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1.
The marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1) changes its affinity for external inorganic carbon used in photosynthesis, depending on the concentration of CO 2 provided during growth. The high affinity for CO 2 + HCO 3− of air-grown cells (K ½ < 80 nanomoles [pH 8.2]) would seem to be the result of the presence of an inducible mechanism which concentrates inorganic carbon (and thus CO 2) within the cells. Silicone-oil centrifugation experiments indicate that the inorganic carbon concentration inside suitably induced cells may be in excess of 1,000-fold greater than that in the surrounding medium, and that this accumulation is dependent upon light energy. The quantum requirements for O 2 evolution appear to be some 2-fold greater for low CO 2-grown cells, compared with high CO 2-grown cells. This presumably is due to the diversion of greater amounts of light energy into inorganic carbon transport in these cells. A number of experimental approaches to the question of whether CO2 or HCO3− is primarily utilized by the inorganic carbon transport system in these cells show that in fact both species are capable of acting as substrate. CO2, however, is more readily taken up when provided at an equivalent concentration to HCO3−. This discovery suggests that the mechanistic basis for the inorganic carbon concentrating system may not be a simple HCO3− pump as has been suggested. It is clear, however, that during steady-state photosynthesis in seawater equilibrated with air, HCO3− uptake into the cell is the primary source of internal inorganic carbon. 相似文献
2.
Synechococcus leopoliensis was grown in HCO 3−-limited chemostats. Growth at 50% the maximum rate occurred when the inorganic carbon concentration was 10 to 15 micromolar (or 5.6 to 8.4 nanomolar CO 2). The O 2 to CO 2 ratios during growth were as high as 192,000 to 1. At growth rates below 80% the maximum rate, essentially all the supplied inorganic carbon was converted to organic carbon, and the cells were carbon limited. Carbon-limited cells used HCO 3− rather than CO 2 for growth. They also exhibited a very high photosynthetic affinity for inorganic carbon in short-term experiments. Cells growing at greater than 80% maximum growth rate, in the presence of high dissolved inorganic carbon, were termed carbon sufficient. These cells had photosynthetic affinities that were about 1000-fold lower than HCO 3−-limited cells and also had a reduced capacity for HCO 3− transport. HCO 3−-limited cells are reminiscent of the air-grown cells of batch culture studies while the carbon sufficient cells are reminiscent of high-CO 2 grown cells. However, the low affinity cells of the present study were growing at CO 2 concentrations less than air saturation. This suggests that supranormal levels of CO 2 not required to induce the physiological changes usually ascribed to high CO 2 cells. 相似文献
3.
The rates of CO 2-dependent O 2 evolution by Chlamydomonas reinhardtii, grown with either air levels of CO 2 or air with 5% CO 2, were measured at varying external pH. Over a pH range of 4.5 to 8.5, the external concentration of CO 2 required for half-maximal rates of photosynthesis was constant, averaging 25 micromolar for cells grown with 5% CO 2. This is consistent with the hypothesis that these cells take up CO 2 but not HCO 3− from the medium and that their CO 2 requirement for photosynthesis reflects the Km(CO 2) of ribulose bisphosphate carboxylase. Over a pH range of 4.5 to 9.5, cells grown with air required an external CO 2 concentration of only 0.4 to 3 micromolar for half-maximal rates of photosynthesis, consistent with a mechanism to accumulate external inorganic carbon in these cells. Air-grown cells can utilize external inorganic carbon efficiently even at pH 4.5 where the HCO 3− concentration is very low (40 nanomolar). However, at high external pH, where HCO 3− predominates, these cells cannot accumulate inorganic carbon as efficiently and require higher concentrations of NaHCO 3 to maintain their photosynthetic activity. These results imply that, at the plasma membrane, CO 2 is the permeant inorganic carbon species in air-grown cells as well as in cells grown on 5% CO 2. If active HCO 3− accumulation is a step in CO 2 concentration by air-grown Chlamydomonas, it probably takes place in internal compartments of the cell and not at the plasmalemma. 相似文献
4.
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. 相似文献
5.
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 Ki 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. 相似文献
6.
The species of inorganic carbon (CO 2 or HCO 3−) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO 2 or HCO 3− transport, during steady state photosynthesis, is achieved by monitoring the changes (by 14C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO 2 or HCO 3− transport) and experimental time-courses of 14C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO 2, rather than HCO 3−, is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO 2 is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO 3− transport, as the incorporation of 14C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO 2 uptake alone. The contribution of HCO 3− to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO 3− concentration. The evidence for direct HCO 3− transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of 14C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO 2, which is partially alleviated by a high extracellular concentration of HCO 3−. 相似文献
7.
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, 12CO 2 readily interfered with 13CO 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. 相似文献
8.
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. 相似文献
9.
Scenedesmus cells grown on high CO 2, when adapted to air levels of CO 2 for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO 2 from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O 2 evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO 2 pump with K 0.5 for O 2 evolution of less than 5 micromolar CO 2 was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO 3− concentrating process between pH 8 to 10 lowered the K 0.5 (DIC) from 7300 micromolar HCO 3− in high CO 2 grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO 3− pump was inhibited by vanadate (K i of 150 micromolar), as if it involved an ATPase linked HCO 3− transporter. The CO 2 pump was formed on low CO 2 by high-CO 2 grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO 3− pump was partially activated on low CO 2 within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO 3− pump at pH 9 had requirements similar to the activation of the CO 2 pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO 2-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO 2 pump and in some algae an additional vanadate-sensitive plasmalemma HCO 3− transporter for a HCO 3− pump. 相似文献
10.
Photosynthetic carbon uptake of the tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers was studied by several methods. Photosynthesis in buffered seawater in media in the range of pH 6 to pH 9 showed an exponentially increasing rate with decreasing pH, thus indicating that free CO 2 was a photosynthetic substrate. However, these experiments were unable to determine whether photosynthesis at alkaline pH also contained some component due to HCO 3− uptake. This aspect was further investigated by studying photosynthetic rates in a number of media of varying pH (7.8-8.61) and total inorganic carbon (0.75-13.17 millimolar). In these media, photosynthetic rate was correlated with free CO 2 concentration and was independent of the HCO 3− concentration in the medium. Short time-course experiments were conducted during equilibration of free CO 2 and HCO 3− after injection of 14C labeled solution at acid or alkaline pH. High initial photosynthetic rates were observed when acidic solutions (largely free CO 2) were used but not with alkaline solutions. The concentration of free CO 2 was found to be a limiting factor for photosynthesis in this plant. 相似文献
11.
Thalli discs of the marine macroalga Ulva lactuca were given inorganic carbon in the form of HCO 3−, and the progression of photosynthetic O 2 evolution was followed and compared with predicted O 2 evolution as based on calculated external formation of CO 2 (extracellular carbonic anhydrase was not present in this species) and its carboxylation (according to the Km(CO 2) of ribulose-1,5-bisphosphate carboxylase/oxygenase), at two different pHs, assuming a photosynthetic quotient of 1. The Km(inorganic carbon) was some 2.5 times lower at pH 5.6 than at the natural seawater pH of 8.2, whereas Vmax was similar under the two conditions, indicating that the unnaturally low pH per se had no adverse effect on U. lactuca's photosynthetic performance. These results, therefore, could be evaluated with regard to differential CO 2 and HCO 3− utilization. The photosynthetic performance observed at the lower pH largely followed that predicted, with a slight discrepancy probably reflecting a minor diffusion barrier to CO 2 uptake. At pH 8.2, however, dehydration rates were too slow to supply CO 2 for the measured photosynthetic response. Given the absence of external carbonic anhydrase activity, this finding supports the view that HCO 3− transport provides higher than external concentrations of CO 2 at the ribulose-1,5-bisphosphate carboxylase/oxygenase site. Uptake of HCO 3− by U. lactuca was further indicated by the effects of potential inhibitors at pH 8.2. The alleged band 3 membrane anion exchange protein inhibitor 4,4′-diisothiocyanostilbene-2,2′disulphonate reduced photosynthetic rates only when HCO 3− (but not CO 2) could be the extracellular inorganic carbon form taken up. A similar, but less drastic, HCO 3−-competitive inhibition of photosynthesis was obtained with Kl and KNO 3. It is suggested that, under ambient conditions, HCO 3− is transported into cells at defined sites either via facilitated diffusion or active uptake, and that such transport is the basis for elevated internal [CO 2] at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation. 相似文献
12.
The role of external carbonic anhydrase in inorganic carbon acquisition and photosynthesis by Chlamydomonas reinhardii at alkaline pH (8.0) was studied. Acetazolamide (50 micromolar) completely inhibited external carbonic anhydrase (CA) activity as determined from isotopic disequilibrium experiments. Under these conditions, photosynthetic rates at low dissolved inorganic carbon (DIC) were far greater than could be maintained by CO 2 supplied from the spontaneous dehydration of HCO 3− thereby showing that C. reinhardii has the ability to utilize exogenous HCO 3−. Acetazolamide increased the concentration of DIC required to half-saturate photosynthesis from 38 to 80 micromolar, while it did not affect the maximum photosynthetic rate. External CA activity was also removed from the cell-wall-less mutant (CW-15) by washing. This had no effect on the photosynthetic kinetics of the algae while the addition of acetazolamide to washed cells (CW-15) increased the K ½DIC from 38 to 80 micromolar. Acetazolamide also caused a buildup of the inorganic carbon pool upon NaHCO 3 addition, indicating that this compound partially inhibited internal CA activity. The effects of acetazolamide on the photosynthetic kinetics of C. reinhardii are likely due to the inhibition of internal rather than a consequence of the inhibition of external CA. Further analysis of the isotopic disequilibrium experiments at saturating concentration of DIC provided evidence consistent with active CO 2 transport by C. reinhardii. The observation that C. reinhardii has the ability to take up both CO 2 and bicarbonate throws into question the role of external CA in the accumulation of DIC in this alga. 相似文献
13.
The possibility of HCO 3− transport into isolated leaf mesophyll cells of Asparagus sprengeri Regel has been investigated. Measurement of the inorganic carbon pool in these cells over an external pH range 6.2 to 8.0, using the silicone-fluid filtration technique, indicated that the pool was larger than predicted by passive 14CO 2 distribution, suggesting that HCO 3− as well as CO 2 crosses the plasmalemma. Intracellular pH values, calculated from the distribution of 14CO 2 between the cells and the medium, were found to be higher (except at pH 8.0) than those previously determined by 5,5-dimethyl[2- 14C]oxazolidine-2,4-dione distribution. It is suggested that the inorganic carbon accumulated above predicted concentrations may be bound to proteins and membranes and thus may not represent inorganic carbon actively accumulated by the cells, inasmuch as in a closed system at constant CO 2 concentration, the photosynthetic rates at pH 7.0 and 8.0 were 5 to 8 times lower than the maximum rate which could be supported by CO 2 arising from the spontaneous dehydration of HCO 3−. Furthermore, CO 2 compensation points of the cells in liquid media at 21% O 2 at pH 7.0 and 8.0, and the K ½ CO 2 (CO 2 concentration supporting the half maximal rate of O 2 evolution) at 2% O 2 at pH 7.0 and 8.0 are not consistent with HCO 3− transport. These results indicate that the principal inorganic carbon species crossing the plasmalemma in these cells is CO 2. 相似文献
15.
Membrane-permeable and impermeable inhibitors of carbonic anhydrase have been used to assess the roles of extracellular and intracellular carbonic anhydrase on the inorganic carbon concentrating system in Chlamydomonas reinhardtii. Acetazolamide, ethoxzolamide, and a membrane-impermeable, dextran-bound sulfonamide were potent inhibitors of extracellular carbonic anhydrase measured with intact cells. At pH 5.1, where CO 2 is the predominant species of inorganic carbon, both acetazolamide and the dextran-bound sulfonamide had no effect on the concentration of CO 2 required for the half-maximal rate of photosynthetic O 2 evolution (K 0.5[CO 2]) or inorganic carbon accumulation. However, a more permeable inhibitor, ethoxzolamide, inhibited CO 2 fixation but increased the accumulation of inorganic carbon as compared with untreated cells. At pH 8, the K 0.5(CO 2) was increased from 0.6 micromolar to about 2 to 3 micromolar with both acetazolamide and the dextran-bound sulfonamide, but to a higher value of 60 micromolar with ethoxzolamide. These results are consistent with the hypothesis that CO 2 is the species of inorganic carbon which crosses the plasmalemma and that extracellular carbonic anhydrase is required to replenish CO 2 from HCO 3− at high pH. These data also implicate a role for intracellular carbonic anhydrase in the inorganic carbon accumulating system, and indicate that both acetazolamide and the dextran-bound sulfonamide inhibit only the extracellular enzyme. It is suggested that HCO 3− transport for internal accumulation might occur at the level of the chloroplast envelope. 相似文献
16.
A simple model based on HCO 3− transport has been developed to relate photosynthesis and inorganic carbon fluxes for the marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1). Predicted relationships between inorganic carbon transport, CO 2 fixation, internal carbonic anhydrase activity, and leakage of CO 2 out of the cell, allow comparisons to be made with experimentally obtained data. Measurements of inorganic carbon fluxes and internal inorganic carbon pool sizes in these cells were made by monitoring time-courses of CO 2 changes (using a mass spectrometer) during light/dark transients. At just saturating CO 2 conditions, total inorganic carbon transport did not exceed net CO 2 fixation by more than 30%. This indicates CO 2 leakage similar to that estimated for C 4 plants. For this leakage rate, the model predicts the cell would need a conductance to CO2 of around 10−5 centimeters per second. This is similar to estimates made for the same cells using inorganic carbon pool sizes and CO2 efflux measurements. The model predicts that carbonic anhydrase is necessary internally to allow a sufficiently fast rate of CO2 production to prevent a large accumulation of HCO3−. Intact cells show light stimulated carbonic anhydrase activity when assayed using 18O-labeled CO2 techniques. This is also supported by low but detectable levels of carbonic anhydrase activity in cell extracts, sufficient to meet the requirements of the model. 相似文献
17.
Mass spectrometric measurements of dissolved free 13CO 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 13CO 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−. 相似文献
18.
This study investigated inorganic carbon accumulation in relation to photosynthesis in the marine dinoflagellate Prorocentrum micans. Measurement of the internal inorganic carbon pool showed a 10-fold accumulation in relation to external dissolved inorganic carbon (DIC). Dextran-bound sulfonamide (DBS), which inhibited extracellular carbonic anhydrase, caused more than 95% inhibition of DIC accumulation and photosynthesis. We used real-time imaging of living cells with confocal laser scanning microscopy and a fluorescent pH indicator dye to measure transient pH changes in relation to inorganic carbon availability. When steady-state photosynthesizing cells were DIC limited, the chloroplast pH decreased from 8.3 to 6.9 and cytosolic pH decreased from 7.7 to 7.1. Re-addition of HCO 3− led to a rapid re-establishment of the steady-state pH values abolished by DBS. The addition of DBS to photosynthesizing cells under steady-state conditions resulted in a transient increase in intracellular pH, with photosynthesis maintained for 6 s, the amount of time needed for depletion of the intracellular inorganic carbon pool. These results demonstrate the key role of extracellular carbonic anhydrase in facilitating the availability of CO 2 at the exofacial surface of the plasma membrane necessary to maintain the photosynthetic rate. The need for a CO 2-concentrating mechanism at ambient CO 2 concentrations may reflect the difference in the specificity factor of ribulose-1,5 bisphosphate carboxylase/oxygenase in dinoflagellates compared with other algal phyla. 相似文献
19.
At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na +. The rate of photosynthesis greatly exceeded the CO 2 supply rate and indicated that HCO 3− was taken up by a Na +-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na + and exceeded the CO 2 supply rate by 8 to 45 times. The apparent photosynthetic affinity ( K½) for DIC was high (6-40 micromolar) and was not markedly affected by Na + concentration, whereas with air-grown cells K½ (DIC) decreased by more than an order of magnitude in the presence of Na +. Lithium, which inhibited Na +-dependent HCO 3− uptake in air-grown cells, had little effect on Na +-independent HCO 3− uptake by standing culture cells. A component of total HCO 3− uptake in standing culture cells was also Na +-dependent with a K½ (Na +) of 4.8 millimolar and was inhibited by lithium. Analysis of 14C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na +-independent CO 2 transport system. The conversion from Na +-independent to Na +-dependent HCO 3− uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO 3− for subsequent photosynthetic fixation. 相似文献
20.
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 2S, 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. 相似文献
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