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.Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-014-9984-9) contains supplementary material, which is available to authorized users.

Summary A combined increase in seawater [CO2] and [H+] was recently shown to induce a shift from photosynthetic HCO3 − to CO2 uptake in Emiliania huxleyi. This shift occurred within minutes, whereas acclimation to ocean acidification (OA) did not affect the carbon source.To identify the driver of this shift, we exposed low‐ and high‐light acclimated E. huxleyi to a matrix of two levels of dissolved inorganic carbon (1400, 2800 μmol kg−1) and pH (8.15, 7.85) and directly measured cellular O2, CO2 and HCO3 − fluxes under these conditions.Exposure to increased [CO2] had little effect on the photosynthetic fluxes, whereas increased [H+] led to a significant decline in HCO3 − uptake. Low‐light acclimated cells overcompensated for the inhibition of HCO3 − uptake by increasing CO2 uptake. High‐light acclimated cells, relying on higher proportions of HCO3 − uptake, could not increase CO2 uptake and photosynthetic O2 evolution consequently became carbon‐limited.These regulations indicate that OA responses in photosynthesis are caused by [H+] rather than by [CO2]. The impaired HCO3 − uptake also provides a mechanistic explanation for lowered calcification under OA. Moreover, it explains the OA‐dependent decrease in photosynthesis observed in high‐light grown phytoplankton.

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Cell division of the coccolithophore Emiliania huxleyi and other phytoplankton typically becomes entrained to diel light/dark cycles under laboratory conditions, with division occurring primarily during dark phases and production occurring during light phases. Under these conditions, increases in cell and biomass concentrations deviate from exponential functions on time scales < 24 h. These deviations lead to significant diel variations in common measurements of phytoplankton physiology such as cellular quotas of particulate organic and inorganic carbon (POC, PIC) and their production rates. Being time-dependent, only the temporal mean of the various values during the day are comparable between experiments. Deviations from exponential growth furthermore imply that increases in cell and biomass concentrations cannot be expressed by the daily growth rate μ 24 h (typically determined from daily increments in cell concentrations). Consequently, conventional calculations of production as the product of a cellular quota (e.g., POC quota) and μ 24 h are mathematically incorrect. To account for this, we here describe short-term changes in cell and biomass concentrations of fastdividing, dilute-batch cultures of E. huxleyi grown under a diel light/dark cycle using linear regression. Based on the derived models, we present calculations for daily means of cellular quotas and production rates. Conventional (time-specific) measurements of cellular quotas and production differ from daily means by up to 65% in our example and, under some circumstances, cause false "effects" of treatments. Intending to reduce errors in ecophysiological studies, we recommend determining daily means-mathematically or by adjusting the experimental setup or sampling times appropriately.

Coccolithophores feature a haplo-diplontic life cycle comprised of diploid cells producing heterococcoliths and haploid cells producing morphologically different holococcoliths. These life cycle phases of each species appear to have distinct spatial and temporal distributions in the oceans, with the heavily-calcified heterococcolithophores (HET) often more prevalent in winter and at greater depths, whilst the lightly-calcified holococcolithophores (HOL) are more abundant in summer and in shallower waters. The haplo-diplontic life cycle may therefore allow coccolithophores to expand their ecological niche, switching between life cycle phases to exploit conditions that are more favourable. However, coccolithophore life cycles remain poorly understood and fundamental information on the physiological differences between life cycle phases is required if we are to better understand the ecophysiology of coccolithophores. In this study, we have examined the physiology of HET and HOL phases of the coccolithophore Coccolithus braarudii in response to changes in light and nutrient availability. We found that the HOL phase was more tolerant to high light than the HET phase, which exhibited defects in calcification at high irradiances. The HET phase exhibited defects in coccolith formation under both nitrate (N) and phosphate (P) limitation, whilst no defects in calcification were detected in the HOL phase. The HOL phase grew to a higher cell density under P-limitation than N-limitation, whereas no difference was observed in the maximum cell density reached by the HET phase at these nutrient concentrations. HET cells grown under a light:dark cycle divided primarily in the dark and early part of the light phase, whereas HOL cells continued to divide throughout the 24 h period. The physiological differences may contribute to the distinct biogeographical distributions observed between life cycle phases, with the HOL phase potentially better adapted to high light, low nutrient regimes, such as those found in seasonally stratified surface waters. Highlights: Coccolithus braarudii life cycle phases exhibit different physiological responses. The heavily-calcified heterococcolithophores (HET) life cycle phase is more sensitive to high light. The lightly-calcified holococcolithophores (HOL) life cycle phase may be better suited to growth under low phosphate availability..