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1.
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
2.
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
3.
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
4.
The active transport of CO 2 in the cyanobacterium Synechococcus UTEX 625 was inhibited by H 2S. Treatment of the cells with up to 150 micromolar H 2S + 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 2S was removed by flushing with N 2. At constant total H 2S + HS − concentrations, inhibition of CO 2 transport increased as the ratio of H 2S to HS − increased, suggesting a direct role for H 2S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H 2S 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 2S. 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. 相似文献
5.
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 + ( K½[Na +]) were calculated. The value of both the low and the high K½(Na +) was dependent upon extracellular DIC concentration. The low K½(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 K½(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 K½(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. 相似文献
6.
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
7.
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
8.
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. 相似文献
9.
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
10.
Mass-spectrometric disequilibrium analysis was applied to investigate CO 2 uptake and HCO 3− transport in cells and chloroplasts of the microalgae Dunaliella tertiolecta and Chlamydomonas reinhardtii, which were grown in air enriched with 5% (v/v) CO 2 (high-Ci cells) or in ambient air (low-Ci cells). High- and low-Ci cells of both species had the capacity to transport CO 2 and HCO 3−, with maximum rates being largely unaffected by the growth conditions. In high- and low-Ci cells of D. tertiolecta, HCO 3− was the dominant inorganic C species taken up, whereas HCO 3− and CO 2 were used at similar rates by C. reinhardtii. The apparent affinities of HCO 3− transport and CO 2 uptake increased 3- to 9-fold in both species upon acclimation to air. Photosynthetically active chloroplasts isolated from both species were able to transport CO 2 and HCO 3−. For chloroplasts from C. reinhardtii, the concentrations of HCO 3− and CO 2 required for half-maximal activity declined from 446 to 33 μm and 6.8 to 0.6 μm, respectively, after acclimation of the parent cells to air; the corresponding values for chloroplasts from D. tertiolecta decreased from 203 to 58 μm and 5.8 to 0.5 μm, respectively. These results indicate the presence of inducible high-affinity HCO 3− and CO 2 transporters at the chloroplast envelope membrane. 相似文献
11.
An O 2 electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O 2 concentrations on photosynthetic O 2 evolution. At pH = 5.5 and 20% O 2, cells grown in HCO 3− medium (low CO 2, pH ≥ 9.0) exhibited a higher affinity for external CO 2 (K ½(CO 2) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO 2 medium (pH = 6.5), (K ½(CO 2) = 94 ± 16 micromolar). With O 2 ≤ 2% in contrast, both types of cells showed a high apparent affinity (K ½(CO 2) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO 2 medium (pH = 6.5). The high-pH, HCO 3−-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO 2 which exceeded the CO 2 externally supplied, and the reverse situation has been observed in high-CO 2-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO 3− (K ½[HCO 3−]) was 0.6 ± 0.2 millimolar, at 20% O 2. But under low O 2 concentrations (≤2%), surprisingly, an inhibition of net O 2 evolution was elicited, which was maximal at low HCO 3− concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO 2 medium, (b) Chara cells are able to accumulate CO 2 internally by means of a process apparently independent of the plasmalemma HCO 3− transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO 3−: pseudocyclic electron flow, sustained by O 2 photoreduction, may produce the additional ATP needed for the HCO 3− transport. 相似文献
12.
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
13.
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
14.
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
15.
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
16.
The pH of the medium during CO 2 uptake into the intracellular inorganic carbon (C i) pool of a high CO 2-requiring mutant (E 1) and wild type of Anacystis nidulans R2 was measured. Experiments were performed under conditions where photosynthetic CO 2 fixation is inhibited. There was an acidification of the medium during CO 2 uptake in the light and an alkalization during CO 2 efflux after darkening. A one to one stoichiometry existed between the amounts of H + appearing in the medium and CO 2 taken up into the intracellular C i pool, regardless of the carbon species transported. The results indicate that (a) CO 2 is taken up simultaneously with an efflux of equimolar H +, probably produced as a result of CO 2 hydration during transport and (b) HCO 3− produced by hydration of CO 2 in the medium was transported into the cells without accompanying net flux of H + or OH −. The influx and efflux of C i during C i transport produced nonequilibrium between CO 2 and HCO 3− in the medium, with the concentration of HCO 3− being higher than that expected under equilibrium conditions. The nonequilibrium was present even under the conditions where the influx of C i is compensated by its efflux. The direction of this nonequilibrium suggested that efflux of HCO 3− occurs during uptake of C i. 相似文献
17.
Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O 2 evolution, dissolved inorganic carbon accumulation, and photosynthetic CO 2 fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO 2-concentrating process. This COS inhibition can be attributed to inhibition of external HCO 3− conversion to CO 2 and OH − by the carbonic anhydrase component of an active CO 2 pump. At a low pH of 5 to 6, COS stimulated O 2 evolution during photosynthesis by algae with low CO 2 in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO 2. Although COS had less effect on HCO 3− accumulation at pH 9 by a HCO 3− pump in Scenedesmus, COS reduced O 2 evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO 2 and H 2S, its inhibition of the CO 2 pump activity and photosynthesis is not accurate, when measured by O 2 evolution, by NaH 14CO 3 accumulation, or by 14CO 2 fixation. 相似文献
18.
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
19.
The nature of the inorganic carbon (C i) species actively taken up by cyanobacteria CO 2 or HCO 3− has been investigated. The kinetics of CO 2 uptake, as well as that of HCO 3− uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO 2 was higher than that for HCO 3−. Though the calculated Vmax was the same in both cases, the maximum rate of uptake actually observed was higher when HCO 3− was supplied. C i uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO 2 was the species supplied. Observations of photosynthetic rate as a function of intracellular C i level (following supply of CO 2 or HCO 3− for 5 seconds) led to the inference that HCO 3− is the species which arrives at the inner membrane surface, regardless of the species supplied. When the two species were supplied simultaneously, mutual inhibition of uptake was observed. On the basis of these and other results, a model is proposed postulating that a carboic anhydrase-like subunit of the Ci transport apparatus binds CO2 and releases HCO3− at or near a membrane porter. The latter transports HCO3− ions to the cell interior. 相似文献
20.
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
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