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PubMed1987

A Mutant of Synechococcus PCC7942 Incapable of Adapting to Low CO(2) Concentration.

Some properties of a mutant (RK1) of Synechococcus PCC7942, which requires high CO(2) for growth, are described. The photosynthetic affinity for inorganic carbon (C(i)) in RK1 was about 40 times lower than that in the wild type (WT) when grown at 3% CO(2) (H-cells) and did not change during 10 hours of exposure to low CO(2) (air containing 0.04% CO(2)). The gas exchange of WT and RK1 cells was measured using an open gas-analysis system. All the measurements were performed at a CO(2) concentration of 400 microliters per liter under the conditions where photosynthetic CO(2) fixation is inhibited. When the suspension of H-cells of WT or RK1 was illuminated, the rate of CO(2) influx from the gas phase into the suspension was low and addition of carbonic anhydrase during illumination released only a small amount of CO(2) from the medium into the gas phase. The rate of CO(2) influx and the amount of CO(2) released by carbonic anhydrase were increased in WT during low CO(2) adaptation. These changes did not occur in RK1 during exposure to low CO(2). Cytoplasmic membrane from H-cells of WT or RK1 contained small amount of 42-kilodalton polypeptide. Exposure of RK1 to low CO(2) did not have significant effect on the amount of 42-kilodalton polypeptide, while the same treatment on WT resulted in a large increase of this polypeptide. The RK1 mutant appears to be defective in its ability to utilize the intracellular C(i) pool for photosynthesis and also to transmit a low CO(2) signal for inducing the functional and compositional changes observed in WT during low CO(2) adaptation.

Journal Article↗
PubMed1988

Chlorophyll a Fluorescence Yield as a Monitor of Both Active CO(2) and HCO(3) Transport by the Cyanobacterium Synechococcus UTEX 625.

Simultaneous measurements have been made of inorganic carbon accumulation (by mass spectrometry) and chlorophyll a fluorescence yield of the cyanobacterium Synechococcus UTEX 625. The accumulation of inorganic carbon by the cells was accompanied by a substantial quenching of chlorophyll a fluorescence. The quenching occurred even when CO(2) fixation was inhibited by iodoacetamide and whether the accumulation of inorganic carbon resulted from either active CO(2) or HCO(3) (-) transport. Measurement of chlorophyll a fluorescence yield of cyanobacteria may prove to be a rapid and convenient means of screening for mutants of inorganic carbon accumulation.

Journal Article↗
PubMed1988

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

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

Journal Article↗
PubMed1988

Role of Photosynthetic Reactions in the Activity of Carbonic Anhydrase in Synechococcus sp. (UTEX 2380) in the Light : Inhibitor Studies Using the O-Exchange in C/O-Labeled Bicarbonate.

The role of the photosystems in the exchange of (18)O between species of inorganic carbon and water was studied in suspensions of the cyanobacterium Synechococcus sp. (UTEX 2380) using membrane-inlet mass spectrometry. This (18)O exchange is caused by the hydration-dehydration cycle of CO(2) and is catalyzed by carbonic anhydrase. We observed the complex (18)O exchange kinetics including dark-light-dark transients in suspensions of whole cells and found these to be identical to the (18)O exchange kinetics of physiologically fully active spheroplast preparations. There was no enhancement effect of inorganic nitrogen on inorganic carbon accumulation. Membrane preparations exhibited no uptake of inorganic carbon and very little carbonic anhydrase activity, although these membranes were photosynthetically fully competent. DCMU, the inhibitor of photosystem II, eliminated almost entirely the (18)O exchange activity of whole cells in the light. But this effect of DCMU could be reversed by addition of the electron donor couple 3,6-diaminodurene/ascorbate, suggesting the involvement of photosystem I in the events leading to (18)O exchange. Iodoacetamide, an inhibitor of CO(2) fixation, enhanced the (18)O exchange in whole cell suspensions and inhibited neither the uptake of inorganic carbon nor the dehydration of bicarbonate in the light. The proton carrier carbonylcyanide m-chlorophenylhydrazone and the inhibitors diethylstilbestrol and N,N' -dicyclohexyl carbodiimide affecting the membrane potential, totally abolished (18)O exchange in the light. From (18) O-labeled inorganic carbon experiments we conclude that one of the roles of photosystem I is to provide the active uptake of inorganic carbon into the cells, where carbonic anhydrase catalyzes the interconversion between CO(2) and HCO(3) (-) resulting in the (18)O exchange from inorganic carbon to water.

Journal Article↗
PubMed1988

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.

Journal Article↗
PubMed1988

Cytoplasmic membrane changes during adaptation of the fresh water cyanobacterium Synechococcus 6311 to salinity.

In this investigation, changes were characterized in cell structure and cytoplasmic membrane organization that occur when the freshwater cyanobacterium Synechococcus 6311 is transferred from 'low salt' (0.03 molar NaCl) to 'high salt' (0.5 molar NaCl) media (i.e. sea water concentration). Cells were examined at several time points after the imposition of the salt stress and compared to control cells, in thin sections and freeze fracture electron microscopy, and by flow cytometry. One minute after exposure to high salt, i.e. 'salt shock', virtually all intracellular granules disappeared, the density of the cytoplasm decreased, and the appearance of DNA material was changed. Glycogen and other granules, however, reappeared by 4 hours after salt exposure. The organization of the cytoplasmic membrane undergoes major reorganization following salt shock. Freeze-fracture electron microscopy showed that small intramembrane particles (diameter 7.5 and 8.5 nanometers) are reduced in number by two- to fivefold, whereas large particles, (diameters 14.5 and 17.5 nanometers) increase two- to fourfold in frequency, compared to control cells grown in low salt medium. The changes in particle size distribution suggest synthesis of new membrane proteins, in agreement with the known increases in respiration, cytochrome oxidase, and sodium proton exchange activity of the cytoplasmic membrane.

Cell Membrane↗
PubMed1988

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.

Journal Article↗
PubMed1989

Ethoxyzolamide Inhibition of CO(2) Uptake in the Cyanobacterium Synechococcus PCC7942 without Apparent Inhibition of Internal Carbonic Anhydrase Activity.

In high inorganic carbon grown (1% CO(2) [volume/volume]) cells of the cyanobacterium Synechococcus PCC7942, the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZ), was found to inhibit the rate of CO(2) uptake and to reduce the final internal inorganic carbon (C(i)) pool size reached. The relationship between CO(2) fixation rate and internal C(i) concentration in high C(i) grown cells was little affected by EZ. This suggests that in intact cells internal CA activity was unaffected by EZ. High C(i) grown cells readily took up CO(2) but had little or no capacity for HCO(3) (-) uptake. These cells appear to possess a CO(2) utilizing C(i) pump that has a CA-like function associated with the transport step such that HCO(3) (-) is the species delivered to the cell interior. This CA-like step may be the site of inhibition by EZ. Low C(i) grown cells possess both CO(2) uptake and HCO(3) (-) uptake activities and EZ inhibited both activities to a similar degree, suggesting that a common step in CO(2) and HCO(3) (-) uptake (such as the C(i) pump) may have been affected. The inhibitor had no apparent effect on internal CO(2)/HCO(3) (-) equilibria (internal CA function) in low C(i) grown cells.

Journal Article↗
PubMed1989

Carbonic Anhydrase Activity Associated with the Cyanobacterium Synechococcus PCC7942.

Intact cells and crude homogenates of high (1% CO(2)) and low dissolved inorganic carbon (C(i)) (30-50 microliters per liter of CO(2)) grown Synechococcus PCC7942 have carbonic anhydrase (CA)-like activity, which enables them to catalyze the exchange of (18)O from CO(2) to H(2)O. This activity was studied using a mass spectrometer coupled to a cuvette with a membrane inlet system. Intact high and low C(i) cells were found to contain CA activity, separated from the medium by a membrane which is preferentially permeable to CO(2). This activity is most apparent in the light, where (18)O-labeled CO(2) species are being taken up by the cells but the effluxing CO(2) has lost most of its label to water. In the dark, low C(i) cells catalyze the depletion of the (18)O enrichment of CO(2) and this activity is inhibited by both ethoxyzolamide and 2-(trifluoromethoxy)carbonyl cyanide. This may occur via a common inhibition of the C(i) pump and the C(i) pump is proposed as a potential site for the exchange of (18)O. CA activity was measurable in homogenates of both cell types but was 5- to 10-fold higher in low C(i) cells. This was inhibited by ethoxyzolamide with an I(50) of 50 to 100 micromolar in both low and high C(i) cells. A large proportion of the internal CA activity appears to be pelletable in nature. This pelletability is increased by the presence of Mg(2+) in a manner similar to that of ribulose bisphosphate carboxylase-oxygenase activity and chlorophyll (thylakoids) and may be the result of nonspecific aggregation. Separation of crude homogenates on sucrose gradients is consistent with the notion that CA and ribulose bisphosphate carboxylase-oxygenase activity may be associated with the same pelletable fraction. However, we cannot unequivocally establish that CA is located within the carboxysome. The sucrose gradients show the presence of separate soluble and pelletable CA activity. This may be due to the presence of separate forms of the enzyme or may arise from the same pelletable association which is unstable during extraction.

Journal Article↗
PubMed1989

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.

Journal Article↗
PubMed1989

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.

Journal Article↗
PubMed1989

Isolation and Characterization of High CO(2)-Requiring-Mutants of the Cyanobacterium Synechococcus PCC7942 : Two Phenotypes that Accumulate Inorganic Carbon but Are Apparently Unable to Generate CO(2) within the Carboxysome.

A total of 24 high CO(2)-requiring-mutants of the cyanobacterium Synechococcus PCC7942 have been isolated and partially characterized. These chemically induced mutants are able to grow at 1% CO(2), on agar media, but are incapable of growth at air levels of CO(2). All the mutants were able to accumulate inorganic carbon (C(i)) to levels similar to or higher than wild type cells, but were apparently unable to generate intracellular CO(2). On the basis of the rate of C(i) release following a light (5 minutes) --> dark transition two extreme phenotypes (fast and slow release mutants) and a number of ;intermediate' mutants (normal release) were identified. Compared to wild-type cells, Type I mutants had the following characteristics: fast C(i) release, normal internal C(i) pool, normal carbonic anhydrase (CA) activity in crude extracts, reduced internal exchange of (18)O from (18)O-labeled CO(2), 1% CO(2) requirement for growth in liquid media, normal affinity of carboxylase for CO(2), and long, rod-like carboxysomes. Type II mutants had the following characteristics: slow C(i) release, increased internal C(i) pool, normal CA activity in crude extracts, normal internal (18)O exchange, a 3% CO(2) requirement for growth in liquid media, high carboxylase activity, normal affinity of carboxylase for CO(2), and normal carboxysome structure but increased in numbers per cell. Both mutant phenotypes appear to have genetic lesions that result in an inability to convert intracellular HCO(3) (-) to CO(2) inside the carboxysome. The features of the type I mutants are consistent with a scenario where carboxysomal CA has been mistargeted to the cytosol. The characteristics of the type II phenotype appear to be most consistent with a scenario where CA activity is totally missing from the cell except for the fact that cell extracts have normal CA activity. Alternatively the type II mutants may have a lesion in their capacity for H(+) import during photosynthesis.

Journal Article↗
PubMed1989

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.

Journal Article↗
PubMed1990

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.

Journal Article↗
PubMed1990

Changes in membrane lipid composition during saline growth of the fresh water cyanobacterium Synechococcus 6311.

Growth of Synechococcus 6311 in the presence of 0.5 molar NaCl is accompanied by significant changes in membrane lipid composition. Upon transfer of the cells from a low salt' (0.015 molar NaCl) to high salt' (0.5 molar NaCl) growth medium at different stages of growth, a rapid decrease in palmitoleic acid (C16:1 delta 9) content was accompanied by a concomitant increase in the amount of the two C18:1 acids (C18:1 delta 9, C18:1 delta 11), with the higher increase in oleic acid C18:1 delta 9 content. These changes began to occur within the first hour after the sudden elevation of NaCl and progressed for about 72 hours. The percentage of palmitic acid (C16:0) and stearic acid (C18:0) remained almost unchanged in the same conditions. High salt-dependent changes within ratios of polar lipid classes also occurred within the first 72 hours of growth. The amount of monogalactosyl diacylglycerol (bilayer-destabilizing lipid) decreased and that of the digalactosyl diacylglycerol (bilayer-stabilizing lipid) increased. Consequently, in the three day old cells, the ratio of monogalactosyl diacylglycerol to digalactosyl diacylglycerol in the membranes of high salt-grown cells was about half of that in the membranes of low salt-grown cells. The total content of anionic lipids (phosphatidylglycerol and sulfoquinovosyl diacylglycerol) was always higher in the isolated membranes and the whole cells from high salt-grown cultures compared to that in the cells and membranes from low salt-grown cultures. All the observed rearrangements in the lipid environment occurred in both thylakoid and cytoplasmic membranes. Similar lipid composition changes, however, to a much lesser extent, were also observed in the aging, low salt-grown cultures. The observed changes in membrane fatty acids and lipids composition correlate with the alterations in electron and ion transport activities, and it is concluded that the rearrangement of the membrane lipid environment is an essential part of the process by which cells control membrane function and stability.

Cell Membrane↗
PubMed1991

Setting of the Circadian N(2)-Fixing Rhythm of the Prokaryotic Synechococcus sp. RF-1 while Its nif Gene Is Repressed.

The N(2)-fixing activity of the prokaryotic Synechococcus sp. RF-1 was repressed in the presence of nitrate. When the cultures in nitrate-containing medium were exposed to diurnal light-dark cycles, an endogenous circadian N(2)-fixing rhythm developed after the cells were transferred to nitrate-free medium and incubated in continuous light. The N(2)-fixing phase of the rhythm coincided with the dark phase of the light-dark cycles that were imposed when the cells were in nitrate-containing medium. The results indicate that after the endogenous N(2)-fixing rhythm has been set, it can be kept latent for at least 38 hours before first manifesting itself.

Journal Article↗
PubMed1991

Circadian Rhythm in Amino Acid Uptake by Synechococcus RF-1.

In the prokaryote Synechococcus RF-1, circadian changes in the uptake of l-leucine and 2-amino isobutyric acid were observed. Uptake rates in the light period were higher than in the dark period for cultures entrained by 12/12 hour light/dark cycles. The periodic changes in l-leucine uptake persisted for at least 72 hours into continuous light (L/L). The rhythm had a free-running period of about 24 hours in L/L at 29 degrees C. A single dark treatment of 12 hours could initiate rhythmic leucine uptake in an L/L culture. The phase of rhythm could be shifted by a pulse of low temperature (0 degrees C). The free-running periodicity was "temperature-compensated" from 21 to 37 degrees C. A 24 hour depletion of extracellular Ca(2+) before the free-running L/L condition reduced the variation in uptake rate but had little effect on the periodicity of the rhythm. The periodicity was also not affected by the introduction of 25 mm NaNO(3). The uptake rates for 20 natural amino acids were studied at 12 hour intervals in cultures exposed to 12/12 hour light/dark cycles. For eight of these amino acids (l-Val, l-Leu, l-Ile, l-Pro, l-Phe, l-Trp, l-Met, and l-Tyr), the light/dark uptake rate ratios had values greater than 3 and the rhythm persisted in L/L.

Journal Article↗
PubMed1991

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.

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