The carbon concentrating mechanism in Chlamydomonas reinhardtii: finding the missing pieces

Highly Active Carbonic Anhydrase of the Thylakoid Lumen of Chlamydomonas reinhardtii

The green unicellular algae Chlamydomonas reinhardtii contains 12-13 carbonic anhydrases (CAs). For a long time, the two closely related α-CAs of the periplasmic membrane CAH1 and CAH2 were considered to be the CAs with the highest CO2 hydration activity. The recombinant protein α-CA CAH3 (rCAH3) from the thylakoid lumen obtained in the present study showed more than three times higher activity compared to CAH1 and more than 11 times higher compared to previous studies with rCAH3. Long-term sustainability of the enzyme was observed at alkaline pH (>8), with maintenance of half of its activity at 4 °C for up to 50 days. Thermostability of rCAH3 indicated the retention of the activity at 20 °C for one hour at pH 9-10 with its ~50% decrease at pH 6-7. However, the residual activity of rCAH3 after incubation at an extremely high temperature (75 °C) for 15 min led to the formation of the double-hump graph with maxima at pH 6 and 9. The enzyme demonstrated high sensitivity to ethoxyzolamide and acetazolamide at nM concentrations, to Zn2+ and Cu2+ cations at 1 mM concentrations, and L-cysteine was able to completely inhibit CA activity of rCAH3 through reduction of sulfhydryl groups. Esterase activity of rCAH3 was well detected with values comparable to those of bovine CAII, but with a maximum at pH 8 instead of pH 9, which is usual for bovine CAII. The results indicated that CAH3 may be the most active CA of C. reinhardtii and that its role in the photosynthetic apparatus function could have been underestimated in previous works.

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

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

Adapting from Low to High: An Update to CO2-Concentrating Mechanisms of Cyanobacteria and Microalgae

The intracellular accumulation of inorganic carbon (C_i) by microalgae and cyanobacteria under ambient atmospheric CO₂ levels was first documented in the 80s of the 20th Century. Hence, a third variety of the CO₂-concentrating mechanism (CCM), acting in aquatic photoautotrophs with the C₃ photosynthetic pathway, was revealed in addition to the then-known schemes of CCM, functioning in CAM and C₄ higher plants. Despite the low affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of microalgae and cyanobacteria for the CO₂ substrate and low CO₂/O₂ specificity, CCM allows them to perform efficient CO₂ fixation in the reductive pentose phosphate (RPP) cycle. CCM is based on the coordinated operation of strategically located carbonic anhydrases and CO₂/HCO₃^- uptake systems. This cooperation enables the intracellular accumulation of HCO₃^-, which is then employed to generate a high concentration of CO₂ molecules in the vicinity of Rubisco's active centers compensating up for the shortcomings of enzyme features. CCM functions as an add-on to the RPP cycle while also acting as an important regulatory link in the interaction of dark and light reactions of photosynthesis. This review summarizes recent advances in the study of CCM molecular and cellular organization in microalgae and cyanobacteria, as well as the fundamental principles of its functioning and regulation.

Urea as a source of nitrogen and carbon leads to increased photosynthesis rates in Chlamydomonas reinhardtii under mixotrophy

Microalgae is a potential source of bioproducts, including feedstock to biofuels. Urea has been pointed as potential N source for microalgae growth. Considering that urea metabolism releases HCO₃^- to the medium, we tested the hypothesis that this carbon source could improve photosynthesis and consequently growth rates of Chlamydomonas reinhardtii. In this sense, the metabolic responses of C. reinhardtii grown with ammonium and urea as nitrogen sources under mixotrophic and autotrophic conditions were investigated. Overall, the mixotrophy led to increased cell growth as well as to a higher accumulation of lipids independent of N source, followed by a decrease in photosynthesis over the growth phases. In mixotrophy, urea stimulates growth in terms of cell number and dry weight. Furthermore, higher photosynthesis was verified in late logarithmic phase compared to ammonium. Under autotrophy conditions, although cell number and biomass were reduced, there was higher production of starch independent of N source. Nonetheless, urea-based autotrophic treatments stimulated biomass production compared to ammonium-based treatment. Under mixotrophy higher input of carbon into the cell from acetate and urea optimized photosynthesis and consequently promoted cell growth. Together, these results suggest urea as alternative source of carbon, improving photosynthesis and cell growth in C. reinhardtii.

Role of Microalgae in Global CO2 Sequestration: Physiological Mechanism, Recent Development, Challenges, and Future Prospective

The rising concentration of global atmospheric carbon dioxide (CO2) has severely affected our planet’s homeostasis. Efforts are being made worldwide to curb carbon dioxide emissions, but there is still no strategy or technology available to date that is widely accepted. Two basic strategies are employed for reducing CO2 emissions, viz. (i) a decrease in fossil fuel use, and increased use of renewable energy sources; and (ii) carbon sequestration by various biological, chemical, or physical methods. This review has explored microalgae’s role in carbon sequestration, the physiological apparatus, with special emphasis on the carbon concentration mechanism (CCM). A CCM is a specialized mechanism of microalgae. In this process, a sub-cellular organelle known as pyrenoid, containing a high concentration of Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), helps in the fixation of CO2. One type of carbon concentration mechanism in Chlamydomonas reinhardtii and the association of pyrenoid tubules with thylakoids membrane is represented through a typical graphical model. Various environmental factors influencing carbon sequestration in microalgae and associated techno-economic challenges are analyzed critically.

Loss of carbonic anhydrase in the thylakoid lumen causes unusual moderate-light-induced rearrangement of the chloroplast in Chlamydomonas reinhardtii as a way of photosystem II photoprotection

Chlamydomonas reinhardtii cells have a single large cup-shaped chloroplast that can lose lobes under high light to prevent photodamage of the photosynthetic apparatus, including photosystem II (PSII). Here, under moderate light treatment, the development of the unusual morphology of the chloroplast is shown for mutant cia3, which is deficient in carbonic anhydrase (EC 4.2.1.1) CAH3 in the thylakoid lumen, while such light intensity is harmless for wild type (WT) cells for hours. Cia3 cells had more activated PSII photoprotective mechanisms and showed a tendency to shift in the balance of the PSII damage-repair cycle, whereas PSII retained the same photosynthetic efficiency as in the WT. These findings allow speculation about the unique PSII photoprotection strategy by rearranging the chloroplast in the absence of CAH3. CAH3, in turn, is suggested to be an important participant of the C. reinhardtii photosynthetic apparatus operation, functioning in close connection with PSII.

Responses of macroalgae to CO2 enrichment cannot be inferred solely from their inorganic carbon uptake strategy

Increased plant biomass is observed in terrestrial systems due to rising levels of atmospheric CO2, but responses of marine macroalgae to CO2 enrichment are unclear. The 200% increase in CO2 by 2100 is predicted to enhance the productivity of fleshy macroalgae that acquire inorganic carbon solely as CO2 (non-carbon dioxide-concentrating mechanism [CCM] species-i.e., species without a carbon dioxide-concentrating mechanism), whereas those that additionally uptake bicarbonate (CCM species) are predicted to respond neutrally or positively depending on their affinity for bicarbonate. Previous studies, however, show that fleshy macroalgae exhibit a broad variety of responses to CO2 enrichment and the underlying mechanisms are largely unknown. This physiological study compared the responses of a CCM species (Lomentaria australis) with a non-CCM species (Craspedocarpus ramentaceus) to CO2 enrichment with regards to growth, net photosynthesis, and biochemistry. Contrary to expectations, there was no enrichment effect for the non-CCM species, whereas the CCM species had a twofold greater growth rate, likely driven by a downregulation of the energetically costly CCM(s). This saved energy was invested into new growth rather than storage lipids and fatty acids. In addition, we conducted a comprehensive literature synthesis to examine the extent to which the growth and photosynthetic responses of fleshy macroalgae to elevated CO2 are related to their carbon acquisition strategies. Findings highlight that the responses of macroalgae to CO2 enrichment cannot be inferred solely from their carbon uptake strategy, and targeted physiological experiments on a wider range of species are needed to better predict responses of macroalgae to future oceanic change.

Effects of microcompartmentation on flux distribution and metabolic pools in Chlamydomonas reinhardtii chloroplasts

Cells and organelles are not homogeneous but include microcompartments that alter the spatiotemporal characteristics of cellular processes. The effects of microcompartmentation on metabolic pathways are however difficult to study experimentally. The pyrenoid is a microcompartment that is essential for a carbon concentrating mechanism (CCM) that improves the photosynthetic performance of eukaryotic algae. Using Chlamydomonas reinhardtii, we obtained experimental data on photosynthesis, metabolites, and proteins in CCM-induced and CCM-suppressed cells. We then employed a computational strategy to estimate how fluxes through the Calvin-Benson cycle are compartmented between the pyrenoid and the stroma. Our model predicts that ribulose-1,5-bisphosphate (RuBP), the substrate of Rubisco, and 3-phosphoglycerate (3PGA), its product, diffuse in and out of the pyrenoid, respectively, with higher fluxes in CCM-induced cells. It also indicates that there is no major diffusional barrier to metabolic flux between the pyrenoid and stroma. Our computational approach represents a stepping stone to understanding microcompartmentalized CCM in other organisms.

Ion and metabolite transport in the chloroplast of algae: lessons from land plants

Chloroplasts are endosymbiotic organelles and play crucial roles in energy supply and metabolism of eukaryotic photosynthetic organisms (algae and land plants). They harbor channels and transporters in the envelope and thylakoid membranes, mediating the exchange of ions and metabolites with the cytosol and the chloroplast stroma and between the different chloroplast subcompartments. In secondarily evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Here, we provide an overview of the current knowledge about ion and metabolite transport in the chloroplast from algae. The main aspects reviewed are localization and activity of the transport proteins from algae and/or of homologues from other organisms including land plants. Most chloroplast transporters were identified in the green alga Chlamydomonas reinhardtii, reside in the envelope and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play role in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and roles in the chloroplast are suggested. A deep knowledge in this field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.

Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead

Bicarbonate plays a fundamental role in the cell pH status in all organisms. In autotrophs, HCO₃− may further contribute to carbon concentration mechanisms (CCM). This is especially relevant in the CO₂-poor habitats of cyanobacteria, aquatic microalgae, and macrophytes. Photosynthesis of terrestrial plants can also benefit from CCM as evidenced by the evolution of C₄ and Crassulacean Acid Metabolism (CAM). The presence of HCO₃− in all organisms leads to more questions regarding the mechanisms of uptake and membrane transport in these different biological systems. This review aims to provide an overview of the transport and metabolic processes related to HCO₃− in microalgae, macroalgae, seagrasses, and terrestrial plants. HCO₃− transport in cyanobacteria and human cells is much better documented and is included for comparison. We further comment on the metabolic roles of HCO₃− in plants by focusing on the diversity and functions of carbonic anhydrases and PEP carboxylases as well as on the signaling role of CO₂/HCO₃− in stomatal guard cells. Plant responses to excess soil HCO₃− is briefly addressed. In conclusion, there are still considerable gaps in our knowledge of HCO₃− uptake and transport in plants that hamper the development of breeding strategies for both more efficient CCM and better HCO₃− tolerance in crop plants.

Pyrenoid functions revealed by proteomics in Chlamydomonas reinhardtii

Organelles are intracellular compartments which are themselves compartmentalized. Biogenic and metabolic processes are localized to specialized domains or microcompartments to enhance their efficiency and suppress deleterious side reactions. An example of intra-organellar compartmentalization is the pyrenoid in the chloroplasts of algae and hornworts. This microcompartment enhances the photosynthetic CO₂-fixing activity of the Calvin-Benson cycle enzyme Rubisco, suppresses an energetically wasteful oxygenase activity of Rubisco, and mitigates limiting CO₂ availability in aquatic environments. Hence, the pyrenoid is functionally analogous to the carboxysomes in cyanobacteria. However, a comprehensive analysis of pyrenoid functions based on its protein composition is lacking. Here we report a proteomic characterization of the pyrenoid in the green alga Chlamydomonas reinhardtii. Pyrenoid-enriched fractions were analyzed by quantitative mass spectrometry. Contaminant proteins were identified by parallel analyses of pyrenoid-deficient mutants. This pyrenoid proteome contains 190 proteins, many of which function in processes that are known or proposed to occur in pyrenoids: e.g. the carbon concentrating mechanism, starch metabolism or RNA metabolism and translation. Using radioisotope pulse labeling experiments, we show that pyrenoid-associated ribosomes could be engaged in the localized synthesis of the large subunit of Rubisco. New pyrenoid functions are supported by proteins in tetrapyrrole and chlorophyll synthesis, carotenoid metabolism or amino acid metabolism. Hence, our results support the long-standing hypothesis that the pyrenoid is a hub for metabolism. The 81 proteins of unknown function reveal candidates for new participants in these processes. Our results provide biochemical evidence of pyrenoid functions and a resource for future research on pyrenoids and their use to enhance agricultural plant productivity. Data are available via ProteomeXchange with identifier PXD004509.

Enhancing growth-relevant metabolic pathways of Arthrospira platensis (CYA-1) with gamma irradiation from 60Co

The biomass yield of Arthrospira mutant ZJU9000 was 176% higher than that of wild type on day 4, and the results of transcriptome sequencing showed that processes related to cell growth were synergistically enhanced in this mutant. The amount of energy for biomass accumulation increased because the efficiency of the photoreaction was enhanced by the elevated levels of chlorophyll a and carotene. The increased biosynthesis rates of ribose phosphate, nucleotides and multiple vitamins increased the production of genetic materials for cell proliferation. Furthermore, the carbon concentration mechanism in mutant ZJU9000 was enhanced, indicating the increased utilization efficiency of CO₂ at low concentration (0.04 vol% in air). The enhancement of these growth-relevant metabolic pathways contributed to the robust growth of Arthrospira mutant ZJU9000.

Growth-relevant metabolic pathways of Arthrospira enhanced after gamma irradiation.

Identification and characterization of a solute carrier, CIA8, involved in inorganic carbon acclimation in Chlamydomonas reinhardtii

The supply of inorganic carbon (Ci) at the site of fixation by Rubisco is a key parameter for efficient CO2 fixation in aquatic organisms including the green alga, Chlamydomonas reinhardtii. Chlamydomonas reinhardtii cells, when grown on limiting CO2, have a CO2-concentrating mechanism (CCM) that functions to concentrate CO2 at the site of Rubisco. Proteins thought to be involved in inorganic carbon uptake have been identified and localized to the plasma membrane or chloroplast envelope. However, current CCM models suggest that additional molecular components are involved in Ci uptake. In this study, the gene Cia8 was identified in an insertional mutagenesis screen and characterized. The protein encoded by Cia8 belongs to the sodium bile acid symporter subfamily. Transcript levels for this gene were significantly up-regulated when the cells were grown on low CO2. The cia8 mutant exhibited reduced growth and reduced affinity for Ci when grown in limiting CO2 conditions. Prediction programs localize this protein to the chloroplast. Ci uptake and the photosynthetic rate, particularly at high external pH, were reduced in the mutant. The results are consistent with the model that CIA8 is involved in Ci uptake in C. reinhardtii.

The potential for co-evolution of CO2-concentrating mechanisms and Rubisco in diatoms

Diatoms are a diverse group of unicellular algae that contribute significantly to global photosynthetic carbon fixation and export in the modern ocean, and are an important source of microfossils for paleoclimate reconstructions. Because of their importance in the environment, diatoms have been a focus of study on the physiology and ecophysiology of carbon fixation, in particular their CO2-concentrating mechanisms (CCMs) and Rubisco characteristics. While carbon fixation in diatoms is not as well understood as in certain model aquatic photoautotrophs, a greater number of species have been examined in diatoms. Recent work has highlighted a large diversity in the function, physiology, and kinetics of both the CCM and Rubisco between different diatom species. This diversity was unexpected since it has generally been assumed that CCMs and Rubiscos were similar within major algal lineages as the result of selective events deep in evolutionary history, and suggests a more recent co-evolution between the CCM and Rubisco within diatoms. This review explores our current understanding of the diatom CCM and highlights the diversity of both the CCM and Rubisco kinetics. We will suggest possible environmental, physiological, and evolutionary drivers for the co-evolution of the CCM and Rubisco in diatoms.

Pyrenoid loss in Chlamydomonas reinhardtii causes limitations in CO2 supply, but not thylakoid operating efficiency

The pyrenoid of the unicellular green alga Chlamydomonas reinhardtii is a microcompartment situated in the centre of the cup-shaped chloroplast, containing up to 90% of cellular Rubisco. Traversed by a network of dense, knotted thylakoid tubules, the pyrenoid has been proposed to influence thylakoid biogenesis and ultrastructure. Mutants that are unable to assemble a pyrenoid matrix, due to expressing a vascular plant version of the Rubisco small subunit, exhibit severe growth and photosynthetic defects and have an ineffective carbon-concentrating mechanism (CCM). The present study set out to determine the cause of photosynthetic limitation in these pyrenoid-less lines. We tested whether electron transport and light use were compromised as a direct structural consequence of pyrenoid loss or as a metabolic effect downstream of lower CCM activity and resulting CO2 limitation. Thylakoid organization was unchanged in the mutants, including the retention of intrapyrenoid-type thylakoid tubules, and photosynthetic limitations associated with the absence of the pyrenoid were rescued by exposing cells to elevated CO2 levels. These results demonstrate that Rubisco aggregation in the pyrenoid functions as an essential element for CO2 delivery as part of the CCM, and does not play other roles in maintenance of photosynthetic membrane energetics.

The diversity of CO2-concentrating mechanisms in marine diatoms as inferred from their genetic content

Marine diatoms are one of the most ecologically significant primary producers in the ocean. Most diatoms use a CO2-concentrating mechanism (CCM) to overcome the scarcity of CO2 in the ocean and limitations of the carbon-fixing enzyme Rubisco. However, the CCMs in model diatoms differ substantially in their genetic make-up and structural organization. To assess the extent of CCM diversity in marine diatoms more generally, we analyzed genome and transcriptome data from 31 diatom strains to identify putative CCM genes, examine the overall CCM architecture, and study CCM development in the context of the evolutionary history of these diatoms. Key CCM genes [carbonic anhydrases (CAs) and solute carrier 4 (SLC4) bicarbonate transporters] identified in the diatoms were placed into groups of likely orthologs by sequence similarity (OrthoMCL) and phylogenetic methods. These analyses indicated that diatoms seem to share similar HCO3- transporters, but possess a variety of CAs that have either undergone extensive diversification within the diatom lineage or have been acquired through horizontal gene transfer. Hierarchical clustering of the diatom species based on their CCM gene content suggests that CCM development is largely congruent with evolution of diatom species, despite some notable differences in CCM genes even among closely related species.

A robust protocol for efficient generation, and genomic characterization of insertional mutants of Chlamydomonas reinhardtii

BACKGROUND

Random insertional mutagenesis of Chlamydomonas reinhardtii using drug resistance cassettes has contributed to the generation of tens of thousands of transformants in dozens of labs around the world. In many instances these insertional mutants have helped elucidate the genetic basis of various physiological processes in this model organism. Unfortunately, the insertion sites of many interesting mutants are never defined due to experimental difficulties in establishing the location of the inserted cassette in the Chlamydomonas genome. It is fairly common that several months, or even years of work are conducted with no result. Here we describe a robust method to identify the location of the inserted DNA cassette in the Chlamydomonas genome.

RESULTS

Insertional mutants were generated using a DNA cassette that confers paromomycin resistance. This protocol identified the cassette insertion site for greater than 80% of the transformants. In the majority of cases the insertion event was found to be simple, without large deletions of flanking genomic DNA. Multiple insertions were observed in less than 10% of recovered transformants.

CONCLUSION

The method is quick, relatively inexpensive and does not require any special equipment beyond an electroporator. The protocol was tailored to ensure that the sequence of the Chlamydomonas genomic DNA flanking the random insertion is consistently obtained in a high proportion of transformants. A detailed protocol is presented to aid in the experimental design and implementation of mutant screens in Chlamydomonas.

Early alterations on photosynthesis-related parameters in Chlamydomonas reinhardtii cells exposed to atrazine: A multiple approach study

Chlamydomonas reinhardtii cells were exposed to a sublethal concentration of the widespread herbicide atrazine for 3h. Physiological cellular parameters, such as chlorophyll a fluorescence and oxidative stress monitored by flow cytometry and pigments levels were altered in microalgal cells exposed to 0.25 μM of atrazine. Furthermore, the effects of this herbicide on C. reinhardtii were explored using "omics" techniques. Transcriptomic analyses, carried out by RNA-Seq technique, displayed 9 differentially expressed genes, related to photosynthesis, between control cultures and atrazine exposed cultures. Proteomic profiles were obtained using iTRAQ tags and MALDI-MS/MS analysis, identifying important changes in the proteome during atrazine stress; 5 proteins related to photosynthesis were downexpressed. The results of these experiments advance the understanding of photosynthetic adjustments that occur during an early herbicide exposure. Inhibition of photosynthesis induced by atrazine toxicity will affect the entire physiological and biochemical states of microalgal cells.

The ins and outs of CO2

It is difficult to distinguish influx and efflux of inorganic C in photosynthesizing tissues; this article examines what is known and where there are gaps in knowledge. Irreversible decarboxylases produce CO2, and CO2 is the substrate/product of enzymes that act as carboxylases and decarboxylases. Some irreversible carboxylases use CO2; others use HCO3(-). The relative role of permeation through the lipid bilayer versus movement through CO2-selective membrane proteins in the downhill, non-energized, movement of CO2 is not clear. Passive permeation explains most CO2 entry, including terrestrial and aquatic organisms with C3 physiology and biochemistry, terrestrial C4 plants and all crassulacean acid metabolism (CAM) plants, as well as being part of some mechanisms of HCO3(-) use in CO2 concentrating mechanism (CCM) function, although further work is needed to test the mechanism in some cases. However, there is some evidence of active CO2 influx at the plasmalemma of algae. HCO3(-) active influx at the plasmalemma underlies all cyanobacterial and some algal CCMs. HCO3(-) can also enter some algal chloroplasts, probably as part of a CCM. The high intracellular CO2 and HCO3(-) pools consequent upon CCMs result in leakage involving CO2, and occasionally HCO3(-). Leakage from cyanobacterial and microalgal CCMs involves up to half, but sometimes more, of the gross inorganic C entering in the CCM; leakage from terrestrial C4 plants is lower in most environments. Little is known of leakage from other organisms with CCMs, though given the leakage better-examined organisms, leakage occurs and increases the energetic cost of net carbon assimilation.

The CO2 concentrating mechanism and photosynthetic carbon assimilation in limiting CO2 : how Chlamydomonas works against the gradient

The CO2 concentrating mechanism (CCM) represents an effective strategy for carbon acquisition that enables microalgae to survive and proliferate when the CO2 concentration limits photosynthesis. The CCM improves photosynthetic performance by raising the CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), simultaneously enhancing carbon fixation and suppressing photorespiration. Active inorganic carbon (Ci) uptake, Rubisco sequestration and interconversion between different Ci species catalyzed by carbonic anhydrases (CAs) are key components in the CCM, and an array of molecular regulatory elements is present to facilitate the sensing of CO2 availability, to regulate the expression of the CCM and to coordinate interplay between photosynthetic carbon metabolism and other metabolic processes in response to limiting CO2 conditions. This review intends to integrate our current understanding of the eukaryotic algal CCM and its interaction with carbon assimilation, based largely on Chlamydomonas as a model, and to illustrate how Chlamydomonas acclimates to limiting CO2 conditions and how its CCM is regulated.

Impacts of CO2 concentration on growth, lipid accumulation, and carbon-concentrating-mechanism-related gene expression in oleaginous Chlorella

Biodiesel production by microalgae with photosynthetic CO2 biofixation is thought to be a feasible way in the field of bioenergy and carbon emission reduction. Knowledge of the carbon-concentrating mechanism plays an important role in improving microalgae carbon fixation efficiency. However, little information is available regarding the dramatic changes of cells suffered upon different environmental factors, such as CO2 concentration. The aim of this study was to investigate the growth, lipid accumulation, carbon fixation rate, and carbon metabolism gene expression under different CO2 concentrations in oleaginous Chlorella. It was found that Chlorella pyrenoidosa grew well under CO2 concentrations ranging from 1 to 20 %. The highest biomass and lipid productivity were 4.3 g/L and 107 mg/L/day under 5 % CO2 condition. Switch from high (5 %) to low (0.03 %, air) CO2 concentration showed significant inhibitory effect on growth and CO2 fixation rate. The amount of the saturated fatty acids was increased obviously along with the transition. Low CO2 concentration (0.03 %) was suitable for the accumulation of saturated fatty acids. Reducing the CO2 concentration could significantly decrease the polyunsaturated degree in fatty acids. Moreover, the carbon-concentrating mechanism-related gene expression revealed that most of them, especially CAH2, LCIB, and HLA3, had remarkable change after 1, 4, and 24 h of the transition, which suggests that Chlorella has similar carbon-concentrating mechanism with Chlamydomonas reinhardtii. The findings of the present study revealed that C. pyrenoidosa is an ideal candidate for mitigating CO2 and biodiesel production and is appropriate as a model for mechanism research of carbon sequestration.

Ions channels/transporters and chloroplast regulation

Ions play fundamental roles in all living cells and their gradients are often essential to fuel transports, to regulate enzyme activities and to transduce energy within and between cells. Their homeostasis is therefore an essential component of the cell metabolism. Ions must be imported from the extracellular matrix to their final subcellular compartments. Among them, the chloroplast is a particularly interesting example because there, ions not only modulate enzyme activities, but also mediate ATP synthesis and actively participate in the building of the photosynthetic structures by promoting membrane-membrane interaction. In this review, we first provide a comprehensive view of the different machineries involved in ion trafficking and homeostasis in the chloroplast, and then discuss peculiar functions exerted by ions in the frame of photochemical conversion of absorbed light energy.

CCM8: the eighth international symposium on inorganic carbon uptake by aquatic photosynthetic organisms

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 CO2 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.