Changes in Chlorophyll a/b Ratio and Products of CO(2) Fixation by Algae Grown in Blue or Red Light

Chlamydomonas and Chlorella were grown for 10 days in white light. 955 μw/cm² blue light (400-500 mμ) or 685 μw/cm² red light (above 600 mμ). Rates of growth in blue or red light were initially slow, but increased over a period of 5 days until normal growth rates were reestablished. During this adaptation period in blue light, total chlorophyll per volume of algae increased 20% while the chlorophyll a/b ratio decreased. In red light no change was observed in the total amount of chlorophyll or in the chlorophyll a/b ratio. After adaptation to growth in blue light and upon exposure to ¹⁴CO₂ with either blue or white light for 3 to 10 minutes, 30 to 36% of the total soluble fixed ¹⁴C accumulated in glycolate-¹⁴C which was the major product. However, with 1 minute experiments, it was shown that phosphate esters of the photosynthetic carbon cycle were labeled before the glycolate. Glycolate accumulation by algae grown in blue light occurred even at low light intensity. After growth of the algae in red light, ¹⁴C accumulated in malate, aspartate, glutamate and alanine, whereas glycolate contained less than 3% of the soluble ¹⁴C fraction.     

PHOSPHORUS LIMITATION AND CARBON METABOLISM IN A UNICELLULAR ALGA: INTERACTION BETWEEN GROWTH RATE AND THE MEASUREMENT OF NET AND GROSS PHOTOSYNTHESIS1

Chlamydomonas reinhardii Dangeard was grown in continuous culture under P limitation at a range of dilution rates. Carbon uptake measurements were performed using double isotope (¹²C/¹⁴C) techniques and the fluxes of carbon in the light and dark were analysed over the range of growth rates. ¹⁴C uptake was shown to be equal to gross photosynthesis only at maximum relative growth rates; at low relative growth rates ¹⁴C uptake approximated net photosynthesis. The altered pattern of C uptake was found to be due to the suppression of dark respiration in the light and the release of ¹⁴C0₂ from respiratory pathways at low relative growth rates. Metabolic channelling of ¹⁴C from photosynthetic pathways to respiratory pathways occurred at low growth rates as the specific activity of the respired CO₂ reached 45% of the input gas mixture. These data are discussed in the light of the controversy concerning the measurement of gross and net photosynthesis in natural populations and in the light of models of ¹⁴C uptake in single celled algae. Existing models are shown to be adequate for high relative growth rates but not for low relative growth rates under P limitation.     

Light stimulation of proline synthesis in water-stressed barley leaves

The effect of light on [¹⁴C]glutamate conversion to free proline during water stress was studied in attached barley (Hordeum vulgare L.) leaves which had been trimmed to 10 cm in length. Plants at the three-leaf stage were stressed by flooding the rooting medium with polyethylene glycol 6000 (osmotic potential-19 bars) for up to 3 d. During this time the free proline content of 10-cm second leaves rose from about 0.02 to 2 mol/leaf while free glutamate content remained steady at about 0.6 mol/leaf. In stressed leaves, the amount of [¹⁴C]glutamate converted to proline in a 3-h period of light or darkness was taken to reflect the in-vivo rate of proline biosynthesis because the following conditions were met: (a) free-glutamate levels were not significantly different in light and darkness; (b) both tracer [¹⁴C]-glutamate and [¹⁴C]proline were rapidly absorbed; (c) rates of [¹⁴C]proline oxidation and incorporation into protein were very slow. As leaf water potential fell, more [¹⁴C]glutamate was converted to proline in both light and darkness, but at any given water potential in the range-12 to-20 bars, illuminated leaves converted twice as much [¹⁴C]glutamate to proline.     

Separation of Light-Induced [C]ent-Kaurene Metabolism and Light-Induced Germination in Grand Rapids Lettuce Seeds

The effect of light on the metabolism of [¹⁴C]kaurene in light-requiring lettuce seeds (Lactuca sativa L. cv Grand Rapids) was investigated. Seeds were soaked in a solution of [¹⁴C]ent-kaurene in methylene chloride with 0.01% Tween-20, dried, and incubated in 20% polyethylene glycol (PEG) to prevent seedling development. Labeled metabolites were extracted and analyzed by high performance liquid chromatography and gas chromatography-radio counting. [¹⁴C]ent-Kaurenol and [¹⁴C]ent-kaurenal were identified in seeds incubated in constant white light, while no ethyl acetate-soluble metabolites were found in seeds incubated in the dark. In time course experiments using acid scarified seeds, metabolism began after 18 hours of incubation and greatly increased after 24 hours of incubation in 20% PEG. By 48 hours, several unidentified, more polar metabolites were found. Germination was induced in seeds imbibed in 20% PEG by 4 hours of red or 4 hours of white light following 20 hours in the dark, and was fully reversed by 2 hours of far red light. However, in metabolism experiments, [¹⁴C]ent-kaurene oxidation was observed only with constant white light. These results indicate that although ent-kaurene oxidation is a light sensitive step in the biosynthesis of gibberellins in Grand Rapids lettuce seeds, ent-kaurene metabolism is not required for light-induced germination.     

Polyunsaturated Fatty Acid Biosynthesis in Cotyledons from Germinating and Developing Cucumis sativus L. Seedlings

Etiolated Cucumis sativus L. cotyledons preferentially catabolized exogenous [1-¹⁴C]oleic acid and [1-¹⁴C]linoleic acid with relatively little incorporation into complex lipids or desaturation of the ¹⁴C-labeled fatty acids. Following a 16-hour exposure to light, the greening cotyledons efficiently desaturated the exogenous ¹⁴C-labeled fatty acids. A small amount of oleate desaturation to linoleate was observed in etiolated tissue, but hardly any linoleate desaturation to α-linolenate was detected. Both oleate and linoleate desaturation showed diurnal variations with maxima at the end of light periods and minima at the end of dark periods. Illumination of etiolated tissue by flashing light, as opposed to continuous light, failed to stimulate either chlorophyll or α-linolenic acid biosynthesis, and both processes could be halted or reversed by 10 micrograms per milliliter cycloheximide. Production of polyunsaturated fatty acids from [1-¹⁴C]acetate, [1-¹⁴C]oleic acid, and [1-¹⁴C]linoleic acid, by greening cucumber cotyledons, was markedly affected by tissue integrity with finely chopped cotyledons having very little capacity for their synthesis and intact seedlings showing the highest rates.     

A study of potassium gradients in the epidermis of intact leaves of Commelina communis L. in relation to stomatal opening

Summary Activated phytochrome lowers the free amino acid pools of all plants investigated by about 15%. The action of red light is preferentially directed to Asp, Glu and Phe. Exogenously supplied Leu-U-¹⁴C is incorporated more quickly into protein of red-light-treated samples compared with dark controls. In contrast, red light decreases the amount of Asp-U-¹⁴C incorporated into protein, but increases the amount of ¹⁴CO₂ respired after feeding with Asp-U-¹⁴C. Red light has no effect on the amount of ¹⁴CO₂ respired after feeding with Leu-U-¹⁴C. Red-light-mediated stimulation of incorporation of Leu-U-¹⁴C into protein occurred within 15 min, well before the red-light-mediated increase in ¹⁴CO₂ production following feeding with Asp-U-¹⁴C could be detected.Abbreviations R red light - FR far-red light - R+FR red immediately followed by far-red light - D dark control - TCA tricarboxylic acid cycle For amino acids as stated in Biochem. J. 126, 773–780 (1972).     

Light-dependent Induction of Polyunsaturated Fatty Acid Biosynthesis in Greening Cucumber Cotyledons

Greening cucumber (Cucumis sativus L.) cotyledons exhibited dramatic increases in the ability to desaturate exogenously added [1-¹⁴C]oleic acid and [1-¹⁴C]linoleic acid within 2 to 3 hours of illumination. These increases were effectively inhibited by 10 micrograms per milliliter cycloheximide. Oleate desaturation remained at a high level in constant light for 5 to 6 days after induction and then declined by about 50%; when returned to the dark, the tissue showed a sharp decrease in conversion of [¹⁴C]oleate to [¹⁴C]linoleate. Linoleate desaturation reached a maximum about 15 hours after induction and declined immediately thereafter while the tissue still was in the light; after induction had peaked return of the tissue to the dark showed a dramatic fall of linoleate desaturation. The changes in desaturation were correlated with the conversion of the principal fatty acid in the etiolated cotyledons, linoleate, to α-linolenate, and with the assembly of the chlorophyll-containing photosynthetic membranes. The incorporation of [1-¹⁴C]acetate into lipids showed no significant light stimulation. The role of light in the regulation of certain aspects of plant metabolism during development is discussed.     

Obligately phototrophic Chloroflexus: primary production in anaerobic hot spring microbial mats

Microbial mats which lack cyanobacteria occur at 50° to 65° C in the sulfide-containing Mammoth Springs of Yellowstone National Park. The principal organisms within these mats are filamentous bacteria which resemble Chloroflexus aurantiacus. The incorporation of [¹⁴C]-HCO ₃ ^- into mat material depended upon both light and sulfide, and was not inhibited when complete natural light was replaced with far-red and infra-red radiation. [¹⁴C]-acetate was incorporated in a light-dependent reaction which was stimulated by, but did not require, sulfide. In situ experiments with microelectrodes demonstrated net sulfide uptake by the mat in the light, and net sulfide production by the mat in the dark, suggesting the operation of a sulfur cycle.Filamentous phototrophic bacteria isolated from the mat were incapable of sustained growth in the presence of O₂.Simultaneous exposure of cultures to light and O₂ caused degradation of bacteriochlorophyll c. The stimulation of light-dependent [¹⁴C]-HCO ₃ ^- -uptake by sulfide was more pronounced in these isolates than in strains of Chloroflexus aurantiacus.     

The origin and turnover of organelle membranes in castor bean endosperm

The origin and turnover of organelle membranes in castor bean (Ricinus communis L. var. Hale) endosperm was examined using choline-¹⁴C as a phospholipid precursor. On sucrose gradients three major particulate fractions were separated; a light membranous fraction (density 1.11-1.13 gram per cm³), the mitochondria (1.18 gram per cm³), and the glyoxysomes (1.24 gram per cm³). Choline-¹⁴C was readily incorporated into lecithin in all three particulate fractions, but the light membranous fraction became labeled first. Incorporation continued into all three fractions for 6 hours, at which time the available choline-¹⁴C had been completely used. Subsequently, ¹⁴C was lost from the three components at distinctly different rates. When an excess of unlabeled choline was added after 1 hour (pulse-chase experiment), incorporation of choline-¹⁴C into glyoxysomes and mitochondria continued for three hours, but at a diminishing rate. This was followed by a period in which the ¹⁴C content of the mitochondria declined at a rate expected, if the half life of lecithin in the membrane were about 50 hours and that of the glyoxysomes 10 hours. These values are close to those calculated from the experiments in which no chase was used. The labeling in the light membrane fraction behaved differently from that of the mitochondria and glyoxysomes following the chase of unlabeled choline. Incorporation continued for only 1 additional hour, and then the ¹⁴C content declined sharply in the subsequent 4 hours. The early kinetics and subsequent interrelationships are those expected if the lecithin in the membranes of mitochondria and glyoxysomes originates in components of the light membrane fraction.     

DISTRIBUTION OF CARBON AMONG PHOTOSYNTHETIC END-PRODUCTS IN PHYTOPLANKTON OF THE EASTERN CANADIAN ARCTIC1

The distribution of ¹⁴C among photosynthetic end-products was examined in eastern Canadian arctic phytoplankton, with particular emphasis on the synthesis of lipids. The pattern of ¹⁴C distribution for phytoplankton at each of three depths was generally similar among populations from 12 stations. About 18% of the total ¹⁴C fixed was incorporated into lipids. At one station, phytoplankton were experimentally subjected to temperature and light conditions different from those in situ: lipid-¹⁴C did not exceed 30% of total ¹⁴C fixed within the temperature range -1.0 to 6.0° C and irradiance range 1 to 700 W · m^?2. It is suggested that low temperatures and low light intensities, even when, maintained for prolonged periods, are not fully sufficient conditions for eliciting high relative rates of ¹⁴C incorporation into lipids. It is possible that differences in species composition may be a factor accounting for different patterns of ¹⁴C distribution between north and south polar phytoplankton under apparently similar environmental conditions.     

The glycollate pathway during the photoassimilation of acetate by Chlorella

Summary When Chlorella pyrenoidosa photoassimilates ³H–¹⁴C-acetate glycollic acid rapidly becomes labelled with both tritium and ¹⁴C. The ³H/¹⁴C ratio was 10 in glycollate, (compared with 4 in the acetate added) and the only other intermediates showing similar ³H/¹⁴C ratios to glycollate were glycerate and serine. This suggests a glycollate pathway for the formation of serine was operating in Chlorella pyrenoidosa during the photoassimilation of acetate. When Chlorella pyrenoidosa assimilated ³H–¹⁴C-acetate in the dark glycollate was not labelled with either ¹⁴C or tritium. Although glycerate and serine both became labelled with ¹⁴C and tritium in the dark they did not show the high ³H/¹⁴C ratios recorded in the light. When cells were aerated with unlabelled 5% CO₂ during the photoassimilation of ³H–¹⁴C-acetate, the ³H/¹⁴C ratios of glycollate, glycerate and serine were slightly decreased. Similarly, under anaerobic conditions in the light the ³H/¹⁴C ratio was decreased compared with aerobic conditions.     

Decomposition and transfer of plant residue 14C between size and density fractions in soil

The aim of our study was to follow the transfer of ¹⁴C-labeled ryegrass between size and density fractions of soil organic matter in a sandy and a loam soil. Our hypotheses were a) that the applied ¹⁴C would be transferred from light and soluble fractions to intermediate and heavy macroorganic matter fractions (>150 m) and finally become stabilized in microaggregates (<150 m), and b) that the physical protection of ¹⁴C associated with microaggregates against decomposition would decrease with increasing saturation of the microaggregates with soil organic matter. Generally, the hypotheses were confirmed. Immediately after application most of the label was present in the soluble and light macroorganic matter fractions. Newly synthesized microbial biomass fed on the labeled components of the fractions. The amounts of ¹⁴C in the soluble and light macroorganic matter fractions decreased rapidly, while the amounts of ¹⁴C in the intermediate and heavy macroorganic matter fractions and in microaggregates remained more or less stable. At the end of the incubation most of the residual soil ¹⁴C was found in the microaggregates. In the sandy soil ¹⁴C was concentrated in the 20–150 m fraction, whereas in the loam a larger proportion was present in the <20 m fraction.The mineralization rates of ¹⁴C-labeled material were similar in the light intermediate and heavy fractions of macroorganic matter and in the microaggregates 0 and 180 days after the application of ¹⁴C-labeled ryegrass. In all fractions, ¹⁴C mineralized more rapidly than total C. The results indicate that considerable amounts of ¹⁴C must have transferred from the soluble and light macroorganic matter fractions and newly synthesized microbial biomass to the intermediate and heavy macroorganic matter fractions and the microaggregates, and that ¹⁴C was not yet physically protected against microbial degradation during the whole incubation period. The degree of physical protection of ¹⁴C against decomposition in the microaggregate fraction <20 m was negatively correlated with the degree of saturation of this particle size fraction with soil organic matter.     

The biosynthesis and content of gamma-aminobutyric acid in the goldifsh retina

Goldfish retinas incubated with L-glutamate-¹⁴C (UL) were found to synthesize γ-aminobutyric acid-¹⁴C (GABA-¹⁴C) The accumulation of newly synthesized GABA was enhanced by physiological stimulation of the retina with flashing light; and this increase was directly proportional to the logarithm of the light intensity. The total GABA content was also higher in light-stimulated than in dark-adapted retinas, although the glutamate content remained unchanged No differences were found in the cell-free activities of glutamate decarboxylase (EC 4 1.1 15) and GABA-glutamate transaminase (EC 2.6.1.19) extracted from light-stimulated and dark-adapted retinas. These findings, together with other physiological and morphologcal evidence, suggest that GABA plays a functional role in synaptic transmission in the goldfish retina     

MOBILIZATION OF β-1,3-GLUCAN AND BIOSYNTHESIS OF AMINO ACIDS INDUCED BY NH4+ ADDITION TO N-LIMITED CELLS OF THE MARINE DIATOM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)

Mobilization of the reserve β-1,3-glucan (chrysolaminaran) in N-limited cells of the marine diatom Skeletonema costatum (Grev.) Cleve (Bacillariophyceae) was investigated. The diatom was grown in pH-regulated batch cultures with a 14:10-h light:dark cycle until N depletion. In a pulse-chase experiment, the cells were first incubated in high light (200 μmol photons·m ^{− 2}·s ^{− 1}) with ¹⁴C-bicarbonate until dissolved inorganic carbon was exhausted. Unlabeled bicarbonate (1 mM) was then added, and the cells were incubated in the dark and subsequently in low light (20 μmol photons·m ^{− 2}·s ^{− 1}) with additions of 40 μM NH₄ ⁺ . In the ¹⁴C pulse phase with high light and N depletion, β-1,3-glucan accumulated and accounted for 85% of incorporated ¹⁴C. In the subsequent ¹⁴C chase phases, added NH₄ ⁺ was assimilated at an N-specific rate of 0.11 h ^{− 1} in both the dark and low light, and in both cases it caused a significant mobilization of β-1,3-glucan (dark, 26%; low light, 19%). Biochemical fractionation of organic ¹⁴C showed that free amino acids were most rapidly labeled in the early stage of NH₄ ⁺ assimilation, whereas proteins and polysaccharides were labeled more rapidly after 1.2 h. Analysis of the cellular free amino acids strongly indicated that de novo biosynthesis was occurring, with a Gln:Glu ratio increasing from 0.4 to 10 within 1.2 h. After the NH₄ ⁺ was exhausted, the cellular pools of glucan and amino acids became constant or slowly decreased. In another experiment, N-limited cells were first incubated in high light until dissolved inorganic carbon was exhausted and were further incubated in high light with 150 μM NH₄ ⁺ under inorganic carbon limitation. Added NH₄ ⁺ was assimilated at an N-specific rate of 0.023 h ^{− 1}, and cellular β-1,3-glucan decreased by 15% within 6 h. Hence, β-1,3-glucan was mobilized during NH₄ ⁺ assimilation, even though inorganic carbon was modifying the metabolic rates. The results provide new evidence of β-1,3-glucan supplying essential precursors for biosynthesis of amino acids and other components in S. costatum in both the dark and subsaturating light and even saturating light under inorganic carbon limitation.     

Carotenoid composition in Zea mays developed at sub-optimal temperature and different light intensities

The content and composition of pigments were examined in the third leaf of Zea mays L. plants grown under controlled environment at near-optimal temperature (24°C) or sub-optimal temperature (14°C) at a light intensity of either 200 or 600 μmol m^?2 s^?1. Compared to leaves grown at 24°C, leaves grown at 14°C showed a large reduction in the chlorophyll (Chl) content, a marked decrease in the Chl a/b ratio, and a large increase in the ratio of total carotenoids/Chl a+b. Leaves grown at 14°C showed a much lower content of β-carotene than leaves grown at 24°C, while the content of the carotenoids of the xanthophyll cycle (violaxanthin [V] + antheraxanthin [A] + zeaxanthin [Z]) was markedly higher in the former leaves as compared to the latter leaves; neoxanthin and lutein were affected by the growth temperature to a much lesser extent. The xanthophylls/β-carotene ratio was about three times higher in leaves grown at 14°C as compared to leaves grown at 24°C. On a chlorophyll basis, the two types of leaves hardly differed in their level of β-carotene, while the levels of the xanthophylls (including lutein and neoxanthin) were higher in 14°C-grown leaves as compared to 24°C-grown leaves. In leaves grown at 14°C, 40 and 56% of the V+A+Z pool was in the form of zeaxanthin at low light intensity and high light intensity, respectively. Only trace amounts of zeaxanthin, if any, were present in leaves grown at 24°C. The changes in the pigment composition induced by growth at sub-optimal temperature were more pronounced at a light intensity of 600 as compared to 200 μmol m^?2 s^?1. In the given range, the light intensity slightly affected the composition of pigments in leaves grown at 24°C. The physiological significance of the modifications to the pigment composition induced by growth at sub-optimal temperature is discussed.     

Photosynthetic products of division synchronized cultures of euglena

Rates and products of photosynthetic ¹⁴CO₂ fixation by division synchronized cultures of Euglena gracilis strain Z were determined over the cycle. Rate of ¹⁴CO₂ fixation doubled in a continuous manner throughout the light phase followed by a slight reduction of photosynthetic capacity in the dark phase. Greater ¹⁴C incorporation into the nucleic acid-polysaccharide fraction occurred with mature cells. Products of ¹⁴CO₂ fixation varied markedly over the cycle: although with mature cells ¹⁴C-labeled sucrose was not detected, with dividing cells this was the main sugar labeled; in young cells ¹⁴C maltose was formed. Cells removed at end of dark phase accumulated ¹⁴C in glycolate, whereas at other stages over the cycle less ¹⁴C was present in glycolate, and this was accompanied by a rapid incorporation of ¹⁴C into glycine and serine. Glycerate was an early and major product of photosynthesis with cells at the mature stage of the cycle.     

Light and dark uptake and loss of 14C: methodological problems with productivity measurements in oceanic waters

Measurements of the uptake and loss of ⁴C in the light and in the dark in the Tasman and Coral Seas have revealed methodological problems with the estimation of productivity in these waters. Rates of productivity estimated without replication, time series incubations and dark controls frequently overestimated the true rates of autotrophic production. The data showed unexpectedly high rates of both uptake and loss in the dark in oligotrophic waters. In oligotrophic oceanic waters, dark incorporation of ¹⁴C sometimes equalled the uptake of ¹⁴C in the light bottle. Rapid uptake of isotope in the dark controls appeared to be the result of rapid bacterial growth and metabolism. This problem was exacerbated by agitation of the sample before or during the incubation. Tropical samples were particularly susceptible to problems arising from the agitation of the samples. Latitudinal gradients of dark uptake and loss were revealed in these incubations. The loss of label during 8–12 hours in the dark (after 12 hr in the light) was as high as 50% in subtropical waters. The loss was frequently unmeasurable (< 10%) in temperate waters. The time course of ¹⁴C uptake indicated active grazing in the bottles and suggested that most of the nighttime losses of label were due to grazing by microheterotrophs. Respiratory losses appeared to be small. Calculated values of the assimilation number (or photosynthetic capacity) which did not correct for dark ¹⁴C uptake were too high to be biochemically realistic. The errors were due to the heterotrophic uptake of label and the lack of dark controls. Rapid release of ¹⁴C in the dark after incubation in the light meant that the estimate of productivity was dependant on the trophic state of the sample and on the period of incubation.     

Ozone treatment affects pigment precursor metabolism in pine seedlings

Five‐week‐old seedlings of Pinus halepensis Mill. and Pinus brutia Ten. were exposed to air polluted with ozone (O₃) (250 nl l^?1, 12 h day^?1 for 4 days) or to ambient air containing ca 10–20 nl l^?1 O₃, in the light (180 μmol m^?2 s^?1 photosynthetic photon flux density [PPFD], 12 h day^?1) and then fed for 24 h in the light (100 μmol m^?2 s^?1 PPFD) with various radioactive precursors of chlorophyll (Chl) and carotene biosynthesis: 5‐[4‐¹⁴C]‐aminolevulinic acid (¹⁴C‐ALA), l ‐[¹⁴C(U)]‐glutamic acid (¹⁴C‐Glu), or d ,l ‐[2‐¹⁴C]‐mevalonic acid (¹⁴C‐MVA). Pigments were then extracted from cotyledons and fully expanded needles. Chl a and carotene were separated by thin‐layer chromatography and high‐performance liquid chromatography and their specific activities were determined. ¹⁴C‐ALA and ¹⁴C‐Glu labels were incorporated into Chl a and carotene. Exposure to O₃ did not inhibit incorporation of ¹⁴C‐ALA into Chl a molecules, but hydrolysis of Chl a showed that O₃ inhibited phytol labelling of Chl a. Labelling of carotene was also inhibited by O₃, but not when ¹⁴C‐MVA was used as the label. These data suggest that O₃ treatment inhibits (directly or indirectly) the biosynthesis of isoprenoids from products of ALA and Glu metabolism in the plastid, but not from MVA in the cytosol. This inhibition was more prominent when ¹⁴C‐ALA was used as the label than when ¹⁴C‐Glu was the labelling precursor. A significant increase in pheophorbide a, a tetrapyrrole component of Chl a labelling, and a concomitant decrease in phytol labelling was observed following incubation of O₃‐treated pine seedlings with ¹⁴C‐ALA and ¹⁴C‐Glu. Stronger inhibition of carotene biosynthesis and activation of Chl a tetrapyrrole labelling by ¹⁴C‐ALA (in comparison with ¹⁴C‐Glu) indicated that exposure to O₃ inhibits the conversion of ALA to Glu as the first step in ALA catabolism. These results also suggested a more intensive Glu metabolism (in comparison with ALA) for carotene biosynthesis in the cytosol, as well as cooperation between two pathways of isopentenyl diphosphate biosynthesis.     

Effect of External pH on the Internal pH of Chlorella saccharophila

The overall internal pH of the acid-tolerant green alga, Chlorella saccharophila, was determined in the light and in the dark by the distribution of 5,5-dimethyl-2-[¹⁴C]oxazolidine-2,4-dione ([¹⁴C]DMO) or [¹⁴C]benzoic acid ([¹⁴C]BA) between the cells and the surrounding medium. [¹⁴C]DMO was used at external pH of 5.0 to 7.5 while [¹⁴C]BA was used in the range pH 3.0 to pH 5.5. Neither compound was metabolized by the algal cells and intracellular binding was minimal. The internal pH of the algae obtained with the two compounds at external pH values of 5.0 and 5.5 were in good agreement. The internal pH of C. saccharophila remained relatively constant at pH 7.3 over the external pH range of pH 5.0 to 7.5. Below pH 5.0, however, there was a gradual decrease in the internal pH to 6.4 at an external pH of 3.0. The maintenance of a constant internal pH requires energy and the downward drift of internal pH with a drop in external pH may be a mechanism to conserve energy and allow growth at acid pH.     

The pathway of carbon dioxide assimilation in rhodospirillum rubrum grown in turbidostat continuous-flow culture

Summary A comparison of light and dark short-term incorporation of [¹⁴C]-carbon dioxide by Rhodospirillum rubrum grown in turbidostat continuous-flow culture at two different steady states on medium containing malate has shown that the labelling of phosphate esters was the main light-dependent process. Thus, the reductive pentose phosphate cycle appears to be the major pathway of carbon dioxide assimilation in the light under these growth conditions.The labelling of glutamate was also light-dependent and was most marked in the most rapidly growing steady state culture.The assimilated [¹⁴C]carbon was transferred to metabolites of the tricarboxylic acid cycle, particularly C₄-dicarboxylic acids, and the transfer involved additional carboxylations which were not light-dependent. The activity of these reactions accounted for initial high rates of carbon dioxide assimilation in the dark.In the dark assimilated [¹⁴C]carbon accumulated in succinate.