Use of Carbon Oxysulfide, a Structural Analog of CO(2), to Study Active CO(2) Transport in the Cyanobacterium Synechococcus UTEX 625

Identification and characterization of a carboxysomal γ-carbonic anhydrase from the cyanobacterium Nostoc sp. PCC 7120

Carboxysomes are proteinaceous microcompartments that encapsulate carbonic anhydrase (CA) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco); carboxysomes, therefore, catalyze reversible HCO3 (-) dehydration and the subsequent fixation of CO2. The N- and C-terminal domains of the β-carboxysome scaffold protein CcmM participate in a network of protein-protein interactions that are essential for carboxysome biogenesis, organization, and function. The N-terminal domain of CcmM in the thermophile Thermosynechococcus elongatus BP-1 is also a catalytically active, redox regulated γ-CA. To experimentally determine if CcmM from a mesophilic cyanobacterium is active, we cloned, expressed and purified recombinant, full-length CcmM from Nostoc sp. PCC 7120 as well as the N-terminal 209 amino acid γ-CA-like domain. Both recombinant proteins displayed ethoxyzolamide-sensitive CA activity in mass spectrometric assays, as did the carboxysome-enriched TP fraction. NstCcmM209 was characterized as a moderately active and efficient γ-CA with a k cat of 2.0 × 10(4) s(-1) and k cat/K m of 4.1 × 10(6) M(-1) s(-1) at 25 °C and pH 8, a pH optimum between 8 and 9.5 and a temperature optimum spanning 25-35 °C. NstCcmM209 also catalyzed the hydrolysis of the CO2 analog carbonyl sulfide. Circular dichroism and intrinsic tryptophan fluorescence analysis demonstrated that NstCcmM209 was progressively and irreversibly denatured above 50 °C. NstCcmM209 activity was inhibited by the reducing agent tris(hydroxymethyl)phosphine, an effect that was fully reversed by a molar excess of diamide, a thiol oxidizing agent, consistent with oxidative activation being a universal regulatory mechanism of CcmM orthologs. Immunogold electron microscopy and Western blot analysis of TP pellets indicated that Rubisco and CcmM co-localize and are concentrated in Nostoc sp. PCC 7120 carboxysomes.

Association between carbonyl sulfide uptake and (18)Δ during gas exchange in C(3) and C(4) leaves

Carbonyl sulfide (COS) and C(18)OO exchange by leaves provide potentially powerful tracers of biosphere-atmosphere CO(2) exchange, and both are assumed to depend on carbonic anhydrase (CA) activity and conductance along the diffusive pathway in leaves. We investigated these links using C(3) and C(4) plants, hypothesizing that the rates of COS and C(18)OO exchange by leaves respond in parallel to environmental and biological drivers. Using CA-deficient antisense lines of C(4) and C(3) plants, COS uptake was essentially eliminated and discrimination against C(18)OO exchange ((18)Δ) greatly reduced, demonstrating CA's key role in both processes. (18)Δ showed a positive linear correlation with leaf relative uptake (LRU; ratio of COS to CO(2) assimilation rates, A(s)/A(c), normalized to their respective ambient concentrations), which reflected the effects of stomatal conductance on both COS and C(18)OO exchange. Unexpectedly, a decoupling between A(s) and (18)Δ was observed in comparing C(4) and C(3) plants, with a large decrease in (18)Δ but no parallel reduction in A(s) in the former. This could be explained by C(4) plants having higher COS concentrations at the CA site (maintaining high A(s) with reduced CA) and a high phosphoenolpyruvate carboxylase/CA activity ratio (reducing (18)O exchange efficiency between CO(2) and water, but not A(s)). Similar A(s) but higher A(c) in C(4) versus C(3) plants resulted in lower LRU values in the former (1.16 ± 0.20 and 1.82 ± 0.18 for C(4) and C(3), respectively). LRU was, however, relatively constant in both plant types across a wide range of conditions, except low light (<191 μmol photon m(-2) s(-1)).

Relationships between carbonyl sulfide (COS) and CO2 during leaf gas exchange

*Carbonyl sulfide (COS) exchange in C(3) leaves is linked to that of CO(2), providing a basis for the use of COS as a powerful tracer of gross CO(2) fluxes between plants and the atmosphere, a critical element in understanding the response of the land biosphere to global change. *Here, we carried out controlled leaf-scale gas-exchange measurements of COS and CO(2) in representative C(3) plants under a range of light intensities, relative humidities and temperatures, CO(2) and COS concentrations, and following abscisic acid treatments. *No 'respiration-like' emission of COS or detectable compensation point, and no cross-inhibition effects between COS and CO(2) were observed. The mean ratio of COS to CO(2) assimilation flux rates, A(s)/A(c), was c. 1.4 pmol micromol(-1) and the leaf relative uptake (assimilation normalized to ambient concentrations, (A(s)/A(c))(C(a)(c)/C(a)(s))) was 1.6-1.7 across species and conditions, with significant deviations under certain conditions. Stomatal conductance was enhanced by increasing COS, which was possibly mediated by hydrogen sulfide (H(2)S) produced from COS hydrolysis, and a correlation was observed between A(s) and leaf discrimination against C(18)OO. *The results provide systematic and quantitative information necessary for the use of COS in photosynthesis and carbon-cycle research on the physiological to global scales.

Carbonic anhydrase metabolism is a key factor in the toxicity of CO2 and COS but not CS2 toward the flour beetle Tribolium castaneum [Coleoptera: Tenebrionidae]

The analogues carbon dioxide (CO(2)), carbonyl sulfide (COS) and carbon disulfide (CS(2)) have been useful as substrate probes for enzyme activities. Here we explored the affinity of the enzyme carbonic anhydrase for its natural substrate CO(2), as well as COS and CS(2) (1) by in vitro kinetic metabolism studies using pure enzyme and (2) through mortality bioassay of insects exposed to toxic levels of each of the gases during carbonic anhydrase inhibition. Hydrolysis of COS to form hydrogen sulfide was catalysed rapidly showing parameters K(m) 1.86 mM and K(cat) 41 s(-1) at 25 degrees C; however, the specificity constant (K(cat)/K(m)) was 4000-fold lower than the reported value for carbonic anhydrase-catalysed hydration of CO(2). Carbonic anhydrase-mediated CS(2) metabolism was a further 65,000-fold lower than COS. Both results demonstrate the deactivating effect toward the enzyme of sulfur substitution for oxygen in the molecule. We also investigated the role of carbonic anhydrases in CO(2), COS and CS(2) toxicity using a specific inhibitor, acetazolamide, administered to Tribolium castaneum (Herbst) larvae via the diet. CO(2) toxicity was greatly enhanced by up to seven-fold in acetazolamide-treated larvae indicating that carbonic anhydrases are a key protective enzyme in elevated CO(2) concentrations. Conversely, mortality was reduced by up to 12-fold in acetazolamide-treated larvae exposed to COS due to reduced formation of toxic hydrogen sulfide. CS(2) toxicity was unaffected by acetazolamide. These results show that carbonic anhydrase has a key role in toxicity of the substrates CO(2) and COS but not CS(2), despite minor differences in chemical formulae.

Inorganic carbon acquisition systems in cyanobacteria

This minireview focuses on the mechanism of inorganic carbon uptake in cyanobacteria and in particular the two CO(2)-uptake systems and two bicarbonate transporters recently identified in Synechocycstis PCC 6803, and their presence in other cyanobacterial strains.

Energy sources for HCO3- and CO2 transport in air-grown cells of synechococcus UTEX 625

Light-dependent inorganic C (Ci) transport and accumulation in air-grown cells of Synechococcus UTEX 625 were examined with a mass spectrometer in the presence of inhibitors or artificial electron acceptors of photosynthesis in an attempt to drive CO2 or HCO3- uptake separately by the cyclic or linear electron transport chains. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the cells were able to accumulate an intracellular Ci pool of 20 mm, even though CO2 fixation was completely inhibited, indicating that cyclic electron flow was involved in the Ci-concentrating mechanism. When 200 m N,N-dimethyl-p-nitrosoaniline was used to drain electrons from ferredoxin, a similar Ci accumulation was observed, suggesting that linear electron flow could support the transport of Ci. When carbonic anhydrase was not present, initial CO2 uptake was greatly reduced and the extracellular [CO2] eventually increased to a level higher than equilibrium, strongly suggesting that CO2 transport was inhibited and that Ci accumulation was the result of active HCO3- transport. With 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-treated cells, Ci transport and accumulation were inhibited by inhibitors of CO2 transport, such as COS and Na2S, whereas Li+, an HCO3--transport inhibitor, had little effect. In the presence of N,N-dimethyl-p-nitrosoaniline, Ci transport and accumulation were not inhibited by COS and Na2S but were inhibited by Li+. These results suggest that CO2 transport is supported by cyclic electron transport and that HCO3- transport is supported by linear electron transport.

Influx and efflux of inorganic carbon during steady-state photosynthesis of air-grown Anabaena variabilis

The inward and outward fluxes of inorganic carbon in illuminated cell suspensions of air-grown Anabaena variabilis were measured by mass spectrometry under conditions of inorganic carbon disequilibrium. The inclusion of 25 mM NaCl significantly enhanced both inward inorganic carbon influx during CO2 fixation and outward CO2 efflux when CO2 fixation was blocked by the Calvin cycle inhibitor, iodoacetamide. At low, steady-state concentrations of inorganic carbon (< 100μM), CO2 fixation was nearly entirely supported by HCO3− transport in the presence of 25 mM NaCl. At approximately 150 μM inorganic carbon, the contributions of CO2 and HCO3− transport to CO2 fixation were about equal. Above this, CO2 transport provided most of the substrate for CO2 fixation. The affinity (K0.5) of photosynthesizing cells for CO2, HCO3− and total inorganic carbon was determined and mean values of 1.7, 9.5, and 8.2 μM, respectively, were determined. Maximum rates of inward CO2 and HCO3− transport and CO2 fixation during steady state were 255.7, 307.3, and 329.1 μmol∙mg−1 Chl∙h−1, respectively. Permeability coefficients for CO2 of 9.8 × 10−8 m∙s−1 and 2.8 × 10−7 m∙s−1 were calculated for the plasma membrane and carboxysomal surface areas, respectively, from the dark efflux rates assuming an internal pH of 7.2. A permeability coefficient for HCO3− across the plasma membrane of 7.6 × 10−9 m∙s−1 was calculated from the dark inorganic carbon efflux corrected for the corresponding dark CO2 efflux. Sodium sulphide (Na2S, 200 μM) blocked CO2 transport. In the presence of 25 mM NaCl, net CO2 efflux was approximately seven times greater than in its absence, when CO2 transport and fixation were both blocked, indicating greater CO2 leakage as a result of larger internal inorganic carbon pools in the presence of NaCl. The rapidity and amount of C16O2 generated from the exchange of 18O from 18O-enriched HCO3− with water in cell suspensions suggested that the internal inorganic carbon pool may be rapidly equilibrated. Key words: Anabaena variabilis, CO2-concentrating mechanism, CO2 transport, HCO3− transport, CO2 efflux, permeability coefficient.

Measurement of the amount and isotopic composition of the CO2 released from the cyanobacterium Synechococcus UTEX 625 after rapid quenching of the active CO2 transport system

Air-grown cells of the cyanobacterium Synechococcus UTEX 625 were suspended in a cuvette connected to a mass spectrometer and supplied with H13C18O3− to investigate the intracellular interconversion between CO2 and HCO3− as determined from the isotopic composition of CO2 appearing in the extracellular medium under a wide variety of experimental conditions. Upon injection of H13C18O3− to the cell suspension in the light, the extracellular [13C16O2] increased. As the CO2 species were 13C labelled, this demonstrated that the 18O-depleted CO2 was originating from the added H13C18O3−. A comparison of the rates of 13C16O16O appearance in the medium with the formation of 13C16O16O from spontaneous dehydration–hydration in the extracellular medium in the presence of cells demonstrated that most of it had to originate from a series of intracellular dehydration–hydration cycles of H13C18O3− that had been recently transported into the cells. During the time course of the experiments both the m/z (mass to charge) = 49 (i.e., 13C18O18O) and 47 (i.e., 13C18O16O) signals decreased constantly, whereas the m/z = 45 signal (i.e.,13C16O2) always increased. Inhibiting CO2 fixation enhanced the amount of CO2 arising in the medium but did not change its isotopic composition, and the CO2 was always fully depleted of 18O. When the CO2 transport system was inhibited by darkening the cells, adding inhibitors such as Na2S or COS, or quenching the uptake of inorganic 13C with an excess of inorganic 12C, the magnitude of the extracellular [13C16O2] was increased but the CO2 species were still always depleted of 18O. Various incubation times of the illuminated cells in the presence of H13C18O3− were used to obtain a variety of internal Ci pool sizes. When the inhibitor (COS) was added, the amount of 13C16O2 arising during the response time of the mass spectrometer was equivalent to the amount of CO2 that would have been present in the whole cell if CO2 and HCO3− were in equilibrium throughout the entire cell volume, but it was at least 40 times higher than the amount of CO2 that would have been present in the cell if the CO2 was confined to the carboxysomes. Experiments were also conducted at pH 9.0 where the spontaneous rate of 13C16O2 production from H13C1803− dehydration–hydration would be negligible, and again the same features were observed. Results show that intracellular HCO3− and CO2 are in rapid equilibrium throughout the entire cell volume. Key words: Synechococcus UTEX 625, cyanobacteria, CO2 leakage, 18O exchange, active CO2 transport, carboxysomes, inorganic C concentrating mechanism.

Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO)

Production and consumption processes in soils contribute to the global cycles of many trace gases (CH4, CO, OCS, H2, N2O, and NO) that are relevant for atmospheric chemistry and climate. Soil microbial processes contribute substantially to the budgets of atmospheric trace gases. The flux of trace gases between soil and atmosphere is usually the result of simultaneously operating production and consumption processes in soil: The relevant processes are not yet proven with absolute certainty, but the following are likely for trace gas consumption: H2 oxidation by abiontic soil enzymes; CO cooxidation by the ammonium monooxygenase of nitrifying bacteria; CH4 oxidation by unknown methanotrophic bacteria that utilize CH4 for growth; OCS hydrolysis by bacteria containing carbonic anhydrase; N2O reduction to N2 by denitrifying bacteria; NO consumption by either reduction to N2O in denitrifiers or oxidation to nitrate in heterotrophic bacteria. Wetland soils, in contrast to upland soils are generally anoxic and thus support the production of trace gases (H2, CO, CH4, N2O, and NO) by anaerobic bacteria such as fermenters, methanogens, acetogens, sulfate reducers, and denitrifiers. Methane is the dominant gaseous product of anaerobic degradation of organic matter and is released into the atmosphere, whereas the other trace gases are only intermediates, which are mostly cycled within the anoxic habitat. A significant percentage of the produced methane is oxidized by methanotrophic bacteria at anoxic-oxic interfaces such as the soil surface and the root surface of aquatic plants that serve as conduits for O2 transport into and CH4 transport out of the wetland soils. The dominant production processes in upland soils are different from those in wetland soils and include H2 production by biological N2 fixation, CO production by chemical decomposition of soil organic matter, and NO and N2O production by nitrification and denitrification. The processes responsible for CH4 production in upland soils are completely unclear, as are the OCS production processes in general. A problem for future research is the attribution of trace gas metabolic processes not only to functional groups of microorganisms but also to particular taxa. Thus, it is completely unclear how important microbial diversity is for the control of trace gas flux at the ecosystem level. However, different microbial communities may be part of the reason for differences in trace gas metabolism, e.g., effects of nitrogen fertilizers on CH4 uptake by soil; decrease of CH4 production with decreasing temperature; or different rates and modes of NO and N2O production in different soils and under different conditions.

Carbon Oxysulfide Inhibition of the CO(2)-Concentrating Process of Unicellular Green Algae

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O(2) evolution, dissolved inorganic carbon accumulation, and photosynthetic CO(2) fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO(2)-concentrating process. This COS inhibition can be attributed to inhibition of external HCO(3) (-) conversion to CO(2) and OH(-) by the carbonic anhydrase component of an active CO(2) pump. At a low pH of 5 to 6, COS stimulated O(2) evolution during photosynthesis by algae with low CO(2) in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO(2). Although COS had less effect on HCO(3) (-) accumulation at pH 9 by a HCO(3) (-) pump in Scenedesmus, COS reduced O(2) evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO(2) and H(2)S, its inhibition of the CO(2) pump activity and photosynthesis is not accurate, when measured by O(2) evolution, by NaH(14)CO(3) accumulation, or by (14)CO(2) fixation.

High Affinity Transport of CO(2) in the Cyanobacterium Synechococcus UTEX 625

The active transport of CO(2) in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO(3) (-) transport. The substrate concentration required to give one half the maximum rate for whole cell CO(2) 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 CO(2) 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 O(2) evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO(2) transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO(2) transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO(2) transport was caused by HCO(3) (-) or by Na(+)-dependent HCO(3) (-) transport. However, (12)CO(2) readily interfered with (13)CO(2) transport. CO(2) transport and Na(+)-dependent HCO(3) (-) transport are separate, independent processes and the high affinity CO(2) transporter is not only responsible for the initial transport of CO(2) into the cell but also for scavenging any CO(2) that may leak from the cell during ongoing photosynthesis.

Carbon Oxysulfide Is an Inhibitor of Both CO(2) and HCO(3) Uptake in the Cyanobacterium Synechococcus PCC7942

Carbon oxysulfide (COS) was reinvestigated as an inhibitor of active inorganic carbon transport in cells of Synechococcus PCC7942 adapted to growth at low inorganic carbon. COS inhibited both CO(2) and HCO(3) (-) transport processes in a reversible (in the short term) and mixed competitive manner. The inhibition of COS was established using both silicone oil centrifugation experiments and O(2)-evolution studies. The K(i) for COS inhibition was 29 micromolar for CO(2) transport and 110 micromolar for HCO(3) (-) transport. These results support a model of inorganic carbon transport with a central CO(2) pump and an inducible HCO(3) (-) utilizing accessory protein which supplies CO(2) to the primary pump.

Properties of a Mutant from Synechocystis PCC6803 Resistant to Acetazolamide, an Inhibitor of Carbonic Anhydrase

A spontaneous mutant of the cyanobacterium Synechocystis PCC6803 was isolated for its resistance to acetazolamide, an inhibitor of carbonic anhydrase. The mutant showed a deficiency in oxygen exchange between CO(2) and H(2)O, a lower level of stable internal CO(2) pool and a decreased capacity to adapt its photosynthetic affinity under limited inorganic carbon regime. The initial rate of uptake of inorganic carbon was identical to that of wild-type cells. It is demonstrated that the mutation affects the carbonic anhydrase activity. This could result from either of two impairments: a deficiency in the enzyme activity detectable by mass spectrometric determinations, or a modification of the cellular compartment in which the enzyme is located, preventing its activity.

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus

Scenedesmus cells grown on high CO(2), when adapted to air levels of CO(2) for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO(2) from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O(2) evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO(2) pump with K(0.5) for O(2) evolution of less than 5 micromolar CO(2) was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO(3) (-) concentrating process between pH 8 to 10 lowered the K(0.5) (DIC) from 7300 micromolar HCO(3) (-) in high CO(2) grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO(3) (-) pump was inhibited by vanadate (K(i) of 150 micromolar), as if it involved an ATPase linked HCO(3) (-) transporter. The CO(2) pump was formed on low CO(2) by high-CO(2) grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO(3) (-) pump was partially activated on low CO(2) within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO(3) (-) pump at pH 9 had requirements similar to the activation of the CO(2) pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO(2)-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO(2) pump and in some algae an additional vanadate-sensitive plasmalemma HCO(3) (-) transporter for a HCO(3) (-) pump.

Salicylhydroxamic Acid (SHAM) Inhibition of the Dissolved Inorganic Carbon Concentrating Process in Unicellular Green Algae

Rates of photosynthetic O(2) evolution, for measuring K(0.5)(CO(2) + HCO(3) (-)) at pH 7, upon addition of 50 micromolar HCO(3) (-) to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K(1)(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO(2) uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O(2) evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO(3)), and the rate of (14)CO(2) fixation with 100 micromolar [(14)C] HCO(3) (-). Salicylhydroxamic acid inhibition of O(2) evolution and (14)CO(2)-fixation was reversed by higher levels of NaHCO(3). Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO(2) accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.

Selective and Reversible Inhibition of Active CO(2) Transport by Hydrogen Sulfide in a Cyanobacterium

The active transport of CO(2) in the cyanobacterium Synechococcus UTEX 625 was inhibited by H(2)S. Treatment of the cells with up to 150 micromolar H(2)S + HS(-) at pH 8.0 had little effect on Na(+)-dependent HCO(3) (-) transport or photosynthetic O(2) evolution, but CO(2) transport was inhibited by more than 90%. CO(2) transport was restored when H(2)S was removed by flushing with N(2). At constant total H(2)S + HS(-) concentrations, inhibition of CO(2) transport increased as the ratio of H(2)S to HS(-) increased, suggesting a direct role for H(2)S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H(2)S and Na(+) -dependent HCO(3) (-) transport, the extracellular CO(2) concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H(2)S. The inhibition of CO(2) transport, therefore, revealed an ongoing leakage from the cells of CO(2) which was derived from the intracellular dehydration of HCO(3) (-) which itself had been recently transported into the cells. Normally, leaked CO(2) is efficiently transported back into the cell by the CO(2) transport system, thus maintaining the extracellular CO(2) concentration near zero. It is suggested that CO(2) transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO(2) lost from the cell. A schematic model describing the relationship between the CO(2) and HCO(3) (-) transport systems is presented.