Photosynthetic Carbon Partitioning Research Articles

Straw-derived biochar incorporation improves seedcotton yield and fiber quality by optimizing photosynthetic carbon and nutrients partitioning and boll formation patterns

Synergistic improvement of cotton (Gossypium hirsutum L.) yield and quality is necessary to meet the demands of the textile industry. Straw or biochar addition has been shown to increase boll number and hence yield, while optimizing reproductive growth processes (e.g., accelerating boll formation during the effective boll-setting period) is essential for improving fiber quality. However, it remains unclear how straw and biochar affect boll formation dynamics and whether they can simultaneously improve cotton yield and fiber quality. In order to reveal the mechanism of simultaneous improvement of cotton yield and quality, this study investigated the responses of plant photosynthetic carbon allocation, bud and boll dynamics, seedcotton yield, and fiber quality to different straw returning modes. A 7- year field experiment was performed with four straw treatments: no straw or biochar addition (CK); straw-derived biochar incorporation (SBI); straw incorporation (SI); and straw mulching (SM). The results suggested that SBI, SI and SM remarkably increased the seedcotton yield (24.5–30.7 %, 12.6–20.4 % and 4.7 %-11.0 %), fiber length (3.6–6.7 %, 4.0–9.5 % and 2.4–7.1 %) and fiber strength (8.0–13.0 %, 6.2–10.7 % and 4.4–5.7 %) from 2019 to 2021, compared with CK. There was a strong consistency between seedcotton yield and fiber strength among straw returning modes, and were highest under SBI, followed by SI and SM. Compared with CK, SBI significantly increased nutrients and photosynthetic carbon accumulation in cotton plant, and enhanced their distribution proportion in reproductive organs. This increased the number of fruiting branches, number of fruiting sites and ratio of fruiting sites to fruiting branches, accelerated budding and reduced the shedding rate of buds and bolls. Finally, SBI obtained the highest boll forming rate during the boll formation stage, as well as the highest effective boll number at the harvest stage. Overall, returning straw-derived biochar into soils optimized the allocation of nutrients and photosynthetic carbon in cotton plant, and accelerated the formation of cotton bolls from peak flowering stage to peak boll setting stage, thereby improving seedcotton yield and fiber quality. This work provides practical guidance for improving seedcotton yield, fiber quality, and the utilization of straw resources.

Two types of growth pattern of the five microalgal species under different nitrogen supplies

Manipulation of nitrogen levels is a viable strategy to improve microalgal biomass production. Here, nitrogen uptake and biochemical characteristics of the five microalgal strains were studied with high and low nitrogen concentrations. Two types of growth pattern were observed, non-influencing growth (type Ⅰ, Nannochloropsis gaditana and Kirchneriella sp.) and inhibitory growth (type Ⅱ, Scenedesmus acuminatus, Scenedesmus obliquus and Chlorella vulgaris). The highest biomass of the type Ⅰ microalgae was obtained with higher nitrogen supply (≥18 mM), whereas type Ⅱ microalgae reached highest biomass under nitrogen-limited conditions (7.2–10.8mM). Type Ⅰ microalgae had the highest nitrogen uptake capacity (N. gaditana) and nitrate reductase (NR) activity (Kirchneriella sp. 3.67 U g−1, 10.8 mM). The lower the nitrogen concentration, the higher the NR activity measured in type Ⅱ microalgae on day 1. The nitrogen replete group (21.6mM) of the type Ⅱ microalgae decreased the nitrogen uptake. The NR activity of S. acuminatus was positively associated with the initial nitrogen supply, and an NR activity threshold effect was observed in S. obliquus and C. vulgaris. The amount of nitrite released by the type Ⅱ microalgae was one order magnitude higher than those released by the type Ⅰ microalgae. In nitrogen-limited conditions, the five microalgae species shifted the photosynthetic carbon partitioning into lipid accumulation. The difference in growth patterns was mainly due to nitrogen uptake capacity, activity of nitrate reductase and the amount of nitrite released.

RNA-seq analysis reveals genes related to photosynthetic carbon partitioning and lipid production in Phaeodactylum tricornutum under alkaline conditions.

Alkaline pH can induce triacylglyceride accumulation in microalgae, however its molecular mechanism remains elusive. Here, we investigated the effect of 2-[N-cyclohexylamino]-ethane-sulfonic acid (CHES) -induced intracellular alkalization on the lipid production in Phaeodactylum tricornutum. Intracellular pH was increased upon CHES treatment, displaying a high BCECF fluorescence ratio. CHES treatment significantly induced lipid accumulation but had no change in cell density and biomass. The expression of genes involved in photoreaction, carbon fixation, glycolysis, pentose phosphate pathway, amino acid catabolism, GS/GOGAT cycle, TCA cycle, triacylglyceride assembly, de novo fatty acid synthesis were up-regulated, while that in amino acid biosynthesis were down-regulated under CHES conditions. Correspondingly, the activity of phosphoribulokinase, carbonic anhydrase, pyruvate dehydrogenase and acetaldehyde dehydrogenase were enhanced by CHES treatment. Chloroplast-specific biological processes were activated by CHES treatment in P. tricornutum, which redirects the flux of carbon into lipid biosynthesis, meanwhile stimulates de novo fatty acid biosynthesis, leading to lipid accumulation under CHES conditions. These indicate an enhancement of intermediate metabolism, resulting in lipid accumulation by CHES.

Synchronised interaction of carbon and nitrogen provides drought tolerance in Cyamopsis tetragonoloba

Drought is a major adverse environmental factor that limits plant productivity. Concomitant assimilation of carbon (C) and nitrogen (N) in illuminated leaves requires the regulated partitioning of reductant and photosynthate to sustain the demands of amino acid and carbohydrate biosynthesis. The short-term exposure to drought and its responses of photosynthesis and photosynthate partitioning to N enrichment was studied in Cyamopsis tetragonoloba (L.) Taub (guar). Four contrasting varieties of guar, HG-563, RGC-986 (drought-tolerant) and RGC-471, Varsha (drought-sensitive) were subjected to 15 days water stress and leaf tissues were studied in order to contemplate the counter strategy engaged under drought conditions. In guar plants, sensors trigger a signalling cascade that activates numerous proteins to induce the expression of certain genes involved in carbon and nitrogen metabolism.The levels of glucose, starch, and Glutamate dehydrogenase (GDH), Aspartate aminotransferase (AspAT), isocitrate dehydrogenase (ICDH), Nitrate Reductase (NR), and Nitrite reductase (NiR) activities were determined biochemically during 15 days drought treatment, at three stages. Further, the carbon (oxidative phosphorylation and TCA cycle) - nitrogen metabolic pathway interaction was studied at the molecular level. The respiratory pathway genes were upregulated, while the nitrogen metabolic pathway genes were downregulated under all stages of drought in guar plants. The physiological, biochemical and molecular data suggests that the high rates of dark respiration was insufficient to provide extra carbon required to assist amino acid synthesis but rather sufficed by a rapid metamorphosis in the photosynthetic carbon partitioning to amino acid metabolism via the nitrogen metabolism. • Drought causes NADH DEHYDROGENASE gene to escalate dark respiration. • Nitrogen assimilation reduces under drought in leguminous Cyamopsis tetragonoloba. • Drought triggers protein kinase cascade signalling in guar plants. • Drought regulates nitrogen metabolic genes in guar. • Drought triggers increase in sugar content at the cost of amino acids.

Phosphate availability regulates flavonoid accumulation associated with photosynthetic carbon partitioning in Cyclocarya paliurus.

Cyclocarya paliurus has traditionally been used as medicine and functional food. This study aims at investigating the flavonoid accumulation in C. paliurus dependent on phosphate (Pi) availability and its potential association with internal carbon partitioning. One-year-old seedlings of C. paliurus were planted in four different Pi levels. Low Pi resulted in low phosphorus content within plants, while the nitrogen content increased. Further analysis revealed that the surplus carbon pool was greater and was allocated to N-metabolism and carbohydrate synthesis under low Pi conditions, as shown by the higher levels of free amino acids, starch, and soluble sugars. Low Pi availability also induced higher enzymatic activities of shikimate dehydrogenase (SDH) and flavonoid 3-hydoxylase (FHT), and higher flavonoid accumulation in leaves. Our results indicated that the surplus carbon induced by low-Pi levels can increase flavonoid synthesis in seedlings of C. paliurus. In addition, growth and biomass accumulation were increased by the elevated Pi levels. As a result, the highest flavonoid yield per plant was obtained under relative low Pi conditions. This study can provide the basis for developing new agricultural practices to maintain high yield while still keeping the quality of medicinal plants and crops.

Photosynthetic Carbon Partitioning and Metabolic Regulation in Response to Very-Low and High CO2 in Microchloropsis gaditana NIES 2587.

Photosynthetic organisms fix inorganic carbon through carbon capture machinery (CCM) that regulates the assimilation and accumulation of carbon around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, few constraints that govern the central carbon metabolism are regulated by the carbon capture and partitioning machinery. In order to divert the cellular metabolism toward lipids and/or biorenewables it is important to investigate and understand the molecular mechanisms of the CO2-driven carbon partitioning. In this context, strategies for enhancement of CO2 fixation which will increase the overall biomass and lipid yields, can provide clues on understanding the carbon assimilation pathway, and may lead to new targets for genetic engineering in microalgae. In the present study, we have focused on the physiological and metabolomic response occurring within marine oleaginous microalgae Microchloropsis gaditana NIES 2587, under the influence of very-low CO2 (VLC; 300 ppm, or 0.03%) and high CO2 (HC; 30,000 ppm, or 3% v/v). Our results demonstrate that HC supplementation in M. gaditana channelizes the carbon flux toward the production of long chain polyunsaturated fatty acids (LC-PUFAs) and also increases the overall biomass productivities (up to 2.0 fold). Also, the qualitative metabolomics has identified nearly 31 essential metabolites, among which there is a significant fold change observed in accumulation of sugars and alcohols such as galactose and phytol in VLC as compared to HC. In conclusion, our focus is to understand the entire carbon partitioning and metabolic regulation within these photosynthetic cell factories, which will be further evaluated through multiomics approach for enhanced productivities of biomass, biofuels, and bioproducts (B3).

Influence of nutrient formulations on growth, lipid yield, carbon partitioning and biodiesel quality potential of Botryococcus sp. and Chlorella sp.

The study was conducted to analyse the influence of three nutrient formulations, namely BG-11 medium, BBM and TAP medium, on growth potential and lipid yield of two microalgal genera (Botryococcus sp. and Chlorella sp.) and to study the roles of N, P and other major nutrients. The study focussed on the general patterns of starch and lipid synthesis and storage and to further assess how photosynthetic carbon partitioning into starch and lipid is altered by conditions in growth media such as N and C presence as seen in BG11 medium which are known to induce neutral lipid production and the lack of it in BBM and TAP medium. BG-11 medium performed better as compared to BBM and TAP medium in terms of biomass productivity and lipid yield. The lipid yield was highest in Botryococcus sp. (63.03% dry wt.) and Chlorella sp. (50.27% dry wt.) at 30th day of incubation. Mean biomass productivity was highest for Botryococcus in BBM medium (6.14mg/L/day) and for Chlorella in BG-11 medium (4.97mg/L/day). Mean lipid productivity (50.78% and 39.36%) was highest in BG11 medium for both Botryococcus and Chlorella species, respectively. A sharp decline in sugar content was observed in the late stationary phase of growth from 30th day to 45th day. Fatty acid methyl ester (FAME) profile of the extracted lipids showed predominantly oleic acid, followed by palmitic acid and stearic acid in both the strains when grown in BG-11 medium. The other biodiesel quality parameters were in accordance with the international standards. A complex relationship was found between chemical composition and biodiesel properties. Proximity analysis indicated that the fuel properties of biodiesels are determined by a number of parameters and by the combination of different chemical compositions. The results provide an insight into organic carbon partitioning into lipid compounds and how the organism's lipid metabolism changes due to N-deplete culturing in TAP medium and inorganic carbon source availability as seen in BG-11 and BBM medium.

Carbon allocation and fate in paddy soil depending on phosphorus fertilization and water management: results of 13C continuous labelling of rice

We grew rice in phosphorus (P) deficient subtropical paddy soil in a field study and used 13CO2 continuous labelling to investigate photosynthetic carbon (C) partitioning and allocation under FLOOD versus WET/DRY conditions, with and without P fertilization (80 mg P kg−1). The plants and soil were sampled after each of three WET/DRY cycles to determine 13C allocation in above- and belowground plant biomass, microbial biomass, the rhizosphere, and bulk soil. Irrespective of water management, P-fertilized plants had higher biomass and P content and more total 13C in the rice-soil system, especially the 13C incorporation into the shoots (51%–96%), than samples without P fertilization. Root and bulk-soil 13C were largely independent of both P fertilization and water management. However, by the third sampling, P fertilization had increased the amount of 13C and microbial biomass 13C in the rhizosphere soil (RS) by 28% (WET/DRY) and 95% (FLOOD), and by 47% (WET/DRY) and 50% (FLOOD), respectively. The WET/DRY condition had significantly higher microbial biomass and 13C contents than FLOOD condition only in the RS. These results indicate that a well-established aboveground plant biomass following P fertilization is required to increase belowground C allocation. Thus, WET/DRY conditions, like FLOOD conditions, can provide moisture sufficient for unhindered P availability in rice-paddy system.

Overexpression of bifunctional fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase leads to enhanced photosynthesis and global reprogramming of carbon metabolism in Synechococcus sp. PCC 7002

Cyanobacteria fix atmospheric CO2 to biomass and through metabolic engineering can also act as photosynthetic factories for sustainable productions of fuels and chemicals. The Calvin Benson cycle is the primary pathway for CO2 fixation in cyanobacteria, algae and C3 plants. Previous studies have overexpressed the Calvin Benson cycle enzymes, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and bifunctional sedoheptulose-1,7-bisphosphatase/fructose-1,6-bisphosphatase (hereafter BiBPase), in both plants and algae, although their impacts on cyanobacteria have not yet been rigorously studied. Here, we show that overexpression of BiBPase and RuBisCO have distinct impacts on carbon metabolism in the cyanobacterium Synechococcus sp. PCC 7002 through physiological, biochemical, and proteomic analyses. The former enhanced growth, cell size, and photosynthetic O2 evolution, and coordinately upregulated enzymes in the Calvin Benson cycle including RuBisCO and fructose-1,6-bisphosphate aldolase. At the same time it downregulated enzymes in respiratory carbon metabolism (glycolysis and the oxidative pentose phosphate pathway) including glucose-6-phosphate dehydrogenase (G6PDH). The content of glycogen was also significantly reduced while the soluble carbohydrate content increased. These results indicate that overexpression of BiBPase leads to global reprogramming of carbon metabolism in Synechococcus sp. PCC 7002, promoting photosynthetic carbon fixation and carbon partitioning towards non-storage carbohydrates. In contrast, whilst overexpression of RuBisCO had no measurable impact on growth and photosynthetic O2 evolution, it led to coordinated increase in the abundance of proteins involved in pyruvate metabolism and fatty acid biosynthesis. Our results underpin that singular genetic modifications in the Calvin Benson cycle can have far broader cellular impact than previously expected. These features could be exploited to more efficiently direct carbons towards desired bioproducts.

Physiological and biochemical changes reveal stress-associated photosynthetic carbon partitioning into triacylglycerol in the oleaginous marine alga Nannochloropsis oculata

Abiotic stresses such as high light (HL) and/or nitrogen deficiency (−N) might induce triacylglycerol (TAG) biosynthesis in microalgae. However, little is known about the changes of lipid classes and the mechanism underlying the enhanced lipid production as imposed by irradiance and nitrogen starvation. In this work HL and −N were applied individually or in combination to Nannochloropsis oculata to investigate the underlying mechanism for stress-associated TAG synthesis. Stress conditions, HL−N in particular, inhibited cell growth and stimulated the accumulation of neutral lipids, predominantly in the form of TAG (402mg/gDW). The membrane polar lipids, however, were attenuated to differential degrees depending on the stress conditions. Over 90% of TAG sn-2 position was occupied by C16 fatty acids, suggesting that N. oculata may involve the prokaryotic pathway mostly for TAG synthesis. Key enzymes of acetyl-CoA carboxylase, diacylglycerol acyltransferase and NADPH involved in lipid biosynthesis were up-regulated approximately 3–13 times by HL−N. Our results altogether demonstrated that the photosynthetically fixed carbon was partitioned into TAG as the major energy reservoir, and irradiance and nitrogen were the two important engineering factors that enhanced lipid production by pushing photosynthetic carbon flux to fatty acid biosynthesis and pulling fatty acids to TAG assembly.

Draft Whole-Genome Sequence of the Biocontrol Agent Trichoderma harzianum T6776.

Trichoderma harzianum T6776 is a promising beneficial isolate whose effects consist of growth promotion, positive response of photosynthetic activity, hormonal signaling, and carbon partitioning in tomato, coupled with biocontrol of plant pathogens. Here, we present the first genome assembly of T6776, providing a useful platform for the scientific community.

Effect of light intensity on physiological changes, carbon allocation and neutral lipid accumulation in oleaginous microalgae

Chlorella sp. and Monoraphidium sp. were the potential microalgal species for lipid production. This study aimed to investigate different light intensities (40, 200, 400μmolphotonm−2s−1) on physiological changes, photosynthetic carbon partitioning and neutral lipid accumulation in both microalgae. Results suggested that under high light (HL, 400μmolphotonm−2s−1), chlorophyll degraded, protein and carbohydrate content decreased; more carbon allocated into lipid as well as most of intracellular space was occupied by lipid bodies. Moreover, with the lipid accumulation, Fv/Fm decreased and ROS scavenging enzyme increased. Membrane lipid reduced dramatic (29.73–37.97%) to format NL (71.66% of total lipid in Chlorella sp. L1 and 60.65% in Monoraphidium dybowskii Y2). The NL productivity under HL (51.36 and 49.71mgL−1d−1) were more than 3 times of those under LL. Additionally, FAME profiles proved that the useful fatty acid components for biodiesel production were enhanced under HL.

The relationship between leaf area growth and biomass accumulation in Arabidopsis thaliana.

Leaf area growth determines the light interception capacity of a crop and is often used as a surrogate for plant growth in high-throughput phenotyping systems. The relationship between leaf area growth and growth in terms of mass will depend on how carbon is partitioned among new leaf area, leaf mass, root mass, reproduction, and respiration. A model of leaf area growth in terms of photosynthetic rate and carbon partitioning to different plant organs was developed and tested with Arabidopsis thaliana L. Heynh. ecotype Columbia (Col-0) and a mutant line, gigantea-2 (gi-2), which develops very large rosettes. Data obtained from growth analysis and gas exchange measurements was used to train a genetic programming algorithm to parameterize and test the above model. The relationship between leaf area and plant biomass was found to be non-linear and variable depending on carbon partitioning. The model output was sensitive to the rate of photosynthesis but more sensitive to the amount of carbon partitioned to growing thicker leaves. The large rosette size of gi-2 relative to that of Col-0 resulted from relatively small differences in partitioning to new leaf area vs. leaf thickness.

Photosynthetic efficiency and carbon partitioning in nitrogen-starved Scenedesmus obliquus

This work investigates how the photoacclimated state at the onset of nitrogen starvation and the light intensity during nitrogen starvation influence carbon partitioning and photosynthetic efficiency in nitrogen-starved Scenedesmus obliquus. Nitrogen-depleted batch cultivations were performed at an incident light intensity of 200 and 1000μmolm−2s−1. These nitrogen-depleted batch cultivations were started with biomass that was photoacclimated to an incident light intensity of either 200 or 1000μmolm−2s−1 under nitrogen replete conditions. During initial nitrogen starvation, fatty acids and starch were produced in a 1:4 ratio. This ratio progressively increased towards only fatty acid synthesis. Hereafter, the initially accumulated starch was degraded and likely used as a substrate for fatty acid synthesis. The ratio between starch and fatty acid synthesis correlated strongly to the biomass nitrogen content. Slightly more carbon was partitioned to starch synthesis at high light intensities. Carbon partitioning was not affected by the photoacclimated state at the onset of nitrogen starvation. The photosynthetic efficiency decreased 2-fold at high versus low light intensities but was not affected by photoacclimation. Likely, the observed carbon partitioning is caused by competition between fatty acid and starch synthesis for a common carbon pre-cursor.

Metabolic and photosynthetic consequences of blocking starch biosynthesis in the green alga Chlamydomonas reinhardtii sta6 mutant.

Upon nutrient deprivation, microalgae partition photosynthate into starch and lipids at the expense of protein synthesis and growth. We investigated the role of starch biosynthesis with respect to photosynthetic growth and carbon partitioning in the Chlamydomonas reinhardtii starchless mutant, sta6, which lacks ADP-glucose pyrophosphorylase. This mutant is unable to convert glucose-1-phosphate to ADP-glucose, the precursor of starch biosynthesis. During nutrient-replete culturing, sta6 does not re-direct metabolism to make more proteins or lipids, and accumulates 20% less biomass. The underlying molecular basis for the decreased biomass phenotype was identified using LC-MS metabolomics studies and flux methods. Above a threshold light intensity, photosynthetic electron transport rates (water → CO2) decrease in sta6 due to attenuated rates of NADPH re-oxidation, without affecting photosystems I or II (no change in isolated photosynthetic electron transport). We observed large accumulations of carbon metabolites that are precursors for the biosynthesis of lipids, amino acids and sugars/starch, indicating system-wide consequences of slower NADPH re-oxidation. Attenuated carbon fixation resulted in imbalances in both redox and adenylate energy. The pool sizes of both pyridine and adenylate nucleotides in sta6 increased substantially to compensate for the slower rate of turnover. Mitochondrial respiration partially relieved the reductant stress; however, prolonged high-light exposure caused accelerated photoinhibition. Thus, starch biosynthesis in Chlamydomonas plays a critical role as a principal carbon sink influencing cellular energy balance however, disrupting starch biosynthesis does not redirect resources to other bioproducts (lipids or proteins) during nutrient-replete culturing, resulting in cells that are susceptible to photochemical damage caused by redox stress.

Molecular mechanisms for photosynthetic carbon partitioning into storage neutral lipids in Nannochloropsis oceanica under nitrogen-depletion conditions

Polysaccharides are a major carbon/energy-reservoir in microalgae, yet their relationship with another form of carbon/energy storage, triacylglycerol (TAG), is poorly understood. Here employing oleaginous microalga Nannochloropsis oceanica as a model, we probed the crosstalk between carbohydrate metabolism and TAG accumulation by tracking the temporal dynamics of lipidomes, monosaccharides and polysaccharides and transcripts of selected genes over 14days under nitrogen-depleted (N−) and nitrogen-replete (N+) conditions. Glucose, galactose and mannitol were the main monosaccharides in IMET1, and laminarin may be the storage polysaccharide that competes for carbon precursors with TAG. Transcriptional expression analysis revealed that the β-1,3-glucan degradation and pyruvate dehydrogenases pathways were the main regulatory components involved in driving carbon flow to TAG synthesis. Furthermore, temporal changes of lipidomes and transcripts of glycerolipid metabolism genes were indicative of possible conversion of membrane lipids to TAG, especially under an early stage of nitrogen deprivation conditions. A carbon partitioning model for N. oceanica was proposed, in which β-1,3-glucan metabolism, acetyl–CoA synthesis and membrane lipid turnover/degradation, in addition to de novo fatty acid synthesis, all contributed to TAG synthesis.

Heterologous Expression of the Mevalonic Acid Pathway in Cyanobacteria Enhances Endogenous Carbon Partitioning to Isoprene

Heterologous expression of the isoprene synthase gene in the cyanobacterium Synechocystis PCC 6803 conferred upon these microorganisms the property of photosynthetic isoprene (C₅H₈) hydrocarbons production. Continuous production of isoprene from CO₂ and H₂O was achieved in the light, occurring via the endogenous methylerythritol-phosphate (MEP) pathway, in tandem with the growth of Synechocystis. This work addressed the issue of photosynthetic carbon partitioning between isoprene and biomass in Synechocystis. Evidence is presented to show heterologous genomic integration and cellular expression of the mevalonic acid (MVA) pathway genes in Synechocystis endowing a non-native pathway for carbon flux amplification to isopentenyl-diphosphate (IPP) and dimethylallyl-diphosphate (DMAPP) precursors of isoprene. Heterologous expression of the isoprene synthase in combination with the MVA pathway enzymes resulted in photosynthetic isoprene yield improvement by approximately 2.5-fold, compared with that measured in cyanobacteria transformed with the isoprene synthase gene only. These results suggest that the MVA pathway introduces a bypass in the flux of endogenous cellular substrate in Synechocystis to IPP and DMAPP, overcoming flux limitations of the native MEP pathway. The work employed a novel chromosomal integration and expression of synthetic gene operons in Synechocystis, comprising up to four genes under the control of a single promoter, and expressing three operons simultaneously. This is the first time an entire biosynthetic pathway with seven recombinant enzymes has been heterologously expressed in a photosynthetic microorganism. It constitutes contribution to the genetic engineering toolkit of photosynthetic microorganisms and a paradigm in the pursuit of photosynthetic approaches for the renewable generation of high-impact products.

Relationship between starch and lipid accumulation induced by nutrient depletion and replenishment in the microalga Parachlorella kessleri

Photosynthetic carbon partitioning into starch and neutral lipids, as well as the influence of nutrient depletion and replenishment on growth, pigments and storage compounds, were studied in the microalga, Parachlorella kessleri. Starch was utilized as a primary carbon and energy storage compound, but nutrient depletion drove the microalgae to channel fixed carbon into lipids as secondary storage compounds. Nutrient depletion inhibited both cellular division and growth and caused degradation of chlorophyll. Starch content decreased from an initial value of 25, to around 10% of dry weight (DW), while storage lipids increased from almost 0 to about 29% of DW. After transfer of cells into replenished mineral medium, growth, reproductive processes and chlorophyll content recovered within 2 days, while the content of both starch and lipids decreased markedly to 3 or less % of DW; this suggested that they were being used as a source of energy and carbon.

Photosynthetic Enzymes and Carbon Partitioning Studies in Different Switchgrass Populations

Switchgrass (Panicum virgatum L.) is a C4 perennial lignocellulosic biomass producing biofuel plant. Several populations of this plant have been already established and their potential use in biofuel production has become a topic of significant discussion. The large variation across various populations of switchgrass warranted an examination of carbon partitioning as well as primary photosynthetic enzymes. Using 13C as a tracer, we were able to examine the assimilation and partitioning of carbon into various carbohydrates, both structural and non‐structural in switchgrass over its growing season. Levels and activities of the primary photosynthetic enzymes (Ribulose 1,5‐bisphosphate carboxylase/oxygenase, Phosphoenolpyruvate carboxylase, NADP – Malic enzyme, Pyruvate Pi Dikinase) involved in the assimilation and fixation of carbon, in a few switchgrass populations were determined. Results from these studies suggest that not only is there a large amount of variation across populations, but also within them.

Carbon partitioning in photosynthesis

The work seeks to raise awareness of a fundamental problem that impacts the renewable generation of fuels and chemicals via (photo)synthetic biology. At issue is regulation of the endogenous cellular carbon partitioning between different biosynthetic pathways, over which the living cell exerts stringent control. The regulation of carbon partitioning in photosynthesis is not understood. In plants, microalgae and cyanobacteria, methods need be devised to alter photosynthetic carbon partitioning between the sugar, terpenoid, and fatty acid biosynthetic pathways, to lower the prevalence of sugar biosynthesis and correspondingly upregulate terpenoid and fatty acid hydrocarbons production in the cell. Insight from unusual but naturally occurring carbon-partitioning processes can help in the design of blueprints for improved photosynthetic fuels and chemicals production.