High Affinity Transport of CO₂ in the Cyanobacterium Synechococcus UTEX 625

Abstract

The active transport of CO2 in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HC03-transport. The substrate concentration required to give one half the maximum rate for whole cell CO2 transport was determined to be 0.4 ± 0.2 micromolar (mean ± standard deviation; n = 7) with a range between 0.2 and 0.66 micromolar. The maximum rates of CO2 transport ranged between 400 and 735 micromoles per milligram of chlorophyll per hour with an average rate of 522 for seven experiments. This rate of transport was about three times greater than the dissolved inorganic carbon saturated rate of photosynthetic 02 evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO2 transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO2 transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO2 transport was caused by HC03-or by Nat-dependent HC03-transport. However, 12CO2 readily interfered with 13CO2 transport. CO2 transport and Na+-dependent HC03-transport are separate, independent processes and the high affinity CO2 transporter is not only responsible for the initial transport of CO2 into the cell but also for scavenging any CO2 that may leak from the cell during ongoing photosynthesis.The efficient assimilation of CO2 by cyanobacteria during photosynthesis depends upon the concurrent operation of a mechanism to concentrate CO2 at the site of carboxylation (15,28). Unlike photosynthetic cells of higher plants which rely primarily on CO2 diffusion (9), cyanobacteria acquire DIC3 from their environment through the active transport of CO2 and 6,18,23). These transport processes lead to the accumulation and maintenance of a large intracellular pool of DIC from which CG2 is fixed by Rubisco (15,28). In cells in which the C02-concentrating mechanism is highly ' Supported by grants from the Natural Sciences and Engineering Research Council of Canada.

Passive Entry of CO2 and Its Energy-dependent Intracellular Conversion to HCO3− in Cyanobacteria Are Driven by a Photosystem I-generated ΔμH+

“…If CO 2 fixation accounted for alterations in [CO 2(dis) ], one would have expected that changes in the CO 2 uptake curve to be the mirror image of those in the O 2 evolution curve, but that is not the case. These data provide supporting evidence for the conclusion that CO 2 uptake does not solely reflect CO 2 fixation and may occur even in its absence (5,6).…”

“…This displacement of [CO 2(dis) ] from equilibrium can be observed in the absence of CO 2 fixation and is largely due to CO 2 uptake, intracellular conversion to HCO 3 Ϫ , and release of the latter into the medium (5,6). The reverse phenomenon has been described in Synechococcus WH 7803 (7) and Nannochloropsis sp.…”

“…On illumination, many photosynthetic microorganisms maintain the concentration of dissolved CO 2 ([CO 2(dis) ]) in their surrounding medium below that expected at chemical equilibrium with HCO 3 Ϫ (1)(2)(3)(4)(5). This displacement of [CO 2(dis) ] from equilibrium can be observed in the absence of CO 2 fixation and is largely due to CO 2 uptake, intracellular conversion to HCO 3 Ϫ , and release of the latter into the medium (5,6).…”

“…The location of the CAlike activity has not been identified and the mode of energization of the active HCO 3 Ϫ accumulation is not understood. Active transport of CO 2 across the plasmalemma has also been suggested (5,18), but it is difficult to distinguish this from diffusion of CO 2 across the plasma membrane with subsequent energy-dependent conversion to HCO 3 Ϫ (6,19). Passive entry of CO 2 across the membrane may occur via aquaporins (20), a possibility examined here by the application of a water channel blocker (WCB), p-chloromercuriphenylsulfonic acid.…”

cemA homologue essential to CO2 transport in the cyanobacterium Synechocystis PCC6803.

“…Both CO2 and HCO3 are the species transported into the cells by this mechanism, and HCO3 appears to be the only species delivered to the interior of the cells (5,6). The cyanobacterial Ci-transport system has a high affinity to CO2 but has a low affinity to HCO3 (4,(6)(7)(8)) (e.g., K2l, values of 1.6 and 141 ,tM for CO2 uptake and HCO3 uptake, respectively, for air-grown cells of Synechococcus; ref. 8).…”