The role of extracellular carbonic anhydrase for accumulation of inorganic carbon in the green alga Chlamydomonas reinhardtii. A comparison between wild‐type and cell‐wall‐less mutant cells

Abstract

The role of extracellular carbonic anhydrase (CAex) for dissolved inorganic carbon (DIC) accumulation in the green alga Chlamydomonas reinhardtii was investigated. It was found that when algal cells were bubbled with ambient air, cell‐wall‐less mutant cells exhibited the same high photosynthetic affinity for CO2 as wild‐type cells despite a 10 times lower activity of CAex. It was also found that the affinity for CO2 was further increased when the total DIC concentration of the algal medium was reduced from that in equilibrium with ambient air to even lower levels. This increased affinity was not correlated with any further increase in the CAex activity.Dextran‐bound sulfonamide (DBS. 100 μM bound ligand) completely inhibited the activity of CAex in intact, low‐DIC grown, wild‐type cells, while photosynthesis at <2 μM CO2(aq) proceeded at a far greater rate than could be maintained by CO2 supplied from the spontaneous dehydration of HCO−3. DBS‐inhibition of CAex, during the induction of the DIC‐accumulating mechanism in previously high‐DIC grown cells, only caused a 50% inhibition of photosynthesis at 10 μM CO2(aq) after 1 h of low‐DIC acclimation. It was also shown that 50 μM acetazolamide (AZ) inhibited photosynthesis at low DIC concentrations to a relatively higher degree than DBS, suggesting that AZ inhibited intracellular CA as well.Taken together, these results suggest that low‐DIC grown cells of C. reinhardtii have the ability to transport HCO−3 across the plasma membrane in addition to the CAex‐mediated, facilitated diffusion and/or transport of CO2. It is also suggested that the relative importance of these two fluxes (CO2 or HCO−3) is dependent on the growth and experimental conditions. Facilitated CO2 uptake seems to be most prevalent, supported by HCO−3‐transport under more or less extreme situations, such as a reduction of CO2 to extremely low concentrations, leakage of CAex to the medium as in cultures of cell‐wall‐less mutant cells or when the activity of CAex has been artificially inhibited.