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

Membrane-Inlet Mass Spectrometry Enables a Quantitative Understanding of Inorganic Carbon Uptake Flux and Carbon Concentrating Mechanisms in Metabolically Engineered Cyanobacteria

Photosynthesis uses solar energy to drive inorganic carbon (Ci) uptake, fixation, and biomass formation. In cyanobacteria, Ci uptake is assisted by carbon concentrating mechanisms (CCM), and CO₂ fixation is catalyzed by RubisCO in the Calvin-Benson-Bassham (CBB) cycle. Understanding the regulation that governs CCM and CBB cycle activities in natural and engineered strains requires methods and parameters that quantify these activities. Here, we used membrane-inlet mass spectrometry (MIMS) to simultaneously quantify Ci concentrating and fixation processes in the cyanobacterium Synechocystis 6803. By comparing cultures acclimated to ambient air conditions to cultures transitioning to high Ci conditions, we show that acclimation to high Ci involves a concurrent decline of Ci uptake and fixation parameters. By varying light input, we show that both CCM and CBB reactions become energy limited under low light conditions. A strain over-expressing the gene for the CBB cycle enzyme fructose-bisphosphate aldolase showed higher CCM and carbon fixation capabilities, suggesting a regulatory link between CBB metabolites and CCM capacity. While the engineering of an ethanol production pathway had no effect on CCM or carbon fixation parameters, additional fructose-bisphosphate aldolase gene over-expression enhanced both activities while simultaneously increasing ethanol productivity. These observations show that MIMS can be a useful tool to study the extracellular Ci flux and how CBB metabolites regulate Ci uptake and fixation.

On the cradle of CCM research: discovery, development, and challenges ahead

Herein, 40 years after its discovery, I briefly and critically survey the development of ideas that propelled research on CO2-concentrating mechanisms (CCMs; a term proposed by Dean Price) of phytoplankton, mainly focusing on cyanobacteria. This is not a comprehensive review on CCM research, but a personal view on the past developments and challenges that lie ahead.

In Synechococcus sp. competition for energy between assimilation and acquisition of C and those of N only occurs when growth is light limited

The carbon-concentrating mechanisms (CCMs) of cyanobacteria counteract the low CO2 affinity and CO2:O2 selectivities of the Rubisco of these photolithotrophs and the relatively low oceanic CO2 availability. CCMs have a significant energy cost; if light is limiting, the use of N sources whose assimilation demands less energy could permit a greater investment of energy into CCMs and inorganic C (Ci) assimilation. To test this, we cultured Synechococcus sp. UTEX LB 2380 under either N or energy limitation, in the presence of NO3- or NH4+. When growth was energy-limited, NH4+-grown cells had a 1.2-fold higher growth rate, 1.3-fold higher dissolved inorganic carbon (DIC)-saturated photosynthetic rate, 19% higher linear electron transfer, 80% higher photosynthetic 1/K1/2(DIC), 2.0-fold greater slope of the linear part of the photosynthesis versus DIC curve, 3.5-fold larger intracellular Ci pool, and 2.3-fold higher Zn quota than NO3--grown cells. When energy was not limiting growth, there were not differences between NH4+- and NO3--grown cells, except for higher linear electron transfer and larger intracellular Ci pool.We conclude that, when energy limits growth, cells that use the cheaper N source divert energy from N assimilation to C acquisition and assimilation; this does not happen when energy is not limiting.

Inorganic carbon acquisition in the acid-tolerant alga Chlorella kessleri

The ability of the freshwater alga, Chlorella kessleri, to maintain a carbon concentrating mechanism when grown at acid pH was investigated. The alga grows over the pH range 4.0-9.0 and was found to take up bicarbonate and CO2 actively when grown at pH 6.0. However, when grown at acid pH (below 5.5), it does not have active CO2 uptake. The acidotolerant species maintained an internal pH of 6.1-7.5 over the external pH range 4.5-7.5, thus the pH difference between the cell interior and the external medium was large enough to allow for the diffusive uptake of CO2 at acid external pH. Mass spectrometric monitoring of O2 and CO2 fluxes by suspensions of C. kessleri, grown at acid pH, and maintained at pH 7.5 showed that the rates of O2 evolution did not exceed those of CO2 uptake. The final CO2 compensation concentrations of 14.0-17.7 µM reached by photosynthetic cells were above the CO2 equilibrium concentration in the external medium, indicating a lack of active CO2 uptake at acid pH. Chlorella kessleri accumulated CO2 with internal concentrations that were 9.9, 18.7 and 22.7-fold that of the external medium for cells grown, respectively, at pH 4.5, 5.0 and 5.5. The ability of C. kessleri cells to accumulate high intracellular concentrations of inorganic carbon at acid pH would provide a sufficiently high concentration of CO2 at the active site of Rubisco thus allowing the alga to maintain growth rates similar to those at alkaline pH.

Detection of a variable intracellular acid-labile carbon pool in Thalassiosira weissflogii (Heterokontophyta) and Emiliania huxleyi (Haptophyta) in response to changes in the seawater carbon system

Accumulation of an intracellular pool of carbon (C(i) pool) is one strategy by which marine algae overcome the low abundance of dissolved CO2 (CO2 (aq) ) in modern seawater. To identify the environmental conditions under which algae accumulate an acid-labile C(i) pool, we applied a (14) C pulse-chase method, used originally in dinoflagellates, to two new classes of algae, coccolithophorids and diatoms. This method measures the carbon accumulation inside the cells without altering the medium carbon chemistry or culture cell density. We found that the diatom Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle] and a calcifying strain of the coccolithophorid Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler] develop significant acid-labile C(i) pools. C(i) pools are measureable in cells cultured in media with 2-30 µmol l(-1) CO2 (aq), corresponding to a medium pH of 8.6-7.9. The absolute C(i) pool was greater for the larger celled diatoms. For both algal classes, the C(i) pool became a negligible contributor to photosynthesis once CO2 (aq) exceeded 30 µmol l(-1) . Combining the (14) C pulse-chase method and (14) C disequilibrium method enabled us to assess whether E. huxleyi and T. weissflogii exhibited thresholds for foregoing accumulation of DIC or reduced the reliance on bicarbonate uptake with increasing CO2 (aq) . We showed that the C(i) pool decreases with higher CO2 :HCO3 (-) uptake rates.

Inactivation of a low temperature-induced RNA helicase in Synechocystis sp. PCC 6803: physiological and morphological consequences

Inactivation of the DEAD box RNA helicase, crhR, has dramatic effects on the physiology and morphology of the photosynthetic cyanobacterium, Synechocystis sp. PCC 6803. These effects are observed at both normal growth temperature (30°C) and under cold stress (20°C), indicating that CrhR performs crucial function(s) at all temperatures. A major physiological effect is the rapid cessation of photosynthesis upon temperature downshift from 30 to 20°C. This defect does not originate from an inability to transport or accumulate inorganic carbon or a deficiency in photosynthetic capacity as the mutant has sufficient electron transport and enzymatic capacity to sustain photosynthesis at 30°C and inorganic carbon (Ci) accumulation at 20°C. Oxygen consumption in the presence of methyl viologen indicated that while electron transport capacity is sufficient to accumulate Ci, the mutant does not possess sufficient activity to sustain carbon fixation at maximal rates. These defects are correlated with severely impaired cell growth and decreased viability, cell size and DNA content at low temperature. The ΔcrhR mutant also progressively accumulates structural abnormalities at low temperature that cannot be attributed solely to reactive oxygen species (ROS)-induced photooxidative damage, suggesting that they are manifestations of pre-existing defects that are amplified over time. The data indicate that the observed physiological and morphological effects are intimately related to crhR mutation, implying that the lack of CrhR RNA unwinding/annealing activity results in the inability to execute one or more vital steps in photosynthesis that are required at all temperatures but are crucial at low temperature.

Physiological characterization and light response of the CO2-concentrating mechanism in the filamentous cyanobacterium Leptolyngbya sp. CPCC 696

We studied the interactions of the CO(2)-concentrating mechanism and variable light in the filamentous cyanobacterium Leptolyngbya sp. CPCC 696 acclimated to low light (15 μmol m(-2) s(-1) PPFD) and low inorganic carbon (50 μM Ci). Mass spectrometric and polarographic analysis revealed that mediated CO(2) uptake along with both active Na(+)-independent and Na(+)-dependent HCO(3)(-) transport, likely through Na(+)/HCO(3)(-) symport, were employed to concentrate Ci internally. Combined transport of CO(2) and HCO(3)(-) required about 30 kJ mol(-1) of energy from photosynthetic electron transport to support an intracellular Ci accumulation 550-fold greater than the external Ci. Initially, Leptolyngbya rapidly induced oxygen evolution and Ci transport to reach 40-50% of maximum values by 50 μmol m(-2) s(-1) PPFD. Thereafter, photosynthesis and Ci transport increased gradually to saturation around 1,800 μmol m(-2) s(-1) PPFD. Leptolyngbya showed a low intrinsic susceptibility to photoinhibition of oxygen evolution up to PPFD of 3,000 μmol m(-2) s(-1). Intracellular Ci accumulation showed a lag under low light but then peaked at about 500 μmol photons m(-2) s(-1) and remained high thereafter. Ci influx was accompanied by a simultaneous, light-dependent, outward flux of CO(2) and by internal CO(2)/HCO(3)(-) cycling. The high-affinity and high-capacity CCM of Leptolyngbya responded dynamically to fluctuating PPFD and used excitation energy in excess of the needs of CO(2) fixation by increasing Ci transport, accumulation and Ci cycling. This capacity may allow Leptolyngbya to tolerate periodic exposure to excess high light by consuming electron equivalents and keeping PSII open.

Evidence for the occurrence of photorespiration in synurophyte algae

The fluxes of CO(2) and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO(2,) only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa, when O(2) evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO(2), indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO(2) was observed similar to the post-illumination burst characteristic of C(3) higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO(2) burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O(2). This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O(2) evolution rates in all four algae, measured by O(2) electrode, were significantly higher (40-50%) in media at low O(2) (4%) than in air-equilibrated (21% O(2)) media, indicating an O(2) inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.

Inorganic carbon acquisition in two green marine Stichococcus species

The mechanism of inorganic carbon (C(i)) uptake was examined in the marine green microalgae Stichococcus cylindricus and Stichococcus minor. External carbonic anhydrase (CA) activity was not detected in either species, by potentiometric assay or by mass spectrometry. Photosynthetic characteristics of C(i) uptake indicate that both species have high apparent affinity for CO(2) with a low K(1/2) (CO(2)) of about 10 µm. The O(2) evolution rates in light exceeded the spontaneous CO(2) formation rate by 2.5-fold in both species, which thus have active bicarbonate uptake. Mass spectrometric monitoring of CO(2) and O(2) fluxes showed that rates of O(2) evolution exceeded those of CO(2) depletion by about three- and twofold in S. minor and S. cylindricus, respectively, and also showed, in cells photosynthesizing at pH 8.2, a rapid depletion of CO(2) upon illumination to a CO(2) compensation concentration of 15.42 and 12.03 µm in S. minor and S. cylindricus, respectively. Both species also exhibit active CO(2) uptake: addition of bovine CA at CO(2) compensation concentration caused a rapid rise in CO(2) as the CO(2) -HCO(3) (-) equilibrium was restored. Accumulation of unfixed C(i) by cells at pH 8.2 was calculated to be 84.33 mm in S. cylindricus, and 30.37 mm in S. minor to give internal accumulations of 23- and 8-fold, respectively, compared to the external C(i) concentration.

PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID-TOLERANT, FREE-LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)(1)

The processes of CO2 acquisition were characterized for the acid-tolerant, free-living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO3 (-) uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO2 formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO2 uptake and O2 evolution by membrane-inlet mass spectrometry demonstrated that air-grown cells reduced the CO2 concentration in the medium to an equilibrium concentration of 15 μM, but AZA-treated cells caused a drop in extracellular CO2 concentration to a compensation concentration of 27 μM at pH 8.0. CO2 -pulsing experiments with cells in the light indicated that the cells do not actively take up CO2 . An internal pool of unfixed inorganic carbon was not detected at the CO2 compensation concentration, probably because of the lack of active CO2 uptake, but was detectable at times before compensation point was reached. These results indicate that this free-living Coccomyxa possesses a CO2 -concentrating mechanism (CCM) due to an active bicarbonate-uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.

Inorganic carbon acquisition in some synurophyte algae

Some characteristics of photosynthesis of three synurophyte algae, Synura petersenii, Synura uvella and Tessellaria volvocina were investigated to determine the mechanism of inorganic carbon (C(i)) uptake. All three species were found to have no external carbonic anhydrase, no capacity for direct bicarbonate uptake and a low whole-cell affinity for C(i). The internal pH of S. petersenii determined using (14)C-benzoic acid and [2-(14)C]-5,5-dimethyloxazolidine-2,4-dione was pH 7.0-7.5, over an external pH range of 5.0-7.5. Thus, the pH difference between the cell interior of S. petersenii and the external medium was large enough, over the alga's growth range, to allow the accumulation of C(i) by the diffusive uptake of CO(2). Monitoring O(2) evolution and CO(2) uptake by suspensions of S. petersenii at pH 7.0 by mass spectrometry did not indicate a rapid uptake of CO(2), and the final CO(2) compensation concentration reached was 24 +/- 0.7 microM. Furthermore, when the cells were darkened, a brief burst of CO(2) occurred before a steady rate of dark respiration was established, suggesting a loss of CO(2) by photorespiration. An examination of the kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in homogenates of cells of S. petersenii, S. uvella and Mallomonas papillosa showed that values of the K(m) (CO(2)) were 28.4, 41.8 and 18.2 microM, respectively. These species lack the characteristics of cells with a CO(2)-concentrating mechanism because the cell affinity for C(i) appears to be determined by the relatively high CO(2) affinity of the Rubisco of these algae.

Mechanism of CO2 acquisition in an acid-tolerant Chlamydomonas

The acid-tolerant green alga Chlamydomonas (UTCC 121) grows in media ranging in pH from 2.5 to 7.0. Determination of the overall internal pH of the cells, using (14)C-benzoic acid (BA) or [2-(14)C]-5,5-dimethyloxazolidine-2,4-dione (DMO), showed that the cells maintain a neutral pH (6.6 to 7.2) over an external pH range of 3.0-7.0. The cells express an external carbonic anhydrase (CA) when grown in media above pH 5.5, and CA increases to a maximum at pH 7.0. Removal of external CA by trypsin digestion or by acetazolamide (AZA) inhibition indicated that CA was essential for photosynthesis at pH 7.0 and that the cells had no capacity for direct bicarbonate uptake. Monitoring of CO(2) uptake and O(2) evolution by mass spectrometry during photosynthesis did not provide any evidence of active CO(2) uptake. The CO(2) compensation concentration of the cells ranged from 9.4 microM at pH 4.5 to 16.2 microM at pH 7.0. An examination of the kinetics of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), in homogenates of cells grown at pH 7.0, showed that the K(m) (CO(2)) was 16.3 microM. These data indicate that the pH between the cell interior and the external medium was large enough at acid pH to allow the accumulation of inorganic carbon (Ci) by the diffusive uptake of CO(2), and the expression of external CA at neutral pH values would maintain an equilibrium CO(2) concentration at the cell surface. This species does not possess a CO(2)-concentrating mechanism because the whole cell affinity for Ci appears to be determined by the low K(m) (CO(2)) Rubisco of the alga.

Involvement of the cynABDS operon and the CO2-concentrating mechanism in the light-dependent transport and metabolism of cyanate by cyanobacteria

The cyanobacteria Synechococcus elongatus strain PCC7942 and Synechococcus sp. strain UTEX625 decomposed exogenously supplied cyanate (NCO-) to CO2 and NH3 through the action of a cytosolic cyanase which required HCO3- as a second substrate. The ability to metabolize NCO- relied on three essential elements: proteins encoded by the cynABDS operon, the biophysical activity of the CO2-concentrating mechanism (CCM), and light. Inactivation of cynS, encoding cyanase, and cynA yielded mutants unable to decompose cyanate. Furthermore, loss of CynA, the periplasmic binding protein of a multicomponent ABC-type transporter, resulted in loss of active cyanate transport. Competition experiments revealed that native transport systems for CO2, HCO3-, NO3-, NO2-, Cl-, PO4(2-), and SO4(2-) did not contribute to the cellular flux of NCO- and that CynABD did not contribute to the flux of these nutrients, implicating CynABD as a novel primary active NCO- transporter. In the S. elongatus strain PCC7942 DeltachpX DeltachpY mutant that is defective in the full expression of the CCM, mass spectrometry revealed that the cellular rate of cyanate decomposition depended upon the size of the internal inorganic carbon (Ci) (HCO3- + CO2) pool. Unlike wild-type cells, the rate of NCO- decomposition by the DeltachpX DeltachpY mutant was severely depressed at low external Ci concentrations, indicating that the CCM was essential in providing HCO3- for cyanase under typical growth conditions. Light was required to activate and/or energize the active transport of both NCO- and Ci. Putative cynABDS operons were identified in the genomes of diverse Proteobacteria, suggesting that CynABDS-mediated cyanate metabolism is not restricted to cyanobacteria.

Mechanisms of inorganic carbon acquisition in twoEuglenaspecies

The mechanism of inorganic carbon uptake was examined in Euglena gracilis Klebs. and the acidophilic species Euglena mutabilis Schmitz. Both species, whether grown in acidic (pH 3.5) or alkaline (pH 7.5) media lack external carbonic anhydrase. Acid-grown E. gracilis was shown to have no capacity for bicarbonate transport, but transport was induced on transfer to alkaline medium (pH 7.5) in the light over a period of 8 h. In contrast, acid-grown E. mutabilis appears to have no capacity for bicarbonate transport even at neutral pH. The overall internal pH of the cells was determined by equilibration with14C-labelled benzoic acid over the pH range 3.5–5.0 and with14C-labelled 5,5-dimethyloxazolidine-2,4-dione over the range pH 5.5–7.5. The acidophilic species maintains an internal pH range of 6.6–6.8 in an external pH range of 3.5–5.5, whereas the acid-tolerant species E. gracilis maintains a neutral internal pH in an external pH range of 3.5–7.5. Measurement, by mass spectrometry, of the fluxes of CO2and O2in photosynthesizing cells at pH 3.5 demonstrated a rapid uptake of CO2by both species that was completely blocked by iodoacetamide, an inhibitor of CO2fixation. Uptake of CO2by E. gracilis, grown at pH 7.5, was not completely inhibited by iodoacetamide and O2evolution was sustained when the cells reached the CO2compensation concentration, indicating a direct uptake of bicarbonate. These data indicate that the acidophilic species, E. mutabilis, takes up CO2by diffusion, whereas the acid-tolerant species, E. gracilis, takes up CO2by diffusion at acid pH levels but has some capacity for active bicarbonate uptake when grown at alkaline pH levels.Key words: acidophilic alga, acidotolerant alga, bicarbonate uptake, CO2uptake, Euglena gracilis, Euglena mutabilis, internal pH.

Photosynthetic utilisation of inorganic carbon and its regulation in the marine diatom Skeletonema costatum

Photosynthetic uptake of inorganic carbon and regulation of photosynthetic CO₂ affinity were investigated in Skeletonema costatum (Grev.) Cleve. The pH independence of K_1/2(CO₂) values indicated that algae grown at either ambient (12 μmol L^-1) or low (3 μmol L^-1) CO₂ predominantly took up CO₂ from the medium. The lower pH compensation point (9.12) and insensitivity of photosynthetic rate to di-isothiocyanatostilbene disulfonic acid (DIDS) indicated that the alga had poor capacity for direct HCO₃^- utilisation. Photosynthetic CO₂ affinity is regulated by the concentration of CO₂ rather than HCO₃^-, CO₃^2- or total dissolved inorganic carbon (DIC) in the medium. The response of photosynthetic CO₂ affinity to changes in CO₂ concentration was most sensitive within the range 3-48 μmol L^-1 CO₂. Light was required for the induction of photosynthetic CO₂ affinity, but not for its repression, when cells were shifted between high (126 μmol L^-1) and ambient (12 μmol L^-1) CO₂. The time needed for cells grown at high CO₂ (126 μmol L^-1) to fully develop photosynthetic CO₂ affinity at ambient CO₂ was approximately 2 h, but acclimation to low or very low CO₂ levels (3 and 1.3 μmol L^-1, respectively) took more than 10 h. Cells grown at low CO₂ (3 μmol L^-1) required approximately 10 h for repression of all photosynthetic CO₂ affinity when transferred to ambient or high CO₂ (12 or 126 μmol L^-1, respectively), and more than 10 h at very high CO₂ (392 μmol L^-1).

Massive light-dependent cycling of inorganic carbon between oxygenic photosynthetic microorganisms and their surroundings

Membrane inlet mass spectrometry indicated massive light-dependent cycling of inorganic carbon between the medium and the cells of various phytoplankton species representing the main groups of aquatic primary producers. These included diatoms, symbiotic and free living dinoflagellates, a coccolithophorid, a green alga and filamentous and single cell cyanobacteria. These organisms could maintain an ambient CO(2) concentration substantially above or below that expected at chemical equilibrium with HCO(3) (-). The coccolithophorid Emiliania huxleyi shifted from net CO(2) uptake to net CO(2) efflux with rising light intensity. Differing responses of CO(2) uptake and CO(2) fixation to changing light intensity supported the notion that these two processes are not compulsorily linked. Simultaneous measurements of CO(2) and O(2) exchange and of the fluorescence parameters in Synechococcus sp. strain PCC 7942, showed that CO(2) uptake can serve as a sensitive probe of the energy status of the photosynthetic reaction centers. However, during transitions in light intensity, changes in CO(2) uptake did not accord with those expected from fluorescence change. Quantification of the net fluxes of CO(2), HCO(3) (-) and of photosynthesis at steady-state revealed that substantial HCO(3) (-) efflux accompanied CO(2) uptake and fixation in the case of 'CO(2) users'. On the other hand, 'HCO(3) (-) users' were characterized by a rate of net CO(2) uptake below that of CO(2) fixation. The results support the notion that entities associated with the CCM function not only in raising the CO(2) concentration at the site of Rubisco; they may also serve as a means of diminishing photodynamic damage by dissipating excess light energy.

Passive entry of CO2 and its energy-dependent intracellular conversion to HCO3- in cyanobacteria are driven by a photosystem I-generated deltamuH+

CO(2) entry into Synechococcus sp. PCC7942 cells was drastically inhibited by the water channel blocker p-chloromercuriphenylsulfonic acid suggesting that CO(2) uptake is, for the most part, passive via aquaporins with subsequent energy-dependent conversion to HCO3(-). Dependence of CO(2) uptake on photosynthetic electron transport via photosystem I (PSI) was confirmed by experiments with electron transport inhibitors, electron donors and acceptors, and a mutant lacking PSI activity. CO(2) uptake was drastically inhibited by the uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP) and ammonia but substantially less so by the inhibitors of ATP formation arsenate and N, N,-dicyclohexylcarbodiimide (DCCD). Thus a DeltamuH(+) generated by photosynthetic PSI electron transport apparently serves as the direct source of energy for CO(2) uptake. Under low light intensity, the rate of CO(2) uptake by a high-CO(2)-requiring mutant of Synechococcus sp. PCC7942, at a CO(2) concentration below its threshold for CO(2) fixation, was higher than that of the wild type. At saturating light intensity, net CO(2) uptake was similar in the wild type and in the mutant IL-3 suggesting common limitation by the rate of conversion of CO(2) to HCO3(-). These findings are consistent with a model postulating that electron transport-dependent formation of alkaline domains on the thylakoid membrane energizes intracellular conversion of CO(2) to HCO3(-).

The photoreduction of H(2)O(2) by Synechococcus sp. PCC 7942 and UTEX 625

It has been claimed that the sole H(2)O(2)-scavenging system in the cyanobacterium Synechococcus sp. PCC 7942 is a cytosolic catalase-peroxidase. We have measured in vivo activity of a light-dependent peroxidase in Synechococcus sp. PCC 7942 and UTEX 625. The addition of small amounts of H(2)O(2) (2.5 microM) to illuminated cells caused photochemical quenching (qP) of chlorophyll fluorescence that was relieved as the H(2)O(2) was consumed. The qP was maximal at about 50 microM H(2)O(2) with a Michaelis constant of about 7 microM. The H(2)O(2)-dependent qP strongly indicates that photoreduction can be involved in H(2)O(2) decomposition. Catalase-peroxidase activity was found to be almost completely inhibited by 10 microM NH(2)OH with no inhibition of the H(2)O(2)-dependent qP, which actually increased, presumably due to the light-dependent reaction now being the only route for H(2)O(2)-decomposition. When (18)O-labeled H(2)O(2) was presented to cells in the light there was an evolution of (16)O(2), indicative of H(2)(16)O oxidation by PS 2 and formation of photoreductant. In the dark (18)O(2) was evolved from added H(2)(18)O(2) as expected for decomposition by the catalase-peroxidase. This evolution was completely blocked by NH(2)OH, whereas the light-dependent evolution of (16)O(2) during H(2)(18)O(2) decomposition was unaffected.

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.

Inorganic Carbon Accumulation Stimulates Linear Electron Flow to Artificial Electron Acceptors of Photosystem I in Air-Grown Cells of the Cyanobacterium Synechococcus UTEX 625

The effect of inorganic carbon (Ci) transport and accumulation on photosynthetic electron transport was studied in air-grown cells of the cyanobacterium Synechococcus UTEX 625. When the cells were depleted of Ci, linear photosynthetic electron flow was almost completely inhibited in the presence of the photosystem I (PSI) acceptor N,N-dimethyl-p-nitrosoaniline (PNDA). The addition of Ci to these cells, in which CO2 fixation was inhibited with glycolaldehyde, greatly stimulated linear electron flow and resulted in increased levels of photochemical quenching and O2 evolution. In aerobic conditions substantial quenching resulted from methyl viologen (MV) addition and further quenching was not observed upon the addition of Ci. In anaerobic conditions MV addition did not result in quenching until Ci was added. Intracellular Ci pools were formed when MV was present in aerobic or anaerobic conditions or PNDA was present in aerobic conditions. There was no inhibitory effect of Ci depletion on electron flow to 2,6-dimethylbenzoquinone and oxidized diaminodurene, which accept electrons from photosystem II. The degree of stimulation of PNDA-dependent O2 evolution varied with the Ci concentration. The extracellular Ci, concentration required for a half-maximum rate (K1/2) was 3.8 [mu]M and the intracellular K1/2 was 1.4 mM for the stimulation of PNDA reduction. These values agreed closely with the K1/2 values of extracellular and intracellular Ci for O2 photoreduction. Linear electron flow to artificial electron acceptors of PSI was enhanced by intracellular Ci, which appeared to exert an effect on PSI or on the intersystem electron transport chain.

Effect of the intracellular inorganic carbon pool on chlorophyll a fluorescence quenching and O2 photoreduction in air-grown cells of the cyanobacterium Synechococcus UTEX 625

Simultaneous measurements were made of O2 exchange, inorganic carbon (Ci) accumulation and assimilation, and chlorophyll a fluorescence of the cyanobacterium Synechococcus UTEX 625. The addition of Ci to cells at the CO2 compensation point resulted in quenching of chlorophyll a fluorescence in the presence or absence of the CO2 fixation inhibitor, iodoacetamide. The magnitude of quenching was related to electron flow to terminal electron acceptors such as CO2 and O2. When photosynthetic CO2 fixation was allowed, the rate of electron transport, as expressed by (F*m – F)/F*m, was highly correlated with the onset of photosynthesis. When CO2 fixation was inhibited by the addition of iodoacetamide, the observed fluorescence quenching was consistent with the enhanced rate of O2 photoreduction that occurred when Ci was added. There was a close correlation (r = 0.98) between the magnitude of O2-dependent fluorescence quenching and the amount of O2 photoreduction. The degree of stimulation of electron flow to O2 photoreduction was dependent on the inorganic carbon concentration. The K1/2 (Ci) for extracellular Ci was 1.36 ± 0.13 μM (mean ± SD, n = 3) and K1/2 (Ci) for the intracellular Ci pool was 1.4 ± 0.18 mM (mean ± SD, n = 3). The reduction of N,N-dimethyl-p-nitrosoaniline was also stimulated by the addition of Ci, whereas the addition of Ci had no effect on the reduction of 2,6-dimethylbenzoquinone and ferricyanide. The results suggest that Ci stimulates electron flow in photosystem I. Key words: cyanobacteria, O2 photoreduction, fluorescence, Ci concentrating mechanism, inorganic carbon pool, linear electron transport, kinetic study.

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.

Oxygen photoreduction and its effect on CO2 accumulation and assimilation in air-grown cells of Synechococcus UTEX 625

Mass spectrometric measurements of 16O2, 18O2, and 13CO2 were used to measure the rates of gross O2 evolution, O2 uptake, and CO2 assimilation in relation to light intensity, temperature, pH, and O2 concentration by air-grown cells of the cyanobacterium Synechococcus UTEX 625. CO2 fixation and O2 photoreduction increased with increased light intensity and, although CO2 fixation was saturated at 250 μmol ∙ m−2 ∙ s−1, O2 photoreduction was not saturated until about 550 μmol ∙ m−2 ∙ s−1. At high light intensity addition of inorganic carbon to the cells stimulated O2 photoreduction 2-fold when CO2, fixation was allowed and 5-fold when CO2, fixation was inhibited with iodoacetamide. The ability of O2, to act as an acceptor of photosynthetically generated reducing power was dependent upon the O2 concentration, and the substrate concentration required for half maximum rate (K½(O2)) was 53.2 ± 4.2 μM (mean ± SD, n = 3). The Q10 for oxygen photoreduction was about 2. A certain amount (10%) of O2 appeared to be required for maximum photosynthesis, as photosynthesis was inhibited under anaerobic conditions, especially at high light intensity. The point of inhibition is unknown but it seemed unlikely to be on CO2 transport or the concentration of intracellular dissolved inorganic carbon (Ci), as the rate of initial CO2 transport was enhanced and the intracellular Q1 pool increased in size under anaerobic conditions. Key words: cyanobacteria, photosynthesis, Ci concentrating mechanism, inorganic carbon pool, O2 photoreduction, electron transport, temperature.

Ethoxyzolamide Differentially Inhibits CO2 Uptake and Na+-Independent and Na+-Dependent HCO3- Uptake in the Cyanobacterium Synechococcus sp. UTEX 625

The effects of ethoxyzolamide (EZ), a carbonic anhydrase inhibitor, on the active CO2 and Na+-independent and Na+-dependent HCO3- transport systems of the unicellular cyanobacterium Synechococcus sp. UTEX 625 were examined. Measurements of transport and accumulation using radiochemical, fluorometric, and mass spectrometric assays indicated that active CO2 transport and active Na+-independent HCO3- transport were inhibited by EZ. However, Na+-independent HCO3- transport was about 1 order of magnitude more sensitive to EZ inhibition than was CO2 transport (50% inhibition = 12 [mu]M versus 80 [mu]M). The data suggest that both the active CO2 (G.D. Price, M.R. Badger [1989] Plant Physiol 89: 37-43) and the Na+ -independent HCO3 - transport systems possessed carbonic anhydrase-like activity as part of their mechanism of action. In contrast, Na+-dependent HCO3- transport was only partially (50% inhibition = 230 [mu]M) and noncompetitively inhibited by EZ. The collective evidence suggested that EZ inhibition of Na+ -dependent HCO3- transport was an indirect consequence of the action of EZ on the CO2 transport system, rather than a direct effect on HCO3- transport. A model is presented in which the core of the inorganic carbon translocating system is formed by Na+-dependent HCO3- transport and the CO2 transport system. It is argued that the Na+-independent HCO3 - utilizing system was not directly involved in translocation, but converted HCO3- to CO2 for use in CO2 transport.

Quantitative evaluation of the role of a putative CO2-scavenging entity in the cyanobacterial CO2-concentrating mechanism

This paper assesses the contribution of a postulated CO2-scavenging system to the efficient operation of the CO2-concentrating mechanism (CCM) in cyanobacteria. A quantitative model for the CCM is presented which incorporates an energy-dependent carbonic anhydrase-like entity located at or near the inner surface of the plasma membrane. This entity, which converts CO2 to HCO3- against the thermodynamic potential, scavenges CO2 leaking outward from the carboxysomes, and, further, converts CO2 entering from the medium to HCO3-, thus maintaining an inward diffusion gradient along which CO2 enters passively. The model resembles our earlier models in postulating that CO2 and HCO3- are not at equilibrium throughout the greater part of the cell, and that CO2 is generated in high concentration at carbonic anhydrase sites within the carboxysomes. The model further takes into account the concentric thylakoid membranes which surround the carboxysomes, and events in the periplasmic space and the unstirred layer surrounding the cell. Implications of the predicted steady state fluxes of CO2 and HCO3-, and of their steady state concentrations in various cellular compartments, are discussed. The plasma membrane carbonic anhydrase-like activity lowers the photosynthetic Km for external Ci, as well as decreasing the inorganic C 'leak', but it may not save on energy expenditure.

Photosynthetic Nitrite Reduction as Influenced by the Internal Inorganic Carbon Pool in Air-Grown Cells of Synechococcus UTEX 625

Photosynthetic reduction of NO2- was studied in air-grown cells of a cyanobacterium, Synechococcus UTEX 625. Addition of NO2- resulted in significant amounts of chlorophyll a fluorescence quenching both in the absence and presence of CO2, fixation inhibitors, glycolaldehyde or iodoacetamide. The degree of NO2- quenching was insensitive to the O2 concentration in the medium. Addition of 100 [mu]M inorganic carbon in the presence of glycolaldehyde and O2, leading to formation of the carbon pool within the cells, resulted in pronounced fluorescence quenching. Removal of O2 from the medium restored the fluorescence yield completely, and the subsequent addition of NO2- quenched 36% of the variable fluorescence. From the response to added 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the quenching by NO2- appeared to be photochemical quenching, and nonphotochemical quenching did not seem to be present. The reduction of NO2- observed on its addition to inorganic carbon-depleted cells remained uninfluenced by O2 or glycolaldehyde. The internal inorganic carbon pool in the cells stimulated NO2- reduction, both in the presence and absence of O2, by 4.8-fold. An increase in NO2- reduction by 0.5-fold was also observed in the presence of O2 during simultaneous assimilation of carbon and nitrogen in inorganic carbon-depleted cells. Contrary to this, under anaerobiosis, NO2- reduction was suppressed when carbon and nitrogen assimilation occurred together.

Active uptake of CO2 during photosynthesis in the green alga Eremosphaera viridis is mediated by a CO2-ATPase

Mass spectrometry was used to investigate the uptake of CO2 in Eremosphaera viridis DeBary. Upon illumination, cells preincubated at pH 7.5 with 100 μM dissolved inorganic carbon (DIC) rapidly depleted almost all the free CO2 from the medium. Rapid equilibrium between HCO 3 (-) and CO2 occurred upon addition of bovine carbonic anhydrase (CA) to the medium, showing that CO2 depletion resulted from a selective uptake of CO2 rather than an uptake of all inorganic carbon species. Glycolaldehyde (10 mM) completely inhibited CO2 fixation but had little effect on CO2 transport. Transfer of glycolaldehyde-treated cells to the dark caused a rapid efflux of CO2 from the unfixed intracellular DIC pool which was found to be at least threeto sixfold higher in concentration than that of the external medium. These results indicate that E. viridis actively transports CO2 against a concentration gradient. No external CA was detected in these cells either by potentiometric or mass-spectrometric assay. In the absence of external CA, the rate of photosynthetic O2 evolution in the pH range 7.5 to 8.0 did not exceed the calculated rate of CO2 supply, indicating a limited capacity for HCO2 uptake in these cells. Electrophysiological measurements indicate that CO2 uptake is electrically silent and thus is not a consequence of H(+)-CO2 symport activity. Microsomal membranes isolated from Eremosphaera showed ATPase activity which was enhanced by CO2. These results indicate that active CO2 uptake is mediated by an ATPase.

Bicarbonate Concentration by Synechocystis PCC6803 : Modulation of Protein Phosphorylation and Inorganic Carbon Transport by Glucose

The ability of the cyanobacterium Synechocystis PCC6803 to transport inorganic carbon in the form of bicarbonate rapidly decreased following a shift from bicarbonate-limited growth to either excess bicarbonate supply or to photoheterotrophic growth on glucose. Nonmetabolizable analogs of glucose did not exert this effect. The rate at which the bicarbonate uptake rate declined was too rapid to be accounted for by dilution of the activity by culture growth and suggested that posttranslational modification may be involved. Several proteins that were unphosphorylated during bicarbonate-limited growth became phosphorylated during the shifts to high CO(2) conditions and to photoheterotrophic growth. A similar alteration in the profile of phosphopolypeptides was observed following a shift into the dark. The changes in protein phosphorylation were not blocked by chloramphenicol or rifampicin.

Na-Independent HCO(3) Transport and Accumulation in the Cyanobacterium Synechococcus UTEX 625

The active transport and intracellular accumulation of HCO(3) (-) by air-grown cells of the cyanobacterium Synechococcus UTEX 625 (PCC 6301) was strongly promoted by 25 millimolar Na(+).Na(+)-dependent HCO(3) (-) accumulation also resulted in a characteristic enhancement in the rate of photosynthetic O(2) evolution and CO(2) fixation. However, when Synechococcus was grown in standing culture, high rates of HCO(3) (-) transport and photosynthesis were observed in the absence of added Na(+). The internal HCO(3) (-) pool reached levels up to 50 millimolar, and an accumulation ratio as high as 970 was observed. Sodium enhanced HCO(3) (-) transport and accumulation in standing culture cells by about 25 to 30% compared with the five- to eightfold enhancement observed with air-grown cells. The ability of standing culture cells to utilize HCO(3) (-) from the medium in the absence of Na(+) was lost within 16 hours after transfer to air-grown culture and was reacquired during subsequent growth in standing culture. Studies using a mass spectrometer indicated that standing culture cells were also capable of active CO(2) transport involving a high-affinity transport system which was reversibly inhibited by H(2)S, as in the case for air-grown cells. The data are interpreted to indicate that Synechococcus possesses a constitutive CO(2) transport system, whereas Na(+)-dependent and Na(+)-independent HCO(3) (-) transport are inducible, depending upon the conditions of growth. Intracellular accumulation of HCO(3) (-) was always accompanied by a quenching of chlorophyll a fluorescence which was independent of CO(2) fixation. The extent of fluorescence quenching was highly dependent upon the size of the internal pool of HCO(3) (-) + CO(2). The pattern of fluorescence quenching observed in response to added HCO(3) (-) and Na(+) in air-grown and standing culture cells was highly characteristic for Na(+)-dependent and Na(+)-independent HCO(3) (-) accumulation. It was concluded that measurements of fluorescence quenching provide an indirect means for following HCO(3) (-) transport and the dynamics of intracellular HCO(3) (-) accumulation and dissipation.

Identification and localization of carbonic anhydrase in two chlorella species

Carbonic anhydrase (CA) activity and localization have been examined in two species of the eukaryotic green alga Chlorella. Mass spectrometric and potentiometric assays of CA activity indicate that C. ellipsoidea contains very little extracellular CA activity whereas C. saccharophila exhibits significant extracellular activity when grown at alkaline pH values. Extracellular CA activity appears to be correlated with the presence of a 36 kilodalton polypeptide that was detected immunologically using a polyclonal antibody directed against the 37 kilodalton Chlamydomonas CA monomer. Both Chlorella species and enzymatically isolated C. ellipsoidea chloroplasts also contain an immunologically similar 38 kilodalton polypeptide that may be a cytosolic or chloroplastic form of CA.

Mass Spectrometric Measurement of Intracellular Carbonic Anhydrase Activity in High and Low C(i) Cells of Chlamydomonas: Studies Using O Exchange with C/O Labeled Bicarbonate

By measuring (18)O exchange from doubly labeled CO(2) ((13)C(18)O(18)O), intracellular carbonic anhydrase activity was studied with protoplasts and chloroplasts isolated from Chlamydomonas reinhardtii grown either on air (low inorganic carbon [C(i)]) or air enriched with 5% CO(2) (high C(i)). Intact low C(i) protoplasts had a 10-fold higher carbonic anhydrase activity than did high C(i) protoplasts. Application of dextran-bound inhibitor and quaternary ammonium sulfanilamide, both known as membrane impermeable inhibitors of carbonic anhydrase, had no influence on the catalysis of (18)O exchange, indicating that cross-contamination with extracellular carbonic anhydrase was not responsible for the observed activity. This intracellular in vivo activity from protoplasts was inhibited by acetazolamide and ethoxyzolamide. Intracellular carbonic anhydrase activity was partly associated with intact chloroplasts isolated from high and low C(i) cells, and the latter had a sixfold greater rate of catalysis. The presence of dextran-bound inhibitor had no effect on chloroplast-associated carbonic anhydrase, whereas 150 micromolar ethoxyzolamide caused a 61 to 67% inhibition of activity. These results indicate that chloroplastic carbonic anhydrase was located within the plastid and that it was relatively insensitive to ethoxyzolamide. Carbonic anhydrase activity in crude homogenates of protoplasts and chloroplasts was about six times higher in the low C(i) than in high C(i) preparations. Further separation into soluble and insoluble fractions together with inhibitor studies revealed that there are at least two different forms of intracellular carbonic anhydrase. One enzyme, which was rather insoluble and relatively insensitive to ethoxyzolamide, is likely an intrachloroplastic carbonic anhydrase. The second carbonic anhydrase, which was soluble and sensitive to ethoxyzolamide, is most probably located in an extrachloroplastic compartment.

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.

Uptake of Inorganic Carbon by Isolated Chloroplasts of the Unicellular Green Alga Chlorella ellipsoidea

Chloroplasts, isolated from protoplasts of the green alga, Chlorella ellipsoidea, were estimated to be 99% intact by the ferricyanide-reduction assay, and gave CO(2) and PGA-dependent rates of O(2) evolution of 64.5 to 150 micromoles per milligram of chlorophyll per hour, that is 30 to 70% of the photosynthetic activity of the parent cells. Intact chloroplasts showed no carbonic anhydrase activity, but it was detected in preparations of ruptured organelles. Rates of photosynthesis, measured in a closed system at pH 7.5, were twice the calculated rate of CO(2) supply from the uncatalyzed dehydration of HCO(3) (-) indicating a direct uptake of bicarbonate by the intact chloroplasts. Mass spectrometric measurements of CO(2) depletion from the medium on the illumination of chloroplasts indicate the lack of an active CO(2) transport across the chloroplast envelope.

Transport and assimilation of inorganic carbon by Lichina pygmaea under emersed and submersed conditions

Photosynthetic O₂ evolution by the upper littoral lichen, Lichina pygmaea (Lightf.) C.Ag., under light-saturated conditions at 5 °C is saturated by the 2 mol m^-3 inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m^-3 inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO₃ ^- . It is concluded that HCO₃ ^- is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO₂ enters the photobiont (Calothrix) cells even during HCO₃ use. pH drift experiments support the notion of HCO₃ ^- use. Emersed photosynthesis at 5 °C is more than half-saturated by 35 Pa (normal atmospheric) CO₂ ; the light- and CO₂ -saturated emersed photosynthetic rate is not significantly different from the light and inorganic C-saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO₃ ^- fluxes to supplement CO₂ movement. The CO₂ compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark ¹⁴ C-inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM-like variation in intracellular titratable acidity. The high value (-11.5%) of δ¹³ C, the low CO₂ compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free-living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.

Glycolaldehyde Inhibits CO(2) Fixation in the Cyanobacterium Synechococcus UTEX 625 without Inhibiting the Accumulation of Inorganic Carbon or the Associated Quenching of Chlorophyll a Fluorescence

When studying active CO(2) and HCO(3) (-) transport by cyanobacteria, it is often useful to be able to inhibit concomitant CO(2) fixation. We have found that glycolaldehyde was an efficient inhibitor of photosynthetic CO(2) fixation in Synechococcus UTEX 625. Glycolaldehyde did not inhibit inorganic carbon accumulation due to either active CO(2) or HCO(3) (-) transport. When glycolaldehyde (10 millimolar) was added to rapidly photosynthesizing cells, CO(2) fixation was stopped within 15 seconds. The quenching of chlorophyll a fluorescence remained high (</= 82% control) when CO(2) fixation was completely blocked by glycolaldehyde. This quenching was relieved upon the addition of a glucose oxidase oxygentrap. This is consistent with our previous finding that q-quenching in the absence of CO(2) fixation was due to O(2) photoreduction. Photosynthetic CO(2) fixation was also inhibited by d,l,-glyceraldehyde but a sixfold higher concentration was required. Glycolaldehyde acted much more rapidly than iodoacetamide (15 seconds versus 300 seconds) and did not cause the onset of net O(2) evolution often observed with iodoacetamide. Glycolaldehyde will be a useful inhibitor when it is required to study CO(2) and HCO(3) (-) transport without the complication of concomitant CO(2) fixation.

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.

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

Carbon oxysulfide (carbonyl sulfide, COS) is a close structural analog of CO(2). Although hydrolysis of COS (to CO(2) and H(2)S) does occur at alkaline pH (>9), at pH 8.0 the rate of hydrolysis is slow enough to allow investigation of COS as a possible substrate and inhibitor of the active CO(2) transport system of Synechococcus UTEX 625. A light-dependent uptake of COS was observed that was inhibited by CO(2) and the ATPase inhibitor diethylstilbestrol. The COS taken up by the cells could not be recovered when the lights were turned off or when acid was added. It was concluded that most of the COS taken up was hydrolyzed by intracellular carbonic anhydrase. The production of H(2)S was observed and COS removal from the medium was inhibited by ethoxyzolamide. Bovine erythrocyte carbonic anhydrase catalysed the stoichiometric hydrolysis of COS to H(2)S. The active transport of CO(2) was inhibited by COS in an apparently competitive manner. When Na(+)-dependent HCO(3) (-) transport was allowed in the presence of COS, the extracellular [CO(2)] rose considerably above the equilibrium level. This CO(2) appearing in the medium was derived from the dehydration of transported HCO(3) (-) and was leaked from the cells. In the presence of COS the return to the cells of this leaked CO(2) was inhibited. These results showed that the Na(+)-dependent HCO(3) (-) transport was not inhibited by COS, whereas active CO(2) transport was inhibited. When COS was removed by gassing with N(2), a normal pattern of CO(2) uptake was observed. The silicone fluid centrifugation method showed that COS (100 micromolar) had little effect upon the initial rate of HCO(3) (-) transport or CO(2) fixation. The steady state rate of CO(2) fixation was, however, inhibited about 50% in the presence of COS. This inhibition can be at least partially explained by the significant leakage of CO(2) from the cells that occurred when CO(2) uptake was inhibited by COS. Neither CS(2) nor N(2)O acted like COS. It is concluded that COS is an effective and selective inhibitor of active CO(2) transport.

The Relationship between Ribulose Bisphosphate Concentration, Dissolved Inorganic Carbon (DIC) Transport and DIC-Limited Photosynthesis in the Cyanobacterium Synechococcus leopoliensis Grown at Different Concentrations of Inorganic Carbon

To examine the factors which limit photosynthesis and their role in photosynthetic adaptation to growth at low dissolved inorganic carbon (DIC), Synechococcus leopoliensis was grown at three concentrations (as signified by brackets) of DIC, high (1000-1800 micromolar), intermediate (200-300 micromolar), and low (10-20 micromolar). In all cell types photosynthesis varied from being ribulose bisphosphate (RuBP)-saturated at low external [DIC] to RuBP-limited at high external [DIC]. The maximum rate of photosynthesis (P(max)) was achieved when the internal concentration of RuBP fell below the active site density of RuBP carboxylase/oxygenase (Rubisco). At rates of photosynthesis below P(max), photosynthetic capacity was limited by the ability of the cell to transport inorganic carbon and to supply CO(2) to Rubisco. Adaptation to low DIC was reflected by a decrease in the [DIC] required to half-saturate photosynthesis. Simultaneous mass-spectrometric measurement of rates of photosynthesis and DIC transport showed that the initial slope of the photosynthesis versus [DIC] curve is identical to the initial slope of the DIC transport versus [DIC] curve. This provided evidence that the enhanced capacity for DIC transport which occurs upon adaptation to low [DIC] was responsible for the increase in the initial slope of the photosynthesis versus [DIC] curve and therefore the decrease in the half saturation constant of photosynthesis with respect to DIC. Levels of RuBP and in vitro Rubisco activity varied only slightly between high and intermediate [DIC] grown cells but fell significantly (65-70%) in low [DIC] grown cells. Maximum rates of photosynthesis followed a similar pattern with P(max) only slightly lower in intermediate [DIC] grown cells than in high [DIC] grown cells, but much lower in low [DIC] grown cells. The changing response of photosynthesis to [DIC] during adaptation to low DIC, may be explained by the interaction between DIC-transport limited and [RuBP]-limited photosynthesis.

Active CO(2) Transport by the Green Alga Chlamydomonas reinhardtii

Mass spectrometric measurements of dissolved free (13)CO(2) were used to monitor CO(2) uptake by air grown (low CO(2)) cells and protoplasts from the green alga Chlamydomonas reinhardtii. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of external carbonic anhydrase reduced the (13)CO(2) concentration in the medium to close to zero. Similar results were obtained with low CO(2) cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO(2) uptake revealed that the removal of CO(2) from the medium in the light was due to selective and active CO(2) transport rather than uptake of total dissolved inorganic carbon. In the light, low CO(2) cells and protoplasts incubated with carbonic anhydrase took up CO(2) at an apparently low rate which reflected the uptake of total dissolved inorganic carbon. No net CO(2) uptake occurred in the dark. Measurement of chlorophyll a fluorescence yield with low CO(2) cells and washed protoplasts showed that variable fluorescence was mainly influenced by energy quenching which was reciprocally related to photosynthetic activity with its highest value at the CO(2) compensation point. During the linear uptake of CO(2), low CO(2) cells and protoplasts incubated with carbonic anhydrase showed similar rates of net O(2) evolution (102 and 108 micromoles per milligram of chlorophyll per hour, respectively). The rate of net O(2) evolution (83 micromoles per milligram of chlorophyll per hour) with washed protoplasts was 20 to 30% lower during the period of rapid CO(2) uptake and decreased to a still lower value of 46 micromoles per milligram of chlorophyll per hour when most of the free CO(2) had been removed from the medium. The addition of carbonic anhydrase at this point resulted in more than a doubling of the rate of O(2) evolution. These results show low CO(2) cells of Chlamydomonas are able to transport both CO(2) and HCO(3) (-) but CO(2) is preferentially removed from the medium. The external carbonic anhydrase is important in the supply to the cells of free CO(2) from the dehydration of HCO(3) (-).

Characterization of the na-requirement in cyanobacterial photosynthesis

The Na(+) requirement for photosynthesis and its relationship to dissolved inorganic carbon (DIC) concentration and Li(+) concentration was examined in air-grown cells of the cyanobacterium Synechococcus leopoliensis UTEX 625 at pH 8. Analysis of the rate of photosynthesis (O(2) evolution) as a function of Na(+) concentration, at fixed DIC concentration, revealed two distinct regions to the response curve, for which half-saturation values for Na(+) (K((1/2))[Na(+)]) were calculated. The value of both the low and the high K((1/2))(Na(+)) was dependent upon extracellular DIC concentration. The low K((1/2))(Na(+)) decreased from 1000 micromolar at 5 micromolar DIC to 200 micromolar at 140 micromolar DIC whereas over the same DIC concentration range the high K((1/2))(Na(+)) decreased from 10 millimolar to 1 millimolar. The most significant increases in photosynthesis occurred in the 1 to 20 millimolar range. A fraction of total photosynthesis, however, was independent of added Na(+) and this fraction increased with increased DIC concentration. A number of factors were identified as contributing to the complexity of interaction between Na(+) and DIC concentration in the photosynthesis of Synechococcus. First, as revealed by transport studies and mass spectrometry, both CO(2) and HCO(3) (-) transport contributed to the intracellular supply of DIC and hence to photosynthesis. Second, both the CO(2) and HCO(3) (-) transport systems required Na(+), directly or indirectly, for full activity. However, micromolar levels of Na(+) were required for CO(2) transport while millimolar levels were required for HCO(3) (-) transport. These levels corresponded to those found for the low and high K((1/2))(Na(+)) for photosynthesis. Third, the contribution of each transport system to intracellular DIC was dependent on extracellular DIC concentration, where the contribution from CO(2) transport increased with increased DIC concentration relative to HCO(3) (-) transport. This change was reflected in a decrease in the Na(+) concentration required for maximum photosynthesis, in accord with the lower Na(+)-requirement for CO(2) transport. Lithium competitively inhibited Na(+)-stimulated photosynthesis by blocking the cells' ability to form an intracellular DIC pool through Na(+)-dependent HCO(3) (-) transport. Lithium had little effect on CO(2) transport and only a small effect on the size of the pool it generated. Thus, CO(2) transport did not require a functional HCO(3) (-) transport system for full activity. Based on these observations and the differential requirement for Na(+) in the CO(2) and HCO(3) (-) transport system, it was proposed that CO(2) and HCO(3) (-) were transported across the membrane by different transport systems.

Active Transport of Inorganic Carbon Increases the Rate of O(2) Photoreduction by the Cyanobacterium Synechococcus UTEX 625

Chlorophyll a fluorescence of Synechococcus UTEX 625 was quenched during the transport of inorganic carbon, even when CO(2) fixation was inhibited by iodoacetamide. Measurements with a pulse modulation fluorometer showed that at least 75% of the quenching was due to oxidation of Q(a), the primary acceptor of photosystem II. Mass spectrometry revealed that transport of inorganic carbon increased the rate of O(2) photoreduction. Hence, O(2) could serve as an electron acceptor to allow oxidation of Q(a) even in the absence of CO(2) fixation.

Simultaneous Transport of CO(2) and HCO(3) by the Cyanobacterium Synechococcus UTEX 625

A mass spectrometer was used to simultaneously follow the time course of photosynthetic O(2) evolution and CO(2) depletion of the medium by cells of the cyanobacterium Synechococcus leopoliensis UTEX 625. Analysis of the data indicated that both CO(2) and HCO(3) (-) were simultaneously and continuously transported by the cells as a source of substrate for photosynthesis. Initiation of HCO(3) (-) transport by Na(+) addition had no effect on ongoing CO(2) transport. This result is interpreted to indicate that the CO(2) and HCO(3) (-) transport systems are separate and distinctly different transport systems. Measurement of CO(2)-dependent photosynthesis indicated that CO(2) uptake involved active transport and that diffusion played only a minor role in CO(2) acquisition in cyanobacteria.