Carbon affinity adaptation of Synechococcus and its phycocyanin mutant to various CO 2 concentrations and light intensities

Abstract

The dependence of the photosynthetic capacity on the CO 2 concentration provided was used as a measurement of the affinity of the photosynthetic apparatus for carbon. “Affinity” includes uptake, transport and incorporation of inorganic carbon. The affinity of the photosynthetic apparatus for CO 2 was tested in cells adapted to high and low CO 2 concentrations and light intensities. The photosynthetic capacity at saturating concentrations of bicarbonate ( V sat) was higher for cells grown under aerated conditions with 3% CO 2 (HC cells) than for those grown under aerated conditions with air (LC cells). The half-saturating bicarbonate concentration ( K 1/2) was lower in LC cells than in HC cells. The effect of various light intensities on V sat and K 1/2 during adaptation was small. Principally the same results were obtained for the wild type and the mutant AN 112, which lacks most of the phycocyanin within the phycobilisome, i.e. its light-harvesting system. This indicates that light intensities can modulate the affinity for CO 2 much less than the CO 2 concentration during the adaptation process. When the CO 2 concentration during adaptation of the mutant cells was reduced from 3% to the level of air, K 1/2 decreased to about 5% within a few hours, whereas V sat approached a level of about 60% of the starting value gradually over many days. When the CO 2 concentration was raised, K 1/2 began to increase after 6 h. V sat decreased quickly and thereafter increased to exceed the original level after 6 h. As a result of the adaptation to different CO 2 concentrations, the kinetics of V sat and K 1/2 are different. This implies that V sat and K 1/2 are controlled by different processes.