Culture conditions, mass spectrometric measurements and acclimation carbonate chemistry

- 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 lmol 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.