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

Abstract

ABSTRACTInduction of the carbon concentrating mechanism (CCM) has been investigated during the acclimation of 5% CO2‐grown Chlamydomonas reinhardtii 2137 mt + cells to well‐defined dissolved inorganic carbon (Ci) limited conditions. The CCM components investigated were active HCO3− transport, active CO2 transport and extracellular carbonic anhydrase (CAext) activity. The CAext 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 CAext 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 Ci concentrations effecting the induction of active Ci transport and CAext 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 Ci concentration eliciting the induction and repression of active Ci 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 Ci transport and CAext activity were 10 and 100 μM, respectively. The induction of CAext and active Ci transport are not correlated temporally, but are regulated by the same critical CO2 concentration in the medium.