Ribulose Bisphosphate Carboxylase Activity Research Articles

Physiological Changes in Portulacaria afra (L.) Jacq. during a Summer Drought and Rewatering

The changes of titratable acidity, enzyme activity, water status, and pigment composition were studied in Portulacaria afra (L.) Jacq. during a normal summer drought and rewatering. Two groups of plants were grown outside under a clear plastic canopy with water stress initiated at 2-week intervals in May 1986. Drought resulted in a linear decrease of fresh weight for 80 days and there was no further fresh weight change for the next 65 days. Nocturnal CO(2) uptake remained measurable for 83 days. Cessation of exogenous CO(2) uptake corresponded to the point where the pressure potential (Psi(p)) became zero. Ribulose-1, 5-bisphosphate (RuBP) and phosphoenolpyruvate carboxylase were reduced to 50% of this activity by the end of the drought period. Phosphoenolpyruvate carboxykinase activity was undetectable after 120 to 140 days of drought. Chlorophyll (Chl) levels decreased with a preferential loss of Chl a over Chl b. Carotenoid content was relatively constant over the course of the drought period. After 145 days of drought, plants responded to rewatering within 24 hours; Psi(p) became positive and daytime CO(2) uptake resumed after 24 hours. After 3 days, RuBP carboxylase activity reached control levels. Activity of the CAM pathway recovered after 5 days, as noted by increased diurnal acid fluctuations. Phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase activity fully recovered within 6 days. Chl levels were greater than control levels within 5 days. Chl a/b ratios took 27 days to return to control levels. The results indicated that P. afra can withstand a normal summer drought by utilizing the CAM and CAM-idling pathway for 130 to 140 days. The plants respond rapidly to rewatering because of the conservation of enzyme activity and the quick recovery of Psi(p).

Rubisco Activase Mediates ATP-Dependent Activation of Ribulose Bisphosphate Carboxylase

The activation level of ribulosebisphosphate carboxylase following preincubation with ribulose 1,5-bisphosphate was increased by ATP and ribulosebisphosphate carboxylase activase in the absence of thylakoids or illumination. Maximal activation was obtained with 0.5 millimolar ATP in the presence of an ATP regenerating system (phosphoenolpyruvate and pyruvate kinase). Without the ATP regenerating system, activation was lower, linearly dependent on ATP concentration up to 1.0 millimolar, and was strongly inhibited by ADP.

Factors Affecting the Activation State and the Level of Total Activity of Ribulose Bisphosphate Carboxylase in Tobacco Protoplasts

The relationship between the activation state and the level of total activatable activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) was examined in tobacco protoplasts. When darkened protoplasts were illuminated, both activation and total activity increased, but at different rates; the t((1/2)) were 2.3 and 6.7 minutes, respectively. The light response of rubisco activation state and total activity, measured after 15 minutes of illumination, were similar but their responses to light transitions and photosynthetic inhibitors were different. When irradiance was reduced from saturating to subsaturating, deactivation of rubisco in protoplasts was immediate, whereas there was little change in total activity during the first 20 minutes following the transition. The light-induced increases in activation state and total activity were inhibited by nigericin, but activation was more sensitive exhibiting a response similar to that of photosynthesis. Treatment of tobacco protoplasts and leaves with methyl viologen at limiting irradiance increased rubisco activation, but inhibited the light-induced increase in total activity. These results indicate that light activation of rubisco is mechanistically distinct from the light-dependent changes in total activity in tobacco, a species containing carboxyarabinitol 1-phosphate, an endogenous inhibitor of total rubisco activity.

Effects of light intensity and nutrient availability on diel patterns of cell metabolism and growth in populations of Synechococcus spp.

Between July 21 and August 8, 1984, phytoplankton were collected from the surface (2 m) and/or chlorophyll maximum of a neritic front, warm-core eddy 84-E and Wilkinson's Basin in the Northwest Atlantic Ocean and incubated up to 38 h in 200-liter vats. Effects of light intensity and nutrient availability on diel patterns of cell metabolism were analyzed in a 0.6- to 1-μm fraction, where Synechococcus spp. represented 80 to 100% of the total photoautotrophs. Populations held under in situ conditions exhibited daytime peaks in photosynthetic potential (Pmax) that were an order of magnitude higher than nighttime Pmax values. Daytime phasing of Pmax peaks had no relationship to asynchronous fluctuations in cellular activities of ribulose 1,5 bisphosphate carboxylase (RUBPCase) or phosphoenol pyruvate carboxylase (PEPCase), or to variations in chlorophyll content. Daytime Pmax peaks were about 12 h out of phase with nighttime maxima in the frequency of dividing cells (FDC). The phase relationship between Pmax and FDC could be altered by manipulating environmental conditions. High light exposure of depp populations did not affect timing of the Pmax peak, but its magnitude increased and coincided with increased RUBPCase activity and chlorophyll photobleaching. In the eddy population, a major shift in the timing of peak Pmax was induced when increased light intensity was accompanied by nutrient enrichment. This change coincided with major increases in cellular chlorophyll and carboxylating enzyme activity. Lowering irradiance and/or increasing nutrient availability elicited different diel pattern in cellular metabolism in surface populations from the eddy and from Wilkinson's Basin that appeared linked to differences in the nutrient status of the cells. Rates of cell division estimated from the percentage of dividing cells in preserved samples were 0.83 divisions d-1 in surface warm-core eddy populations, supporting the view that carbon and nitrogen turnover rates in oligotrophic waters can be sufficient to promote near optimal growth of Synechococcus spp.

A genetic locus essential for formate-dependent growth of Bradyrhizobium japonicum.

A genetic locus essential for the formate-dependent growth of Bradyrhizobium japonicum was isolated by complementation of ethyl methanesulfonate-induced mutants with a cosmid gene library of B. japonicum DNA. Three related cosmids containing 18.7 kilobase pairs of B. japonicum DNA in common were identified as being able to restore formate-dependent growth capability to mutants lacking either ribulosebisphosphate carboxylase or both ribulosebisphosphate carboxylase and phosphoribulokinase activities. To further localize the complementing gene(s), a series of four deletions spanning a total of 16.1 kilobase pairs were introduced into the B. japonicum chromosome. Each resulting deletion mutant lacked formate dehydrogenase activity and lacked ribulosebisphosphate carboxylase activity and immunologically detectable protein. Three of the four also lacked phosphoribulokinase activity. Two other mutants in which the deletion-bearing recombinant plasmid had integrated into the chromosome also lacked ribulosebisphosphate carboxylase activity and protein and phosphoribulokinase activities. The genetic locus defined by these mutants could contain the structural genes for these enzymes or a regulatory gene(s) controlling their expression or both.

Consequence of Absence of Nitrate Reductase Activity on Photosynthesis in Nicotiana plumbaginifolia Plants

Chlorate-resistant Nicotiana plumbaginifolia (cv Viviani) mutants were found to be deficient in the nitrate reductase apoprotein (NR(-)nia). Because they could not grow with nitrate as sole nitrogen source, they were cultivated as graftings on wild-type Nicotiana tabacum plants. The grafts of mutant plants were chlorotic compared to the grafts of wild type. Mutant leaves did not accumulate nitrogen and nitrate but contained less malate and more glutamine than wild leaves. They exhibited a slight increase of the proportion of the light-harvesting chlorophyll a/b protein complexes and a lowering of the efficiency of energy transfer between these complexes and the active centers. After a 3 second (14)CO(2) pulse, the total (14)C incorporation of the mutant leaves was approximately 20% of that of the control. The (14)C was essentially recovered in ribulose bisphosphate in these plants. It was consistent with a decline of ribulose bisphosphate carboxylase activity observed in the mutant. After a 3 second (14)CO(2) pulse followed by a 60 second chase with normal CO(2), (14)C was mainly accumulated in starch which was labeled more in the mutant than in the wild type. These results confirm the observation that in the nitrate reductase deficient leaves, chloroplasts were loaded with large starch inclusions preceding disorganization of the photosynthetic apparatus.

Variations in photosynthetic, anatomical, and enzymatic leaf traits and correlations with growth in recently selected Populus hybrids

Several photosynthetic, anatomical, and enzymatic leaf traits were studied on 1- and 2-year-old fast growing Populus clones representing interspecific hybrids of P. deltoides, P. trichocarpa, and P. maximowiczii. Growth performance of the clones was studied with container-grown plants and for 5 years in the field. Considerable variation in photosynthetic, anatomical, and enzymatic leaf traits was found, but variation in growth differences among the clonal groups was minimal. Photosynthetic photon flux density saturated net photosynthesis of 1-year-old container-grown plants was significantly correlated with shoot height growth, but none of the other leaf traits measured showed a significant correlation with any of the growth characteristics. Clonal groups could be segregated by taxonomic and genetic affinities with hierarchical clustering and principal component analysis. Although enzymatic and biochemical traits (area leaf weight, ribulose-bisphosphate carboxylase and phosphoenolpyruvate carboxylase activities, protein content) and adaxial stomatal frequency can be used to discriminate among clonal groups, no significant regression of these leaf traits on growth performance was observed.

Minor Physiological Response to Elevated CO2 by the CAM Plant Agave vilmoriniana

One-year-old plants of the CAM leaf succulent Agave vilmoriniana Berger were grown outdoors at Riverside, California. Potted plants were acclimated to CO(2)-enrichment (about 750 microliters per liter) by growth for 2 weeks in an open-top polyethylene chamber. Control plants were grown nearby where the ambient CO(2) concentration was about 370 microliters per liter. When the plants were well watered, CO(2)-induced differences in stomatal conductances and CO(2) assimilation rates over the entire 24-hour period were not large. There was a large nocturnal acidification in both CO(2) treatments and insignificant differences in leaf chlorophyll content. Well watered plants maintained water potentials of -0.3 to -0.4 megapascals. When other plants were allowed to dry to water potentials of -1.2 to -1.7 megapascals, stomatal conductances and CO(2) uptake rates were reduced in magnitude, with the biggest difference in Phase IV photosynthesis. The minor nocturnal response to CO(2) by this species is interpreted to indicate saturated, or nearly saturated, phosphoenolpyruvate carboxylase activity at current atmospheric CO(2) concentrations. CO(2)-enhanced diurnal activity of ribulose bisphosphate carboxylase activity remains a possibility.

Drought Stress and Elevated CO2 Effects on Soybean Ribulose Bisphosphate Carboxylase Activity and Canopy Photosynthetic Rates

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 K(m)(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.

Isolation, identification, and synthesis of 2-carboxyarabinitol 1-phosphate, a diurnal regulator of ribulose-bisphosphate carboxylase activity

The diurnal change in activity of ribulose 1,5-bisphosphate (Rbu-1,5-P(2)) carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing); EC 4.1.1.39] of leaves of Phaseolus vulgaris is regulated (in part) by mechanisms that control the level of an endogenous inhibitor that binds tightly to the activated (carbamoylated) form of Rbu-1,5-P(2) carboxylase. This inhibitor was extracted from leaves and copurified with the Rbu-1,5-P(2) carboxylase of the leaves. Further purification by ion-exchange chromatography, adsorption to purified Rbu-1,5-P(2) carboxylase, barium precipitation, and HPLC separation yielded a phosphorylated compound that was a strong inhibitor of Rbu-1,5-P(2) carboxylase. The compound was analyzed by GC/MS, (13)C NMR, and (1)H NMR and shown to be 2-carboxyarabinitol 1-phosphate [(2-C-phosphohydroxymethyl)-D-ribonic acid]. Verification of structure was obtained by comparison of the inhibitory activity of the isolated compound with that of 2-carboxy-D-arabinitol 1-phosphate synthesized in vitro. This compound (but not 2-carboxy-D-arabinitol 5-phosphate) inhibited Rbu-1,5-P(2) carboxylase in a way that was kinetically identical to that of the isolated, naturally occurring compound. The structure of the isolated compound differs from the Rbu-1,5-P(2) carboxylase transition-state analogue 2-carboxyarabinitol 1,5-bisphosphate only by the lack of the C-5 phosphate group. This difference results in a higher binding constant for the monophosphate (K(d) = 32 nM) compared with the bisphosphate (K(d) < 10 pM). The less tightly bound compound acts in a light-dependent, reversible regulation of Rbu-1,5-P(2) carboxylase activity in vivo.

The effect of ammonium and nitrate on CO2 assimilation, RuBP and PEP carboxylase activity and dry matter production in wheat

Photosynthetic(14)CO2 assimilation, ribulose 1, 5-bisphosphate carboxylase (RuBPC), phosphoenol pyruvate carboxylase (PEPC) and dry matter (DM) production were examined in wheat under varying levels and forms of nitrogen.(14)CO2 assimilation increased gradually after germination reaching a peak value at anthesis, followed by a sharp decline. A similar pattern was observed for both the carboxylases, RuBPC and PEPC activities. Increase in nitrogen levels, in general, brought about a significant increase over the control (zero-nitrogen) in(14)CO2 assimilation, RuBPC, PEPC activities and DM production. There were no significant differences in RuBPC activity and(14)CO2 assimilation with respect to the forms of nitrogen. Significantly higher PEPC activity and DM was observed in plants supplied with nitrate-nitrogen (NO3-N), as compared to those supplied with ammonium-nitrogen (NH4-N). The significance of PEPC activity in C3 photosynthesis is discussed in relation to DM distribution.

Photosynthesis and Activity of Ribulose Bisphosphate Carboxylase of Wheat and Maize Seedlings during and following Exposure to O2-Low, CO2-Free N2

Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO(2)-free air for 3 hours with either 1% O(2) in N(2) or N(2)-only and then returned to normal air of 340 microliters per liter CO(2), 21% O(2) in N(2). Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO(2)-free treatments as was photosynthetic CO(2) fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N(2) only treatment. During the CO(2)-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO(2)-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO(2)-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO(2) fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO(2) fixed per millogram protein per minute. This suggests that in these seedlings considerable inactive carboxylase must be stored in a form not activatable in extracts by CO(2) and Mg(2+). Possible mechanisms of regulation of photosynthesis by the ribulose bisphosphate carboxylase must consider not only the amount of active enzyme, but the amount of enzyme which the plant can make activatable upon demand.

Effects of Photoinhibition on Photosynthetic Carbon Metabolism in Intact Isolated Spinach Chloroplasts.

Pools of intermediates of the Calvin cycle were measured during photosynthetic 14CO2 fixation by intact isolated spinach chloroplasts. Photoinhibitibn (illumination for 8 min with 4000 �mol m-2 s-1 light in the absence of bicarbonate) decreased the subsequently measured rate of CO2 fixation. Individual compounds were differently affected: the ribulose-1,5-bisphosphate (RuBP) pool was drastically lowered, while that of fructose-1,6-bisphosphate (FBP) was increased, suggesting that photoinhibition causes a limitation in RuBP regeneration. An increase in FBP and decrease in RuBP were not observed during photosynthesis in low light at rates of CO2 fixation comparable to those in photoinhibited chloroplasts. This indicates that changes of the metabolite pools induced by photoinhibition were not due solely to decreased rates of electron transport. Activities of RuBP carboxylase and fructose-1,6-bisphosphatase (FBPase) were decreased by the photoinhibitory treatment. However, the activity of both enzymes in photoinhibited chloroplasts was still well in excess of that required to sustain the measured rates of carbon flux. Photoinhibition largely abolished the light-induced proton gradient across the chloroplast envelope. The concomitant acidification of the chloroplast stroma could inhibit FBPase activity. It is concluded that photoinhibition does not result in irreversible modification of the FBPase protein but that its activity may be decreased by changes in pH and possibly other factors in the chloroplast stroma.

Inactivation of the photosynthetic carbon reduction cycle in isolated mesophyll protoplasts subjected to freezing stress

Isolated mesophyll protoplasts from Valerianella locusta L. were subjected to freeze-thaw cycles. Subsequently, steady-state pool sizes of (14)C-labeled intermediates of the photosynthetic carbon reduction cycle were determined by high performance liquid chromatography. Protoplasts in which CO2 fixation was inhibited by preceding freezing stress, showed a strong increase in the proportion of fructose-1,6-bisphosphate, sedoheptulose-1,7-bisphosphate and triose phosphates. These results indicate an inhibition of the activities of stromal fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase. Furthermore, freezing stress caused a slight increase in the proportion of labeled ribulose-1,5-bisphosphate, which may be based on an inhibition or ribulose bisphosphate carboxylase activity. It was shown earlier (Rumich-Bayer and Krause 1986) that freezing-thawing readily affects photosynthetic CO2 assimilation independently of thylakoid inactivation. The present results are interpreted in terms of an inhibition of the light-activation system of the photosynthetic carbon reduction cycle, caused by freezing stress.

Effect of heavy metal ions on barley ribulose-1,5-bisphosphate carboxylase

Heavy metal ions (Cu2+, Cd2+, Pb2+, and Zn2+) in concentrations of 10-4 mol l-1 inhibit barley ribulose-1,5-bisphosphate carboxylase (RuBPC) (EC 4.1.1.39). The enzyme is inhibited most strongly by Cd2+ and Cu2+ and only very little by Zn2+-ions. Cu2+ and Zn2+-ions in concentrations of the order of 10-6 mol l-1 stimulate the RuBPC activity. The reaction of metal ions with the protein molecule of the enzyme is time dependent. The activation of the enzyme by Cu2+ and Zn2+-ions is the highest after 10 min incubation. Cd2+-ions in concentrations of 10-6 mol l-1 lead to a time dependent irreversible inhibition of the enzyme.

CARBOXYLATING ENZYMES OF SOYBEAN COTYLEDONS

SummaryThe in vitro activities of ribulose‐1,5‐bisphosphate (RuBP) carboxylase and phosphoenol pyruvate (PEP) carboxylase enzymes extracted from soybean [Glycine max (L.) Merrill cv. Amsoy'71] cotyledons are correlated with cotyledon chlorophyll development. High levels of PEP carboxylase activity were found in the spongy mesophyll tissues of young cotyledons before greening occurred, but as the cotyledons developed, the activity of this enzyme fell rapidly and 13 d after sowing it had almost disappeared. RuBP carboxylase activity, in contrast, was concentrated in the palisade mesophyll and increased with chlorophyll content, reaching a peak on day 10. RuBP carboxylase activity then declined and by the onset of senescence was very low. In cotyledons whose senescence had been arrested by epicotyl removal, chlorophyll recovery was found to precede that of the activity of both carhoxylase enzymes. After a lag of 5 d, both enzymes showed increased activity in the palisade mesophyll, RuBP carboxylase activity being greater than that of PEP carboxylase.

Photosynthesis and Growth of Water Hyacinth under CO2 Enrichment

Water hyacinth (Eichhornia crassipes [Mart.] Solms) plants were grown in environmental chambers at ambient and enriched CO(2) levels (330 and 600 microliters CO(2) per liter). Daughter plants (ramets) produced in the enriched CO(2) gained 39% greater dry weight than those at ambient CO(2), but the original mother plants did not. The CO(2) enrichment increased the number of leaves per ramet and leaf area index, but did not significantly increase leaf size or the number of ramets formed. Flower production was increased 147%. The elevated CO(2) increased the net photosynthetic rate of the mother plants by 40%, but this was not maintained as the plants acclimated to the higher CO(2) level. After 14 days at the elevated CO(2), leaf resistance increased and transpiration decreased, especially from the adaxial leaf surface. After 4 weeks in elevated as compared to ambient CO(2), ribulose bisphosphate carboxylase activity was 40% less, soluble protein content 49% less, and chlorophyll content 26% less; whereas starch content was 40% greater. Although at a given CO(2) level the enriched CO(2) plants had only half the net photosynthetic rate of their counterparts grown at ambient CO(2), they showed similar internal CO(2) concentrations. This suggested that the decreased supply of CO(2) to the mesophyll, as a result of the increased stomatal resistance, was counterbalanced by a decreased utilization of CO(2). Photorespiration and dark respiration were lower, such that the CO(2) compensation point was not altered. The photosynthetic light and CO(2) saturation points were not greatly changed, nor was the O(2) inhibition of photosynthesis (measured at 330 microliters CO(2) per liter). It appears that with CO(2) enrichment the temporary increase in net photosynthesis produced larger ramets. After acclimation, the greater total ramet leaf area more than compensated for the lower net photosynthetic rate on a unit leaf area basis, and resulted in a sustained improvement in dry weight gain.

Ribulose bisphosphate carboxylase, protein and nitrogen in Scots pine seedlings cultivated at different nutrient levels

Seedlings of Scots pine (Pinus sylvestris L.) of a northern provenance were cultivated in nutrient solution for 10 weeks in a climate chamber. The nutrient solution (renewed by solution exchange) contained 2.5, 10 or 50 mg N I−1. All other essential elements were added in optimal proportion to the nitrogen.Seedlings cultivated at 10 and 50 mg N I−1 were similar with respect to all characteristics studied. Seedlings cultivated at 2.5 mg N I−1 showed a lower growth rate, especially for the shoot, and an altered morphology, with high root:shoot ratios and long, slender roots. The nitrogen concentrations in shoot and needles as well as in whole seedlings were not significantly affected by the nitrogen supply, while the nitrogen concentrations in the roots were somewhat lower at 2.5 mg N I−1.Ribulose bisphosphate carboxylase (EC 4.1.1.39) activity and the concentrations of carboxylase, total and soluble protein and of chlorophyll in the needles were consistently much lower for seedlings cultivated at 2.5 mg N I−1, than for seedlings grown at higher nutrient levels. A close correlation was observed between activity and concentration of the carboxylase (r=0.95). Carboxylase activity and protein were more sensitive to a low nutrient supply than was chlorophyll.The data show how activity and concentration of ribulose bisphosphate carboxylase and the concentrations of soluble and total protein and of chlorophyll in needles of pine seedlings can be negatively affected by the nutrient supply, also when the nitrogen concentrations in the needles are close to those observed at optimal nutrient supply. It is suggested that pine seedlings store assimilated non‐protein nitrogen in the needles when protein synthesis is under restraint. The nitrogen concentration in needles and seedlings could not be used as a measure of the physiological state of the seedlings.

Activities of sulfur-oxidizing bacteria at the 21�N East Pacific Rise vent site

The incorporation of inorganic and organic carbon into cell material, as well as the activities of carboxylating enzymes (ribulose bisphosphate carboxylase and phosphoenol pyruvate carboxylase), were measured in waters emitted from “warm” vents at the 21°N ocean spreading site (depth 2 600 m) of the East Pacific Rise. Both obligately and facultatively chemoautotrophic bacteria were present and comprised a significant but variable portion of the total microbial population as indicated by comparisons of microscopic cell counts with liquid enrichments and colony counts on media containing reduced sulfur compounds. The proportion of chemoautotrophic, sulfur-oxidizing bacteria maximally reached 79% of the total microbial population based on ribulose bisphosphate carboxylase activity. Variability of chemoautotrophic activity occurred between vents at different locations, but was also observed in one individual vent. Maximum rates of CO2 incorporation in warm vent water were similar to levels measured previously in the O2/H2S interface of the Black Sea. In shipboard experiments, these rates were virtually unaffected by in-situ pressures (ca 260 atm). Rate measurements at various temperatures, as well as the observed mesophilic characteristics of all isolates obtained, suggest that the microbial, chemoautotrophic activity decreases rapidly as freshly emitted vent water is diluted with cold, ambient, deep-sea water.

Étude comparative du métabolisme photosynthétique des feuilles et des gousses de soja Glycine max

Changes in photosynthetic carbon fixation processes were comparatively studied in soybean leaves and pods harvested at different growth stages. Newly fully expanded leaves exhibited both the highest 14CO2 assimilation rate and the maximum ribulose bisphosphate carboxylase levels. Amino acids biosynthesis was more important in young tissues and this result agreed with the evolution of nitrate reductase activities. The radiocarbon distribution in glycine and serine suggested that photorespiratory metabolism increases with leaf age; the activity of glycolate oxidase was found to be significantly lower in younger leaves than in mature ones. Net photosynthesis, chlorophyll content, and ribulose bisphosphate carboxylase activity were low in isolated pods compared to leaves. However, the study of photosynthate translocations within the pod revealed that the pod wall could contribute to the carbon nutrition of the seeds. Soluble compounds labelled after 14CO2 incorporation and glycolate oxidase activity measurement indicated that organic acids biosynthesis and photorespiratory metabolism are relatively higher in pods than in leaves.