Photosynthetic Carbon Dioxide Fixation Research Articles

Multiple levels of transcriptional regulation control glycolate metabolism in Paracoccus denitrificans.

The hydroxyacid glycolate is a highly abundant carbon source in the environment. Glycolate is produced by unicellular photosynthetic organisms and excreted at petagram scales to the environment, where it serves as growth substrate for heterotrophic bacteria. In microbial metabolism, glycolate is first oxidized to glyoxylate by the enzyme glycolate oxidase. The recently described β-hydroxyaspartate cycle (BHAC) subsequently mediates the carbon-neutral assimilation of glyoxylate into central metabolism in ubiquitous Alpha- and Gammaproteobacteria. Although the reaction sequence of the BHAC was elucidated in Paracoccus denitrificans, little is known about the regulation of glycolate and glyoxylate assimilation in this relevant alphaproteobacterial model organism. Here, we show that regulation of glycolate metabolism in P. denitrificans is surprisingly complex, involving two regulators, the IclR-type transcription factor BhcR that acts as an activator for the BHAC gene cluster, and the GntR-type transcriptional regulator GlcR, a previously unidentified repressor that controls the production of glycolate oxidase. Furthermore, an additional layer of regulation is exerted at the global level, which involves the transcriptional regulator CceR that controls the switch between glycolysis and gluconeogenesis in P. denitrificans. Together, these regulators control glycolate metabolism in P. denitrificans, allowing the organism to assimilate glycolate together with other carbon substrates in a simultaneous fashion, rather than sequentially. Our results show that the metabolic network of Alphaproteobacteria shows a high degree of flexibility to react to the availability of multiple substrates in the environment.IMPORTANCEAlgae perform ca. 50% of the photosynthetic carbon dioxide fixation on our planet. In the process, they release the two-carbon molecule glycolate. Due to the abundance of algae, massive amounts of glycolate are released. Therefore, this molecule is available as a source of carbon for bacteria in the environment. Here, we describe the regulation of glycolate metabolism in the model organism Paracoccus denitrificans. This bacterium uses the recently characterized β-hydroxyaspartate cycle to assimilate glycolate in a carbon- and energy-efficient manner. We found that glycolate assimilation is dynamically controlled by three different transcriptional regulators: GlcR, BhcR, and CceR. This allows P. denitrificans to assimilate glycolate together with other carbon substrates in a simultaneous fashion. Overall, this flexible and multi-layered regulation of glycolate metabolism in P. denitrificans represents a resource-efficient strategy to make optimal use of this globally abundant molecule under fluctuating environmental conditions.

Particle-attached bacteria act as gatekeepers in the decomposition of complex phytoplankton polysaccharides

BackgroundMarine microalgae (phytoplankton) mediate almost half of the worldwide photosynthetic carbon dioxide fixation and therefore play a pivotal role in global carbon cycling, most prominently during massive phytoplankton blooms. Phytoplankton biomass consists of considerable proportions of polysaccharides, substantial parts of which are rapidly remineralized by heterotrophic bacteria. We analyzed the diversity, activity, and functional potential of such polysaccharide-degrading bacteria in different size fractions during a diverse spring phytoplankton bloom at Helgoland Roads (southern North Sea) at high temporal resolution using microscopic, physicochemical, biodiversity, metagenome, and metaproteome analyses.ResultsProminent active 0.2–3 µm free-living clades comprised Aurantivirga, “Formosa”, Cd. Prosiliicoccus, NS4, NS5, Amylibacter, Planktomarina, SAR11 Ia, SAR92, and SAR86, whereas BD1-7, Stappiaceae, Nitrincolaceae, Methylophagaceae, Sulfitobacter, NS9, Polaribacter, Lentimonas, CL500-3, Algibacter, and Glaciecola dominated 3–10 µm and > 10 µm particles. Particle-attached bacteria were more diverse and exhibited more dynamic adaptive shifts over time in terms of taxonomic composition and repertoires of encoded polysaccharide-targeting enzymes. In total, 305 species-level metagenome-assembled genomes were obtained, including 152 particle-attached bacteria, 100 of which were novel for the sampling site with 76 representing new species. Compared to free-living bacteria, they featured on average larger metagenome-assembled genomes with higher proportions of polysaccharide utilization loci. The latter were predicted to target a broader spectrum of polysaccharide substrates, ranging from readily soluble, simple structured storage polysaccharides (e.g., laminarin, α-glucans) to less soluble, complex structural, or secreted polysaccharides (e.g., xylans, cellulose, pectins). In particular, the potential to target poorly soluble or complex polysaccharides was more widespread among abundant and active particle-attached bacteria.ConclusionsParticle-attached bacteria represented only 1% of all bloom-associated bacteria, yet our data suggest that many abundant active clades played a pivotal gatekeeping role in the solubilization and subsequent degradation of numerous important classes of algal glycans. The high diversity of polysaccharide niches among the most active particle-attached clades therefore is a determining factor for the proportion of algal polysaccharides that can be rapidly remineralized during generally short-lived phytoplankton bloom events.9xVFsdNaK4F9f5nWnV_ZPhVideo

Microalgae-material hybrid for enhanced photosynthetic energy conversion: a promising path towards carbon neutrality.

Photosynthetic energy conversion for high-energy chemicals generation is one of the most viable solutions in the quest for sustainable energy towards carbon neutrality. Microalgae are fascinating photosynthetic organisms, which can directly convert solar energy into chemical energy and electrical energy. However, microalgal photosynthetic energy has not yet been applied on a large scale due to the limitation of their own characteristics. Researchers have been inspired to couple microalgae with synthetic materials via biomimetic assembly and the resulting microalgae-material hybrids have become more robust and even perform new functions. In the past decade, great progress has been made in microalgae-material hybrids, such as photosynthetic carbon dioxide fixation, photosynthetic hydrogen production, photoelectrochemical energy conversion and even biochemical energy conversion for biomedical therapy. The microalgae-material hybrid offers opportunities to promote artificially enhanced photosynthesis research and synchronously inspires investigation of biotic-abiotic interface manipulation. This review summarizes current construction methods of microalgae-material hybrids and highlights their implication in energy and health. Moreover, we discuss the current problems and future challenges for microalgae-material hybrids and the outlook for their development and applications. This review will provide inspiration for the rational design of the microalgae-based semi-natural biohybrid and further promote the disciplinary fusion of material science and biological science.

Carbon Dioxide Production in Bedrock Beneath Soils Substantially Contributes to Forest Carbon Cycling

AbstractSoils are widely considered the primary terrestrial organic matter pool mediating carbon transactions with the atmosphere and groundwater. Because soils are both a host and a product of rhizosphere activity, they are thought to mark the location where photosynthetic fixation of carbon dioxide (CO2) is balanced by the oxidation of organic matter. However, in many terrestrial environments, the rhizosphere extends below soils and into fractured bedrock, and it is unknown if the resulting biological and hydrologic dynamics in bedrock have a significant impact on carbon cycling. Here we show substantial production of CO2 in weathered bedrock at 4–8 m below the thin soils (<0.5 m thick) of a Northern California forest using innovative monitoring technology for sampling gases and water in fractured rock. The deep CO2 production supports a persistent upward flux of CO2(g) year‐round from bedrock to soil, constituting between 2% and 29% of the average daily CO2 efflux from the land surface. When water is rapidly transported across the fractured bedrock vadose zone, nearly 50% of the CO2 produced in the bedrock dissolves into water, promoting water‐rock interaction and export of dissolved inorganic carbon (DIC) from the unsaturated zone to groundwater, constituting as much as 80% of the DIC exiting the hillslope. Such CO2 production in weathered bedrock is subject to unique moisture, temperature, biological, and mineralogical conditions which are currently missing from terrestrial carbon models.

Global trends in the crop protection industry.

With the world’s human population expected to reach nearly 10 billion by 2050 and the negative impact of climate change on agriculture, maintenance of a stable global food supply is under significant threat. Given these dynamics, this article outlines the current market trends in the crop protection industry, including corporate consolidations and agrochemical regulations, comments on new technologies and the situation surrounding research and development (R&D) in the industry. It also highlights how agrochemicals contribute to building a more sustainable society. 1. Environmental trends (population growth, grain demand, and climate change) According to an estimate by the Food and Agriculture Organization (FAO), the world population has reached 7.55 billion in 2017 and will reach 9.8 billion by 2050. Accompanying this growth is a demand for grain that is expected to increase at least 2.2-fold from 1970 to 2020 (2.5 billion tons). In contrast to needed increases in grain demand to parallel population growth, the amount of cultivated area in the world in 2015 was virtually the same as it was in 1965, with no rapid increase expected in the future. Therefore, combined with the predicted population growth, it appears that cultivated area per person will continue to decline worldwide. Climate change is also looming as a major threat to grain production. On the one hand, temperatures rise could extend the growing season in certain areas, and it is possible that increased carbon dioxide levels may facilitate photosynthetic carbon dioxide fixation in certain types of plants, potentially leading to increases in grain productivity. Especially in areas in the Eurasian continent and high-latitude regions of North America, increased temperatures are expected to enhance grain crop yields. Many other areas, however, are likely to witness lower grain crop yields due to the influence of climate change. Moreover, many of the areas predicted to suffer reduced crop yields are currently major grain-producing regions. This means climate change threatens to greatly affect crop production as a whole. Across the world, the loss of yield in 2050 compared to 2000 is expected to be 24% for maize, 11% for rice, and 3% for wheat.1) Open in a separate window Fig. 1. Cultivated area worldwide and per person. Source: FAOSTAT (http://www.fao.org/faostat/en/#home) Consequently, it is obvious that it will not be possible to maintain the balance of supply and demand of grain down the road at current rates of grain production. In addition to the negative effects of climate change, it is unreasonable to expect a rapid expansion of cultivated area synchronously with the increasing demand for grain production that results from population growth. Under such environmental conditions, agricultural productivity per unit cultivated area must be increased to provide sufficient food to support the world’s population. Furthermore, it is imperative to use agricultural materials, including agrochemicals, more effectively than ever before and adopt innovative technological approaches to overcome these problems.

Identification of Rubisco anxiolytic-like peptides (rALPs) by comprehensive analysis of spinach green leaf protein digest

Rubisco, an enzyme for photosynthetic carbon dioxide fixation, is a major green leaf protein and known as the most abundant protein on the Earth. We found that Rubisco digested mimicking gastrointestinal enzymatic conditions exhibited anxiolytic-like effects after oral administration in mice. Based on a comprehensive peptide analysis of the digest using nanoLC-Orbitrap-MS and the structure-activity relationship of known anxiolytic-like peptides, we identified SYLPPLTT, SYLPPLT and YHIEPV [termed Rubisco anxiolytic-like peptide (rALP)-1, rALP-1(1–7) and rALP-2, respectively], which exhibited potent anxiolytic-like effects after oral administration. The anxiolytic-like effects of rALP-1/rALP-1(1–7) were blocked by a serotonin 5-HT1A receptor antagonist, whereas rALP-2-induced effects were inhibited by a δ-opioid receptor antagonist. In conclusion, novel Rubisco-derived anxiolytic-like peptides, rALP-1/rALP-1(1–7) and rALP-2, act via independent neural pathways.

Research Article: Soil respiration in upper Great Lakes old-growth forest ecosystems

Forests, through photosynthetic fixation of carbon dioxide, serve as natural carbon (C) sinks, thereby offsetting a substantial fraction of anthropogenic greenhouse gas emissions. However, forests of the upper Great Lakes region are on average getting older, prompting uncertainty in the region's future capacity to sequester C. Several studies demonstrate higher-than-expected rates of C sequestration in biomass and soils, or net ecosystem production (NEP), of older forests, with declining C losses from soils a proposed mechanism sustaining NEP as forests age. Forest plots spanning nearly 200 years of ecosystem development and including three different old-growth forest types were used to examine changes with age in growing season soil respiration (Rs) - the C emitted from soils by roots and soil microbes. How heterotrophic soil respiration (Rh) - the C emitted from soils by microbes only - varies among old-growth forest types and with soil environment was also examined. Mean growing season Rs and soil temperature were significantly lower in old-growth forests, with the former declining as forests aged. Laboratory incubations and scaled estimates of Rh suggest significantly higher rates in more tree species diverse mixed canopy old-growth forests than in less diverse conifer and deciduous old-growth forest stands. Soil moisture was an important driver of Rh in all old-growth forest types, particularly in the top, carbon-rich organic horizon. Based on the results of this study, it can be concluded that declining C emissions from the soils of old-growth forests may contribute to unexpectedly high rates of forest C sequestration as forests age.

Identification and characterization of multiple rubisco activases in chemoautotrophic bacteria.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) is responsible for almost all biological CO2 assimilation, but forms inhibited complexes with its substrate ribulose-1,5-bisphosphate (RuBP) and other sugar phosphates. The distantly related AAA+ proteins rubisco activase and CbbX remodel inhibited rubisco complexes to effect inhibitor release in plants and α-proteobacteria, respectively. Here we characterize a third class of rubisco activase in the chemolithoautotroph Acidithiobacillus ferrooxidans. Two sets of isoforms of CbbQ and CbbO form hetero-oligomers that function as specific activases for two structurally diverse rubisco forms. Mutational analysis supports a model wherein the AAA+ protein CbbQ functions as motor and CbbO is a substrate adaptor that binds rubisco via a von Willebrand factor A domain. Understanding the mechanisms employed by nature to overcome rubisco's shortcomings will increase our toolbox for engineering photosynthetic carbon dioxide fixation.

Isocitrate lyase activity and antioxidant responses in copper-stressed cultures of Chlamydomonas reinhardtii (Volvocales, Chlorophyceae)

Sztrum A.A., Sabatini S.E. and Rodríguez M.C. 2012. Isocitrate lyase activity and antioxidant responses in copper-stressed cultures of Chlamydomonas reinhardtii (Volvocales, Chlorophyceae). Phycologia 51: 135–143. DOI: 10.2216/10-56.1The effect of copper-imposed oxidative stress on isocitrate lyase (ICL) activity was studied in a Chlamydomonas reinhardtii wild type and a copper-tolerant pleiotropic mutant strain. Suspension batch bioassays were performed adding copper chloride under moderate irradiation. Two harvest points were selected to represent the acute and restoration phases and oxidative stress indicators were measured: antioxidative metabolites, phenolic compounds, ascorbic acid, abundance of thiobarbituric reactive substances, and activities of detoxifying enzymes such as catalase and ascorbate peroxidase. Copper stress under moderate irradiance favours acetate assimilation through enhanced ICL activity, especially in the acute phase. Though the mutant strain exhibited at least twofold ICL activity in the acute phase compared to the wild type, there was no significant difference in copper tolerance between strains. The higher ICL activity under copper stress may be an adaptation to promote mixotrophic organic carbon assimilation when photosynthetic carbon dioxide fixation is impaired. Thus, the higher ICL activity has the potential to be a useful indicator in acetate-flagellate bioassays.

Characterization of photosynthetic carbon dioxide fixation ability of indigenous Scenedesmus obliquus isolates

Photosynthetic mitigation of CO 2 with microalgae is promising from many aspects, thereby becoming a popular research topic. In this study, seven out of twenty-two indigenous Scenedesmus obliquus isolates obtained from southern Taiwan were selected for detailed study on their CO 2 fixation ability. Among them, 2 strains, namely, S. obliquus CNW-N and AS-6-1, displayed high cell growth rate and CO 2 removal ability when they were grown on 20% CO 2. The 2 strains show a high specific growth rate of 1.019 and 1.065 d −1, respectively, along with a high biomass concentration (2.63 and 1.90 g L −1, respectively). The biomass productivity of S. obliquus CNW-N and AS-6-1 was 201.4 and 150.7 mg L −1 d −1, respectively and the CO 2 consumption rate could reach 390.2 and 290.2 mg L −1 d −1, respectively, which are higher than that reported by most relevant studies.

Characterization and role of carbonic anhydrase in the calcification process of the azooxanthellate coral Tubastrea aurea

In zooxanthellate corals, the photosynthetic fixation of carbon dioxide and the precipitation of CaCO_3 are intimately linked both spatially and temporally making it difficult to study carbon transport mechanisms involved in each pathway. When studying Tubastrea aurea, a coral devoid of zooxanthellae, we can focus on carbon transport mechanisms involved only in the calcification process. We performed this study to characterize T. aurea carbonic anhydrase and to determine its role in the calcification process. We have shown that inhibition of tissular carbonic anhydrase activity affects the calcification rate. We have measured the activity of this enzyme both in the tissues and in the organix matrix extracted from the skeleton. Our results indicate that organic matrix proteins, which are synthesized by the calcifying tissues, are not only structural proteins, but they also play a crucial catalytic role by eliminating the kinetic barrier to interconversion of inorganic carbon at the calcification site. By immunochemistry we have demonstrated the presence of a protein both in the tissues and in the organic matrix, which shares common features with prokaryotic carbonic anhydrases.

Carbohydrates for fermentation

Biomass accumulated by the photosynthetic fixation of carbon dioxide is the only renewable carbon source, and hence, the only renewable raw material for the chemical industry. Carbohydrates are the main constituents of biomass and occur as cell wall and storage carbohydrates, transportation carbohydrates and glycoconjugates. Cellulose, hemicelluloses and starch in particular as well as pectin, inulin and saccharose to a smaller extent are the most abundant carbohydrates. Glucose is the most important monosaccharide and monomer of polysaccharides in natural carbohydrates. Thus, it is the most abundant organic compound on earth. Production of pulp from wood cellulose, applications of starch for paper making as well as uses of glucose and saccharose for fermentation are the most important chemical and technical uses of carbohydrates. Carbohydrates used as fermentation feedstock are essential for the chemical industry. Their importance is steadily growing due to the increasing implementation of biotechnological processes.

A redox-regulated chloroplast protein phosphatase binds to starch diurnally and functions in its accumulation.

Starch is the ultimate storage molecule formed in the photosynthetic fixation of carbon dioxide by chloroplasts. Starch accumulates during the day and is degraded at night to intermediates that are exported to heterotrophic organs. The mechanism by which diurnal cycles control the transitory biosynthesis and degradation of chloroplast starch has long remained a mystery. We now report evidence that a dual-specificity protein phosphatase, DSP4, binds to starch granules during the day and dissociates at night. Disruption of the DSP4 gene resulted in a dramatic increase in the level of starch in mutant Arabidopsis plants. Moreover, although composition was apparently unchanged, the morphology of the starch granule was significantly altered compared to the wild type counterpart. Two regulatory factors linked to light (i.e., pH and redox status) changed both the activity and the starch-binding capacity of DSP4. The results further revealed that DSP4 represents a major fraction of granule-bound phosphatase activity during the day but not at night. Our study suggests that DSP4 acts as a bridge between light-induced redox changes and protein phosphorylation in the regulation of starch accumulation.

Atmospheric dynamics in the “Laboratory Biosphere” with wheat and sweet potato crops

Laboratory Biosphere is a 40-m 3 closed life system equipped with 12,000 W of high pressure sodium lamps over planting beds with 5.37 m 2 of soil. Atmospheric composition changes due to photosynthetic fixation of carbon dioxide and corresponding production of oxygen or the reverse, respiration, are observed in short timeframes, e.g., hourly. To focus on inherent characteristics of the crop as distinct from its area or the volume of the chamber, we report fixation and respiration rates in mmol h −1 m −2 of planted area. An 85-day crop of USU Apogee wheat under a 16-h lighted/8-h dark regime peaked in fixation rate at about 100 mmol h −1 m −2 approximately 24 days after planting. Light intensity was about 840 μmol m −2 s −1. Dark respiration peaked at about 31 mmol h −1 m −2 at the same time. Thereafter, both fixation and respiration declined toward zero as harvest time approached. A residual soil respiration rate of about 1.9 mmol h −1 m −2 was observed in the dark closed chamber for 100 days after the harvest. A 126-day crop of Tuskegee TU-82-155 sweet potato behaved quite differently. Under a 680 μmol m −2 s −1, 18-h lighted/6-h dark regime, fixation during lighted hours rose to a plateau ranging from about 27 to 48 mmol h −1 m −2 after 42 days and dark respiration settled into a range of 12–23 mmol h −1 m −2. These rates continued unabated until the harvest at 126 days, suggesting that tuber biomass production might have continued at about the same rate for some time beyond the harvest time that was exercised in this experiment. In both experiments CO 2 levels were allowed to range widely from a few hundred to about 3000 ppm, which permitted observation of fixation rates both at varying CO 2 concentrations and at each number of days after planting. This enables plotting the fixation rate as a function of both variables. Understanding the atmospheric dynamics of individual crops will be essential for design and atmospheric management of more complex CELSS which integrate the simultaneous growth of several crops as in a sustainable remote life support system.

Photosynthetic evolution in parasitic plants: insight from the chloroplast genome

Despite the enormous diversity in plant form, structure and growth environment across the seed-bearing plants (angiosperms and gymnosperms), the chloroplast genome has, with few exceptions, remained remarkably conserved. This conservation suggests the existence of universal evolutionary selection pressures associated with photosynthesis-the primary function of chloroplasts. The stark exceptions to this conservation occur in parasitic angiosperms, which have escaped the dominant model by evolving the capacity to obtain some or all of their carbon (and nutrients) from their plant hosts. The consequence of this evolution to parasitism is a relaxation of the evolutionary constraints associated with the need to maintain photosynthetic function, the very function that drove early stages of the ancient symbiotic relationship that produced the contemporary chloroplast. Extreme examples of reductionism among parasitic angiosperms reveals major alterations in chloroplast function with the loss of photosynthetic capacity and, with that, massive alterations in chloroplast genome content. This review highlights emerging patterns in reported gene loss and gene retention in the chloroplast genomes of parasitic plants. Some gene losses appear to occur in the early stages of parasitic evolution, even before the loss of photosynthetic capacity, like the chlororespiratory (ndh) genes. This contrasts with unexpected gene retentions, like that of the rbcL gene responsible for photosynthetic carbon dioxide fixation, and belies current understanding of gene function. The review relates gene retention to current knowledge of protein function and gene processing that has implications to broader aspects of genome conservation in organelles.

Following the path of carbon in photosynthesis: a personal story.

Chronological recognition of the intermediates and mechanisms involved in photosynthetic carbon dioxide fixation is delineated. Sam Ruben and Martin Kamen's development of application of radioactive carbon for the study of carbon dioxide fixation provided impetus and techniques for following the path of carbon in photosynthesis. Discovery The identity of the primary carboxylation enzyme and its identity with the major protein of photosynthetic tissues ('Fraction 1' protein of Sam Wildman) is reviewed. Memories are dimmed by sixty years of exciting discoveries exploration in newer fields [see Benson 2002 (Annu Rev Plant Biol 53: 1-25), for research and perspectives beyond the early Berkeley days].

TRM1, a YY1-like suppressor of rbcS-m3 expression in maize mesophyll cells.

The genes rbcS and rbcL encode, respectively, the small and large subunits of the photosynthetic carbon dioxide fixation enzyme ribulose bisphosphate carboxylase/oxygenase. There is a single rbcL gene in each chloroplast chromosome; a family of rbcS genes is located in the nuclear genome. These two genes are not expressed in mesophyll cells but are in adjacent bundle-sheath cells of leaves of the C4 plant Zea mays. Two regions of the maize gene rbcS-m3 are required for suppressing expression in mesophyll cells. One region is just beyond the translation termination site in the 3' region, and the other is several hundred base pairs upstream of the transcription start site. A binding site for a protein with limited homology to the viral, yeast, and mammalian transcription repressor-activator YY1 (Yin-Yang I), has now been identified in the 3' region. A maize gene for a protein with zinc fingers homologous to those of YY1 has been isolated, characterized, and expressed in Escherichia coli. The gene is designated trm1 (transcription repressor-maize 1). The protein TRM1 binds to the YY1-like site and, in addition, TRM1 binds to two sequence regions in the 5' region of the gene that have no homology to the YY1 site. Mutagenesis or deletion of any of these three sequences eliminates repression of rbcS-m3 reporter genes in mesophyll cells.

Inhibitory effect of hypergravity on photosynthetic carbon dioxide fixation in Euglena gracilis

Photosynthesis, the conversion of light energy into chemical energy, is a critical biological process, whereby plants synthesize carbohydrates from light, carbon dioxide (CO2) and water. The influence of gravity on this biological process, however, is not well understood. Thus, centrifugation was used to alter the gravity environment of Euglena gracilis grown on nutritive agar plates illuminated with red and blue light emitting diodes. The results showed that hypergravity (up to 10xg) had an inhibitory effect on photosynthetic CO2 fixation. Chlorophyll accumulation per cell was essentially unaffected by treatment; however, Chl a/Chl b ratios decreased in hypergravity when compared to 1xg controls. Photosynthesis in Euglena appears to have limited tolerance for even moderate changes in gravitational acceleration.

(18)O and mass spectrometry in chlorophyll research: Derivation and loss of oxygen atoms at the periphery of the chlorophyll macrocycle during biosynthesis, degradation and adaptation.

Chlorophylls, magnesium-containing tetrapyrrolic pigments of photosynthesis, are widely-distributed in Nature and participate in both light harvesting and in the transduction of light energy to chemical energy for the photosynthetic fixation of carbon dioxide. We briefly discuss the extensive role of various isotopic labelling techniques in elucidating the pathway of tetrapyrrole-pigment biosynthesis and we acknowledge the classic and meticulous research of David Shemin who, approximately 50 years ago, introduced isotopic tracer techniques with (15)N and (14)C isotopes to study the biosynthesis of the carbon/nitrogen macrocycle of haem, an iron tetrapyrrole. The main focus of this review is the application of mass spectrometry and (18)O labelling to the study of the incorporation of oxygen atoms from molecular oxygen or water into the periphery of the chlorophyll macrocycle during biosynthesis and their loss during degradation and light acclimation. In particular, we review the mechanism of formation of the isocyclic ring of chlorophylls, in higher plants, green algae and various photosynthetic bacteria, which concomitantly incurs formation of the 13(1)-oxo group that is present in all photosynthetically-active chlorophylls. In addition we discuss the formation of the ubiquitous 13(3)- and 17(3)-carboxyl groups and also the formation of the 7-formyl group of chlorophyll b and the 3-acetyl group of bacteriochlorophyll a.

Effect of Water Stress on Pyruvaté, Pi Dikinase and Phosphoenol Pyruvate Carboxylase Activities in the Leaves of Two Cultivars of Sorghum (Sorghum bicolor L.)

AbstractThe effect of withholding water on the specific activities of pyruvate, orthophosphate dikinase (PPDK EC 2.7.9.1.) and phosphoenol pyruvate carboxylase (PEPC, EC 4.1.1.31) was studied in two cultivars of Sorghum bicolor L. different in their sensitivity to water deficit. The drought sensitive cultivar ICSV 1063 and the more resistant cultivar MIGSOR were subjected to water deficit in controlled conditions. The results showed a decrease in both enzyme specific activities, higher for drought sensitive cultivar ICSV 1063, when leaf water potential (Ψ) was lowered to ‐2.3 MPa. Following rewatering, enzyme specific activities increased in both cultivars, with the increase being more significant in MIGSOR. Relative water content (RWC) decreased significantly for water‐stressed ICSV 1063 cultivar, lower for MIGSOR and returned quickly to that of control plants upon rehydration. Net photosynthesis showed a decrease for water‐stressed plants, higher for ICSV 1063. At minimal Ψ, net photosynthesis was completely inhibited, with the stomata being closed After rehydration, MIGSOR showed a better recovery in photosynthesis but never reached the initial values of day 0. Water stress had a striking effect, both on net photosynthesis by regulation of stomatal aperture and on PPDK and PEPC activities, although the enzymes were still active when photosynthesis ceased. Therefore the level of PPDK and PEPC activities may contribute to the limitation of photosynthetic carbon dioxide fixation.