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1.
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
2.
The rate of O 2 evolution and alkalization of the medium in low CO 2− grown Anabaena variabilis was observed as affected by the pH in the medium. Both rates are severely inhibited by pH values higher than 9.5, but the latter is more sensitive to this treatment. This finding, as well as the lag observed in alkalization of the medium, but not in O 2 evolution, following the addition of HCO 3− indicates that the transport of HCO 3 and OH − (or H +) are not compulsorily coupled. The inhibition of photosynthesis by strongly alkaline pH is attributed to an alteration of the internal pH and, hence, the rate of carboxylation. This conclusion is supported by data showing that the rate of O 2 evolution is affected by pH more strongly at saturating [HCO 3−] than at limiting [HCO 3−]. Also, the rate of O 2 evolution at saturating light intensity is affected by pH more strongly than is the initial slope of the curve against light intensity or the rate of dark respiration. 相似文献
3.
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
4.
Carbon oxysulfide (carbonyl sulfide, COS) is a close structural analog of CO 2. Although hydrolysis of COS (to CO 2 and H 2S) 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 2S was observed and COS removal from the medium was inhibited by ethoxyzolamide. Bovine erythrocyte carbonic anhydrase catalysed the stoichiometric hydrolysis of COS to H 2S. 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 2O acted like COS. It is concluded that COS is an effective and selective inhibitor of active CO 2 transport. 相似文献
5.
The possibility of HCO 3− transport in the blue-green alga (cyanobacterium) Coccochloris peniocystis has been investigated. Coccochloris photosynthesized most rapidly in the pH range 8 to 10, where most of the inorganic C exists as HCO 3−. If photosynthesis used only CO 2 from the external solution the rate of photosynthesis would be limited by the rate of HCO 3− dehydration to CO 2. Observed rates of photosynthesis at alkaline pH were as much as 48-fold higher than could be supported by spontaneous dehydration of HCO 3− in the external solution. Assays for extracellular carbonic anhydrase were negative. The evidence strongly suggests that HCO 3− was a direct C source for photosynthesis. 相似文献
6.
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
7.
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
8.
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
9.
Rates of photosynthetic O 2 evolution, for measuring K 0.5(CO 2 + HCO 3−) at pH 7, upon addition of 50 micromolar HCO 3− to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K1(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO 2 uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O 2 evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO 3), and the rate of 14CO 2 fixation with 100 micromolar [ 14C] HCO 3−. Salicylhydroxamic acid inhibition of O 2 evolution and 14CO 2-fixation was reversed by higher levels of NaHCO 3. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO 2 accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related. 相似文献
10.
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
11.
Light-dependent inorganic C (C i) 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 CO 2 or HCO 3− 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 C i pool of 20 mm, even though CO 2 fixation was completely inhibited, indicating that cyclic electron flow was involved in the C i-concentrating mechanism. When 200 μm N,N-dimethyl- p-nitrosoaniline was used to drain electrons from ferredoxin, a similar C i accumulation was observed, suggesting that linear electron flow could support the transport of C i. When carbonic anhydrase was not present, initial CO 2 uptake was greatly reduced and the extracellular [CO 2] eventually increased to a level higher than equilibrium, strongly suggesting that CO 2 transport was inhibited and that C i accumulation was the result of active HCO 3− transport. With 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated cells, C i transport and accumulation were inhibited by inhibitors of CO 2 transport, such as COS and Na 2S, whereas Li +, an HCO 3−-transport inhibitor, had little effect. In the presence of N,N-dimethyl- p-nitrosoaniline, C i transport and accumulation were not inhibited by COS and Na 2S but were inhibited by Li +. These results suggest that CO 2 transport is supported by cyclic electron transport and that HCO 3− transport is supported by linear electron transport. 相似文献
12.
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
13.
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. 相似文献
14.
We have measured the exchange of 18O between CO 2 and H 2O in stirred suspensions of Chlorella vulgaris (UTEX 263) using a membrane inlet to a mass spectrometer. The depletion of 18O from CO 2 in the fluid outside the cells provides a method to study CO 2 and HCO 3− kinetics in suspensions of algae that contain carbonic anhydrase since 18O loss to H 2O is catalyzed inside the cells but not in the external fluid. Low-CO 2 cells of Chlorella vulgaris (grown with air) were added to a solution containing 18O enriched CO 2 and HCO 3− with 2 to 15 millimolar total inorganic carbon. The observed depletion of 18O from CO 2 was biphasic and the resulting 18C content of CO 2 was much less than the 18O content of HCO 3− in the external solution. Analysis of the slopes showed that the Fick's law rate constant for entry of HCO 3− into the cell was experimentally indistinguishable from zero (bicarbonate impermeable) with an upper limit of 3 × 10 −4 s −1 due to our experimental errors. The Fick's law rate constant for entry of CO 2 to the sites of intracellular carbonic anhydrase was large, 0.013 per second, but not as great as calculated for no membrane barrier to CO 2 flux (6 per second). The experimental value may be explained by a nonhomogeneous distribution of carbonic anhydrase in the cell (such as membrane-bound enzyme) or by a membrane barrier to CO 2 entry into the cell or both. The CO 2 hydration activity inside the cells was 160 times the uncatalyzed CO 2 hydration rate. 相似文献
15.
The inorganic carbon (C i) accumulation and the intracellular location of carbonic anhydrase (CA, EC 4.2.1.1) in the halotolerant unicellular alga Dunaliella salina have been investigated. The rate of HCO 3 -dependent O 2 evolution was determined by growth conditions. Algae grown under high CO 2 conditions (5% CO 2 in air, v/v; high C i cells) had a very low affinity for HCO 3? at pH 7.0 and 8.2, whereas algae grown under low CO 2 conditions (0.03% CO 2 in air; low C i cells) showed a high affinity for HCO 3? at both pH values and were sensitive to Dextran-bound sulfonamide (DBS), an inhibitor of extracellular CA. The photosynthetic rate or HCO 4? dependent O 2 evolution was always higher at pH 7.0 than at pH 8.2. Ethoxyzolamide (EZ), an inhibitor of total (extacellular plus intracellular) CA activity, strongly inhibited photosynthesis at both pH values. During adaptation from high to low CO 2 conditions CA activity increased in chloroplasts in a process dependent on the novo protein synthesis. Carbonic anhydrase activity was found in the supernatant and pellet fractions of chloroplast homogenates. The rate of photosynthesis of chloroplasts from low C i cells was higher at pH 7.0 than at pH 8.2. The alkalinization of the growth medium, which took place only in the presence of C i, was partially inhibited by DBS and completely by EZ. We suggest that in D. salina CO 2 is the general form of C i transported across the plasma membrane and the chloroplast envelope and that bicarbonate enters the cell mainly, although not entirely, by an ‘indirect’ mechanism after dehydration to CO 2. 相似文献
16.
It has been proposed that many marine macroalgae are able to utilize HCO
3
–
for photosynthesis and growth, and that energy-dependent ion pumping is involved in this process. We have therefore studied the light-dependent alkalization of the surrounding medium by two species of marine macroscopic brown algae, Fucus serratus L. and Laminaria saccharina (L.) Lamour. with the aim of investigating the role of extracellular carbonic anhydrase (EC 4.2.1.1.) in the assimilation of inorganic carbon from the seawater medium. In particular, the influence of membrane-impermeable or slowly permeable carbonic-anhydrase inhibitors on the rate of alkalization of the seawater has been investigated. Inhibition of the alkalization rate occurred in both species at an alkaline pH (pH 8.0) but no inhibition was observed at an acidic pH (pH 6.0). The alkalization was found to be light-dependent and inhibited by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea and, thus, correlated with photosynthesis. Alkalization by macroalgae has previously been shown to be proportional to inorganiccarbon uptake. We suggest that alkalization of the medium at alkaline pH in both of the species examined is mainly the consequence of an extracellular reaction. The reaction is catalyzed by extracellular carbonic anhydrase which converts HCO
3
–
to OH – and CO 2; CO 2 is then taken up through the plasmalemma. However, we do not exclude the involvement of other mechanisms of inorganic-carbon uptake.Abbreviations AZ
acetazolamide
- CA
carbonic anhydrase
- CAext
extracellular carbonic anhydrase
- C i
inorganic carbon
- DBS
dextran-bound sulfonamide
- DCMU
3-(3,4-dichloro-phenyl)-1,1-dimethylurea
- PPFD
photosynthetic photon flux density
This study was carried out with financial support by SAREC (Swedish Agency for Research Cooperation with Developing Countries), Carl Trygger's Fund for Scientific Research (Sweden), SJFR (Swedish Council for Forestry and Agricultural Research) and CICYT (Spain). Z. Ramazanov is an invited professor of Ministerio de Educación y Ciencia, Spain. 相似文献
17.
In C 4 plants carbonic anhydrase catalyzes the critical first step of C 4 photosynthesis, the hydration of CO 2 to bicarbonate. The maximum activity of this enzyme in C 4 leaf extracts, measured by H + production with saturating CO 2 and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C 3 leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO 2 concentrations was apparently just sufficient to prevent the rate of conversion of CO 2 to HCO 3− from limiting C 4 photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities. 相似文献
18.
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
19.
An experimental system consisting of a gas exchange column linked to an assimilation chamber has been developed to record continuously the free dissolved CO 2 concentration in seawater containing marine plants. From experiments performed on the red macroalga Chondrus crispus (Rhodophyta, Gigartinales), this measurement is in agreement with the free CO 2 concentration calculated from the resistance to CO 2 exchanges in a biphasic system (gas and liquid) as earlier reported. The response time of this apparatus is short enough to detect, in conditions of constant pH, a photosynthesis-caused gradient between free CO 2 and HCO 3− pools which half-equilibrates in 25 seconds. Abolished by carbonic anhydrase, the magnitude of this gradient increases with decreasing time of seawater transit from the chamber to the column apparatus. But its maximum magnitude (0.35 micromolar CO 2) is negligible compared to the difference between air and free CO 2 (11.4 micromolar CO 2). This illustrates the extent of the physical limiting-step occurring at the air-water interface when inorganic carbon consumption in seawater is balanced by dissolving gaseous CO 2. The direction of this small free CO 2/HCO 3− gradient indicates that HCO 3− is consumed during photosynthesis. 相似文献
20.
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
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