⁴C disequilibrium assay, conducted over a range of ecologically relevant pH values (7.9–8.7). In agreement with previous studies, we observed decreased calcification and stimulated biomass production in diploid cells under high pCO₂, but no CO₂-dependent changes in biomass production for haploid cells. In both life-cycle stages, the relative contributions of CO₂ and HCO₃ ⁻ uptake depended strongly on the assay pH. At pH values ≤ 8.1, cells preferentially used CO₂ (≥ 90 % CO₂), whereas at pH values ≥ 8.3, cells progressively increased the fraction of HCO₃ ⁻ uptake (~45 % CO₂ at pH 8.7 in diploid cells; ~55 % CO₂ at pH 8.5 in haploid cells). In contrast to the short-term effect of the assay pH, the pCO₂ acclimation history had no significant effect on the carbon uptake behavior. A numerical sensitivity study confirmed that the pH-modification in the ¹⁴C disequilibrium method yields reliable results, provided that model parameters (e.g., pH, temperature) are kept within typical measurement uncertainties. Our results demonstrate a high plasticity of E. huxleyi to rapidly adjust carbon acquisition to the external carbon supply and/or pH, and provide an explanation for the paradoxical observation of high CO₂ sensitivity despite the apparently high HCO₃ ⁻ usage seen in previous studies.