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1.
A mass spectrometer with a membrane inlet was used to monitor light-driven O 2 evolution, O 2 uptake, and CO 2 uptake in suspensions of algae ( Scenedesmus obliquus). We observed the following. ( a) The rate of O 2 uptake, which, in the presence of iodoacetamide, replaces the uptake of CO 2, showed a distinct plateau (V max) beyond ~30% O 2 and was half-maximal at ~8% O 2. We concluded that this light-driven O 2 uptake process, which does not involve carbon compounds, is saturated at lower O 2 concentrations than are photorespiration and glycolate formation. ( b) In the absence of inhibitor, O 2 evolution was relatively unaffected by the presence or absence of CO 2. During the course of CO 2 depletion, electron flow to CO 2 was replaced by an equivalent flow to O 2. ( c) There was a distinct delay between the cessation of CO 2 uptake and the increase in O 2 uptake. We ascribe this delay to the transient utilization of another electron acceptor—possibly bicarbonate or another bound form of CO 2. 相似文献
2.
Concurrent O 2 evolution, O 2 uptake, and CO 2 uptake by illuminated maize ( Zea mays) leaves were measured using 13CO 2 and 18O 2. Considerable O 2 uptake occurred during active photosynthesis. At CO 2 compensation, O 2 uptake increased. Associated with this increase was a decrease in O 2 release such that a stoichiometric exchange of O 2 occurred. The rate of O 2 exchange at CO 2 compensation was directly related to O 2 concentration in the atmosphere at least up to 8% (v/v). 相似文献
3.
Light-dependent O 2 exchange was measured in intact, isolated soybean ( Glycine max. var. Williams) cells using isotopically labeled O 2 and a mass spectrometer. The dependence of O 2 exchange on O 2 and CO 2 was investigated at high light in coupled and uncoupled cells. With coupled cells at high O 2, O 2 evolution followed similar kinetics at high and low CO 2. Steady-state rates of O 2 uptake were insignificant at high CO 2, but progressively increased with decreasing CO 2. At low CO 2, steady-state rates of O 2 uptake were 50% to 70% of the maximum CO 2-supported rates of O 2 evolution. These high rates of O 2 uptake exceeded the maximum rate of O 2 reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O 2-uptake process ( i.e. photorespiration). Rates of O2 exchange in uncoupled cells were half-saturated at 7% to 8% O2. Initial rates (during induction) of O2 exchange in uninhibited cells were also half-saturated at 7% to 8% O2. In contrast, steady-state rates of O2 evolution and O2 uptake (at low CO2) were half-saturated at 18% to 20% O2. O2 uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O2 for photoreductant. These results suggest that two mechanisms (O2 reduction and photorespiration) are responsible for the light-dependent O2 uptake observed in uninhibited cells under CO2-limiting conditions. The relative contribution of each process to the rate of O2 uptake appears to be dependent on the O2 level. At high O2 concentrations (≥40%), photorespiration is the major O2-consuming process. At lower (ambient) O2 concentrations (≤20%), O2 reduction accounts for a significant portion of the total light-dependent O2 uptake. 相似文献
4.
The nature of the different processes of O 2 uptake involved in the light in the red macroalga Chondrus crispus Stackhouse ( Rhodophyta, Gigartinales) was investigated. At limiting CO 2, INH (2.5 mM) did not alter the O 2 uptake rate. Glycolate was not excreted and did not accumulate within the cells. KCN reduced the rate of O 2 uptake in the light by 76% at limiting CO 2 and by 43% at saturating CO 2, but caused > 95% inhibition of O 2 evolution. DCMU (5 μM) totally blocked the photosynthetic electron transport chain, but allowed a residual O 2 uptake of 3.0±0.6 μmol O 2 .h ?1.g ?1 FW, irrespective of the CO 2 concentration. In saturating CO 2, a high light intensity pretreatment significantly stimulated the rate of O 2 uptake compared to net O 2 evolution, suggesting the persistence, in the light, of mitochondrial respiration. Irrespective of the CO 2 concentration, the optimum temperature for O 2 evolution was 17°C whereas dark O 2 uptake increased linearly with temperature. In contrast, O 2 uptake in the light showed an optimum at 17°C in limiting CO 2, and 21–25° C in saturating CO 2; its Q 10 was 2.4 at limiting CO 2, a value close to that of RuBP oxygenase, and 3.1 at saturating CO 2, a value close to that of dark respiration. It is concluded that: 1) mitochondrial respiration and Mehler reaction are both involved at all CO 2 concentrations, 2) RuBP oxygenase activity cannot account for more than 45%, and Mehler reaction for less than 20%, of the total O 2 uptake observed in the light at limiting CO 2. 相似文献
5.
A mass spectrometer with a membrane inlet system was used to monitor directly gaseous components in a suspension of algae. Using labeled oxygen, we observed that during the first 20 seconds of illumination after a dark period, when no net O 2 evolution or CO 2 uptake was observed, O 2 evolution was normal but completely compensated by O 2 uptake. Similarly, when CO 2 uptake was totally or partially inhibited, O 2 evolution proceeded at a high (near maximal) rate. Under all conditions, O 2 uptake balanced that fraction of the O 2 evolution which could not be accounted for by CO 2 uptake. 相似文献
6.
A mass spectrometric method combining 16O/ 18O and 12C/ 13C isotopes was used to quantify the unidirectional fluxes of O 2 and CO 2 during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O 2 uptake and CO 2 evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O 2 (61 micromoles of O 2 per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O 2 per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO 2 in darkness at a rate of 27 micromoles of CO 2 per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O 2 evolution and CO 2 fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phospho enolpyruvate carboxylase activity. Under light, CO 2 evolution by guard cell protoplasts was sharply decreased (37%), while O 2 uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO 2 assimilation and may be dissipated for other purposes such as ion uptake. 相似文献
7.
The basis of inhibition of photosynthesis by single acute O 3 exposures was investigated in vivo using analyses based on leaf gas exchange measurements. The fully expanded second leaves of wheat plants ( Triticum aestivum L. cv Avalon) were fumigated with either 200 or 400 nanomoles per mole O 3 for between 4 and 16 hours. This reduced significantly the light-saturated rate of CO 2 uptake and was accompanied by a parallel decrease in stomatal conductance. However, the stomatal limitation, estimated from the relationship between CO 2 uptake and the internal CO 2 concentration, only increased significantly during the first 8 hours of exposure to 400 nanomoles per mole O 3; no significant increase occurred for any of the other treatments. Analysis of the response of CO 2 uptake to the internal CO 2 concentration implied that the predominant factor responsible for the reduction in light-saturated CO 2 uptake was a decrease in the efficiency of carboxylation. This was 58 and 21% of the control value after 16 hours at 200 and 400 nanomoles per mole O 3, respectively. At saturating concentrations of CO 2, photosynthesis was inhibited by no more than 22% after 16 hours, indicating that the capacity for regeneration of ribulose bisphosphate was less susceptible to O 3. Ozone fumigations also had a less pronounced effect on light-limited photosynthesis. The maximum quantum yield of CO 2 uptake and the quantum yield of oxygen evolution showed no significant decline after 16 hours with 200 nanomoles per mole O 3, requiring 8 hours at 400 nanomoles per mole O 3 before a significant reduction occurred. The photochemical efficiency of photosystem II estimated from the ratio of variable to maximum chlorophyll fluorescence and the atrazine-binding capacity of isolated thylakoids demonstrated that photochemical reactions were not responsible for the initial inhibition of CO 2 uptake. The results suggest that the apparent carboxylation efficiency appears to be the initial cause of decline in photosynthesis in vivo following acute O 3 fumigation. 相似文献
8.
In spinach ( Spinacia oleracea) and barley ( Hordeum vulgare) leaves, chlorophyll a fluorescence and O 2 evolution have been measured simultaneously following re-illumination after a dark interval or when steady state photosynthesis has been perturbed by changes in the gas phase. In high CO 2 concentrations, both O 2 and fluorescence can display marked dampening oscillations that are antiparallel but slightly out of phase (a rise or fall in fluorescence anticipating a corresponding fall or rise in O 2 by about 10 to 15 seconds). Infrared gas analysis measurements showed that CO 2 uptake behaved like O 2 evolution both in the period of oscillation (about 1 minute) and in its relation to fluorescence. In the steady state, oscillations were initiated by increases in CO 2 or by increases or decreases in O 2. Oscillations in O 2 or CO 2 did not occur without associated oscillations in fluorescence and the latter were a sensitive indicator of the former. The relationship between such oscillations in photosynthetic carbon assimilation and chlorophyl a fluorescence is discussed in the context of the effect of ATP or NADPH consumption on known quenching mechanisms. 相似文献
9.
Wheat was cultivated in a small phytotronic chamber. 18O 2 was used to measure the O 2 uptake by the plant, which was recorded simultaneously with the O 2 evolution, net CO 2 uptake, and transpiration. At normal atmospheric CO 2 concentration, photorespiration, measured as O 2 uptake, was as important as the net photosynthesis. The level of true O 2 evolution was independent of CO 2 concentration and stayed nearly equal to the sum of net CO 2 photosynthesis and O 2 uptake. We conclude that at a given light intensity, O 2 and CO 2 compete for the reducing power produced at constant rate by the light reactions of photosynthesis. 相似文献
10.
Photosynthetic O 2 production and photorespiratory O 2 uptake were measured using isotopic techniques, in the C 3 species Hirschfeldia incana Lowe., Helianthus annuus L., and Phaseolus vulgaris L. At high CO 2 and normal O 2, O 2 production increased linearly with light intensity. At low O 2 or low CO 2, O 2 production was suppressed, indicating that increased concentrations of both O 2 and CO 2 can stimulate O 2 production. At the CO 2 compensation point, O 2 uptake equaled O 2 production over a wide range of O 2 concentrations. O 2 uptake increased with light intensity and O 2 concentration. At low light intensities, O 2 uptake was suppressed by increased CO 2 concentrations so that O 2 uptake at 1,000 microliters per liter CO 2 was 28 to 35% of the uptake at the CO 2 compensation point. At high light intensities, O 2 uptake was stimulated by low concentrations of CO 2 and suppressed by higher concentrations of CO 2. O 2 uptake at high light intensity and 1000 microliters per liter CO 2 was 75% or more of the rate of O 2 uptake at the compensation point. The response of O 2 uptake to light intensity extrapolated to zero in darkness, suggesting that O 2 uptake via dark respiration may be suppressed in the light. The response of O 2 uptake to O 2 concentration saturated at about 30% O 2 in high light and at a lower O 2 concentration in low light. O 2 uptake was also observed with the C 4 plant Amaranthus edulis; the rate of uptake at the CO 2 compensation point was 20% of that observed at the same light intensity with the C 3 species, and this rate was not influenced by the CO 2 concentration. The results are discussed and interpreted in terms of the ribulose-1,5-bisphosphate oxygenase reaction, the associated metabolism of the photorespiratory pathway, and direct photosynthetic reduction of O 2. 相似文献
11.
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
12.
Oxygen inhibition of photosynthesis and CO 2 evolution during photorespiration were compared in high CO 2-grown and air-grown Chlorella pyrenoidosa, using the artificial leaf technique at pH 5.0. High CO 2 cells, in contrast to air-grown cells, exhibited a marked inhibition of photosynthesis by O 2, which appeared to be competitive and similar in magnitude to that in higher C 3 plants. With increasing time after transfer to air, the photosynthetic rate in high CO 2 cells increased while the O 2 effect declined. Photorespiration, measured as the difference between 14CO 2 and 12CO 2 uptake, was much greater and sensitive to O 2 in high CO 2 cells. Some CO 2 evolution was also present in air-grown algae; however, it did not appear to be sensitive to O 2. True photosynthesis was not affected by O 2 in either case. The data indicate that the difference between high CO 2 and air-grown algae could be attributed to the magnitude of CO 2 evolution. This conclusion is discussed with reference to the oxygenase reaction and the control of photorespiration in algae. 相似文献
13.
The submerged aquatic plant Isoetes howellii Engelmann possesses Crassulacean acid metabolism (CAM) comparable to that known from terrestrial CAM plants. Infrared gas analysis of submerged leaves showed Isoetes was capable of net CO 2 uptake in both light and dark. CO 2 uptake rates were a function of CO 2 levels in the medium. At 2,500 microliters CO 2 per liter (gas phase, equivalent to 1.79 milligrams per liter aqueous phase), Isoetes leaves showed continuous uptake in both the light and dark. At this CO 2 level, photosynthetic rates were light saturated at about 10% full sunlight and were about 3-fold greater than dark CO 2 uptake rates. In the dark, CO 2 uptake rates were also a function of length of time in the night period. Measurements of dark CO 2 uptake showed that, at both 2,500 and 500 microliters CO 2 per liter, rates declined during the night period. At the higher CO 2 level, dark CO 2 uptake rates at 0600 h were 75% less than at 1800 h. At 500 microliters CO 2 per liter, net CO 2 uptake in the dark at 1800 h was replaced by net CO 2 evolution in the dark at 0600 h. At both CO 2 levels, the overnight decline in net CO 2 uptake was marked by periodic bursts of accelerated CO 2 uptake. CO 2 uptake in the light was similar at 1% and 21% O 2, and this held for leaves intact as well as leaves split longitudinally. Estimating the contribution of light versus dark CO 2 uptake to the total carbon gain is complicated by the diurnal flux in CO 2 availability under field conditions. 相似文献
14.
C 4 grasses of the NAD‐ME type ( Astrebla lappacea, Eleusine coracana, Eragrostis superba, Leptochloa dubia, Panicum coloratum, Panicum decompositum) and the NADP‐ME type ( Bothriochloa bladhii, Cenchrus ciliaris, Dichanthium sericeum, Panicum antidotale, Paspalum notatum, Pennisetum alopecuroides, Sorghum bicolor) were used to investigate the role of O 2 as an electron acceptor during C 4 photosynthesis. Mass spectrometric measurements of gross O 2 evolution and uptake were made concurrently with measurements of net CO 2 uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40 °C. In all C 4 grasses gross O 2 uptake increased with increasing irradiance at very high CO 2 partial pressures ( pCO 2) and was on average 18% of gross O 2 evolution. Gross O 2 uptake at high irradiance and high pCO 2 was on average 3.8 times greater than gross O 2 uptake in the dark. Furthermore, gross O 2 uptake in the light increased with O 2 concentration at both high CO 2 and the compensation point, whereas gross O 2 uptake in the dark was insensitive to O 2 concentration. This suggests that a significant amount of O 2 uptake may be associated with the Mehler reaction, and that the Mehler reaction varies with irradiance and O 2 concentration. O 2 exchange characteristics at high pCO 2 were similar for NAD‐ME and NADP‐ME species. NAD‐ME species had significantly greater O 2 uptake and evolution at the compensation point particularly at low irradiance compared to NADP‐ME species, which could be related to different rates of photorespiratory O 2 uptake. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and gross O 2 evolution at high light and high pCO 2. 相似文献
15.
Unidirectional O 2 fluxes were measured with 18O 2 in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O 2, O 2 uptake was maximum at 60 microliters per liter CO 2. At lower CO 2 concentrations, it was strongly inhibited, as was photosynthetic O 2 evolution. At 2% O 2, there remained a substantial O 2 uptake, even at high CO 2 level; the O 2 evolution was inhibited at CO 2 concentrations under 330 microliters per liter. The O 2 uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO 2. The behavior of the O 2 uptake and the study of the CO 2 compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis. 相似文献
16.
Photosynthetic CO 2 and O 2 exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using 18O 2 to trace O 2 uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18°C, 330 microliters per liter CO 2, 21% O 2) the net photosynthetic rate was around 40 micromoles CO 2 per gram dry weight per hour in H. cupressiforme and 50 micromoles CO 2 per gram dry weight per hour in D. scoparium. The CO 2 compensation point lay between 45 and 55 microliters per liter CO 2 and the enhancement of net photosynthesis by 3% O 2versus 21% O 2 was 40 to 45%. The ratio of O 2 uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO 2 showed a high apparent Km (CO 2) even in nonsaturating light. On the other hand, O 2 uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O 2 concentration (saturating level as low as 30% O 2), and by 65% by decreasing the CO 2 concentration to the compensation point. Although O 2 is a competitive inhibitor of CO 2 uptake it could not replace CO 2 completely as an electron acceptor, and electron flow, expressed as gross O 2 production, was inhibited by both high O 2 and low CO 2 levels. At high CO 2, O 2 uptake was 70% lower than the maximum at the CO 2 compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction. 相似文献
17.
We found similarities between the effects of low night temperatures (5°C–10°C) and slowly imposed water stress on photosynthesis in grapevine ( Vitis vinifera L.) leaves. Exposure of plants growing outdoors to successive chilling nights caused light- and CO 2-saturated photosynthetic O 2 evolution to decline to zero within 5 d. Plants recovered after four warm nights. These photosynthetic responses were confirmed in potted plants, even when roots were heated. The inhibitory effects of chilling were greater after a period of illumination, probably because transpiration induced higher water deficit. Stomatal closure only accounted for part of the inhibition of photosynthesis. Fluorescence measurements showed no evidence of photoinhibition, but nonphotochemical quenching increased in stressed plants. The most characteristic response to both stresses was an increase in the ratio of electron transport to net O 2 evolution, even at high external CO 2 concentrations. Oxygen isotope exchange revealed that this imbalance was due to increased O 2 uptake, which probably has two components: photorespiration and the Mehler reaction. Chilling- and drought-induced water stress enhanced both O 2 uptake processes, and both processes maintained relatively high rates of electron flow as CO 2 exchange approached zero in stressed leaves. Presumably, high electron transport associated with O 2 uptake processes also maintained a high ΔpH, thus affording photoprotection. 相似文献
18.
This study examines the effect of antimycin A and nitrite on 14CO 2 fixation in intact chloroplasts isolated from spinach ( Spinacia oleracea L.) leaves. Antimycin A (2 micromolar) strongly inhibited CO 2 fixation but did not appear to inhibit or uncouple linear electron transport in intact chloroplasts. The addition of small quantities (40-100 micromolar) of nitrite or oxaloacetate, but not NH 4Cl, in the presence of antimycin A restored photosynthesis. Antimycin A inhibition, and the subsequent restoration of photosynthetic activities by nitrite or oxaloacetate, was observed over a wide range of CO 2 concentration, light intensity, and temperature. High O 2 concentration (up to 240 micromolar) did not appear to influence the extent of the inhibition by antimycin A, nor the subsequent restoration of photosynthetic activity by nitrite or oxaloacetate. Studies of O 2 exchanges during photosynthesis in cells and chloroplasts indicated that 2 micromolar antimycin A stimulated O 2 uptake by about 25% while net O 2 evolution was inhibited by 76%. O 2 uptake in chloroplasts in the presence of 2 micromolar antimycin A was 67% of total O 2 evolution. These results suggest that only a small proportion of the O 2 uptake measured was directly linked to ATP generation. The above evidence indicates that cyclic photophosphorylation is the predominant energy-balancing reaction during photosynthesis in intact chloroplasts. On the other hand, pseudocyclic O 2 uptake appears to play only a minimal role. 相似文献
19.
A closed system consisting of an assimilation chamber furnished with a membrane inlet from the liquid phase connected to a mass spectrometer was used to measure O 2 evolution and uptake by Chlamydomonas reinhardtii cells grown in ambient (0.034% CO 2) or CO 2-enriched (5% CO 2) air. At pH = 6.9, 28°C and concentrations of dissolved inorganic carbon (DIC) saturating for photosynthesis, O 2 uptake in the light (U o) equaled O 2 production (E o) at the light compensation point (15 micromoles photons per square meter per second). E o and U o increased with increasing photon fluence rate (PFR) but were not rate saturated at 600 micromoles photons per square meter per second, while net O 2 exchange reached a saturation level near 500 micromoles photons per square meter per second which was nearly the same for both, CO 2-grown and air-grown cells. Comparison of the U o/E o ratios between air-grown and CO 2-grown C. reinhardtii showed higher values for air-grown cells at light intensities higher than light compensation. For both, air-grown and CO 2-grown algae the rates of mitochondrial O 2 uptake in the dark measured immediately before and 5 minutes after illumination were much lower than U o at PFR saturating for net photosynthesis. We conclude that noncyclic electron flow from water to NADP + and pseudocyclic electron flow via photosystem I to O 2 both significantly contribute to O 2 exchange in the light. In contrast, mitochondrial respiration and photosynthetic carbon oxidation cycle are regarded as minor O 2 consuming reactions in the light in both, air-grown and CO 2-grown cells. It is suggested that the “extra” O 2 uptake by air-grown algae provides ATP required for the energy dependent CO 2/HCO 3− concentrating mechanism known to be present in these cells. 相似文献
20.
Glycolate and ammonia excretion plus oxygen exchanges were measured in the light in l-methionine- dl-sulfoximine treated air-grown Chlamydomonas reinhardii. At saturating CO 2 (between 600 and 700 microliters per liter CO 2) neither glycolate nor ammonia were excreted, whereas at the CO 2 compensation concentration (<10 microliters per liter CO 2) treated algae excreted both glycolate and ammonia at rates of 37 and 59 nanomoles per minute per milligram chlorophyll, respectively. From the excretion values we calculate the amount of O 2 consumed through the glycolate pathway. The calculated value was not significantly different from the component of O 2 uptake sensitive to CO 2 obtained from the difference between O 2 uptake of the CO 2 compensation point and at saturating CO 2. This component was about 40% of stationary O 2 uptake measured at the CO 2 compensation point. From these data we conclude that glyoxylate decarboxylation in air-grown Chlamydomonas represents a minor pathway of metabolism even in conditions where amino donors are deficient and that processes other than glycolate pathway are responsible for the O 2 uptake insensitive to CO 2. 相似文献
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