Carbon-concentrating Mechanism Research Articles

Metabolomics of related C3 and C4 Flaveria species indicate differences in the operation of photorespiration under fluctuating light.

C3 photosynthesis can be complemented with a C4 carbon concentrating mechanism (CCM) to minimize photorespiratory losses. C4 photosynthesis is often more efficient than C3 under steady-state conditions. However, the C4 CCM depends on inter-cellular metabolite concentration gradients, which must increase following increases in light intensity and could decrease rates of C4 photosynthesis under fluctuating light. Additionally, incomplete flux through photorespiration could prove beneficial to C4 assimilation during light induction of the CCM. Here, we compare metabolic profiles in the closely related C3 Flaveria robusta and C4 Flaveria bidentis during a light transient from low to high light to determine if these non-steady state accumulation patterns provide insight to the induction of the metabolite gradients needed to drive C4 intermediate transport and if there is incomplete cycling of photorespiratory intermediates. In these C3 and C4 species, metabolite steady-state pool sizes suggest that C4 transport acids maintain concentration gradients across the bundle sheath and mesophyll cell types under these light fluctuations. However, there was incomplete flux through photorespiration in the C4 F. bidentis, which could reduce photorespiratory CO2 loss via glycine decarboxylation and help maintain higher rates of assimilation during following induction periods.

Developing a European aquatic macrophyte transfer function for reconstructing past lake-water chemistry

Quantitative paleoecological reconstructions using biological proxies, such as diatoms, Cladocera, and chironomids, have revolutionized paleolimnology and have greatly contributed to the understanding of the past local and regional environmental changes, as well as to nature conservation. While macrophytes are good ecological indicators, they have rarely been used to reconstruct past lake-water chemistry. The present study investigates which environmental variable best explains aquatic plant community composition in Finnish, Polish, and Swedish lakes for its further use in quantitative paleoenvironmental reconstructions. The method involved the creation of a modern macrophyte-environment calibration dataset, calculation of modern calibration functions using simple averaging regression, and final reconstruction of past environmental conditions in Lake Linówek (NE Poland) from a fossil assemblage using weighted averaging calibration. The data demonstrate that conductivity and alkalinity best explained macrophyte community composition in our dataset. Species “optima” for alkalinity were influenced by the presence/absence of carbon concentrating mechanisms (CCMs), enabling the utilization of HCO3− as a carbon source. Quantitative paleoenvironmental reconstruction indicates that past water conductivity and alkalinity fluctuated depending on internal lake processes and the supply of basic ions to the lake from the catchment related to climate and soil development in the watershed during the late Glacial (∼14,500–11,700 calibrated years before the present; cal BP) and the Holocene (11,700 cal BP–recent). We conclude that macrophytes can be successfully used for past lake-water chemistry reconstruction. Furthermore, calculated modern calibration functions for conductivity and alkalinity can be used in nature conservation for determining habitat requirements of numerous endangered macrophyte species as a basis for successful (re)introductions.

Lobelia Lakes' Vegetation and Its Photosynthesis Pathways Concerning Water Parameters and the Stable Carbon Isotopic Composition of Plants' Organic Matter.

Most of the aquatic vegetation produces organic substances via the C3 photosynthetic pathway (mosses, isoetids-Lobelia dortmanna L., Luronium natans (L.) Raf., and vascular plants) or Crassulacean acid metabolism (CAM, e.g., Littorella uniflora (L.) Asch. and Isoëtes lacustris L.) or by their ability to use HCO3- via carbon concentration mechanisms (CCMs-some elodeids and charophytes). Differentiating these predominant photosynthetic pathways in aquatic vegetation based on their organic matter (OM) carbon stable isotopes (δ13CORG) is a complex task, in contrast to terrestrial plants. This study investigates the OM deposition, characterized by δ13CORG values in 10 macrophyte species with different photosynthetic pathways (C3, CAM, and CCM) collected from 14 softwater Lobelia lakes in northern Poland. The higher δ13CORG values distinguish the CCM group, indicating their use of 13C-enriched HCO3¯ in photosynthesis. CAM species show slightly higher δ13CORG values than C3, particularly in lower pH lakes. Principal component analysis of isotopic and environmental data did not yield clear distinctions by the groups, but still, they significantly differ in light of analyzed parameters and isotopic signals (PRMANOVA = 5.08, p < 0.01; K-W H = 27.01, p < 0.001). The first two PCA dimensions showed that the water pH and Ca2+ concentration positively influenced δ13C values. The influence of light conditions on δ13CORG values revealed by third PCA components seems to also be important. In summary, northern Polish Lobelia lakes serve as a key differentiation point between vegetation employing CCMs and those relying on C3/CAM photosynthesis without HCO3- utilization, providing insights into transitions in plant communities within these ecosystems.

Usage of Chlorella and diverse microalgae for CO2 capture - towards a bioenergy revolution.

To address climate change threats to ecosystems and the global economy, sustainable solutions for reducing atmospheric carbon dioxide (CO2) levels are crucial. Existing CO2 capture projects face challenges like high costs and environmental risks. This review explores leveraging microalgae, specifically the Chlorella genus, for CO2 capture and conversion into valuable bioenergy products like biohydrogen. The introduction section provides an overview of carbon pathways in microalgal cells and their role in CO2 capture for biomass production. It discusses current carbon credit industries and projects, highlighting the Chlorella genus's carbon concentration mechanism (CCM) model for efficient CO2 sequestration. Factors influencing microalgal CO2 sequestration are examined, including pretreatment, pH, temperature, irradiation, nutrients, dissolved oxygen, and sources and concentrations of CO2. The review explores microalgae as a feedstock for various bioenergy applications like biodiesel, biooil, bioethanol, biogas and biohydrogen production. Strategies for optimizing biohydrogen yield from Chlorella are highlighted. Outlining the possibilities of further optimizations the review concludes by suggesting that microalgae and Chlorella-based CO2 capture is promising and offers contributions to achieve global climate goals.

The genomic potential of photosynthesis in piconanoplankton is functionally redundant but taxonomically structured at a global scale.

Carbon fixation is a key metabolic function shaping marine life, but the underlying taxonomic and functional diversity involved is only partially understood. Using metagenomic resources targeted at marine piconanoplankton, we provide a reproducible machine learning framework to derive the potential biogeography of genomic functions through the multi-output regression of gene read counts on environmental climatologies. Leveraging the Marine Atlas of Tara Oceans Unigenes, we investigate the genomic potential of primary production in the global ocean. The latter is performed by ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) and is often associated with carbon concentration mechanisms in piconanoplankton, major marine unicellular photosynthetic organisms. We show that the genomic potential supporting C4 enzymes and RUBISCO exhibits strong functional redundancy and important affinity toward tropical oligotrophic waters. This redundancy is taxonomically structured by the dominance of Mamiellophyceae and Prymnesiophyceae in mid and high latitudes. These findings enhance our understanding of the relationship between functional and taxonomic diversity of microorganisms and environmental drivers of key biogeochemical cycles.

In search of the pH limit of growth in halo-alkaliphilic cyanobacteria.

Cyanobacteria have many biotechnological applications. Increasing their cultivation pH can assist in capturing carbon dioxide and avoiding invasion by other organisms. However, alkaline media may have adverse effects on cyanobacteria, such as reducing the Carbon-Concentrating Mechanism's efficiency. Here, we cultivated two halo-alkaliphilic cyanobacteria consortia in chemostats at pH 10.2-11.4. One consortium was dominated by Ca. Sodalinema alkaliphilum, the other by a species of Nodosilinea. These two cyanobacteria dominate natural communities in Canadian and Asian alkaline soda lakes. We show that increasing the pH decreased biomass yield. This decrease was caused, in part, by a dramatic increase in carbon transfer to heterotrophs. At pH 11.4, cyanobacterial growth became limited by bicarbonate uptake, which was mainly ATP dependent. In parallel, the higher the pH, the more sensitive cyanobacteria became to light, resulting in photoinhibition and upregulation of DNA repair systems.

The occurrence of positive selection on BicA transporter of Microcystis aeruginosa

The rapid growth of cyanobacteria, particularly Microcystis aeruginosa, poses a significant threat to global water security. The proliferation of toxic Microcystis aeruginosa raises concerns due to its potential harm to human health and socioeconomic impacts. Dense blooms contribute to spatiotemporal inorganic carbon depletion, promoting interest in the roles of carbon-concentrating mechanisms (CCMs) for competitive carbon uptake. Despite the importance of HCO3− transporters, genetic evaluations and functional predictions in M. aeruginosa remain insufficient. In this study, we explored the diversity of HCO3− transporters in the genomes of 46 strains of M. aeruginosa, assessing positive selection for each. Intriguingly, although the Microcystis BicA transporter became a partial gene in 23 out of 46 genomic strains, we observed significant positive sites. Structural analyses, including predicted 2D and 3D models, confirmed the structural conservation of the Microcystis BicA transporter. Our findings suggest that the Microcystis BicA transport likely plays a crucial role in competitive carbon uptake, emphasizing its ecological significance. The ecological function of the Microcystis BicA transport in competitive growth during cyanobacterial blooms raises important questions. Future studies require experimental confirmation to better understand the role of the Microcysits BicA transporter in cyanobacterial blooms dynamics.

Comparative analysis of gene regulatory networks identifies conserved regulators in seed plants

Abstract Gene regulatory networks (GRNs) based on transcription factors (TFs) control development and environmental responses. In this study, GRNs were inferred computationally using random forest decision tree-based regression. Networks were constructed for the grasses barley, maize, wheat, brachypodium, sorghum and rice. When compared with Arabidopsis thaliana and alga networks, they show substantial conservation. The degree of conservation depends on phylogenetic closeness. The processes are conserved between all species include basic cellular functions while the processes conserved only in the grasses are annotated with more specific gene ontology terms. In the three species with a carbon concentration mechanism, photorespiration is partially dissociated from photosynthetic regulation. In contrast, in the C4 species, the regulation of C4 genes is associated with photosynthetic regulation. The comparative analyses reveal conserved TFs that control photosynthesis in seed plants but not in the alga. The analyses resulted in a pipeline for the general transfer of information between the small dicot A. thaliana and the commercially relevant grasses.

Transcriptomic Analysis Reveals the Effect of Urea on Metabolism of Nannochloropsis oceanica.

The eukaryotic microalga Nannochloropsis oceanica represents a promising bioresource for the production of biofuels and pharmaceuticals. Urea, a crucial nutrient for the photosynthetic N. oceanica, stimulates the accumulation of substances such as lipids, which influence growth and physiology. However, the specific mechanisms by which N. oceanica responds and adapts to urea addition remain unknown. High-throughput mRNA sequencing and differential gene expression analysis under control and urea-added conditions revealed significant metabolic changes. This involved the differential expression of 2104 genes, with 1354 being upregulated and 750 downregulated, resulting in the reprogramming of crucial pathways such as carbon and nitrogen metabolism, photosynthesis, and lipid metabolism. The results specifically showed that genes associated with photosynthesis in N. oceanica were significantly downregulated, particularly those related to light-harvesting proteins. Interestingly, urea absorption and transport may depend not only on specialized transport channels such as urease but also on alternative transport channels such as the ABC transporter family and the CLC protein family. In addition, urea caused specific changes in carbon and lipid metabolism. Genes associated with the Calvin cycle and carbon concentration mechanisms were significantly upregulated. In lipid metabolism, the expression of genes associated with lipases and polyunsaturated fatty acid synthesis was highly activated. Furthermore, the expression of several genes involved in the tricarboxylic acid cycle and folate metabolism was enhanced, making important contributions to energy supply and the synthesis and modification of genes and macromolecules. Our observations indicate that N. oceanica actively and dynamically regulates the redistribution of carbon and nitrogen after urea addition, providing references for further research on the effects of urea on N. oceanica.

Hydrogen isotope fractionation is controlled by CO2 in coccolithophore lipids

Hydrogen isotope ratios (δ2H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of 2H/1H fractionation in algal lipids by systematically manipulating temperature, light, and CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (αalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the αalkenone with increasing CO2(aq) and confirm αalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ2H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with 2H-richer intracellular water, increasing αalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of αalkenone to CO2(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO2 at the Rubisco site, but rather that chloroplast CO2 varies with external CO2(aq). The pervasive inverse correlation of αalkenone with CO2(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, αalkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.

Changes in the limiting nutrients and dominant phytoplankton communities of three major European rivers: Response to catchment lithologies and human activities

Catchment lithologies and human activities can cause variations in riverine carbon (C), nitrogen (N), and phosphorus (P) loads, and the ratio of these nutrients may affect the primary productivity of rivers and their phytoplankton communities. We conducted a comparative study of biogeochemical variations in three major European rivers—the Rhine, Danube, and Seine—which reveals the impacts of different carbonate outcrops and human impacts (agricultural activities and sewage input) on the changes in the limiting nutrients and phytoplankton community compositions of flowing water systems. N limitation was observed in the Rhine and Danube owing to their low average NO3– loads (4.04–6.45 mg/L). Although the extensive outcrops of carbonate rocks in the Seine catchment result in high average HCO3– concentrations of ∼ 4.68 mmol/L, the elevated inorganic C input did not match the high average NO3– loads (∼19.38 mg/L), driven by intensive agricultural activity and urban sewage input, demonstrating a potential C limitation on productivity. Our results show that the productivity of the Seine River, determined by the chlorophyll-a concentration, was significantly related to the dissolved inorganic carbon (DIC) concentration (coefficient of determination: R2 = 0.68, P < 0.05). Our results demonstrate that changes in DIC content can significantly alter the dominant phytoplankton species in the water column. The high inputs of DIC and aqueous CO2(aq) to the Seine lead to Bacillariophyta proportions that are 3.96–8.81 % and 19.87–22.56 % higher than those of the Rhine and Danube in the wet and dry seasons, respectively. This can be explained by the difference in the efficiency of carbon concentrating mechanisms (CCMs) among phytoplankton species. Therefore, to mitigate river eutrophication and improve the quality of the aquatic environment, we suggest that future water–carbon management in the flowing water systems with abundant nutrients should consider C-limitation alleviation and its potential effect for reducing the abundance of harmful algae.

Modification of Living Diatom, Thalassiosira weissflogii, with a Calcium Precursor through a Calcium Uptake Mechanism: A Next Generation Biomaterial for Advanced Delivery Systems.

The diatom's frustule, characterized by its rugged and porous exterior, exhibits a remarkable biomimetic morphology attributable to its highly ordered pores, extensive surface area, and unique architecture. Despite these advantages, the toxicity and nonbiodegradable nature of silica-based organisms pose a significant challenge when attempting to utilize these organisms as nanotopographically functionalized microparticles in the realm of biomedicine. In this study, we addressed this limitation by modulating the chemical composition of diatom microparticles by modulating the active silica metabolic uptake mechanism while maintaining their intricate three-dimensional architecture through calcium incorporation into living diatoms. Here, the diatom Thalassiosira weissflogii was chemically modified to replace its silica composition with a biodegradable calcium template, while simultaneously preserving the unique three-dimensional (3D) frustule structure with hierarchical patterns of pores and nanoscale architectural features, which was evident by the deposition of calcium as calcium carbonate. Calcium hydroxide is incorporated into the exoskeleton through the active mechanism of calcium uptake via a carbon-concentrating mechanism, without altering the microstructure. Our findings suggest that calcium-modified diatoms hold potential as a nature-inspired delivery system for immunotherapy through antibody-specific binding.

Improving Crop Yield through Increasing Carbon Gain and Reducing Carbon Loss.

Photosynthesis is a process where solar energy is utilized to convert atmospheric CO2 into carbohydrates, which forms the basis for plant productivity. The increasing demand for food has created a global urge to enhance yield. Earlier, the plant breeding program was targeting the yield and yield-associated traits to enhance the crop yield. However, the yield cannot be further improved without improving the leaf photosynthetic rate. Hence, in this review, various strategies to enhance leaf photosynthesis were presented. The most promising strategies were the optimization of Rubisco carboxylation efficiency, the introduction of a CO2 concentrating mechanism in C3 plants, and the manipulation of photorespiratory bypasses in C3 plants, which are discussed in detail. Improving Rubisco's carboxylation efficiency is possible by engineering targets such as Rubisco subunits, chaperones, and Rubisco activase enzyme activity. Carbon-concentrating mechanisms can be introduced in C3 plants by the adoption of pyrenoid and carboxysomes, which can increase the CO2 concentration around the Rubisco enzyme. Photorespiration is the process by which the fixed carbon is lost through an oxidative process. Different approaches to reduce carbon and nitrogen loss were discussed. Overall, the potential approaches to improve the photosynthetic process and the way forward were discussed in detail.

The redox-sensitive R-loop of the carbon control protein SbtB contributes to the regulation of the cyanobacterial CCM

SbtB is a PII-like protein that regulates the carbon-concentrating mechanism (CCM) in cyanobacteria. SbtB proteins can bind many adenyl nucleotides and possess a characteristic C-terminal redox sensitive loop (R-loop) that forms a disulfide bridge in response to the diurnal state of the cell. SbtBs also possess an ATPase/ADPase activity that is modulated by the redox-state of the R-loop. To investigate the R-loop in the cyanobacterium Synechocystis sp. PCC 6803, site-specific mutants, unable to form the hairpin and permanently in the reduced state, and a R-loop truncation mutant, were characterized under different inorganic carbon (Ci) and light regimes. Growth under diurnal rhythm showed a role of the R-loop as sensor for acclimation to changing light conditions. The redox-state of the R-loop was found to impact the binding of the adenyl-nucleotides to SbtB, its membrane association and thereby the CCM regulation, while these phenotypes disappeared after truncation of the R-loop. Collectively, our data imply that the redox-sensitive R-loop provides an additional regulatory layer to SbtB, linking the CO2-related signaling activity of SbtB with the redox state of cells, mainly reporting the actual light conditions. This regulation not only coordinates CCM activity in the diurnal rhythm but also affects the primary carbon metabolism.

An Evidence of Carbonic Anhydrase Activity in Native Microalgae for CO2 Capture Application.

A promising alternative for effective carbon capture has been found in microalgae because of their high photosynthetic capacity and quick growth. The carbon concentration mechanism of many microalgae is heavily reliant on the enzyme carbonic anhydrase (CA), which catalyze the production of bicarbonate from carbon dioxide. In this study, microalgal samples were collected, characterized, and cultured under controlled conditions for their optimal growth of cultures I-IX. The CA activity was investigated using a standard method; the Wilbur-Anderson assay was used to calculate CA activity in microalgal cultures. The comparative study was then used to measure the activity rate of the collected microalgae. Among the tested, culture I, VI, and IX showed a high enzyme activity rate of 4.15, 4.0, and 4.2µg·mL-1, respectively. To determine the rate of carbon dioxide hydration, the method involved tracking the pH change in a reaction mixture. In addition, genetic analysis facilitates the identification of key genes involved in CA activity and other metabolic processes, which enhance the knowledge of microalgal physiology, and enables genetic engineering efforts in the future studies. Overall, this investigation emphasizes the significance of studying unknown microalgal culture and their potential CA activity for industrial and bio-energy applications.

Salinity Mitigates the Negative Effect of Elevated Temperatures on Photosynthesis in the C3-C4 Intermediate Species Sedobassia sedoides.

The adaptation of plants to combined stresses requires unique responses capable of overcoming both the negative effects of each individual stress and their combination. Here, we studied the C3-C4 (C2) halophyte Sedobassia sedoides in response to elevated temperature (35 °C) and salinity (300 mM NaCl) as well as their combined effect. The responses we studied included changes in water-salt balance, light and dark photosynthetic reactions, the expression of photosynthetic genes, the activity of malate dehydrogenase complex enzymes, and the antioxidant system. Salt treatment led to altered water-salt balance, improved water use efficiency, and an increase in the abundance of key enzymes involved in intermediate C3-C4 photosynthesis (i.e., Rubisco and glycine decarboxylase). We also observed a possible increase in the activity of the C2 carbon-concentrating mechanism (CCM), which allowed plants to maintain high photosynthesis intensity and biomass accumulation. Elevated temperatures caused an imbalance in the dark and light reactions of photosynthesis, leading to stromal overreduction and the excessive generation of reactive oxygen species (ROS). In response, S. sedoides significantly activated a metabolic pathway for removing excess NADPH, the malate valve, which is catalyzed by NADP-MDH, without observable activation of the antioxidant system. The combined action of these two factors caused the activation of antioxidant defenses (i.e., increased activity of SOD and POX and upregulation of FDI), which led to a decrease in oxidative stress and helped restore the photosynthetic energy balance. Overall, improved PSII functioning and increased activity of PSI cyclic electron transport (CET) and C2 CCM led to an increase in the photosynthesis intensity of S. sedoides under the combined effect of salinity and elevated temperature relative to high temperature alone.

Characterization of a novel γ-type carbonic anhydrase, Sjγ-CA2, in Saccharina japonica: Insights into carbon concentration mechanism in macroalgae

Carbonic anhydrase (CA) is a crucial component of CO2-concentrating mechanism (CCM) in macroalgae. In Saccharina japonica, an important brown seaweed, 11 CAs, including 5 α-, 3 β-, and 3 γ-CAs, have been documented. Among them, one α-CA and one β-CA were localized in the periplasmic space, one α-CA was found in the chloroplast, and one γ-CA was situated in mitochondria. Notably, the known γ-CAs have predominantly been identified in mitochondria. In this study, we identified a chloroplastic γ-type CA, Sjγ-CA2, in S. japonica. Based on the reported amino acid sequence of Sjγ-CA2, the epitope peptide for monoclonal antibody production was selected as 165 Pro-305. After purification and specificity identification, anti-SjγCA2 monoclonal antibody was employed in immunogold electron microscopy. The results illustrated that Sjγ-CA2 was localized in the chloroplasts of both gametophytes and sporophytes of S. japonica. Subsequently, immunoprecipitation coupled with LC-MS/MS analysis revealed that Sjγ-CA2 mainly interacted with photosynthesis-related proteins. Moreover, the first 65 amino acids at N-terminal of Sjγ-CA2 was identified as the chloroplast transit peptide by the transient expression of GFP-SjγCA2 fused protein in tabacco. Real-time PCR results demonstrated an up-regulation of the transcription of Sjγ-CA2 gene in response to high CO2 concentration. These findings implied that Sjγ-CA2 might contribute to minimizing the leakage of CO2 from chloroplasts and help maintaining a high concentration of CO2 around Rubisco.

The bHLH transcription factor OsPRI1 activates the Setaria viridisPEPC1 promoter in rice.

Photosynthetic efficiency is reduced by the dual role of Rubisco, which acts either as a carboxylase or as an oxygenase, the latter leading to photorespiration. C4 photosynthesis evolved as a carbon-concentrating mechanism to reduce photorespiration. To engineer C4 into a C3 plant, it is essential to understand how C4 genes, such as phosphoenolpyruvate carboxylase (PEPC1), are regulated to be expressed at high levels and in a cell-specific manner. Yeast one-hybrid screening was used to show that OsPRI1, a rice bHLH transcription factor involved in iron homeostasis, binds to the Setaria viridis PEPC1 promoter. This promoter drives mesophyll-specific gene expression in rice. The role of OsPRI1 in planta was characterized using a rice line harbouring SvPEPC1pro ::GUS. We show that OsPRI1 activates the S. viridis PEPC1 promoter by binding to an N-box in the proximal promoter, and that GUS activity is highly reduced in SvPEPC1pro ::GUS lines when OsPRI1 is mutated. Cross-species comparisons showed that the SvPRI1 homolog binds to the SvPEPC1 promoter but the maize ZmPRI1 does not bind to the ZmPEPC1 promoter. Our results suggest that elements of the iron homeostasis pathway were co-opted to regulate PEPC1 gene expression during the evolution of some but not all C4 species.

CAH3 from Chlamydomonas reinhardtii: Unique Carbonic Anhydrase of the Thylakoid Lumen.

CAH3 is the only carbonic anhydrase (CA) present in the thylakoid lumen of the green algae Chlamydomonas reinhardtii. The monomer of the enzyme has a molecular weight of ~29.5 kDa with high CA activity. Through its dehydration activity, CAH3 can be involved either in the carbon-concentrating mechanism supplying CO2 for RuBisCO in the pyrenoid or in supporting the maximal photosynthetic activity of photosystem II (PSII) by accelerating the removal of protons from the active center of the water-oxidizing complex. Both proposed roles are considered in this review, together with a description of the enzymatic parameters of native and recombinant CAH3, the crystal structure of the protein, and the possible use of lumenal CA as a tool for increasing biomass production in higher plants. The identified involvement of lumenal CAH3 in the function of PSII is still unique among green algae and higher plants and can be used to understand the mechanism(s) of the functional interconnection between PSII and the proposed CA(s) of the thylakoid lumen in other organisms.

Inorganic carbon sensing and signalling in cyanobacteria

AbstractCyanobacteria utilize CO2 and HCO3− as inorganic carbon (Ci) sources. In low Ci, like in ambient air, cyanobacteria efficiently collect Ci using a carbon concentrating mechanism (CCM). The CCM includes bicarbonate transporters SbtA, BicA and BCT1; the specialized NDH complexes NDH‐13 and NDH‐14, which convert CO2 to HCO3− in the cytoplasm; and carboxysomes that are protein shell encapsulated ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBisCo) and carbonic anhydrase containing bodies in which the first reaction of carbon fixation occurs. Ci‐dependent regulation of bicarbonate transporters and specialized NDH complexes, especially the regulation of the SbtA transporter, are well understood. CcmR (also called NdhR), CyAbrB2, CmpR and RbcR act as transcription factors regulating CCM genes. Ci signalling molecules detecting the metabolic status of the cells include 2‐oxoglutarate, which accumulates when the Ci/nitrogen ratio of the cell is high, and 2‐phosphoglycolate, the first intermediate of the photorespiration pathway, whose accumulation indicates low Ci. These signalling molecules act as corepressors and coactivators of the CcmR repressor protein, whereas 2‐phosphoglycolate and ribulose‐1,5‐bisphosphate activate transcription activator CmpR. In addition, bicarbonate or CO2 activates the adenylyl cyclase that produces cAMP, and ATP/ADP/AMP provide information about the energy status of the cell. Less is known about the molecular mechanisms regulating carboxysome dynamics or how production, activity and degradation of photosynthetic complexes are regulated by prevailing Ci conditions or which mechanisms adjust cell division according to Ci. This minireview summarizes the present knowledge about molecular mechanisms regulating cyanobacterial acclimation to prevailing Ci.