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Plant physiologists set about comprehending the genesis of the C4 photosynthetic pathway after its discovery by Hatch and Slack. They discovered that a sophisticated combination of morphological and biochemical adaptations allowed the plant to concentrate CO2 around RuBisCO to achieve maximum efficiency. We categorize the evolutionary events leading to C4 photosynthesis, beginning with anoxygenic photosynthesis and the evolution of RuBisCO to the cooling of Earth by the Great Oxygenation Event that led to the oxygenic photosynthesis. The evolutionary descent of the C4 plants is a phenomenon that occurred around 30 million years ago. Due to industrialization and population growth, improved photosynthetic efficiency and carbon fixation of C4 plants could contest the current global scenario of rising CO2 concentration. C3 crops engineered with C4 traits, implemented on a large scale, could impact the climate globally. Here we discuss the various strategies used to introduce C4 traits in the C3 plants and the potential techniques to be considered for successful hybridization. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Allometric relationships among the dimensions of leaves and their cells hold across diverse eudicotyledons, but have remained untested in the leaves of grasses. We hypothesised that geometric (proportional) allometries of cell sizes across tissues and of leaf dimensions would arise due to the coordination of cell development and that of cell functions such as water, nutrient and energy transport, and that cell sizes across tissues would be associated with light‐saturated photosynthetic rate. We tested predictions across 27 globally distributed C3 and C4 grass species grown in a common garden. We found positive relationships among average cell sizes within and across tissues, and of cell sizes with leaf dimensions. Grass leaf anatomical allometries were similar to those of eudicots, with exceptions consistent with the fewer cell layers and narrower form of grass leaves, and the specialised roles of epidermis and bundle sheath in storage and leaf movement. Across species, mean cell sizes in each tissue were associated with light‐saturated photosynthetic rate per leaf mass, supporting the functional coordination of cell sizes. These findings highlight the generality of evolutionary allometries within the grass lineage and their interlinkage with coordinated development and function. Summary statement: Allometries among leaf cell sizes and of cell sizes with leaf dimensions and photosynthetic rate hold across grass species, supporting their basis in developmental and functional coordination. [ABSTRACT FROM AUTHOR]

Algae play critical roles in the carbon dioxide (CO 2) exchange between the water bodies and the atmosphere. However, the effects of prokaryotic and eukaryotic algae on carbon utilization, CO 2 flux, and the underlying mechanisms remain poorly understood. Therefore, this study investigated the differences in carbon preferences and CO 2 fluxes under different algal dominance days. Our research revealed that dissolved inorganic carbon (DIC) concentration fluctuations had a limited effect on the relative abundance of algae. However, shifts in dominant algal phyla induced changes in DIC, with Cyanobacteria preferring HCO 3 − and Chlorophyta preferring CO 2. Analysis of the water chemistry balance indicated that the growth of Chlorophyta had a 15.59 times greater effect on CO 2 sinks compared with that of Cyanobacteria. During the Cyanobacteria dominance days, the lower DIC concentration did not result in a reduction in CO 2 emissions. However, increases in the dissolved organic carbon concentration provided a favorable environment for Cyanobacteria, which promoted CO 2 emissions. The CCM model indicated that the growth of Chlorophyta resulted in CO 2 uptake rates at least 3.57 times higher and CO 2 leakage rates up to 0.97 times lower compared to Cyanobacteria, accelerating CO 2 transport into the cell. Overall, CO 2 sink was stronger on Chlorophyta dominance days than on Cyanobacteria dominance days. This study emphasized the influence of algal phyla on CO 2 fluxes, revealing the significant CO 2 sink associated with Chlorophyta. Further research should investigate how to manipulate environmental factors to favor Chlorophyta growth and effectively reduce CO 2 emissions. [Display omitted] • Cyanobacteria and Chlorophyta showed different preferences for inorganic carbon. • Higher DOC concentrations during Cyanobacteria dominance days promoted CO 2 emissions. • Chlorophyta had stronger CO 2 uptake & lower leakage rates in different chambers. • Higher carbon sink flux was observed during Chlorophyta dominance days. [ABSTRACT FROM AUTHOR]

Incorporating C4 photosynthetic traits into C3 crops is a rational approach for sustaining future demands for crop productivity. Using classical plant breeding, engineering this complex trait is unlikely to achieve its target. Therefore, it is critical and timely to implement novel biotechnological crop improvement strategies to accomplish this goal. However, a fundamental understanding of C3, C4, and C3–C4 intermediate metabolism is crucial for the targeted use of biotechnological tools. This review assesses recent progress towards engineering C4 photosynthetic traits in C3 crops. We also discuss lessons learned from successes and failures of recent genetic engineering attempts in C3 crops, highlighting the pros and cons of using rice as a model plant for short‐, medium‐ and long‐term goals of genetic engineering. This review provides an integrated approach towards engineering improved photosynthetic efficiency in C3 crops for sustaining food, fibre and fuel production around the globe. [ABSTRACT FROM AUTHOR]

Summary: C2 photosynthesis is a carbon concentrating mechanism that can increase net CO2 assimilation by capturing, concentrating and re‐assimilating CO2 released by photorespiration. Empirical and modelling studies indicate that C2 plants assimilate more carbon than C3 plants under high temperature, bright light, and low CO2 conditions. I argue that engineering C2 photosynthesis into C3 crops is a promising approach to improve photosynthetic performance under these – and temporally heterogeneous – environments, and review the modifications that may re‐create a C2 phenotype in C3 plants. Although a C2 engineering program would encounter many of the same challenges faced by C4 engineering programmes, the simpler leaf anatomical requirements make C2 engineering a feasible approach to improve crops in the medium term. [ABSTRACT FROM AUTHOR]

Membrane permeabilities to CO2 and HCO3− constrain the function of CO2 concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO2 but effectively impermeable to HCO3−. Here, CO2 and HCO3− membrane permeabilities were measured using an 18O‐exchange technique on two species of haptophyte algae, Emiliania huxleyi and Calcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO2 (0.006–0.02 cm · s−1) but minimally permeable to HCO3−. Increased temperature and CO2 generally increased CO2 membrane permeabilities in both species, possibly due to changes in lipid composition or CO2 channel proteins. Changes in CO2 membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO2 were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret 18O‐exchange data assumes that carbonic anhydrase, which catalyzes 18O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate 18O‐exchange data and then inferring plasma‐membrane CO2 permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO2 permeabilities are minimal estimates but should be quite accurate under most conditions. [ABSTRACT FROM AUTHOR]

Background: Pyropia yezoensis (Rhodophyta) is widely cultivated in East Asia and plays important economic, ecological and research roles. Although inorganic carbon utilization of P. yezoensis has been investigated from a physiological aspect, the carbon concentration mechanism (CCM) of P. yezoensis remains unclear. To explore the CCM of P. yezoensis, especially during its different life stages, we tracked changes in the transcriptome, photosynthetic efficiency and in key enzyme activities under different inorganic carbon concentrations. Results: Photosynthetic efficiency demonstrated that sporophytes were more sensitive to low carbon (LC) than gametophytes, with increased photosynthesis rate during both life stages under high carbon (HC) compared to normal carbon (NC) conditions. The amount of starch and number of plastoglobuli in cells corresponded with the growth reaction to different inorganic carbon (Ci) concentrations. We constructed 18 cDNA libraries from 18 samples (three biological replicates per Ci treatment at two life cycles stages) and sequenced these using the Illumina platform. De novo assembly generated 182,564 unigenes, including approximately 275 unigenes related to CCM. Most genes encoding internal carbonic anhydrase (CA) and bicarbonate transporters involved in the biophysical CCM pathway were induced under LC in comparison with NC, with transcript abundance of some PyCAs in gametophytes typically higher than that in sporophytes. We identified all key genes participating in the C4 pathway and showed that their RNA abundances changed with varying Ci conditions. High decarboxylating activity of PEPCKase and low PEPCase activity were observed in P. yezoensis. Activities of other key enzymes involved in the C4-like pathway were higher under HC than under the other two conditions. Pyruvate carboxylase (PYC) showed higher carboxylation activity than PEPC under these Ci conditions. Isocitrate lyase (ICL) showed high activity, but the activity of malate synthase (MS) was very low. Conclusion: We elucidated the CCM of P. yezoensis from transcriptome and enzyme activity levels. All results indicated at least two types of CCM in P. yezoensis, one involving CA and an anion exchanger (transporter), and a second, C4-like pathway belonging to the PEPCK subtype. PYC may play the main carboxylation role in this C4-like pathway, which functions in both the sporophyte and gametophyte life cycles. [ABSTRACT FROM AUTHOR]

Highlights • Karenia brevis is not affected by changes in environmentally relevant CO 2 concentrations. • Karenia brevis maintains an efficient and regulated CO 2 concentrating mechanism (CCM). • The CCM can sustain dense blooms Karenia brevis. • Rising CO 2 can potentially elevate the negative socioeconomic effects of Karenia brevis. Abstract Karenia brevis is a marine dinoflagellate commonly found in the Gulf of Mexico and important both ecologically and economically due to its production of the neurotoxin brevetoxin, which can cause respiratory illness in humans and widespread death of marine animals. K. brevis strains have previously shown to be sensitive to changes in CO 2 , both in terms of growth as well as toxin production. Our study aimed to understand this sensitivity by measuring underlying mechanisms, such as photosynthesis, carbon acquisition, and photophysiology. K. brevis (CCFWC-126) did not show a significant response in growth, cellular composition of carbon and nitrogen, nor in photosynthetic rates between p CO 2 concentrations of 150, 400, or 780 µatm. However, a strong response in its acquisition of inorganic carbon was found. Half saturation values for CO 2 increased from 1.5 to 3.3 µM, inorganic carbon preference switched from HCO 3 − to CO 2 (14–56% CO 2 usage), and external carbonic anhydrase activity was downregulated by 23% when comparing low and high p CO 2. We conclude that K. brevis must employ an efficient and regulated CO 2 concentrating mechanism (CCM) to maintain constant carbon fixation and growth across p CO 2 levels. No statistically significant correlation between CO 2 and brevetoxin content was found, yet a positive trend with enhanced p CO 2 was detected. This study is the first explaining how this socioeconomically important species is able to efficiently supply inorganic carbon for photosynthesis, which can potentially prolong bloom situations. This study also highlights that elevated CO 2 concentrations, as projected for a future ocean, can affect underlying physiological processes of K. brevis , some of which could lead to increases in cellular brevetoxin production and therefore increased impacts on coastal ecosystems and economies. [ABSTRACT FROM AUTHOR]

Carbon concentrating mechanism (CCM) and photorespiration (PR) are interlinked and co-regulated in Chlamydomonas reinhardtii, but conditions where co-regulation alters are not sufficiently explored. Here, we uncover that PR gene transcripts, like CCM transcripts, are induced even in the dark when both processes are not active. Such diurnal cycles show that transcript levels peak in the middle of 12 h day, decline by early part of 12-h dark followed by their onset again at mid-dark. Interestingly, the onset in the mid-dark phase is sensitive to high CO2, implying that the active carbon sensing mechanism operates even in the dark. The rhythmic alterations of both CCM and PR transcript levels are unlinked to circadian clock: the "free-running state" reveals no discernible rhythmicity in transcript changes. Only continuous light leads to high transcript levels but no detectable transcripts were observed in continuous dark. Asynchronous continuous light cultures, upon shifting to low from high CO2 exhibit only transient induction of PR transcripts/proteins while CCM transcript induction is stable, indicating the loss of co-regulation between PR and CCM gene transcription. Lastly, we also describe that both CCM and PR transcripts/proteins are induced in low CO2 even in mixotrophic cultures, but only in high light, the same being attenuated in high CO2, implying that high light is a mandatory "trigger" for CCM and PR induction in low CO2 mixotrophy. Our study provides comprehensive analyses of conditions where CCM and PR were differently regulated, setting a paradigm for a detailed mechanistic probing of these responses. [ABSTRACT FROM AUTHOR]

Autotrophic bacteria rely on various mechanisms to increase intracellular concentrations of inorganic forms of carbon (i.e., bicarbonate and CO 2 ) in order to improve the efficiency with which they can be converted to organic forms. Transmembrane bicarbonate transporters and carboxysomes play key roles in accumulating bicarbonate and CO 2 , but other regulatory elements of carbon concentration mechanisms in bacteria are less understood. In this study, after analyzing the genomic regions around α-type carboxysome operons, we characterize a protein that is conserved across these operons but has not been previously studied. On the basis of a series of apo- and ligand-bound crystal structures and supporting biochemical data, we show that this protein, which we refer to as the carboxysome-associated PII protein (CPII), represents a new and distinct subfamily within the broad superfamily of previously studied PII regulatory proteins, which are generally involved in regulating nitrogen metabolism in bacteria. CPII undergoes dramatic conformational changes in response to ADP binding, and the affinity for nucleotide binding is strongly enhanced by the presence of bicarbonate. CPII therefore appears to be a unique type of PII protein that senses bicarbonate availability, consistent with its apparent genomic association with the carboxysome and its constituents. [ABSTRACT FROM AUTHOR]

Aquatic photosynthetic microorganisms, cyanobacteria and microalgae, account for almost half of the world's photosynthesis. They absorb carbon di oxide (CO2) as the major substrate to support photosynthesis, the beginning of energy flow into living organisms and one of the primary processes comprising the global carbon cycle. Among all photosynthetic mechanisms, inorganic carbon transport into ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is one of the major limiting steps in photosynthetic carbon fixation which involves active transport of HCO3−, CO2and/or H+, or an energized biochemical mechanism. In fact, a unique system “carbon concentrating mechanism (CCM)” manages the inorganic carbon assimilation, accumulation of CO2around RuBisCO, and utilization in algal cells. However, the information on mechanism of CO2uptake and fixation inside the algal cells is limited. In order to make strategies for enhancement of CO2fixation, understanding of CCM is crucial. Thus, this review provides an overview of advances in CCM research, the comparative state of the art and reports on the CO2uptake model in cyanobacteria and microalgae. The review also discusses the challenges and future perspectives associated with algal CCM research. [ABSTRACT FROM AUTHOR]

Transgenic potato ( Solanum tuberosum L. cv. Desiree) plants expressing components of a novel cyanobacterial photorespiratory glycolate catabolism pathway were developed. Transgenic plant expressing glcD1 (glycolate dehydrogenase I) gene was referred to as synGDH and transgenic plants expressing gcl (glyoxylate carboligase) and tsr (tartronic semialdehyde reductase) genes simultaneously were designated as synGT. Both synGDH and synGT plants showed stable gene transformation, integration and expression. Enhanced glyoxylate contents in synGDH plants were detected as compared to synGT and non-transgenic (NT) plants. Phenotypic evaluation revealed that synGDH plants accumulated 11 % higher dry weight, while, tuber weight was 38 and 16 % higher than NT and synGT, respectively. Upon challenging the plants in high temperature and high light conditions synGDH plants maintained higher Fv/ Fm and showed less bleaching of chlorophyll as compared to synGT and NT plants. These results indicate that genetic transformation of complete pathway in one plant holds promising outcomes in terms of biomass accumulation to meet future needs for food and energy. [ABSTRACT FROM AUTHOR]

Photorespiration and oxygenic photosynthesis are intimately linked processes. It has been shown that under the present day atmospheric conditions cyanobacteria and all eukaryotic phototrophs need functional photorespiration to grow autotrophically. The question arises as to when this essential partnership evolved, i.e. can we assume a coevolution of both processes from the beginning or did photorespiration evolve later to compensate for the generation of 2-phosphoglycolate (2PG) due to Rubisco's oxygenase reaction? This question is mainly discussed here using phylogenetic analysis of proteins involved in the 2PG metabolism and the acquisition of different carbon concentrating mechanisms (CCMs). The phylogenies revealed that the enzymes involved in the photorespiration of vascular plants have diverse origins, with some proteins acquired from cyanobacteria as ancestors of the chloroplasts and others from heterotrophic bacteria as ancestors of mitochondria in the plant cell. Only phosphoglycolate phosphatase was found to originate from Archaea. Notably glaucophyte algae, the earliest branching lineage of Archaeplastida, contain more photorespiratory enzymes of cyanobacterial origin than other algal lineages or land plants indicating a larger initial contribution of cyanobacterial-derived proteins to eukaryotic photorespiration. The acquisition of CCMs is discussed as a proxy for assessing the timing of periods when photorespiratory activity may have been enhanced. The existence of CCMs also had marked influence on the structure and function of photorespiration. Here, we discuss evidence for an early and continuous coevolution of photorespiration, CCMs and photosynthesis starting from cyanobacteria via algae, to land plants. [ABSTRACT FROM AUTHOR]

The gammaproteobacterium Thiomicrospira crunogena XCL-2 is an aerobic sulfur-oxidizing hydrothermal vent chemolithoautotroph that has a CO concentrating mechanism (CCM), which generates intracellular dissolved inorganic carbon (DIC) concentrations much higher than extracellular, thereby providing substrate for carbon fixation at sufficient rate. This CCM presumably requires at least one active DIC transporter to generate the elevated intracellular concentrations of DIC measured in this organism. In this study, the half-saturation constant ( K) for purified carboxysomal RubisCO was measured (276 ± 18 µM) which was much greater than the K of whole cells (1.03 µM), highlighting the degree to which the CCM facilitates CO fixation under low CO conditions. To clarify the bioenergetics powering active DIC uptake, cells were incubated in the presence of inhibitors targeting ATP synthesis (DCCD) or proton potential (CCCP). Incubations with each of these inhibitors resulted in diminished intracellular ATP, DIC, and fixed carbon, despite an absence of an inhibitory effect on proton potential in the DCCD-incubated cells. Electron transport complexes NADH dehydrogenase and the bc complex were found to be insensitive to DCCD, suggesting that ATP synthase was the primary target of DCCD. Given the correlation of DIC uptake to the intracellular ATP concentration, the ABC transporter genes were targeted by qRT-PCR, but were not upregulated under low-DIC conditions. As the T. crunogena genome does not include orthologs of any genes encoding known DIC uptake systems, these data suggest that a novel, yet to be identified, ATP- and proton potential-dependent DIC transporter is active in this bacterium. This transporter serves to facilitate growth by T. crunogena and other Thiomicrospiras in the many habitats where they are found. [ABSTRACT FROM AUTHOR]

The transition of C3, via C2 towards C4 photosynthesis is an important example of stepwise evolution of a complex genetic trait. A common feature that was gradually emphasized during this trajectory is the evolution of a CO2 concentration mechanism around Rubisco. In C2 plants, this mechanism is based on tissue- specific accumulation of glycine decarboxylase (GDC) in bundle sheath (BS) cells, relative to global expression in the cells of C3 leaves. This limits photorespiratory CO2 release to BS cells. Because BS cells are surrounded by photosynthetically active mesophyll cells, this arrangement enhances the probability of re-fixation of CO2. The restriction of GDC to BS cells was mainly achieved by confinement of its P-subunit (GLDP). Here, we provide a mechanism for the establishment of C2-type gene expression by studying the upstream sequences of C3 Gldp genes in Arabidopsis thaliana. Deletion of 59 bp in the upstream region of AtGldp1 restricted expression of a reporter gene to BS cells and the vasculature without affecting diurnal variation. This region was named the 'M box'. Similar results were obtained for the AtGldp2 gene. Fusion of the M box to endogenous or exogenous promoters supported mesophyll expression. Nucleosome densities at the M box were low, suggesting an open chromatin structure facilitating transcription factor binding. In silico analysis defined a possible consensus for the element that was conserved across the Brassicaceae, but not in Moricandia nitens, a C2 plant. Collective results provide evidence that a simple mutation is sufficient for establishment of C2-specific gene expression in a C3 plant. [ABSTRACT FROM AUTHOR]

Inorganic carbon (C i ) is the major sink for photosynthetic reductant in organisms capable of oxygenic photosynthesis. In the absence of abundant C i , the cyanobacterium Synechocystis sp. strain PCC6803 expresses a high affinity C i acquisition system, the CO 2 -concentrating mechanisms (CCM), controlled by the transcriptional regulator CcmR and the metabolites NADP + and α-ketoglutarate, which act as co-repressors of CcmR by modulating its DNA binding. The CCM thus responds to internal cellular redox changes during the transition from C i -replete to C i -limited conditions. However, the actual changes in the metabolic state of the NADPH/NADP + system that occur during the transition to C i -limited conditions remain ill-defined. Analysis of changes in the redox state of cells experiencing C i limitation reveals systematic changes associated with physiological adjustments and a trend towards the quinone and NADP pools becoming highly reduced. A rapid and persistent increase in F 0 was observed in cells reaching the C i -limited state, as was the induction of photoprotective fluorescence quenching. Systematic changes in the fluorescence induction transients were also observed. As with Chl fluorescence, a transient reduction of the NADPH pool (‘M’ peak), is assigned to State 2→ State 1 transition associated with increased electron flow to NADP + . This was followed by a characteristic decline, which was abolished by C i limitation or inhibition of the Calvin –Benson–Bassham (CBB) cycle and is thus assigned to the activation of the CBB cycle. The results are consistent with the proposed regulation of the CCM and provide new information on the nature of the Chl and NADPH fluorescence induction curves. [ABSTRACT FROM AUTHOR]

Microalgae are a diverse group of photosynthetic organisms that can be exploited for the production of different compounds, ranging from crude biomass and biofuels to high value-added biochemicals and synthetic proteins. Traditionally, algal biotechnology relies on bioprospecting to identify new highly productive strains and more recently, on forward genetics to further enhance productivity. However, it has become clear that further improvements in algal productivity for biotechnology is impossible without combining traditional tools with the arising molecular genetics toolkit. We review recent advantages in developing high throughput screening methods, preparing genome-wide mutant libraries, and establishing genome editing techniques. We discuss how algae can be improved in terms of photosynthetic efficiency, biofuel and high value-added compound production. Finally, we critically evaluate developments over recent years and explore future potential in the field. [ABSTRACT FROM AUTHOR]

Symbiotic dinoflagellate algae residing inside coral tissues supply the host with the majority of their energy requirements through the translocation of photosynthetically fixed carbon. The algae, in turn, rely on the host for the supply of inorganic carbon. Carbon must be concentrated as CO2 in order for photosynthesis to proceed, and here we show that the coral host plays an active role in this process. The host-derived symbiosome membrane surrounding the algae abundantly expresses vacuolar H+-ATPase (VHA), which acidifies the symbiosome space down to pH ~4. Inhibition of VHA results in a significant decrease in average H+ activity in the symbiosome of up to 75% and a significant reduction in O2 production rate, a measure of photosynthetic activity. These results suggest that host VHA is part of a previously unidentified carbon concentrating mechanism for algal photosynthesis and provide mechanistic evidence that coral host cells can actively modulate the physiology of their symbionts. [ABSTRACT FROM AUTHOR]

Symbiotic dinoflagellate algae residing inside coral tissues supply the host with the majority of their energy requirements through the translocation of photosynthetically fixed carbon. The algae, in turn, rely on the host for the supply of inorganic carbon. Carbon must be concentrated as CO2 in order for photosynthesis to proceed, and here we show that the coral host plays an active role in this process. The host-derived symbiosome membrane surrounding the algae abundantly expresses vacuolar H+-ATPase (VHA), which acidifies the symbiosome space down to pH ∼4. Inhibition of VHA results in a significant decrease in average H+ activity in the symbiosome of up to 75% and a significant reduction in O2 production rate, a measure of photosynthetic activity. These results suggest that host VHA is part of a previously unidentified carbon concentrating mechanism for algal photosynthesis and provide mechanistic evidence that coral host cells can actively modulate the physiology of their symbionts. [ABSTRACT FROM AUTHOR]

As all plastids that have been investigated so far can be traced back to endosymbiotic uptake of cyanobacteria by heterotrophic host cells, they accordingly show a high similarity regarding photosynthesis, which includes both the photosystems and the biochemical reactions around the CO 2 fixation via the Calvin–Bassham cycle. Major differences between the different algal and plant groups may include the presence or absence of carbon concentrating mechanisms, pyrenoids, Rubisco activases, carbonic anhydrases as well as differences in the regulation of the Calvin–Bassham cycle. This review describes the diversity of primary carbon fixation steps in algae and plants and the respective regulatory mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the catalytic activity and inhibition of the β-class carbonic anhydrase (CA, EC 4.2.1.1) CahB1, from the relict cyanobacterium Coleofasciculus chthonoplastes (previously denominated Microcoleus chthonoplastes). The enzyme showed good activity as a catalyst for the CO2 hydration, with a k cat of 2.4×105 s−1 and a k cat/K m of 6.3×107 M−1 s−1. A range of inorganic anions and small molecules were investigated as inhibitors of CahB1. Perchlorate and tetrafluoroborate did not inhibit the enzyme (K Is >200mM) whereas selenate and selenocyanide were ineffective inhibitors too, with K Is of 29.9–48.61mM. The halides, pseudohalides, carbonate, bicarbonate, trithiocarbonate and a range of heavy metal ions-containing anions were submillimolar–millimolar inhibitors (K Is in the range of 0.15–0.90mM). The best CahB1 inhibitors were N,N-diethyldithiocarbamate, sulfamate, sulfamide, phenylboronic acid and phenylarsonic acid, with K Is in the range of 8–75μM, whereas acetazolamide inhibited the enzyme with a K I of 76nM. This is the first kinetic and inhibition study of a cyanobacterial CA. As these enzymes are widespread in many cyanobacteria, being crucial for the carbon concentrating mechanism which assures substrate to RubisCO for the CO2 fixation by these organisms, a detailed kinetic/inhibition study may be essential for a better understanding of this superfamily of metalloenzymes and for potential biotechnological applications in biomimetic CO2 capture processes. [Copyright &y& Elsevier]

Threat of global warming due to carbon dioxide (CO) emissions has stimulated research into carbon sequestration and emissions reduction technologies. Alkaline scrubbing allows CO to be captured as bicarbonate, which can be photochemically fixed by microalgae. The carbon concentrating mechanism (CCM), of which external carbonic anhydrase is a key component, allows organisms to utilise this bicarbonate. In order to select a suitable strain for this application, a screening tool is required. The current method for determining carbonic anhydrase activity, the Wilbur and Anderson assay, was found to be unsuitable as a screening tool as the associated error was unacceptably large and tests on whole cells were inconclusive. This paper presents the development of a new, whole cell assay to measure inorganic carbon uptake and external carbonic anhydrase activity, based on classical pH drift experiments. Spirulina platensis was successfully used to develop a correlation between the specific carbon uptake (C) and the specific pH change (dpH). The relationship is described by the following: C[mmol C (g dry algae) h] = 0.064 × (dpH). Inhibitor and salt dissociation tests validated the activity and presence of external carbonic anhydrase and allowed correlation between the Wilbur and Anderson assay and the new whole cell assay. Screening tests were conducted on S. platensis, Scenedesmus sp., Chlorella vulgaris and Dunaliella salina that were found to have carbon uptake rates of 5.76, 5.86, 3.86 and 2.15 mmol C (g dry algae) h, respectively. These results corresponded to the species' known bicarbonate utilisation abilities and validated the use of the assay as a screening tool. [ABSTRACT FROM AUTHOR]

Oxygenic photosynthesis would not be possible without photorespiration in the present day O2-rich atmosphere. It is now generally accepted that cyanobacteria-like prokaryotes first evolved oxygenic photosynthesis, which was later conveyed via endosymbiosis into a eukaryotic host, which then gave rise to the different groups of algae and streptophytes. For photosynthetic CO2 fixation, all these organisms use Rubis CO, which catalyses both the carboxylation and the oxygenation of ribulose 1,5-bisphosphate. One of the reaction products of the oxygenase reaction, 2-phosphoglycolate (2 PG), represents the starting point of the photorespiratory C2 cycle, which is considered largely responsible for recapturing organic carbon via conversion to the Calvin-Benson cycle ( CBC) intermediate 3-phosphoglycerate, thereby detoxifying critical intermediates. Here we discuss possible scenarios for the evolution of this process toward the well-defined 2 PG metabolism in extant plants. While the origin of the C2 cycle core enzymes can be clearly dated back towards the different endosymbiotic events, the evolutionary scenario that allowed the compartmentalised high flux photorespiratory cycle is uncertain, but probably occurred early during the algal radiation. The change in atmospheric CO2/ O2 ratios promoting the acquisition of different modes for inorganic carbon concentration mechanisms, as well as the evolutionary specialisation of peroxisomes, clearly had a dramatic impact on further aspects of land plant photorespiration. [ABSTRACT FROM AUTHOR]

Abstract: Many photosynthetic organisms have developed inorganic carbon (Ci) concentrating mechanisms (CCMs) that increase the CO2 concentration within the vicinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). Several CCMs, such as four carbon (C4) and crassulacean acid metabolism (CAM), bicarbonate accumulation systems and capsular structures around RubisCO have been described in great detail. These systems are believed to have evolved several times as mechanisms that acclimate organisms to unfavourable growth conditions. Based on recent experimental evidence we propose the occurrence of another more general CCM system present in all plants. This basal CCM (bCCM) is supposed to be composed of mitochondrial carbonic anhydrases (a β-type carbonic anhydrase and the γ-type carbonic anhydrase domain of the mitochondrial NADH dehydrogenase complex) and probably further unknown components. The bCCM is proposed to reduce leakage of CO2 from plant cells and allow efficient recycling of mitochondrial CO2 for carbon fixation in chloroplasts. [Copyright &y& Elsevier]

Abstract: Bacterial microcompartments (BMCs) are polyhedral bodies, composed entirely of proteins, that function as organelles in bacteria; they promote subcellular processes by encapsulating and co-localizing targeted enzymes with their substrates. The best-characterized BMC is the carboxysome, a central part of the carbon-concentrating mechanism that greatly enhances carbon fixation in cyanobacteria and some chemoautotrophs. Here we report the first structural insights into the carboxysome of Prochlorococcus, the numerically dominant cyanobacterium in the world''s oligotrophic oceans. Bioinformatic methods, substantiated by analysis of gene expression data, were used to identify a new carboxysome shell component, CsoS1D, in the genome of Prochlorococcus strain MED4; orthologs were subsequently found in all cyanobacteria. Two independent crystal structures of Prochlorococcus MED4 CsoS1D reveal three features not seen in any BMC-domain protein structure solved to date. First, CsoS1D is composed of a fused pair of BMC domains. Second, this double-domain protein trimerizes to form a novel pseudohexameric building block for incorporation into the carboxysome shell, and the trimers further dimerize, forming a two-tiered shell building block. Third, and most strikingly, the large pore formed at the 3-fold axis of symmetry appears to be gated. Each dimer of trimers contains one trimer with an open pore and one whose pore is obstructed due to side-chain conformations of two residues that are invariant among all CsoS1D orthologs. This is the first evidence of the potential for gated transport across the carboxysome shell and reveals a new type of building block for BMC shells. [Copyright &y& Elsevier]

1. The ecophysiological significance of Crassulacean acid metabolism (CAM) in the invasive aquatic macrophyte Crassula helmsii was studied in an English soft-water lake. The extent and the contribution of CAM to the carbon budget was examined in spring (April) and summer (July) along a depth gradient (0.5–2.2 m), covering the growth range of C. helmsii in the lake. 2. Significant in situ CAM activity (30–80 meq kg−1 FW) was present in all specimens, although it decreased with depth and hence correlated with the decline in photon irradiance. Potential CAM activity (60–161 meq kg−1 FW), measured after exposure to low concentrations of CO2 in the day and high concentrations at night, were on average 2.7-times greater than in situ CAM activity. Overall CAM activity increased from April to July, which is consistent with higher potential carbon limitation caused by increased temperature and light availability. 3. CAM activity in C. helmsii appeared to be carbon-limited at night because night-time carbon-fixation increased at raised, compared to ambient, concentrations of CO2. 4. The high in situ CAM activity in C. helmsii was reflected in the contribution of CAM to the total carbon budget which, independent of depth and season, ranged from 18% to 42%. The amount of CO2 taken up in the night via CAM was 0.74 to 2.94 times the amount of CO2 lost in respiration, thus emphasizing the importance of CAM in refixation of potentially lost respiratory CO2. 5. The onset of decarboxylation in the morning appeared to be under circadian control as there was a delay of up to 5.5 h between the start of the light period and a decline in cell acidity level. 6. There was little variation in δ13C content (−21.69 to 23.49‰) with season or depth suggesting, along with the estimated contribution to the carbon-budget, that CAM is important for the whole population of C. helmsii. CAM may confer a competitive advantage in relation to growth, which may be one of the reasons for the invasiveness of this species. [ABSTRACT FROM AUTHOR]

Estimates of the effect of increased global atmospheric CO2 levels on oceanic primary productivity depend on the physiological responses of contemporary phytoplankton populations. However, microalgal populations will possibly adapt to rising CO2 levels in such a way that they become genetically different from contemporary populations. The unknown properties of these future populations introduce an undefined error into predictions of C pool dynamics, especially the presence and size of the biological C pump. To address the bias in predictions introduced by evolution, we measured the kinetics of CO2 uptake in populations of Chlamydomonas reinhardtii that had been selected for growth at high CO2 for 1000 generations. Following selection at high CO2, the populations were unable to induce high-affinity CO2 uptake, and one line had a lower rate of net CO2 uptake. We attribute this to conditionally neutral mutations in genes affecting the C concentrating mechanism (CCM). Lower affinity CO2 uptake, in addition to smaller population sizes, results in a significant reduction in net CO2 uptake of about 38% relative to contemporary populations under the same conditions. This shows how predictions about the properties of communities in the future can be influenced by the effect of natural selection. [ABSTRACT FROM AUTHOR]

Cyanobacteria show high metabolic plasticity by re-allocating macromolecular resources in response to variations in both environmental inorganic carbon (Ci) and light. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Nägeli across a 50-fold range of growth irradiance at either a dissolved [Ci] <0.1 mM, sufficient to induce strongly the carbon-concentrating mechanism (CCM) or a dissolved [Ci] of ∼4 mM, sufficient to strongly induce the CCM to basal constitutive activity. There was no detectable growth cost of acclimation to low Ci across the entire range of irradiance and growth was nearly light saturated at 50 l mol photons·m−2·s−1. Cells acclimated to low Ci significantly re-allocated macromolecular resources to support their CCM, while maintaining near homeostatis of metabolic flux per unit photosynthetic complex. Changing growth irradiance also drove re-organization of the photosynthetic machinery to balance excitation flux and metabolic demands, but flux per complex varied widely across the range of tolerable growth irradiances. Across the range of growth irradiance, low Ci cells had significantly less phycocyanin than high Ci cells, which corresponded to a lower PSII absorbance capacity. Furthermore, low Ci cells maintained more PSI per cell−1 than high Ci cells under high growth irradiance. Low Ci cells could therefore maintain more of their PSII reaction centers open at high growth irradiance than could high Ci cells, which experienced a significant PSII closure. Thus, acclimation to growth under high available Ci actually constrained acclimation to high light by restricting electron transport downstream from PSII in S. elongatus. [ABSTRACT FROM AUTHOR]

The interactive effects of inorganic carbon status, temperature and light on chlorosis induced by nitrogen deficiency, and the roles of Clp proteases in this process were investigated. In wild-type cultures grown in high or ambient CO2, following transfer to media lacking combined nitrogen, phycocyanin per cell dropped primarily through dilution of the pigment through cell division, and also suffered variable degrees of net degradation. When grown at high CO2 (5%), chlorophyll (Chl) suffered net degradation to a greater extent than phycocyanin. In marked contrast, growth at ambient CO2 resulted in Chl per cell dropping through dilution. Conditions that drove net Chl degradation in the wild-type resulted in little or no net Chl degradation in aclpPIinactivation mutant, with Chl content dropping largely through growth dilution in the mutant. The chlorotic response of aclpPIIinactivation strain was nearly the same as that of wild-type, although phycocyanin degradation may have been slightly accelerated in the former. [ABSTRACT FROM AUTHOR]

Metabolic processes usually involve diffusion of compounds in addition to their metabolic reactions. Such processes are adequately described by reaction–diffusion models (in the form partial differential equations) which are usually difficult and tedious to solve. Compartmental models (ordinary differential equations) are much easier to analyse but may be inadequate since they do not allow for spatial gradients. However, a compartmental model can be considered as the limit of a reaction–diffusion model for very fast diffusion (all diffusion coefficients Dj→∞). A compartmental model mc is termed “associated” to the reaction–diffusion model mrd if mc is the limit of mrd for all Dj→∞. From the analytical solutions of a reaction–diffusion model and its associated compartmental model the extent of a diffusion gradient of mrd can be estimated by means of parameters from both mc and mrd. This approach is extended to more complicated models that cannot be solved analytically. Gradients can be neglected and, consequently, the compartmental description be used, if the characteristic length s of the diffusion path is small compared with the distance a particle travels in time Te, where Te is the characteristic time for the compartmental model. This ratio of lengths can also be expressed as the ratio of two times, namely the residence time s2/DX and the turnover time XC/v, where XC and v are the steady-state concentration of X and its import rate, respectively, for the associated compartmental model. Characteristic times are given for several simple reaction–diffusion systems in rectangular and spherical geometries. Intracellular gradients of HCO3− and CO2 are calculated for some flux situations relevant to photosynthetic carbon fixation in green microalgae. [Copyright &y& Elsevier]

ABSTRACT Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO2 to air and related to H2CO3 dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis (K0·5[DIC]) value in high CO2-grown cells was 1009 mmol m-3 but was reduced three-fold by the addition of bovine carbonic anhydrase (CA), whereas in air-grown cells K0·5[DIC] was 71 mmol m-3, irrespective of the presence of CA. The maximum rate of photosynthesis (Pmax) values varied between 300 and 500 μmol O2 mg Chl-1 h-1 regardless of growth pCO2. Bicarbonate dehydration kinetics in artificial seawater were re-examined to evaluate the direct HCO3- uptake as a substrate for photosynthesis. The uncatalysed CO2 formation rate in artificial seawater of 31·65°/oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m-3 min-1 at 100 mmol m-3 DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air- and high CO2-grown cells, respectively. These data indicate that even high CO2-grown cells of P. tricornutum can take up both CO2 and HCO3- as substrates for photosynthesis and HCO3- use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO2-grown cells to air. The development of high-affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO2-grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K0·5[DIC] values in acclimated cells appeared to be correlated only with [CO2(aq)] in the medium but not to HCO3-, CO32-, total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum, is [CO2(aq)] in the medium. [ABSTRACT FROM AUTHOR]

• Responses of R. raciborskii to different pH/CO 2 scenarios were tested;. • Bicarbonate availability governed growth and saxitoxin production in the cyanobacterium;. • R. raciborskii ITEP-A1 is a bicarbonate-user (potentially expresses the BicA transporter);. • Dissolved inorganic carbon and pH influenced saxitoxins production and toxicity in R. raciborskii ITEP-A1. Raphidiopsis raciborskii is a widely distributed, potentially toxic cyanobacterium described as a tropical-subtropical species. However, its occurrence in temperate regions has been expanding. Understanding the environmental factors underlying the expansion and colonization success of Raphidiopsis has been the object of numerous studies. However, less is known regarding its responses to pH and inorganic carbon in water. Thus, the aim of the present study was to investigate the effects of changes in pH and dissolved inorganic carbon on growth and saxitoxins production in the strain R. raciborskii ITEP-A1. We incubated batch cultures with different unbuffered and buffered pH (neutral-acid and alkaline) and inorganic carbon availability (CO 2 -rich air bubbling and the addition of NaHCO 3) to assess the effect of these factors on the growth, toxin production as well as saxitoxins composition of the cyanobacterium. The carbon concentrating mechanism (CCM) system of ITEP-A1 was also characterized by an in silico analysis of its previously sequenced genome. The growth and saxitoxins production of R. raciborskii were affected. The addition of sodium bicarbonate and air bubbling enhanced the growth of the cyanobacterium in alkaline pH. In contrast, saxitoxins production and relative toxicity were decreased. Moreover, significant changes in the cellular composition of saxitoxins were strongly related to pH changes. ITEP-A1 potentially expresses the low-flux bicarbonate transporter BicA, an efficient CCM which uptakes most of its carbon from HCO 3 −. Hence, increasing the diffusion of CO 2 in alkaline eutrophic lakes is likely to increase R. raciborskii dominance , but produce less toxic blooms. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Many autotrophic microorganisms are likely to adapt to scarcity in dissolved inorganic carbon (DIC; CO2 + HCO3- + CO32-) with CO2 concentrating mechanisms (CCM) that actively transport DIC across the cell membrane to facilitate carbon fixation. Surprisingly, DIC transport has been well studied among cyanobacteria and microalgae only. The deep-sea vent gammaproteobacterial chemolithoautotroph Thiomicrospira crunogena has a low-DIC inducible CCM, though the mechanism for uptake is unclear, as homologs to cyanobacterial transporters are absent. To identify the components of this CCM, proteomes of T. crunogena cultivated under low- and high-DIC conditions were compared. Fourteen proteins, including those comprising carboxysomes, were at least 4-fold more abundant under low-DIC conditions. One of these proteins was encoded by Tcr_0854; strains carrying mutated copies of this gene, as well as the adjacent Tcr_0853, required elevated DIC for growth. Strains carrying mutated copies of Tcr_0853 and Tcr_0854 overexpressed carboxysomes and had diminished ability to accumulate intracellular DIC. Based on reverse transcription (RT)-PCR, Tcr_0853 and Tcr_0854 were cotranscribed and upregulated under low-DIC conditions. The Tcr_0853-encoded protein was predicted to have 13 transmembrane helices. Given the mutant phenotypes described above, Tcr_0853 and Tcr_0854 may encode a two-subunit DIC transporter that belongs to a previously undescribed transporter family, though it is widespread among autotrophs from multiple phyla. [ABSTRACT FROM AUTHOR]