Acclimation of wild‐type cells and CO 2 ‐insensitive mutants of the green alga Chlorella ellipsoidea to elevated [CO 2 ]

CO 2 ‐insensitive mutants of the green alga Chlorella ellipsoidea were previously shown to be unable to repress an inorganic carbon‐concentrating mechanism (CCM) when grown under 5% CO 2 . When air‐grown, wild‐type (WT) cells were transferred to 5% CO 2 , an abrupt drop of P max to 43% the original level of air‐grown cells was observed within the initial 12 h. Photosynthetic affinities of WT cells to dissolved inorganic carbon (DIC) were maintained at high levels for the initial 4 d of acclimation, and then decreased gradually to lower levels over the next 6 d. In contrast to WT cells, the CO 2 ‐insensitive mutant, ENU16, exhibited a constant P max at maximum levels and a low K 1/2 [DIC] throughout the acclimation period. The rapid P max drop within 12 h of acclimation in WT cells was significantly reduced by treatment with 0.5 m m of 6‐ethoxybenzothiazole‐2‐sulphonamide (EZA), a specific membrane‐permeable inhibitor of carbonic anhydrase (CA), suggesting the participation of internal CAs in the temporary drop in P max in WT cells. WT and ENU16 cells were grown in controlled equilibrium [CO 2 ], and the photosynthetic rate of each acclimated cell type was measured under equilibrated growth [DIC] conditions. In WT cells acclimated to 0.14–0.4% [CO 2 ], K 1/2 [DIC] values increased as [CO 2 ] increased, and the photosynthetic rates at growth DIC conditions were shown to decrease to about 70% the P max level in this intermediate [CO 2 ] range. Such decreases in the net photosynthetic rates were not observed in ENU16. These results suggest that algal primary production could be depressed significantly under moderately enriched CO 2 conditions as a result of acquiring intermediate affinities for DIC because of their sensitive responses to changes in the ambient [CO 2 ].