Strong shift from HCO(3)(−) to CO(2) uptake in Emiliania huxleyi with acidification: new approach unravels acclimation versus short-term pH effects

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 pCO(2) levels (380 and 950 μatm). In these differently acclimated cells, the photosynthetic carbon source was assessed by a (14)C 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 pCO(2), but no CO(2)-dependent changes in biomass production for haploid cells. In both life-cycle stages, the relative contributions of CO(2) and HCO(3) (−) uptake depended strongly on the assay pH. At pH values ≤ 8.1, cells preferentially used CO(2) (≥ 90 % CO(2)), whereas at pH values ≥ 8.3, cells progressively increased the fraction of HCO(3) (−) uptake (~45 % CO(2) at pH 8.7 in diploid cells; ~55 % CO(2) at pH 8.5 in haploid cells). In contrast to the short-term effect of the assay pH, the pCO(2) acclimation history had no significant effect on the carbon uptake behavior. A numerical sensitivity study confirmed that the pH-modification in the (14)C 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 CO(2) sensitivity despite the apparently high HCO(3) (−) usage seen in previous studies. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11120-014-9984-9) contains supplementary material, which is available to authorized users.