Acclimation of wild-type cells and CO₂-insensitive mutants of the green alga Chlorella ellipsoidea to elevated [CO₂]
2007
OCHIAI, TERUHIKO | COLMAN, BRIAN | MATSUDA, YUSUKE
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 Pmax 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 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 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], K₁/₂[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].
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