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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]

Bicarbonate species in the aqueous phase is the primary source for CO for the growth of microalgae. The potential of carbon dioxide (CO) fixation by Chlorella pyrenoidosa in enriched bicarbonate medium was evaluated. In the present study, effects of parameters such as pH, sodium bicarbonate concentration and inoculum size were assessed for the removal of CO by C. pyrenoidosa under mixotrophic condition. Central composite design tool from response surface methodology was used to validate statistical methods in order to study the influence of these parameters. The obtained results reveal that the maximum removal of CO was attained at pH 8 with sodium bicarbonate concentration of 3.33 g/l, and inoculum size of 30 %. The experimental results were statistically significant with R value of 0.9527 and 0.960 for CO removal and accumulation of chlorophyll content, respectively. Among the various interactions, interactive effects between the parameters pH and inoculum size was statistically significant ( P < 0.05) for CO removal and chlorophyll accumulation. Based on the studies, the application of C. pyrenoidosa as a potential source for carbon dioxide removal at alkaline pH from bicarbonate source is highlighted. [ABSTRACT FROM AUTHOR]

The PII superfamily consists of widespread signal transduction proteins found in all domains of life. In addition to canonical PII proteins involved in C/N sensing, structurally similar PII-like proteins evolved to fulfill diverse, yet poorly understood cellular functions. In cyanobacteria, the bicarbonate transporter SbtA is co-transcribed with the conserved PII-like protein, SbtB, to augment intracellular inorganic carbon levels for efficient CO 2 fixation. We identified SbtB as a sensor of various adenine nucleotides including the second messenger nucleotides cyclic AMP (cAMP) and c-di-AMP. Moreover, many SbtB proteins possess a C-terminal extension with a disulfide bridge of potential redox-regulatory function, which we call R-loop. Here, we reveal an unusual ATP/ADP apyrase (diphosphohydrolase) activity of SbtB that is controlled by the R-loop. We followed the sequence of hydrolysis reactions from ATP over ADP to AMP in crystallographic snapshots and unravel the structural mechanism by which changes of the R-loop redox state modulate apyrase activity. We further gathered evidence that this redox state is controlled by thioredoxin, suggesting that it is generally linked to cellular metabolism, which is supported by physiological alterations in site-specific mutants of the SbtB protein. Finally, we present a refined model of how SbtB regulates SbtA activity, in which both the apyrase activity and its redox regulation play a central role. This highlights SbtB as a central switch point in cyanobacterial cell physiology, integrating not only signals from the energy state (adenyl-nucleotide binding) and the carbon supply via cAMP binding but also from the day/night status reported by the C-terminal redox switch.

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]

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]

Agricultural yields have kept pace with the rising demands in the recent past as a result of the breeding and improved farming practices, but these practices alone will not be able to meet the demands of the future. The focus is now on the enhancement of the photosynthetic machinery. In photosynthesis, the rate limiting step is the one catalyzed by RuBisCO- Ribulose-1,5-bisphosphate carboxylase/oxygenase (4.1.1.39), which, because of its loose specificity and low turnover rate, is the primary target of most research programs directed towards improved photosynthesis. The other avenues of photosynthetic machinery that are under investigation to enhance it include-improved stomatal regulation and membrane permeability, RuBisCO with high specificity for CO and higher catalytic turnover; bypass of photorespiration and introduction of carbon concentrating mechanism (CCM) into the C plants. Carbon concentrating mechanisms cause accumulation of carbon dioxide in vicinity of RuBisCO producing a high CO/O ratio and hence an environment more suitable for carboxylation reactions than oxygenation reactions. This article includes the basic details of the major naturally occurring CCMs in various photosynthetic organisms to identify the knowledge gaps in each which could help study the prospects of its possible introduction into a non-native system as C plants which are devoid of any CCM. [ABSTRACT FROM AUTHOR]

The photosynthetic, unicellular green alga, Chlamydomonas reinhardtii, lives in environments that often contain low concentrations of CO and HCO, the utilizable forms of inorganic carbon (C). C. reinhardtii possesses a carbon concentrating mechanism (CCM) which can provide suitable amounts of C for growth and development. This CCM is induced when the CO concentration is at air levels or lower and is comprised of a set of proteins that allow the efficient uptake of C into the cell as well as its directed transport to the site where Rubisco fixes CO into biomolecules. While several components of the CCM have been identified in recent years, the picture is still far from complete. To further improve our knowledge of the CCM, we undertook a mutagenesis project where an antibiotic resistance cassette was randomly inserted into the C. reinhardtii genome resulting in the generation of 22,000 mutants. The mutant collection was screened using both a published PCR-based approach (Gonzalez-Ballester et al. ) and a phenotypic growth screen. The PCR-based screen did not rely on a colony having an altered growth phenotype and was used to identify colonies with disruptions in genes previously identified as being associated with the CCM-related gene. Eleven independent insertional mutations were identified in eight different genes showing the usefulness of this approach in generating mutations in CCM-related genes of interest as well as identifying new CCM components. Further improvements of this method are also discussed. [ABSTRACT FROM AUTHOR]

The articles in this special issue of Photosynthesis Research arose from the presentations given at the Eighth International Symposium on Inorganic Carbon Uptake by Aquatic Photosynthetic Organisms held from May 27 to June 1, 2013 in New Orleans, Louisiana USA. The meeting covered all the aspects of CO concentrating mechanisms (CCMs) present in photosynthetic bacteria, microalgae and macrophytes, and spanned disciplines from the molecular biology of CCMs to the importance of CCMs in aquatic ecosystems. The publications in this special issue represent our current understanding of CCMs and highlight recent advances in the field. The influences of CCMs on algal biofuel production as well as recent efforts to use the CCM to improve crop plants are also explored. [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]

This review presents an overview of the two ways that cyanobacteria, algae, and plants have adapted to high O and low CO concentrations in the environment. First, the process of photorespiration enables photosynthetic organisms to recycle phosphoglycolate formed by the oxygenase reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Second, there are a number of carbon concentrating mechanisms that increase the CO concentration around Rubisco which increases the carboxylase reaction enhancing CO fixation. This review also presents possibilities for the beneficial modification of these processes with the goal of improving future crop yields. [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]

Incorporating C 4 photosynthetic traits into C 3 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 C 3 , C 4 , and C 3 -C 4 intermediate metabolism is crucial for the targeted use of biotechnological tools. This review assesses recent progress towards engineering C 4 photosynthetic traits in C 3 crops. We also discuss lessons learned from successes and failures of recent genetic engineering attempts in C 3 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 C 3 crops for sustaining food, fibre and fuel production around the globe., (© 2022 German Society for Plant Sciences, Royal Botanical Society of the Netherlands.)

Abstract: Coccolithophorid algae (Haptophycea) are mainly marine unicellular phytoplankton. The coccolithophorids are of global interest as they can fix carbon by photosynthesis as well as in calcium carbonate (coccoliths). They are the largest carbon sinks and one of the largest primary producers on the planet. They can also produce high amounts of lipids which have a high potential application as a renewable fuel and alternative food source. This paper reviews current knowledge on coccolithophorid algae photosynthesis and calcification and their potential industrial applications. [Copyright &y& Elsevier]

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]

The CO acquisition was analyzed in Chlamydomonas acidophila at pH 2.4 in a range of medium P and Fe concentrations and at high and low CO condition. The inorganic carbon concentrating factor (CCF) was related to cellular P quota ( Q), maximum CO-uptake rate by photosynthesis ( $$ V_{{{ \max },{\text{O}}_{ 2} }} $$), half saturation constant for CO uptake ( K), and medium Fe concentration. There was no effect of the medium Fe concentration on the CCF. The CCF increased with increasing Q in both high and low CO grown algae, but maximum Q was 6-fold higher in the low CO cells. In high CO conditions, the CCF was low, ranging between 0.8 and 3.5. High CCF values up to 9.1 were only observed in CO-limited cells, but P- and CO-colimited cells had a low CCF. High CCF did not relate with a low K as all CO-limited cells had a low K (<4 μM CO). High C-pools in cells with high Q suggested the presence of an active CO-uptake mechanism. The CCF also increased with increasing $$ V_{{{ \max },{\text{O}}_{ 2} }} $$ which reflect an adaptation to the nutrient in highest demand (CO) under balanced growth conditions. It is proposed that the size of the CCF in C. acidophila is more strongly related to porter density for CO uptake (reflected in $$ V_{{{ \max },{\text{O}}_{ 2} }} $$) and less- to high-affinity CO uptake (low K) at balanced growth. In addition, high CCF can only be realized with high Q. [ABSTRACT FROM AUTHOR]

The fluxes of CO and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa, when O evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO, indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO was observed similar to the post-illumination burst characteristic of C higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O. This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O evolution rates in all four algae, measured by O electrode, were significantly higher (40-50%) in media at low O (4%) than in air-equilibrated (21% O) media, indicating an O inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species. [ABSTRACT FROM AUTHOR]

Cyanobacteria (as well as many chemoautotrophs) actively pump inorganic carbon (in the form of HCO) into the cytosol in order to enhance the overall efficiency of carbon fixation. The success of this approach is dependent upon the presence of carboxysomes-large, polyhedral, cytosolic bodies which sequester ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and carbonic anhydrase. Carboxysomes seem to function by allowing ready passage of HCO into the body, but hindering the escape of evolved CO, promoting the accumulation of CO in the vicinity of RubisCO and, consequently, efficient carbon fixation. This selectivity is mediated by a thin shell of protein, which envelops the carboxysome's enzymatic core and uses narrow pores to control the passage of small molecules. In this review, we summarize recent advances in understanding the organization and functioning of these intriguing, and ecologically very important molecular machines. [ABSTRACT FROM AUTHOR]

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]

The availability of inorganic carbon (C i) as the source for photosynthesis is fluctuating in aquatic environments. Despite the involvement of transcriptional regulators CmpR and NdhR in regulating genes encoding C i transporters at limiting CO 2 , the C i -sensing mechanism is largely unknown among cyanobacteria. Here we report that a cAMP-dependent transcription factor SyCRP1 mediates C i response in Synechocystis. The mutant ∆ sycrp1 exhibited a slow-growth phenotype and reduced maximum rate of bicarbonate-dependent photosynthetic electron transport (V max) compared to wild-type at the scarcity of CO 2. The number of carboxysomes was decreased significantly in the ∆ sycrp1 at low CO 2 consistent with its reduced V max. The DNA microarray analysis revealed the upregulation of genes encoding C i transporters in ∆sycrp1. The membrane-localized SyCRP1 was released into the cytosol in wild-type cells shifted from low to high CO 2 or upon cAMP treatment. Soluble His-tagged SyCRP1 was shown to target DNA-binding sites upstream of the C i -regulated genes sbtA and ccmK 3. In addition, cAMP enhanced the binding of SyCRP1 to its target sites. Our data collectively suggest that the C i is sensed through the second messenger cAMP releasing membrane-bound SyCRP1 into cytoplasm under sufficient CO 2 conditions. Hence, SyCRP1 is a possible regulator of carbon concentrating mechanism, and such a regulation might be mediated via sensing C i levels through cAMP in Synechocystis. • It has been known that cAMP receptor proteins (CRP) play a crucial role in regulation of gene expression in bacteria. • Transcription regulators of CCM genes were decribed previously, however C i -sensing mechanism in cyanobacteria is unclear. • A membrane associated Synechocystis CRP (SyCRP1), releases to cytosol when bound to CAMP to regulate CCM. • Thus, SyCRP1 mediates C i response through the signalling molecule cAMP in Synechocytis. [ABSTRACT FROM AUTHOR]

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]

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]

The availability of inorganic carbon (C i ) as the source for photosynthesis is fluctuating in aquatic environments. Despite the involvement of transcriptional regulators CmpR and NdhR in regulating genes encoding C i transporters at limiting CO 2 , the C i -sensing mechanism is largely unknown among cyanobacteria. Here we report that a cAMP-dependent transcription factor SyCRP1 mediates C i response in Synechocystis. The mutant ∆sycrp1 exhibited a slow-growth phenotype and reduced maximum rate of bicarbonate-dependent photosynthetic electron transport (V max ) compared to wild-type at the scarcity of CO 2 . The number of carboxysomes was decreased significantly in the ∆sycrp1 at low CO 2 consistent with its reduced V max . The DNA microarray analysis revealed the upregulation of genes encoding C i transporters in ∆sycrp1. The membrane-localized SyCRP1 was released into the cytosol in wild-type cells shifted from low to high CO 2 or upon cAMP treatment. Soluble His-tagged SyCRP1 was shown to target DNA-binding sites upstream of the C i -regulated genes sbtA and ccmK3. In addition, cAMP enhanced the binding of SyCRP1 to its target sites. Our data collectively suggest that the C i is sensed through the second messenger cAMP releasing membrane-bound SyCRP1 into cytoplasm under sufficient CO 2 conditions. Hence, SyCRP1 is a possible regulator of carbon concentrating mechanism, and such a regulation might be mediated via sensing C i levels through cAMP in Synechocystis., (Copyright © 2022 Elsevier B.V. All rights reserved.)

An acid pH in the lumen of chloroplast thylakoids is necessary in order to derive the required amount of CO2 to account for the observed rates of carbon fixation. We point out that the endosymbiotic derivation of the chloroplast from a cyanobacterium would have resulted in the lumen of the thylakoid having an acid pH. The thylakoids of cyanobacteria are continuous with the plasma membrane, resulting in the lumen of the thylakoid being open to the outside of the cell. Endosymbiosis resulted in the cyanobacterium being taken up into a food vacuole of a protozoan. The vacuole would have had an acid pH, probably around pH 5, so the endosymbiotic bacterium would have been surrounded by an environment with an acidic pH. The lumen of the thylakoids would have been at an acid pH since they were open to the exterior of the cell, and to the contents of the vacuole. [ABSTRACT FROM AUTHOR]

We have assessed the effect of vanadate as an inhibitor of plasma-membrane ATPase on photosynthesis and the ATP/ADP ratio in Chlamydomonas reinhardtii CW-92 (a mutant strain lacking a cell wall). This effect was compared in low-CO-adapted cells grown in media bubbled with air containing 400 or 70 μL · L CO. Evidence is presented indicating that cells grown at 70 μL · L CO have a higher rate of photosynthetic O evolution than cells grown at 400 μL · L CO, at limiting carbon concentrations. Extracellular and intracellular carbonic-anhydrase activities were, however, similar in cells grown in both of the low-carbon conditions. Vanadate inhibited, to a different extent, the HCO-dependent O evolution in cells grown at 400 and 70 μL · L CO. At 400 μM vanadate, inhibition reached 70-75% in cells grown at 400 μL · L but only 50% in those grown at 70 μL · L CO. The ATP/ADP ratios determined with and without vanadate at limiting concentrations of dissolved inorganic carbon indicated that more ATP was hydrolysed in algae grown at 70 μL · L than in those grown at 400 μL · L CO. We conclude that the maximal capacity to accumulate dissolved inorganic carbon is inversely related to the CO concentration in the medium. Activation and - or synthesis of vanadate-sensitive ATPase may be the major explanation for the higher capacity for HCO-dependent O evolution in cells grown under limited CO concentrations. [ABSTRACT FROM AUTHOR]

In reef-building corals (order Scleractinia) and giant clams (phylum Molluca), V-type H + -ATPase (VHA) in host cells is part of a carbon concentrating mechanism (CCM) that regulates photosynthetic rates of their symbiotic algae. Here, we show that VHA plays a similar role in the sea anemone Anemonia majano , a member of the order Actinaria and sister group to the Scleractinia, which in contrast to their colonial calcifying coral relatives is a solitary, soft-bodied taxa. Western blotting and immunofluorescence revealed that VHA was abundantly present in the host-derived symbiosome membrane surrounding the photosymbionts. Pharmacological inhibition of VHA activity in individual anemones resulted in an approximately 80% decrease of photosynthetic O 2 production. These results extend the presence of a host-controlled VHA-dependent CCM to non-calcifying cnidarians of the order Actiniaria, suggesting it is widespread among photosymbiosis between aquatic invertebrates and Symbiodiniaceae algae., (© 2022 The Authors.)

In algae, it is well established that the pyrenoid, a component of the carbon-concentrating mechanism (CCM), is essential for efficient photosynthesis at low CO 2 . However, the signal that triggers the formation of the pyrenoid has remained elusive. Here, we show that, in Chlamydomonas reinhardtii , the pyrenoid is strongly induced by hyperoxia, even at high CO 2 or bicarbonate levels. These results suggest that the pyrenoid can be induced by a common product of photosynthesis specific to low CO 2 or hyperoxia. Consistent with this view, the photorespiratory by-product, H 2 O 2 , induced the pyrenoid, suggesting that it acts as a signal. Finally, we show evidence for linkages between genetic variations in hyperoxia tolerance, H 2 O 2 signaling, and pyrenoid morphologies., Competing Interests: PN, JT, OT, JB, NN, EP, BL, SW, AW, BS, GM, MF, DH No competing interests declared, JW Joseph Weissman is affiliated with ExxonMobil. The author has no financial interests to declare, DK Reviewing editor, eLife, (© 2021, Neofotis et al.)

Microalgae are promising alternative feedstocks that can be used to produce biofuels and replace conventional fossil fuels. In order to increase the lipid production of microalgae, strain development via genetic engineering is required. To date, metabolic engineering studies have mainly focused on lipid synthesis pathways. However, the complicated metabolic pathways that exist in many organelles along with limitations of genetic engineering tools have made it difficult to obtain desirable microalgal strains. As an alternative strategy, we tried to increase carbon flux into fatty acid synthesis by overexpressing the NADP-dependent malic enzyme, NsME1, in Nannochloropsis salina. We found that the biomass and fatty acid methyl ester (FAME) contents were increased in NsME1 overexpressing transformants, such that the FAME yield of the top-producing NsME1 transformant was 53% higher than the wild type. To understand the effects of NsME1 on lipid production, we analyzed the total carbon concentration and NADPH/NADP ratio, which were found to be enhanced in the transformants. We also investigated mRNA expression levels of genes involved in C4-like carbon concentrating mechanism and fatty acid synthesis by quantitative real-time PCR, and confirmed their positive contribution to fatty acid production. Taken together, our results demonstrate that overexpression of NsME1 could simultaneously improve the carbon concentration and reducing power in cells, thereby increasing the lipid and FAME yields of Nannochloropsis salina. We suggest that NsME1 can serve as a promising genetic target for enhancing lipid-based bio-products in Nannochloropsis and potentially other industrial microalgae. • NsME1 overexpression affected total carbon concentration in N. salina culture. • NsME1 overexpression increased NADPH/NADP ratio. • NsME1 overexpression increased dry cell weight and fatty acid contents. [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]

Hygrophila polysperma is a heterophyllous amphibious plant. The growth of H. polysperma in submerged conditions is challenging due to the low CO 2 environment, increased resistance to gas diffusion, and bicarbonate ion (HCO 3 - ) being the dominant dissolved inorganic carbon source. The submerged leaves of H. polysperma have significantly higher rates of underwater photosynthesis compared with the terrestrial leaves. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonate (DIDS), an anion exchanger protein inhibitor, and ethoxyzolamide (EZ), an inhibitor of internal carbonic anhydrase, repressed underwater photosynthesis by the submerged leaves. These results suggested that H. polysperma acclimates to the submerged condition by using HCO 3 - for photosynthesis. H. polysperma transports HCO 3 - into the leaf by a DIDS-sensitive HCO 3 - transporter and converted to CO 2 by carbonic anhydrase. Additionally, proteome analysis revealed that submerged leaves accumulated fewer proteins associated with C4 photosynthesis compared with terrestrial leaves. This finding suggested that H. polysperma is capable of C4 and C3 photosynthesis in the terrestrial and submerged leaves, respectively. The ratio of phosphoenol pyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the submerged leaves was less than that in the terrestrial leaves. Upon anatomical observation, the terrestrial leaves exhibited a phenotype similar to the Kranz anatomy found among C4 plants; however, chloroplasts in the bundle sheath cells were not located adjacent to the vascular bundles, and the typical Kranz anatomy was absent in submerged leaves. These results suggest that H. polysperma performs proto-Kranz type photosynthesis in a terrestrial environment and shifts from a proto-Kranz type in terrestrial leaves to a HCO 3 - use photosynthesis in the submerged environments., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Horiguchi, Matsumoto, Nemoto, Inokuchi and Hirotsu.)

C 2 photosynthesis is a carbon concentrating mechanism that can increase net CO 2 assimilation by capturing, concentrating and re-assimilating CO 2 released by photorespiration. Empirical and modelling studies indicate that C 2 plants assimilate more carbon than C 3 plants under high temperature, bright light, and low CO 2 conditions. I argue that engineering C 2 photosynthesis into C 3 crops is a promising approach to improve photosynthetic performance under these - and temporally heterogeneous - environments, and review the modifications that may re-create a C 2 phenotype in C 3 plants. Although a C 2 engineering program would encounter many of the same challenges faced by C 4 engineering programmes, the simpler leaf anatomical requirements make C 2 engineering a feasible approach to improve crops in the medium term., (© 2020 The Author New Phytologist © 2020 New Phytologist Foundation.)