Active Transport of CO2 by the Cyanobacterium Synechococcus UTEX 625 1: Measurement by Mass Spectrometry

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Mass spectrometry has been used to confirm the presence of an active transport system for CO2 in Synechococcus UTEX 625. Cells were incubated at pH 8.0 in 100 micromolar KHCO3 in the absence of Na+ (to prevent HCO3− transport). Upon illumination the cells rapidly removed almost all the free CO2 from the medium. Addition of carbonic anhydrase revealed that the CO2 depletion resulted from a selective uptake of CO2, rather than a total uptake of all inorganic carbon species. CO2 transport stopped rapidly (<3 seconds) when the light was turned off. Iodoacetamide (3.3 millimolar) completely inhibited CO2 fixation but had little effect on CO2 transport. In iodoacetamide poisoned cells, transport of CO2 occurred against a concentration gradient of about 18,000 to 1. Transport of CO2 was completely inhibited by 10 micromolar diethylstilbestrol, a membrane-bound ATPase inhibitor. Studies with DCMU and PSI light indicated that CO2 transport was driven by ATP produced by cyclic or pseudocyclic photophosphorylation. Low concentrations of Na+ (<100 microequivalents per liter), but not of K+, stimulated CO2 transport as much as 2.4-fold. Unlike Na+-dependent HCO3− transport, the transport of CO2 was not inhibited by high concentrations (30 milliequivalents per liter) of Li+. During illumination, the CO2 concentration in the medium remained far below its equilibrium value for periods up to 15 minutes. This could only happen if CO2 transport was continuously occurring at a rapid rate, since the continuing dehydration of HCO3− to CO2 would rapidly raise the CO2 concentration to its equilibrium value if transport ceased. Measurement of the rate of dissolved inorganic carbon accumulation under these conditions indicated that at least part of the continuing CO2 transport was balanced by HCO3− efflux.

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