Active Transport of CO(2) by the Cyanobacterium Synechococcus UTEX 625 : Measurement by Mass Spectrometry

Abstract

Mass spectrometry has been used to confirm the presence of an active transport system for CO(2) in Synechococcus UTEX 625. Cells were incubated at pH 8.0 in 100 micromolar KHCO(3) in the absence of Na(+) (to prevent HCO(3) (-) transport). Upon illumination the cells rapidly removed almost all the free CO(2) from the medium. Addition of carbonic anhydrase revealed that the CO(2) depletion resulted from a selective uptake of CO(2), rather than a total uptake of all inorganic carbon species. CO(2) transport stopped rapidly (<3 seconds) when the light was turned off. Iodoacetamide (3.3 millimolar) completely inhibited CO(2) fixation but had little effect on CO(2) transport. In iodoacetamide poisoned cells, transport of CO(2) occurred against a concentration gradient of about 18,000 to 1. Transport of CO(2) was completely inhibited by 10 micromolar diethylstilbestrol, a membrane-bound ATPase inhibitor. Studies with DCMU and PSI light indicated that CO(2) transport was driven by ATP produced by cyclic or pseudocyclic photophosphorylation. Low concentrations of Na(+) (<100 microequivalents per liter), but not of K(+), stimulated CO(2) transport as much as 2.4-fold. Unlike Na(+)-dependent HCO(3) (-) transport, the transport of CO(2) was not inhibited by high concentrations (30 milliequivalents per liter) of Li(+). During illumination, the CO(2) concentration in the medium remained far below its equilibrium value for periods up to 15 minutes. This could only happen if CO(2) transport was continuously occurring at a rapid rate, since the continuing dehydration of HCO(3) (-) to CO(2) would rapidly raise the CO(2) 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 CO(2) transport was balanced by HCO(3) (-) efflux.