Acclimation of wild-type cells and CO2-insensitive mutants of the green alga Chlorella ellipsoidea to elevated [CO2]

Abstract

CO2-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% CO2. When air-grown, wild-type (WT) cells were transferred to 5% CO2, 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 CO2-insensitive mutant, ENU16, exhibited a constant Pmax at maximum levels and a low K1/2[DIC] throughout the acclimation period. The rapid Pmax drop within 12 h of acclimation in WT cells was significantly reduced by treatment with 0.5 mm 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 Pmax in WT cells. WT and ENU16 cells were grown in controlled equilibrium [CO2], 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% [CO2], K 1/2[DIC] values increased as [CO2] increased, and the photosynthetic rates at growth DIC conditions were shown to decrease to about 70% the Pmax level in this intermediate [CO2] 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 CO2 conditions as a result of acquiring intermediate affinities for DIC because of their sensitive responses to changes in the ambient [CO2]. © 2007 The Authors.