Productivity and selective accumulation of carotenoids of the novel extremophile microalga <Emphasis Type="Italic">Chlamydomonas acidophila</Emphasis> grown with different carbon sources in batch systems

Cultivation of extremophile microorganisms has attracted interest due to their ability to accumulate high-value compounds. Chlamydomonas acidophila is an acidophile green microalga isolated by our group from Tinto River, an acidic river that flows down from the mining area in Huelva, Spain. This microalga accumulates high concentrations of lutein, a very well-known natural antioxidant. The aim of this study is to assess use of different carbon sources (CO₂, glucose, glycerol, starch, urea, and glycine) for efficient growth of and carotenoid production by C. acidophila. Our results reveal that growth of the microalga on different carbon sources resulted in different algal biomass productivities, urea being as efficient as CO₂ when used as sole carbon source (~20 g dry biomass m^–2 day^–1). Mixotrophic growth on glucose was also efficient in terms of biomass production (~14 g dry biomass m^–2 day^–1). In terms of carotenoid accumulation, mixotrophic growth on urea resulted in even higher productivity of carotenoids (mainly lutein, probably via α-carotene) than obtained with photoautotrophic cultures (70% versus 65% relative abundance of lutein, respectively). The accumulated lutein concentrations of C. acidophila reported in this work (about 10 g/kg dry weight, produced in batch systems) are among the highest reported for a microalga. Glycerol and glycine seem to enhance β-carotene biosynthesis, and when glycine is used as carbon source, zeaxanthin becomes the most accumulated carotenoid in the microalga. Strategies for production of lutein and zeaxanthin are suggested based on the obtained results.     

Both Forward and Reverse TCA Cycles Operate in Green Sulfur Bacteria

The anoxygenic green sulfur bacteria (GSBs) assimilate CO₂ autotrophically through the reductive (reverse) tricarboxylic acid (RTCA) cycle. Some organic carbon sources, such as acetate and pyruvate, can be assimilated during the phototrophic growth of the GSBs, in the presence of CO₂ or HCO₃⁻. It has not been established why the inorganic carbonis required for incorporating organic carbon for growth and how the organic carbons are assimilated. In this report, we probed carbon flux during autotrophic and mixotrophic growth of the GSB Chlorobaculum tepidum. Our data indicate the following: (a) the RTCA cycle is active during autotrophic and mixotrophic growth; (b) the flux from pyruvate to acetyl-CoA is very low and acetyl-CoA is synthesized through the RTCA cycle and acetate assimilation; (c) pyruvate is largely assimilated through the RTCA cycle; and (d) acetate can be assimilated via both of the RTCA as well as the oxidative (forward) TCA (OTCA) cycle. The OTCA cycle revealed herein may explain better cell growth during mixotrophic growth with acetate, as energy is generated through the OTCA cycle. Furthermore, the genes specific for the OTCA cycle are either absent or down-regulated during phototrophic growth, implying that the OTCA cycle is not complete, and CO₂ is required for the RTCA cycle to produce metabolites in the TCA cycle. Moreover, CO₂ is essential for assimilating acetate and pyruvate through the CO₂-anaplerotic pathway and pyruvate synthesis from acetyl-CoA.     

Mutual influence of light and CO2 on carbon sequestration via cultivating mixotrophic alga Auxenochlorella protothecoides UMN280 in an organic carbon-rich wastewater

This study focusses on the assimilation of carbon in concentrated municipal wastewater rich in organic carbon using the mixotrophic microalga Auxenochlorella protothecoides UMN280 with the addition of supplemental CO₂. The entire growth period of A. protothecoides UMN280 can be characterized by three phases: first, a phase where algae grew in a mixotrophic-dominated mode; second, a transition phase; and last, a phase where algae grew in a photoautotrophic-dominated mode. In this study, it was found that light intensity had a strong effect on algal biomass production; the culture system would transfer from a mixotrophic-dominated mode to a photoautotrophic-dominated mode quicker under higher light intensities. The addition of CO₂ exhibited an important role in the photoautotrophic-dominated cultivation stage. At certain level of irradiance and certain range of CO₂ injection rate, higher CO₂ injection rate would result in a higher level of carbon fixation. It is clearly beneficial to inject exogenous CO₂ in the mixotrophic wastewater algae production system when a light source is available, such as during daylight hours.     

Acetate and Bicarbonate Assimilation and Metabolite Formation in Chlamydomonas reinhardtii: A 13C-NMR Study

Cellular metabolite analyses by ¹³C-NMR showed that C. reinhardtii cells assimilate acetate at a faster rate in heterotrophy than in mixotrophy. While heterotrophic cells produced bicarbonate and CO₂ ^aq, mixotrophy cells produced bicarbonate alone as predominant metabolite. Experiments with singly ¹³C-labelled acetate (¹³CH₃-COOH or CH₃-¹³COOH) supported that both the ¹³C nuclei give rise to bicarbonate and CO₂ ^aq. The observed metabolite(s) upon further incubation led to the production of starch and triacylglycerol (TAG) in mixotrophy, whereas in heterotrophy the TAG production was minimal with substantial accumulation of glycerol and starch. Prolonged incubation up to eight days, without the addition of fresh acetate, led to an increased TAG production at the expense of bicarbonate, akin to that of nitrogen-starvation. However, such TAG production was substantially high in mixotrophy as compared to that in heterotrophy. Addition of mitochondrial un-coupler blocked the formation of bicarbonate and CO₂ ^aq in heterotrophic cells, even though acetate uptake ensued. Addition of PSII-inhibitor to mixotrophic cells resulted in partial conversion of bicarbonate into CO₂ ^aq, which were found to be in equilibrium. In an independent experiment, we have monitored assimilation of bicarbonate via photoautotrophy and found that the cells indeed produce starch and TAG at a much faster rate as compared to that in mixotrophy and heterotrophy. Further, we noticed that the accumulation of starch is relatively more as compared to TAG. Based on these observations, we suggest that acetate assimilation in C. reinhardtii does not directly lead to TAG formation but via bicarbonate/CO₂ ^aq pathways. Photoautotrophic mode is found to be the best growth condition for the production of starch and TAG and starch in C. reinhardtii.     

Carbon assimilation pathways in sulfate reducing bacteria. Formate,carbon dioxide,carbon monoxide,and acetate assimilation by Desulfovibrio baarsii

Desulfovibrio baarsii is a sulfate reducing bacterium, which can grown on formate plus sulfate as sole energy source and formate and CO₂ as sole carbon sources. It is shown by ¹⁴C labelling studies that more than 60% of the cell carbon is derived from CO₂ and the rest from formate. The cells thus grow autotrophically. Labelling studies with [¹⁴C]acetate, ¹⁴CO and [¹⁴C]formate indicate that CO₂ fixation does not proceed via the Calvin cycle. The labelling patterns of alanine, aspartate, glutamate, and glucosamine indicate that acetate (or activated acetic acid) is an early intermediate in formate and CO₂ assimilation; the methyl group of acetate is derived from formate, and the carboxyl group from CO₂ via CO; pyruvate is formed from acetyl-CoA by reductive carboxylation. The capacity to synthesize an acetate unit from two C₁-compounds obviously distinguishes D. baarsii from those Desulfovibrio species, which require acetate as a carbon source in addition to CO₂.     

The induction of the CO2-concentrating mechanism is correlated with the formation of the starch sheath around the pyrenoid of Chlamydomonas reinhardtii

The pyrenoid is a prominent proteinaceous structure found in the stroma of the chloroplast in unicellular eukaryotic algae, most multicellular algae, and some hornworts. The pyrenoid contains the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase and is sometimes surrounded by a carbohydrate sheath. We have observed in the unicellular green alga Chlamydomonas reinhardtii Dangeard that the pyrenoid starch sheath is formed rapidly in response to a decrease in the CO₂ concentration in the environment. This formation of the starch sheath occurs coincidentally with the induction of the CO₂-concentrating mechanism. Pyrenoid starch-sheath formation is partly inhibited by the presence of acetate in the growth medium under light and low-CO₂ conditions. These growth conditions also partly inhibit the induction of the CO₂-concentrating mechanism. When cells are grown with acetate in the dark, the CO₂-concentrating mechanism is not induced and the pyrenoid starch sheath is not formed even though there is a large accumulation of starch in the chloroplast stroma. These observations indicate that pyrenoid starch-sheath formation correlates with induction of the CO₂-concentrating mechanism under low-CO₂ conditions. We suggest that this ultrastructural reorganization under lowCO₂ conditions plays a role in the CO₂-concentrating mechanism C. reinhardtii as well as in other eukaryotic algae.     

Mixotrophic chain elongation with syngas and lactate as electron donors

Feeding microbial communities with both organic and inorganic substrates can improve sustainability and feasibility of chain elongation processes. Sustainably produced H₂, CO₂, and CO can be co-fed to microorganisms as a source for acetyl-CoA, while a small amount of an ATP-generating organic substrate helps overcome the kinetic hindrances associated with autotrophic carboxylate production. Here, we operated two semi-continuous bioreactor systems with continuous recirculation of H₂, CO₂, and CO while co-feeding an organic model feedstock (lactate and acetate) to understand how a mixotrophic community is shaped during carboxylate production. Contrary to the assumption that H₂, CO₂, and CO support chain elongation via ethanol production in open cultures, significant correlations (p < 0.01) indicated that relatives of Clostridium luticellarii and Eubacterium aggregans produced carboxylates (acetate to n-caproate) while consuming H₂, CO₂, CO, and lactate themselves. After 100 days, the enriched community was dominated by these two bacteria coexisting in cyclic dynamics shaped by the CO partial pressure. Homoacetogenesis was strongest when the acetate concentration was low (3.2 g L⁻¹), while heterotrophs had the following roles: Pseudoramibacter, Oscillibacter, and Colidextribacter contributed to n-caproate production and Clostridium tyrobutyricum and Acidipropionibacterium spp. grew opportunistically producing n-butyrate and propionate, respectively. The mixotrophic chain elongation community was more efficient in carboxylate production compared with the heterotrophic one and maintained average carbon fixation rates between 0.088 and 1.4 g CO₂ equivalents L⁻¹ days⁻¹. The extra H₂ and CO consumed routed 82% more electrons to carboxylates and 50% more electrons to carboxylates longer than acetate. This study shows for the first time long-term, stable production of short- and medium-chain carboxylates with a mixotrophic community.     

Kohlenstoffassimilation von Chlamydobotrys (Volvocales)

Zusammenfassung Mit Hilfe C¹⁴-markierten Acetats und Bicarbonats wurde die Kohlenstoffassimilation aus Acetat und Kohlendioxyd von Chlamydobotrys unter verschiedenen Bedingungen untersucht. Der überwiegende Teil des aus Acetat in zelleigenes Material eingebauten Kohlenstoffs stammt aus der Methylgruppe, während entwickeltes Kohlendioxyd vorwiegend aus der Carboxylgruppe freigesetzt wird. Verglichen mit der Kohlenstofffixierung unter Anaerobiose steigern aerobe Bedingungen die Kohlenstoffassimilation aus Acetat im Licht um rund 25%. Im Dunkeln wird nur 10% des Kohlenstoffs aus Acetat in zelleigene Substanz eingebaut, das sind nur 3% des unter gleichen Bedingungen im Licht assimilierten.Zur Zeit optimaler Acetatassimilation findet nur eine sehr schwache Kohlendioxydfixierung statt, diese steigt mit dem Alter der Kultur. Die CO₂-Assimilation nimmt bei Acetatmangel zu; der aus CO₂ unter optimalen Bedingungen fixierte Kohlenstoff beträgt nur 5% des unter optimalen Verhältnissen aus Acetat assimilierten.

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Summary The carbon assimilation from radioactive labeled acetate and from NaH¹⁴CO₃ by Chlamydobotrys was studied under different conditions. Most of the carbon assimilated from acetate in the light comes from the methyl group, while the CO₂ produced was derived mainly from the carboxyl carbon. In light under aerobic conditions the carbon assimilation from acetate is higher by 25% than under anaerobic conditions. In the dark only 10% of the total acetate-carbon utilized are incorporated into cell material; that is only 3% of the total acetate carbon incorporated in light. During the period of high acetate assimilation the photosynthetic fixation of CO₂ is extremely weak. At acetate deficiency CO₂-fixation rises, but reaches only 5% of the carbon assimilated from acetate under optimum conditions.

     

自养微生物同化CO_2的分子生态研究及同化碳在土壤中的转化

大气中CO2浓度持续升高和全球气候变暖是亟待解决的重大环境问题。自养微生物在环境中广泛分布,能直接参与CO2的同化,因此研究自养微生物同化CO2的分子生态学机制具有重大的科学意义。以往对自养微生物的研究多针对基因组DNA,从DNA水平揭示了不同生态系统中碳同化自养微生物的种群结构和多样性,但这些微生物在生态系统中的具体功能有待进一步的研究。近年来,随着转录组学研究技术和稳定同位素探针技术(SIP)的发展,自养微生物同化CO2的生态机理研究不断深入,这些研究明确揭示了碳同化自养微生物是河流、湖泊和海洋生态系统中CO2固定作用的驱动者,并新发现了一些具有CO2同化功能的微生物群落。基于国内外有关研究进展,从DNA和RNA水平上对自养微生物同化CO2的分子机理以及稳定同位素探针技术(SIP)在碳同化微生物研究中的应用进行了分析和总结,初步展望了RNA-SIP技术在陆地生态系统碳同化微生物分子生态学研究中的前景。同时,探讨了陆地生态系统同化碳的转化和稳定性机理,以期为深入了解生态系统碳循环过程和应对气候变化提供理论依据。     

Relationships between Carbon Assimilation, Partitioning, and Export in Leaves of Two Soybean Cultivars

To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO₂ assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour interval. Assimilate export was estimated from CO₂ assimilation and leaf dry matter accumulation. Chamber-grown `Amsoy 71' and `Wells' plants were subjected on the day of the measurements to one of six photosynthetic photon flux densities in order to vary CO₂ assimilation rates.

Rate of accumulation of leaf dry matter and rate of export both increased as CO₂ assimilation rate increased in each cultivar.

Starch concentrations were greater in Amsoy 71 than in Wells at all CO₂ assimilation rates. At low CO₂ assimilation rates, export rates in Amsoy 71 were maintained in excess of 1.0 milligram CH₂O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, export rate continued to decline with decreasing CO₂ assimilation rate. The low leaf starch concentration in Wells at low CO₂ assimilation rates may have limited export by limiting carbon from starch remobilization.

Both cultivars exhibited positive correlations between CO₂ assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.

     

Comparison of unitrophic and mixotrophic substrate metabolism by acetate-adapted strain of Methanosarcina barkeri

We examined the unitrophic metabolism of acetate and methanol individually and the mixotrophic utilization of these compounds by using detailed ¹⁴C-labeled tracer studies in a strain of Methanosarcina barkeri adapted to grow on acetate as the sole carbon and energy source. The substrate consumption rate and methane production rate were significantly lower on acetate alone than during the unitrophic or mixotrophic metabolism of methanol. Cell yields (in grams per mole of substrate) were identical during exponential growth on acetate and exponential growth on methanol. During unitrophic metabolism of acetate, the methyl moiety accounted for the majority of the CH₄ produced, but 14% of the CO₂ generated originated from the methyl moiety. This correlated with the concurrent reduction of equivalent amounts of the C-1 of acetate to CH₄. ¹⁴CH₄ was also produced from added ¹⁴CO₂, although to a lesser extent than from reduction of the C-1 of acetate. During mixotrophic metabolism, methanol and acetate were catabolized simultaneously. The rates of ¹⁴CH₄ and ¹⁴CO₂ generation from [2-¹⁴C]acetate were logarithmic and higher in mixotrophic than in unitrophic cultures at substrate concentrations of 50 mM. A comparison of the oxidoreductase activities in cell extracts of the acetate-adapted strain grown on acetate and of strain MS grown on methanol or on H₂ plus CO₂ indicated that the pyruvate, α-ketoglutarate, and isocitrate dehydrogenase activities remained constant, whereas the CO dehydrogenase activity was significantly higher (5,000 nmol/min per mg of protein) in the acetate-adapted strain. These results suggested that a significant intramolecular redox pathway is possible for the generation of CH₄ from acetate, that energy metabolism from acetate by M. barkeri is not catabolite repressed by methanol, and that the acetate-adapted strain is a metabolic mutant with derepressed CO dehydrogenase activity.     

Improving carbohydrate production of Chlorella sorokiniana NIES‐2168 through semi‐continuous process coupled with mixotrophic cultivation

Biofuels from microalgae is now a hot issue of great potential. However, achieving high starch productivity with photoautotrophic microalgae is still challenging. A feasible approach to enhance the growth and target product of microalgae is to conduct mixotrophic cultivation. The appropriate acetate addition combined with CO₂ supply as dual carbon sources (i.e., mixotrophic cultivation) could enhance the cell growth of some microalgae species, but the effect of acetate‐mediated mixotrophic culture mode on carbohydrate accumulation in microalgae remains unclear. Moreover, there is still lack of the information concerning how to increase the productivity of carbohydrates from microalgae under acetate‐amended mixotrophic cultivation and how to optimize the engineering strategies to achieve the goal. This study was undertaken to develop an optimal acetate‐contained mixotrophic cultivation system coupled with effective operation strategies to markedly improve the carbohydrate productivity of Chlorella sorokiniana NIES‐2168. The optimal carbohydrate productivity of 695 mg/L/d was obtained, which is the highest value ever reported. The monosaccharide in the accumulated carbohydrates is mainly glucose (i.e., 85–90%), which is very suitable for bio‐alcohols fermentation. Hence, by applying the optimal process developed in this study, C. sorokiniana NIES‐2168 has a high potential to serve as a feedstock for subsequent biofuels conversion.     

Influence of Spectral Range and Carbon and Nitrogen Sources on Oxygen Evolution and Emerson Enhancement in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii was grown in medium with different carbon (acetate, CO₂, or both), and nitrogen (ammonium chloride, peptone, urea) sources and under light of different spectral composition. The light-dark cycles were found more suitable for mixotrophic growth than continuous irradiation. Both blue (BR) and red (RR) radiations decreased photosynthetic capacity of mixotrophic cells compared to “white light” (WL). Effect of RR was associated with photon distribution favouring photosystem 1 (PS1) suggesting increased cyclic phosphorylation. Mixotrophic growth in 10 mM NH₄Cl increased photosynthetic oxygen evolution compared to standard concentration of 5 mM NH₄Cl used for growing C. reinhardtii. Autotrophic growth stimulated the photosynthetic capacity compared to mixotrophic one. However, higher photosynthetic capacity was achieved for mixotrophic cells by growing them at high NH₄ ⁺/K⁺ ratio and high phosphate concentration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Starch and Sucrose Synthesis in Phaseolus vulgaris as Affected by Light, CO(2), and Abscisic Acid

Phaseolus vulgaris L. leaves were subjected to various light, CO₂, and O₂ levels and abscisic acid, then given a 10 minute pulse of ¹⁴CO₂ followed by a 5 minute chase with unlabeled CO₂. After the chase period, very little label remained in the ionic fractions (presumed to be mostly carbon reduction and carbon oxidation cycle intermediates and amino acids) except at low CO₂ partial pressure. Most label was found in the neutral, alcohol soluble fraction (presumed sucrose) or in the insoluble fraction digestable by amyloglucosidase. Sucrose formation was linearly related to assimilation rate (slope = 0.35). Starch formation increased linearly with assimilation rate (slope = 0.56) but did not occur if the assimilation rate was below 4 micromoles per square meter per second. Neither abscisic acid, nor high CO₂ in combination with low O₂ (thought to disrupt control of carbon metabolism) caused significant perturbations of the sucrose/starch formation ratio. These studies indicate that the pathways for starch and sucrose synthesis both are controlled by the rate of net CO₂ assimilation, with sucrose the preferred product at very low assimilation rates.     

Non-phototrophic CO 2 fixation by soil microorganisms

Although soils are generally known to be a net source of CO₂ due to microbial respiration, CO₂ fixation may also be an important process. The non-phototrophic fixation of CO₂ was investigated in a tracer experiment with ¹⁴CO₂ in order to obtain information about the extent and the mechanisms of this process. Soils were incubated for up to 91 days in the dark. In three independent incubation experiments, a significant transfer of radioactivity from ¹⁴CO₂ to soil organic matter was observed. The process was related to microbial activity and could be enhanced by the addition of readily available substrates such as acetate. CO₂ fixation exhibited biphasic kinetics and was linearly related to respiration during the first phase of incubation (about 20–40 days). The fixation amounted to 3–5% of the net respiration. After this phase, the CO₂ fixation decreased to 1–2% of the respiration. The amount of carbon fixed by an agricultural soil corresponded to 0.05% of the organic carbon present in the soil at the beginning of the experiment, and virtually all of the fixed CO₂ was converted to organic compounds. Many autotrophic and heterotrophic biochemical processes result in the fixation of CO₂. However, the enhancement of the fixation by addition of readily available substrates and the linear correlation with respiration suggested that the process is mainly driven by aerobic heterotrophic microorganisms. We conclude that heterotrophic CO₂ fixation represents a significant factor of microbial activity in soils.     

Autotrophic growth of four Sulfolobus strains on tetrathionate and the effect of organic nutrients

Autotrophic growth yields of four strains of Sulfolobus using tetrathionate as sole energy substrate fell in the range 6.2–7.8 g dry weight (mol tetrathionate oxidized)^-1. Autotrophic organisms lacked ribulose 1,5-bis-phosphate carboxylase, but contained pyruvate and phosphoenolpyruvate carboxylases. S. brierleyi and strains B6-2 and LM exhibited mixotrophic growth, with tetrathionate oxidation, CO₂-fixation and organic substrate assimilation occurring concurrently, using media containing glucose or acetate. Yeast extract or succinate supported heterotrophic growth and showed strain-dependent repression of one or both of tetrathionate oxidation and CO₂-fixation resulting in biphasic growth. All four carbon atoms of succinate were assimilated to cell-carbon during growth. Acetate was the major source of cell-carbon during mixotrophic growth. These observations are not inconsistent with the possibility of a reductive carboxylic acid cycle in these organisms. Radiorespirometric analysis of glucose oxidation indicated CO₂ release to occur by means of an Entner-Doudoroff pathway (followed by pyruvate decarboxylation) and oxidative pentose phosphate pathway reactions. There was little evidence from the glucose radiorespirometry of the large-scale use of an oxidative tricarboxylic acid cycle for terminal oxidation of acetate derived from pyruvate. These results demonstrate the considerable metabolic versatility of Sulfolobus strains and show that there is significant variation among them.Abbreviations PIPES Piperazine-N,N-bis (2-ethane sulphonic acid)     

Diurnal changes in allocation and partitioning of recently assimilated carbon in loblolly pine seedlings

We examined diurnal fluctuations in acquisition and partitioning of recently assimilated ¹⁴CO₂, and in subsequent allocation and partitioning to roots of loblolly pine (Pinus taeda L.) seedlings. Nonmycorrhizal seedlings were grown under optimal nutrient conditions in continuously flowin solution culture. Shoots of 15-week-old loblolly pine seedlings were labeled with ¹⁴CO₂ for 30 min at four separate labeling times: 1000, 1200, 1400 and 1600 h. Six whole plant harvests were conducted during a 48 h chase period, i.e. 0, 4, 8 12, 24 and 48 h after the end of the labeling and evacuation periods. Although assimilation of ¹⁴CO₂ was constant between 1000 and 1400 h, there were significant differences in partitioning of ¹⁴C-labeled assimilate in needles of all age classes. The highest percentage of recently assimilated ¹⁴CO₂ in the ethanol-soluble fraction of photosynthesizing tissue was observed near the beginning and end of the photoperiod. Partitioning of ¹⁴C in the ethanol-soluble fraction declined between the 1000 and 1400 h labeling eriods, and was accompanied by an increase in partitioning of recently assimilated ¹⁴CO₂ toward starch and a decrease in respiratory losses. These data suggest that most of the ¹⁴CO₂ assimilated at 1000 h was used to support shoot metabolic activities and possibly restore soluble sugar reserves. Peak starch accumulation in needles during the 1400 h labeling period, concomitant with minimal respiratory loss, indicated that photosynthate production exceeded demand and export out of source leaves. A possible feedback regulation of photosynthesis by starch and/or sugar accumulation may be responsible for the observed decline in assimilation of ¹⁴CO₂ during the 1600 h labeling period. Net accumulation of recently assimilated ¹⁴CO₂ in roots was correlated with assimilation rate of ¹⁴CO₂, but independent of partitioning of recently assimilated carbon in photosynthetic tissue. However, the percentage of total seedling ¹⁴C allocated to roots was essentially the same throughout the 48 h chase, regardless of time of labeling and assimilation rate. The data suggest a strong diurnal regulation of starch and soluble sugars synthesized from recently assimilated carbon in needles of loblolly pine seedlings that was independent of assimilation rate. Allocation and transport of recently assimilated carbon to roots of loblolly pine seedlings were not subject to short-term fluctuations in supply and demand.     

Glyphosate inhibits photosynthesis and allocation of carbon to starch in sugar beet leaves

Application of glyphosate (N-[phosphonomethyl] glycine) to exporting leaves of sugar beet (Beta vulgaris, L.) during the day lowered stomatal conductance and carbon fixation. Allocation of newly fixed carbon to foliar starch accumulation was nearly completely inhibited, being decreased by the same amount as net carbon fixation. In contrast, decreasing net carbon fixation in untreated leaves by lowering CO₂ concentration caused starch accumulation to decrease, but only in the same proportion as net carbon fixation. Shikimate level increased 50-fold in treated leaves but the elevated rate of carbon accumulation in shikimate was only 4% of the decrease in the rate of starch accumulation. Application of steady state labeling with ¹⁴CO₂ to exporting leaves confirmed the above changes in carbon metabolism, but revealed no other major daytime differences in the ¹⁴C-content of amino acids or other compounds between treated and control leaves. Less ¹⁴C accumulated in treated leaves because of decreased fixation, not increased export. The proportion of newly fixed carbon allocated to sucrose increased, maintaining export at the level in control leaves. Returning net carbon exchange to the rate before treatment restored starch accumulation fully and prevented a decrease in export during the subsequent dark period.     

Influence of elevated CO2 and nitrogen nutrition on photosynthesis and nitrate photo-assimilation in maize (Zea mays L.)

Measurements of CO₂ and O₂ gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO₂ inhibits nitrate (NO₃ ^– ) photo‐assimilation in the C₄ plant, maize (Zea mays L.). The assimilatory quotient (AQ), the ratio of net CO₂ assimilation to net O₂ evolution, decreases as NO₃ ^– photo‐assimilation increases so that the difference in AQ between the ammonium‐ and nitrate‐fed plants (ΔAQ) provided an in planta estimate of NO₃ ^– photo‐assimilation. In fully expanded maize leaves, NO₃ ^– photo‐assimilation was detectable only under high light and was not affected by CO₂ treatments. Furthermore, CO₂ assimilation and O₂ evolution were higher under NO₃ ^– than ammonia (NH₄⁺) regardless of CO₂ levels. In conclusion, NO₃ ^– photo‐assimilation in maize primarily occurred at high light when reducing equivalents were presumably not limiting. Nitrate photo‐assimilation enhanced C₄ photosynthesis, and in contrast to C₃ plants, elevated CO₂ did not inhibit foliar NO₃^– photo‐assimilation.