Active Transport of CO₂ by the Cyanobacterium Synechococcus UTEX 625

Abstract

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 . Upon illumination the ceUls 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. lodoacetamide (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 HC03-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 HC03-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.Photosynthesis by cyanobacteria can occur when the CO2 concentration in the extracellular medium is so low that CO2 fixation via Rubisco2 could not occur were it not for the presence of 'CO2-concentrating' mechanisms (1,2,9,13,16,19,21,25,28 (1,2,9,18,29). For a given DIC concentration, the rate of DIC accumulation was faster under the nonequilibrium conditions (high CO,/HCO3-) than under equilibrium conditions (high HCO3-/C0.), thus indicating a lower Kmn for CO2 transport than for HCO3 transport (1. 2, 9, 29).Miller and Canvin (17) provided further evidence for a CO,-transport capacity, distinct from the HCO3-transport capacity, when they made use of the observation that HCO-transport in rapidly growing cells of Synechococcus UTEX 625 is inhibited by the absence of Na+ in the extracellular medium (8,17,22). Cells that were incubated in the absence of Na + were stimulated to accumulate normal levels of intracellular DIC by the addition of CA (17). It was postulated that, in the absence of the CA, the rate of supply of CO2 to the CO,-transport system was limited by the rate of HCO3-dehydration to CO2 in the extracellular medium. The DIC transport occurring in the presence of CA was not inhibited by the addition of Li+, whereas the Na+-dependent...

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…Iodoacetamide has been used as an inhibitor of carbon fixation (Miller et al 1988;Miller and Canvin 1989), however when added during steady state photosynthesis, it inhibits CO 2 very slowly and may induce O 2 uptake in the light (Miller and Canvin 1989). An alternative inhibitor of CO 2 fixation is D, L-glyceraldehyde (Stokes and Walker 1972), which at very high concentrations (>25 mM) completely inhibits CO 2 fixation (Shelp and Canvin 1989) and blocks the conversion of triose-P to ribulose-1,5-bisphosphate (Stokes and Walker 1972).…”

Selective and Reversible Inhibition of Active CO2 Transport by Hydrogen Sulfide in a Cyanobacterium

“…(m/e = 32, 34, 44, and 45, respectively) in cell suspensions was determined with a magnetic sector mass spectrometer (VG Gas Analysis, Middlewich, England; model MM 14-80 SC) equipped with a membrane inlet system (7,17). The output signal from the mass spectrometer was directed to an external amplification/attenuation and frequency-cutoff unit and subsequently to a Fisher Recordall 5000 strip chart recorder.…”

“…8,13,20) and Anabaena variabilis ( 11) requires extracellular Na+, with an optimum [Na+] of around 25 mm for Synechococcus cells at pH 8. The active transport ofCO2 by air-grown cells of Synechococcus also requires Na+ (8,14,17), but the optimum [Na+] for this process is around 100 ,uM. At pH 8, the differential requirement for Na+ provides a convenient means to distinguish between the two transport processes.…”

“…During steady state photosynthesis in Synechococcus leopoliensis UTEX 625 both species of DIC are transported simultaneously and continuously in light dependent processes (7). Although cyanobacteria rapidly remove CO2 from the medium (2,7,17,28), it is HCO3-transport which provides the bulk of the DIC to the intracellular pool when the extracellular pH is alkaline and the [DIC] low (2,6,7,10,16 …”

“…Cyanobacteria possess mechanisms for the active transport of CO2 and HC03-that lead to the accumulation of a large intracellular pool of DIC3 (2,6,10,11,13,16,17,28) which serves as the immediate source of CO2 for photosynthesis (10,16). During steady state photosynthesis in Synechococcus leopoliensis UTEX 625 both species of DIC are transported simultaneously and continuously in light dependent processes (7).…”