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1.
Statistical analysis of Km (CO 2) values of ribulose-1,5-bisphosphate (RuBP) carboxylase from 35 C 4 grass species shows that the mean value for PEP-carboxykinase (PCK) type C 4 species (41.4±s.e. 2.2 μM CO 2) is significantly different from that of NAD-malic enzyme (NAD-ME) type species (55.3±3.1 μM CO 2) or NADP-malic enzyme (NADP-ME type species (52.5±s.e. 2.0μM CO 2). These C 4 type differences remain detectable within both the eu-panicoid and chloridoid grass subfamilies. By contrast, no between-subfamily differences were found within C 4 types. Variation in Km (CO 2) values of RuBP carboxylase may be related to in vivo differences in CO 2 concentration at the enzyme site, mediated perhaps by differences in CO 2-leakiness of C 4 leaf ‘photosynthetic carbon reduction’ (PCR or ‘Kranz’) tissue. 相似文献
2.
Free ribulose bisphosphate (RuBP 4?) rather than its magnesium complex (RuBP-Mg 2?) was the apparent substrate for spinach ribulose bisphosphate carboxylase/oxygenase. The apparent K m for total RuBP (pH 8.0 at 30° C) increased with increasing Mg 2+ concentrations from 11.6 μM at 13.33 mM Mg 2+ to 32.6 μM at 40.33 mM Mg 2+. Similarly the apparent K m for RuBP-Mg 2? complex increased with increasing Mg 2+ from 9.4 μM at 13.33 mM Mg 2+ to 29.7 μM at 40.33 mM Mg 2+. However, the K m values for uncomplexed RuBP 4? were independent of the (saturating) concentration of Mg 2+ (K m=2.2 μM). The V max did not vary with the changing concentrations of Mg 2+. In contrast, the K m for total RuBP remained constant with varying Mg 2+ concentrations (K m=59.5 μM) for the enzyme from R. rubrum. The apparent K m for the RuBP-Mg 2? complex decreased with increasing Mg 2+ concentrations from 16.0 μM at 7.5 mM Mg 2+ to 5.9 μM at 27.5 mM Mg 2+. The initial velocity for the C. vinosum enzyme was also found to be independent of the (saturating) concentration of Mg 2+ when total RuBP was varied in the assay. Thus the response to total RuBP by these two bacterial enzymes, which markedly differ in structure, was closely similar. 相似文献
3.
Free ribulose hisphosphate (RuBP 4?) rather than its magnesium complex (RuBP-Mg 2?) was the apparent substrate for spinach ribulose bisphosphate carboxylase/oxygenase. The apparent K m for total RuBP (pH 8.0 at 30° C) increased with increasing Mg 2+ concentrations from 11.6 μM at 13.33 mM Mg 2+ to 32.6 μM at 40.33 mM Mg 2+. Similarly the apparent K m for RuBP-Mg 2? complex increased with increasing Mg 2+ from 9.4 μM at 13.33 mM Mg 2+ to 29.7 μM at 40.33 mM Mg 2+. However, the K m values for uncomplexed RuBP 4? were independent of the (saturating) concentration of Mg 2+ (K m=2.2 μM). The V max did not vary with the changing concentrations of Mg 2+. In contrast, the K m for total RuBP remained constant with varying Mg 2+ concentrations (K m=59.5 μM) for the enzyme from R. rubrum. The apparent K m for the RuBP-Mg 2? complex decreased with increasing Mg 2+ concentrations from 16.0 μM at 7.5 mM Mg 2+ to 5.9 μM at 27.5 mM Mg 2+. The initial velocity for the C. vinosum enzyme was also found to be independent of the (saturating) concentration of Mg 2+ when total RuBP was varied in the assay. Thus the response to total RuBP by these two bacterial enzymes, which markedly differ in structure, was closely similar. 相似文献
4.
Ribulosebisphosphate carboxylase can exist in two forms having different kinetic properties. The fraction of enzyme present in each of the two forms is determined by both the absolute and the relative amounts of substrates and other effector molecules in solution with the enzyme. High CO 2 levels induce formation of an active CO 2 form of the enzyme while high ribulosebisphosphate (RuBP) levels cause formation of a much less active form. The CO 2 form is characterized by a high Km(RuBP), low Km(CO2), and relatively high V. The ribulosebisphosphate form has comparatively lower Km(RuBP) and V and higher Km(CO2). CO 2 appears to bind before RuBP in the reaction sequence catalyzed by the CO 2 form; this catalytic binding order is apparently reversed in the RuBP form. Steady state rates of enzyme reaction reflect the contributions of both these forms. A brief model, based on the cooperative effects of binding at a small number of catalytic or activator sites in the multimeric enzyme, is presented to account for the changes in enzyme activity with varying substrate and effector molecule concentrations. 相似文献
5.
Ribulosediphosphate carboxylase, partially purified from corn leaves, demonstrates a low K m(CO 2) of 19 μM if stabilized with ribose-5-phosphate during extraction. It also exhibits a ribulosediphosphate dependent uptake of oxygen, similar to that observed with spinach carboxylase. The low K m(CO 2) is similar to the apparent K m(CO 2) for photosynthesis by intact corn tissue and requires reconsideration of the hypothesis that CO 2 is concentrated in the bundle sheath cell by the C 4 pathway during photosynthesis. 相似文献
6.
The Km(CO 2) ancl V max of ribulose 1,5-bisphosphate (RuBP) carboxylaseand its protein ratio to total soluble protein from Oryza speciesincluding cultivars (25 varieties) and wild types (11 species,21 strains) were surveyed. Their variabilities among cultivarsof O. sativa were very small. The averages of the Km(CO 2) andV max values and the ratio of carboxylase to soluble protein,and their standard errors were 10.2?1.0µM, 1.72?0.13units.mg 1(pH 8.0 and 25?C) and 52?2%, respectively. However, some differencesseemed to exist based on genome constitution in the Oryza genus.RuBP carboxylases from the species with the AgAg genome, O.graberrima and O. breviligulate, exhibited low Km(CO 2) values(8.0?0.8 µM). High V max was associated with the CC genome,O. eichingeri and O. officinalis (2.08?0.15 units.mg 1).A higher ratio of RuBP carboxylase protein to soluble proteinwas found for the AA genome, O. sativa and O. perennis. (Received September 24, 1986; Accepted April 15, 1987) 相似文献
7.
The low activity of ribulose bisphosphate carboxylase from darkened soybean ( Glycine max [L.] Merr. cv. Bragg) leaves was not raised to the level of that from leaves in the light by CO 2 and Mg 2+, even after a 4-h incubation. The extract of darkened leaves, unlike the extract from illuminated leaves, was not fully CO 2/Mg 2+-activatable after Sephadex gel filtration in the absence of Mg 2+. (NH 4) 2SO 4 fractionation eliminated the inhibition effect found in the dark extracts resulting in similar rates for the extracts obtained from leaves in the dark and light. Although the Vmax values of the gel-filtered extracts from dark and light leaves differed by 3-fold, the Km(CO 2)-values were the same (12.7 μM), as were the Km(RuBP)-values (250 μM). These data support the hypothesis that for soybean leaves in the dark a tightly-binding inhibitor renders much of the ribulose bisphosphate carboxylase enzyme catalytically non-functional. 相似文献
8.
The enzymic properties of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase purified from rice ( Oryza sativa L.) leaves were studied. Rice RuBPcarboxylase, activated by preincubation with CO 2 and Mg 2+ like other higher plant carboxylases, had an activation equilibrium constant ( KcKMg) of 1.90 × 10 5 to 2.41 × 10 5 micromolar 2 (pH 8.2 and 25°C). Kinetic parameters of carboxylation and oxygenation catalyzed by the completely activated enzyme were examined at 25°C and the respective optimal pHs. The Km(CO 2), Km(RuBP), and Vmax values for carboxylation were 8 micromolar, 31 micromolar, and 1.79 units milligram −1, respectively. The Km(O 2), Km(RuBP), and Vmax values for oxygenation were 370 micromolar, 29 micromolar, and 0.60 units milligram −1, respectively. Comparison of rice leaf RuBP carboxylase with other C3 plant carboxylases showed that it had a relatively high affinity for CO2 but the lowest catalytic turnover number (Vmax) among the species examined. 相似文献
9.
Partially purified ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) was isolated from diploid and tetraploid cultivars of ryegrass ( Lolium perenne L.) using two separate methods. The apparent Km (CO 2) values for the enzymes prepared by either method did not differ significantly between diploid and tetraploid when assayed by two separate techniques. The unpurified enzymes from freshly lysed (and fully functional) protoplasts of both diploid and tetraploid cultivars gave virtually identical apparent Km (CO 2) values. There was no indication of large differences in affinity for CO 2 of illuminated intact protoplasts from the two cultivars. 相似文献
10.
The enzyme-catalyzed activation of ribulosebisphosphate carboxylase/oxygenase (rubisco) was investigated in an illuminated reconstituted system containing thylakoid membranes, rubisco, ribulosebisphosphate (RuBP), MgCl 2, carbonic anhydrase, catalase, the artificial electron acceptor pyocyanine, and partially purified rubisco activase. Optimal conditions for light-induced rubisco activation were found to include 100 micrograms per milliliter rubisco, 300 micrograms per milliliter rubisco activase, 3 millimolar RuBP, and 6 millimolar free Mg 2+ at pH 8.2. The half-time for rubisco activation was 2 minutes, and was 4 minutes for rubisco deactivation. The rate of rubisco deactivation was identical in the presence and absence of activase. The Kact(CO 2) of rubisco activation in the reconstituted system was 4 micromolar CO 2, compared to a Kact(CO 2) of 25 to 30 micromolar CO 2 for the previously reported spontaneous CO 2/Mg 2+ activation mechanism. The activation process characterized here explains the high degree of rubisco activation at the physiological concentrations of 10 micromolar CO 2 and 2 to 4 millimolar RuBP found in intact leaves, conditions which lead to almost complete deactivation of rubisco in vitro. 相似文献
11.
A survey of the Km(CO 2) values of ribulose-1,5-bisphosphate carboxylase from 60 grass species shows that enzyme from C 3 grasses consistently exhibits lower Km(CO 2) than does that from C 4 grasses. Systematically ordered variation in Km(CO 2) of ribulose-1,5-bisphosphate carboxylases from C 3 and C 4 grasses is also apparent and, among C 4 grasses, this shows some correlation with C 4 types. 相似文献
12.
Changes in photosynthesis and the ribulose 1,5-bisphosphate (RuBP) carboxylase level were examined in the 12th leaf blades of rice ( Oryza sativa L.) grown under different N levels. Photosynthesis was determined using an open infrared gas analysis system. The level of RuBP carboxylase was measured by rocket immunoelectrophoresis. These changes were followed with respect to changes in the activities of RuBP carboxylase, ribulose 5-phosphate kinase, NADP-glyceraldehyde 3-phosphate dehydrogenase, and 3-phosphoglyceric acid kinase. RuBP carboxylase activity was highly correlated with the net rate of photosynthesis (r = 0.968). Although high correlations between the activities of other enzymes and photosynthesis were also found, the activity per leaf of RuBP carboxylase was much lower than those of other enzymes throughout the leaf life. The specific activity of RuBP carboxylase on a milligram of the enzyme protein basis remained fairly constant (1.16 ± 0.07 micromoles of CO2 per minute per milligram at 25°C) throughout the experimental period. Kinetic parameters related to CO2 fixation were examined using the purified carboxylase. The Km(CO2) and Vmax values were 12 micromolar and 1.45 micromoles of CO2 per minute per milligram, respectively (pH 8.2 and 25°C). The in vitro specific activity calculated at the atomospheric CO2 level from the parameters was comparable to the in situ true photosynthetic rate per milligram of the carboxylase throughout the leaf life. The results indicated that the level of RuBP carboxylase protein can be a limiting factor in photosynthesis throughout the life span of the leaf. 相似文献
13.
Ribulose-1,5-bisphosphate (RuBP) pool size was determined at regular intervals during the growing season to understand the effects of tropospheric ozone concentrations, elevated atmospheric carbon dioxide concentrations and their interactions on the photosynthetic limitation by RuBP regeneration. Soybean ( Glycine max [L.] Merr. cv. Essex) was grown from seed to maturity in open-top field chambers in charcoal-filtered air (CF) either without (22 nmol O 3 mol ?1) or with added O 3 (83 nmol mol ?1) at ambient (AA, 369 μmol CO 2 mol ?1) or elevated CO 2 (710 μmol mol ?1). The RuBP pool size generally declined with plant age in all treatments when expressed on a unit leaf area and in all treatments but CF-AA when expressed per unit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) binding site. Although O 3 in ambient CO 2 generally reduced the RuBP pool per unit leaf area, it did not change the RuBP pool per unit Rubisco binding site. Elevated CO 2, in CF or O 3-fumigated air, generally had no significant effect on RuBP pool size, thus mitigating the negative O 3 effect. The RuBP pools were below 2 mol mol ?1 binding site in all treatments for most of the season, indicating limiting RuBP regeneration capacity. These low RuBP pools resulted in increased RuBP regeneration via faster RuBP turnover, but only in CF air and during vegetative and flowering stages at elevated CO 2. Also, the low RuBP pool sizes did not always reflect RuBP consumption rates or the RuBP regeneration limitation relative to potential carboxylation (%RuBP). Rather, %RuBP increased linearly with decrease in the RuBP pool turnover time. These data suggest that amelioration of damage from O 3 by elevated atmospheric CO 2 to the RuBP regeneration may be in response to changes in the Rubisco carboxylation. 相似文献
14.
Photosynthesis in two cultivars of Triticum aestivum was compared with photosynthesis in two lines having the same nuclear genomes but with cytoplasms derived from T. boeoticum. The in-vitro specific activity of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) isolated from lines with T. boeoticum cytoplasm was only 71% of that of normal T. aestivum. By contrast, the RuBPCase activities calculated from the CO 2-assimilation rate at low partial pressures of CO 2, p(CO 2), were the same for all lines for a given RuBPCase content. This indicates that both types of RuBPCase have the same turnover numbers in-vivo of 27.5 mol CO 2·(mol enzyme) –1·s –1 (23°). The rate of CO 2 assimilation measured at normal p(CO 2), p
a
=340 bar, and high irradiance could be quantitatively predicted from the amount of RuBPCase protein. The maximum rate of RuBP regeneration could also predict the rate of CO 2 assimilation at normal ambient conditions. Therefore, the maximum capacities for RuBP carboxylation and RuBP regeneration appear to be well-balanced for normal ambient conditions. As photosynthetic capacity declined with increasing leaf age, the capacities for RuBP carboxylation and RuBP regeneration declined in parallel.Abbreviations PAR
photosynthetically active radiation
- RuBP(Case)
ribulose-1,5-bisphosphate (carboxylase) 相似文献
15.
Like many enzymes, the biogenesis of the multi-subunit CO 2-fixing enzyme ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) in different organisms requires molecular chaperones. When expressed in Escherichia coli, the large (L) subunits of the Rubisco from the archaeabacterium Methanococcoides burtonii assemble into functional dimers (L 2). However, further assembly into pentamers of L 2 (L 10) occurs when expressed in tobacco chloroplasts or E. coli producing RuBP. In vitro analyses indicate that the sequential assembly of L 2 into L 10 (via detectable L 4 and L 6 intermediates) occurs without chaperone involvement and is stimulated by protein rearrangements associated with either the binding of substrate RuBP, the tight binding transition state analog carboxyarabinitol-1,5-bisphosphate, or inhibitory divalent metal ions within the active site. The catalytic properties of L 2 and L 10 M. burtonii Rubisco (MbR) were indistinguishable. At 25 °C they both shared a low specificity for CO 2 over O 2 (1.1 mol·mol −1) and RuBP carboxylation rates that were distinctively enhanced at low pH (∼4 s −1 at pH 6, relative to 0.8 s −1 at pH 8) with a temperature optimum of 55 °C. Like other archaeal Rubiscos, MbR also has a high O 2 affinity ( Km(O 2) = ∼2.5 μ m). The catalytic and structural similarities of MbR to other archaeal Rubiscos contrast with its closer sequence homology to bacterial L 2 Rubisco, complicating its classification within the Rubisco superfamily. 相似文献
16.
Ribulose diphosphate carboxylase was found to exist in two distinct kinetic forms in spinach leaf extracts. One form displayed an apparent Km for CO 2 in excess of 200 μm and is likely to be the form purified and studied by many previous workers. However, if leaf extracts were prepared in the presence of Mg 2+ and atmospheric levels of CO 2, the recently described high-affinity form was obtained. It had a Km for CO 2 of about 20 μm, was quite stable even at 25 °C, and its properties were consistent with it being the form which operates in photosynthesis in vivo. Mg 2+ was also able to convert the high- Km (CO 2) form to the low- Km (CO 2) form when it was added to an extract which had been prepared in its absence. Mg 2+ was more effective in causing this conversion if bicarbonate was added as well. This activating effect of bicarbonate is a probable cause of previously reported apparent homotropic effects of bicarbonate on ribulose diphosphate carboxylase activity. It is possible that the apparently high- Km (CO 2) form is not intrinsically active and appears to have activity only by virtue of the low- Km (CO 2) form produced by contact with Mg 2+ and bicarbonate (or CO 2) during the course of the assay. Extracts prepared with ribose 5-phosphate in the absence of Mg 2+ also showed low- Km (CO 2) carboxylase activity initially, but the presence of this sugar phosphate was deleterious during storage at 25 °C, where it promoted conversion to the apparently high- Km (CO 2) form.Effects on the affinity of ribulose diphosphate carboxylase for CO 2 were paralleled by effects on the activity of the associated ribulose diphosphate oxygenase. Treatments which produced the low- Km (CO 2) form of the carboxylase also resulted in high oxygenase activity, and it is possible that the apparently high- Km (CO 2) form of the carboxylase has little, if any, oxygenase activity associated with it.The carboxylase and oxygenase activities of the low- Km (CO 2) form showed broad and quite similar responses to pH variation, and the oxygenase had a Km for O 2 of 0.22 mm.The stability of the low- Km (CO 2) form in the presence of Mg 2+ and bicarbonate was quite sufficient for it to be partially purified by Sepharose chromatography. The significance of the low- Km (CO 2) form is discussed with respect to activation of photosynthesis by Mg 2+. 相似文献
17.
2-Carboxy-d-hexitol 1,6-bisphosphate (CHBP) has been prepared from d-fructose 1,6-bisphosphate and cyanide. DEAE-Sephadex chromatography separated the reaction products into two fractions which were identified as CHBP and CHBP-lactone. CHBP is presumably a mixture of two diastereomers, 2-carboxy-d-glucitol 1,6-bisphosphate and 2-carboxy-d-mannitol 1,6-bisphosphate, but an attempt to separate these compounds was not successful. The material in the CHBP-lactone peak had no effect on d-ribulose 1,5-bisphosphate (RuBP) carboxylase. However, CHBP was a potent reversible inhibitor of RuBP carboxylases. This compound displayed an inhibition constant ( Ki at pH 8.0 and 30 °C) of 1–2 μm with the enzymes from spinach and barley, while the Ki was 60–70 μm with bacterial RuBP carboxylases from Pseudomonas oxalaticus and Rhodospirillum rubrum. The mode of inhibition was competitive with respect to RuBP for all the carboxylases, and noncompetitive with respect to CO 2 for the enzymes from spinach, P. oxalaticus and R. rubrum. The results indicate that, in the binding of certain organic phosphates by RuBP carboxylases, there may be a fundamental difference between the enzymes isolated from microbial and from higher plant sources. RuBP oxygenase activities from spinach and P. oxalaticus were also inhibited by CHBP, with Ki values which were similar to those obtained with the carboxylase activity of the same enzymes. The mode of inhibition of the oxygenase activities was also competitive with respect to RuBP. Thus, it seems that the binding of CHBP is similar for the carboxylase and oxygenase reactions of the same enzyme. 相似文献
18.
Trypsin digestion reduces the sizes of both the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) from the green alga Chlamydomonas reinhardtii. Incubation of either CO 2/Mg 2+ -activated or nonactivated enzyme with the transition-state analogue carboxyarabinitol bisphosphate protects a trypsin-sensitive site of the large subunit, but not of the small subunit. Incubation of the nonactivated enzyme with ribulosebisphosphate (RuBP) provided the same degree of protection. Thus, the very tight binding that is a characteristic of the transitionstate analogue is apparently not required for the protection of the trypsin-sensitive site of the large subunit. Mutant enzymes that have reduced CO 2/O 2 specificities failed to bind carboxyarabinitol bisphosphate tightly. However, their large-subunit trypsin-sensitive sites could still be protected. The K
m values for RuBP were not significantly changed for the mutant enzymes, but the V
max values for carboxylation were reduced substantially. These results indicate that the failure of the mutant enzymes to bind the transition-state analogue tightly is primarily the consequence of an impairment in the second irreversible binding step. Thus, in all of the mutant enzymes, defects appear to exist in stabilizing the transition state of the carboxylation step, which is precisely the step proposed to influence the CO 2/O 2 specificity of Rubisco.Abbreviations and Symbols CABP
2-carboxyarabinitol 1,5-bisphosphate
- enol-RuBP
2,3-enediolate of ribulose 1,5-bisphosphate
-
K
c
K
m for CO 2
-
K
o
K
m for O 2
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate
-
V
c
V
max for carboxylation
-
V
o
V
max for oxygenation
Paper No. 9313, Journal Series, Nebraska Agricultural Research DivisionThis work was supported by National Science Foundation grant DMB-8703820. We thank Drs. Archie Portis and Raymond Chollet for their helpful comments, and also thank Dr. Chollet for graciously providing CABP and [ 14C]CABP. 相似文献
19.
Soybean ( Glycine max [L.] cv Bragg) was grown at 330 or 660 microliters CO 2 per liter in outdoor, controlled-environment chambers. When the plants were 50 days old, drought stress was imposed by gradually reducing irrigation each evening so that plants wilted earlier each succeeding day. On the ninth day, as the pots ran out of water CO 2 exchange rate (CER) decreased rapidly to near zero for the remainder of the day. Both CO 2-enrichment and drought stress reduced the total (HCO 3−/Mg 2+-activated) extractable ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity, as expressed on a chlorophyll basis. In addition, drought stress when canopy CER values and leaf water potentials were lowest, reduced the initial (nonactivated) RuBPCase activity by 50% compared to the corresponding unstressed treatments. This suggests that moderate to severe drought stress reduces the in vivo activation state of RuBPCase, as well as lowers the total activity. It is hypothesized that stromal acidification under drought stress causes the lowered initial RuBPCase activities. The Km(CO 2) values of activated RuBPCase from stressed and unstressed plants were similar; 15.0 and 12.6 micromolar, respectively. RuBP levels were 10 to 30% lower in drought stressed as compared to unstressed treatments. However, RuBP levels increased from near zero at night to around 150 to 200 nanomoles per milligram chlorophyll during the day, even as water potentials and canopy CERs decreased. This suggests that the rapid decline in canopy CER cannot be attributed to drought stress induced limitations in the RuBP regeneration capability. Thus, in soybean leaves, a nonstomatal limitation of leaf photosynthesis under drought stress conditions appears due, in part, to a reduction of the in vivo activity of RuBPCase. Because initial RuBPCase activities were not reduced as much as canopy CER values, this enzymic effect does not explain entirely the response of soybean photosynthesis to drought stress. 相似文献
20.
Abstract Some characteristics of photosynthetic inorganic carbon uptake by Palmaria palmata, a marine red macroalga, have been measured under physiological conditions in artificial seawater. The apparent affinity of thallus for CO 2 [K 1/2(CO 2)] at pH 8.0 and 15°C was 21.4±3.0mmol m ?3 CO 2 under air, and 25.7±70mmol m ?3 CO 2 under N 2. The corresponding values of Vmax were 2.98 ± 0.42 and 3.65±0.87 mmol O 2 evolved g Chr ?1 s ?l. The apparent K m(CO 2) of isolated ribulose bisphosphate carboxylase was determined at pH 8.0 and 30 °C to be 30.2 mmol m ?3 CO 2, and the corresponding value of V max was 19.67 μniol CO 2 g protein ?1 s ?1. The CO 2 compensation points of the thallus were measured in artificial seawater at pH 8.0 under air and N 2, using a gas-chromatographic method. The values were relatively low, rising from 10 cm 3 m ?3 at 15°C, to 35 cm 3 m ?3 at 25°C, but were not affected by the O 2 concentration. The lack of an effect of O 2 on photosynthesis and on compensation point indicates that there is little photorespiratory CO 2 loss in this macroalga. The high affinity of the thallus for CO 2, and the low CO 2 compensation concentrations, are consistent with the occurrence of bicarbonate uptake in this alga. 相似文献
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