Changes in the CO2 Concentrating Mechanism During the Cell Cycle in Dunaliella tertiolecta

Abstract

Abstract:Synchronised cells of Dunaliella tertiolecta were used to investigate the expression of the CO2 concentrating mechanism over the cell cycle during growth in either ambient air (low Ci cells) or air enriched with 5% CO2 (high Ci cells). The cultures were analysed for extracellular carbonic anhydrase activity, affinity of photosynthesis for inorganic carbon (Ci) and the ability to accumulate Ci. In high Ci cells, carbonic anhydrase activity changed between 2 − 4 units mg−1 Chi during the light‐dark rhythm showing no clear periodicity. Similarly, the apparent affinity for Ci remained rather constant over the cell cycle. This was judged from the Ci concentrations required for half maximum rate of photosynthesis (K1/2(Ci)) of 72 − 80μM. In the same cells the accumulation ratio of internal Ci versus external Ci ranged between 5 and 9.5 without a clear rhythm. In contrast, these parameters showed distinct periodical changes in synchronised low Ci cells. Carbonic anhydrase activity changed from 10 to 350 units mg−1 Chl with maximum and minimum activities occurring in the middle and at the end of the light period, respectively. The K1/2(Ci) values showed similar periodicity ranging between 13 − 36μM. In addition the accumulation ratio increased up to 30 in the middle of illumination and decreased to its lowest level of 12 at the end of the light period. These results indicate the presence of a common step in regulating the induction of the measured parameters and that light is not an absolute requirement for the induction of the CO2 concentrating mechanism in synchronous low CO2 grown cells of Dunaliella tertiolecta.