The induction of inorganic carbon transport and external carbonic anhydrase in Chlamydomonas reinhardtii is regulated by external CO2 concentration

Induction of the carbon concentrating mechanism (CCM) has been investigated during the acclimation of 5% CO2-grown Chlamdomonas reinhardtii 2137 mt + cells to well-defined dissolved inorganic carbon (C-i) limited conditions. The CCM components investigated were active HCO3- transport, active CO2 transport and extracellular carbonic anhydrase (CA(ext)) activity The CA(ext) activity increased 10-fold,within 6 h of acclimation to 0.035% CO2 and there was a further slight increase over the next 18 h. The CA(ext) activity also increased substantially after an 8 h lag period during acclimation to air in darkness, Active CO2 and HCO3- uptake by C, reinhardtii cells were induced within 2 h of acclimation to air, but active CO2 transport was induced prior to active HCO3- transport. Similar results were obtained during acclimation to air in darkness. The critical C-i concentrations effecting the induction of active C-i transport and CA(ext) activity were determined by allowing cells to acclimate to various inflow CO2 concentrations in the range 0.035-0.84% at constant pH, The total C-i concentration eliciting the induction and repression of active C-i transport was higher during acclimation at pH 7.5 than at pH 5.5, but the external CO2 concentration was the same at both pHs of acclimation. The concentration of external CO2 required for the full induction and repression of C-i transport and CA(ext) activity were 10 and 100 mu M, respectively. The induction of CA(ext) and active C-i transport are not correlated temporally, but are regulated by, the same critical CO2 concentration in the medium.