Molecular and Structural Changes in Chlamydomonas under Limiting CO2 (A Possible Mitochondrial Role in Adaptation)

Dramatic Changes in Mitochondrial Subcellular Location and Morphology Accompany Activation of the CO2 Concentrating Mechanism

Dynamic changes in intracellular ultrastructure can be critical for the ability of organisms to acclimate to environmental conditions. Microalgae, which are responsible for ~50% of global photosynthesis, compartmentalize their Rubisco into a specialized structure known as the pyrenoid when the cells experience limiting CO2 conditions; this compartmentalization appears to be a component of the CO2 Concentrating Mechanism (CCM), which facilitates photosynthetic CO2 fixation as environmental levels of inorganic carbon (Ci) decline. Changes in the spatial distribution of mitochondria in green algae have also been observed under CO2 limiting conditions, although a role for this reorganization in CCM function remains unclear. We used the green microalgae Chlamydomonas reinhardtii to monitor changes in the position and ultrastructure of mitochondrial membranes as cells transition between high CO2 (HC) and Low/Very Low CO2 (LC/VLC). Upon transferring cells to VLC, the mitochondria move from a central to a peripheral location, become wedged between the plasma membrane and chloroplast envelope, and mitochondrial membranes orient in parallel tubular arrays that extend from the cell's apex to its base. We show that these ultrastructural changes require protein and RNA synthesis, occur within 90 min of shifting cells to VLC conditions, correlate with CCM induction and are regulated by the CCM master regulator CIA5. The apico-basal orientation of the mitochondrial membrane, but not the movement of the mitochondrion to the cell periphery, is dependent on microtubules and the MIRO1 protein, which is involved in membrane-microtubule interactions. Furthermore, blocking mitochondrial electron transport in VLC acclimated cells reduces the cell's affinity for inorganic carbon. Overall, our results suggest that CIA5-dependent mitochondrial repositioning/reorientation functions in integrating cellular architecture and energetics with CCM activities and invite further exploration of how intracellular architecture can impact fitness under dynamic environmental conditions.

Energy crosstalk between photosynthesis and the algal CO2-concentrating mechanisms

Microalgal photosynthesis is responsible for nearly half of the CO2 annually captured by Earth's ecosystems. In aquatic environments where the CO2 availability is low, the CO2-fixing efficiency of microalgae greatly relies on mechanisms - called CO2-concentrating mechanisms (CCMs) - for concentrating CO2 at the catalytic site of the CO2-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While the transport of inorganic carbon (Ci) across membrane bilayers against a concentration gradient consumes part of the chemical energy generated by photosynthesis, the bioenergetics and cellular mechanisms involved are only beginning to be elucidated. Here, we review the current knowledge relating to the energy requirement of CCMs in the light of recent advances in photosynthesis regulatory mechanisms and the spatial organization of CCM components.

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.

Intelligent image-activated sorting of Chlamydomonas reinhardtii by mitochondrial localization

Organelle positioning in cells is associated with various metabolic functions and signaling in unicellular organisms. Specifically, the microalga Chlamydomonas reinhardtii repositions its mitochondria, depending on the levels of inorganic carbon. Mitochondria are typically randomly distributed in the Chlamydomonas cytoplasm, but relocate toward the cell periphery at low inorganic carbon levels. This mitochondrial relocation is linked with the carbon-concentrating mechanism, but its significance is not yet thoroughly understood. A genotypic understanding of this relocation would require a high-throughput method to isolate rare mutant cells not exhibiting this relocation. However, this task is technically challenging due to the complex intracellular morphological difference between mutant and wild-type cells, rendering conventional non-image-based high-event-rate methods unsuitable. Here, we report our demonstration of intelligent image-activated cell sorting by mitochondrial localization. Specifically, we applied an intelligent image-activated cell sorting system to sort for C. reinhardtii cells displaying no mitochondrial relocation. We trained a convolutional neural network (CNN) to distinguish the cell types based on the complex morphology of their mitochondria. The CNN was employed to perform image-activated sorting for the mutant cell type at 180 events per second, which is 1-2 orders of magnitude faster than automated microscopy with robotic pipetting, resulting in an enhancement of the concentration from 5% to 56.5% corresponding to an enrichment factor of 11.3. These results show the potential of image-activated cell sorting for connecting genotype-phenotype relations for rare-cell populations, which require a high throughput and could lead to a better understanding of metabolic functions in cells.

Mitochondrial carbonic anhydrases are needed for optimal photosynthesis at low CO2 levels in Chlamydomonas

Chlamydomonas reinhardtii can grow photosynthetically using CO2 or in the dark using acetate as the carbon source. In the light in air, the CO2 concentrating mechanism (CCM) of C. reinhardtii accumulates CO2, enhancing photosynthesis. A combination of carbonic anhydrases (CAs) and bicarbonate transporters in the CCM of C. reinhardtii increases the CO2 concentration at Ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco) in the chloroplast pyrenoid. Previously, CAs important to the CCM have been found in the periplasmic space, surrounding the pyrenoid and inside the thylakoid lumen. Two almost identical mitochondrial CAs, CAH4 and CAH5, are also highly expressed when the CCM is made, but their role in the CCM is not understood. Here, we adopted an RNAi approach to reduce the expression of CAH4 and CAH5 to study their possible physiological functions. RNAi mutants with low expression of CAH4 and CAH5 had impaired rates of photosynthesis under ambient levels of CO2 (0.04% CO2 [v/v] in air). These strains were not able to grow at very low CO2 (<0.02% CO2 [v/v] in air), and their ability to accumulate inorganic carbon (Ci = CO2 + HCO3-) was reduced. At low CO2 concentrations, the CCM is needed to both deliver Ci to Rubisco and to minimize the leak of CO2 generated by respiration and photorespiration. We hypothesize that CAH4 and CAH5 in the mitochondria convert the CO2 released from respiration and photorespiration as well as the CO2 leaked from the chloroplast to HCO3- thus "recapturing" this potentially lost CO2.

Orchestral manoeuvres in the light: crosstalk needed for regulation of the Chlamydomonas carbon concentration mechanism

The inducible carbon concentration mechanism (CCM) in Chlamydomonas reinhardtii has been well defined from a molecular and ultrastructural perspective. Inorganic carbon transport proteins, and strategically located carbonic anhydrases deliver CO2 within the chloroplast pyrenoid matrix where Rubisco is packaged. However, there is little understanding of the fundamental signalling and sensing processes leading to CCM induction. While external CO2 limitation has been believed to be the primary cue, the coupling between energetic supply and inorganic carbon demand through regulatory feedback from light harvesting and photorespiration signals could provide the original CCM trigger. Key questions regarding the integration of these processes are addressed in this review. We consider how the chloroplast functions as a crucible for photosynthesis, importing and integrating nuclear-encoded components from the cytoplasm, and sending retrograde signals to the nucleus to regulate CCM induction. We hypothesize that induction of the CCM is associated with retrograde signals associated with photorespiration and/or light stress. We have also examined the significance of common evolutionary pressures for origins of two co-regulated processes, namely the CCM and photorespiration, in addition to identifying genes of interest involved in transcription, protein folding, and regulatory processes which are needed to fully understand the processes leading to CCM induction.

Photosynthetic Carbon Partitioning and Metabolic Regulation in Response to Very-Low and High CO2 in Microchloropsis gaditana NIES 2587

Photosynthetic organisms fix inorganic carbon through carbon capture machinery (CCM) that regulates the assimilation and accumulation of carbon around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, few constraints that govern the central carbon metabolism are regulated by the carbon capture and partitioning machinery. In order to divert the cellular metabolism toward lipids and/or biorenewables it is important to investigate and understand the molecular mechanisms of the CO2-driven carbon partitioning. In this context, strategies for enhancement of CO2 fixation which will increase the overall biomass and lipid yields, can provide clues on understanding the carbon assimilation pathway, and may lead to new targets for genetic engineering in microalgae. In the present study, we have focused on the physiological and metabolomic response occurring within marine oleaginous microalgae Microchloropsis gaditana NIES 2587, under the influence of very-low CO2 (VLC; 300 ppm, or 0.03%) and high CO2 (HC; 30,000 ppm, or 3% v/v). Our results demonstrate that HC supplementation in M. gaditana channelizes the carbon flux toward the production of long chain polyunsaturated fatty acids (LC-PUFAs) and also increases the overall biomass productivities (up to 2.0 fold). Also, the qualitative metabolomics has identified nearly 31 essential metabolites, among which there is a significant fold change observed in accumulation of sugars and alcohols such as galactose and phytol in VLC as compared to HC. In conclusion, our focus is to understand the entire carbon partitioning and metabolic regulation within these photosynthetic cell factories, which will be further evaluated through multiomics approach for enhanced productivities of biomass, biofuels, and bioproducts (B3).

Carbon Supply and Photoacclimation Cross Talk in the Green Alga Chlamydomonas reinhardtii

Photosynthetic organisms are exposed to drastic changes in light conditions, which can affect their photosynthetic efficiency and induce photodamage. To face these changes, they have developed a series of acclimation mechanisms. In this work, we have studied the acclimation strategies of Chlamydomonas reinhardtii, a model green alga that can grow using various carbon sources and is thus an excellent system in which to study photosynthesis. Like other photosynthetic algae, it has evolved inducible mechanisms to adapt to conditions where carbon supply is limiting. We have analyzed how the carbon availability influences the composition and organization of the photosynthetic apparatus and the capacity of the cells to acclimate to different light conditions. Using electron microscopy, biochemical, and fluorescence measurements, we show that differences in CO2 availability not only have a strong effect on the induction of the carbon-concentrating mechanisms but also change the acclimation strategy of the cells to light. For example, while cells in limiting CO2 maintain a large antenna even in high light and switch on energy-dissipative mechanisms, cells in high CO2 reduce the amount of pigments per cell and the antenna size. Our results show the high plasticity of the photosynthetic apparatus of C. reinhardtii This alga is able to use various photoacclimation strategies, and the choice of which to activate strongly depends on the carbon availability.

Introducing an algal carbon-concentrating mechanism into higher plants: location and incorporation of key components

Many eukaryotic green algae possess biophysical carbon-concentrating mechanisms (CCMs) that enhance photosynthetic efficiency and thus permit high growth rates at low CO2 concentrations. They are thus an attractive option for improving productivity in higher plants. In this study, the intracellular locations of ten CCM components in the unicellular green alga Chlamydomonas reinhardtii were confirmed. When expressed in tobacco, all of these components except chloroplastic carbonic anhydrases CAH3 and CAH6 had the same intracellular locations as in Chlamydomonas. CAH6 could be directed to the chloroplast by fusion to an Arabidopsis chloroplast transit peptide. Similarly, the putative inorganic carbon (Ci) transporter LCI1 was directed to the chloroplast from its native location on the plasma membrane. CCP1 and CCP2 proteins, putative Ci transporters previously reported to be in the chloroplast envelope, localized to mitochondria in both Chlamydomonas and tobacco, suggesting that the algal CCM model requires expansion to include a role for mitochondria. For the Ci transporters LCIA and HLA3, membrane location and Ci transport capacity were confirmed by heterologous expression and H(14) CO3 (-) uptake assays in Xenopus oocytes. Both were expressed in Arabidopsis resulting in growth comparable with that of wild-type plants. We conclude that CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. As expression of individual Ci transporters did not enhance Arabidopsis growth, stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.

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.

Reciprocal regulation of protein synthesis and carbon metabolism for thylakoid membrane biogenesis

A subunit of the chloroplast pyruvate dehydrogenase complex, which serves as a metabolic enzyme, also has a dual function as an RNA-binding protein and influences mRNA translation.

Metabolic control of gene expression coordinates the levels of specific gene products to meet cellular demand for their activities. This control can be exerted by metabolites acting as regulatory signals and/or a class of metabolic enzymes with dual functions as regulators of gene expression. However, little is known about how metabolic signals affect the balance between enzymatic and regulatory roles of these dual functional proteins. We previously described the RNA binding activity of a 63 kDa chloroplast protein from Chlamydomonas reinhardtii, which has been implicated in expression of the psbA mRNA, encoding the D1 protein of photosystem II. Here, we identify this factor as dihydrolipoamide acetyltransferase (DLA2), a subunit of the chloroplast pyruvate dehydrogenase complex (cpPDC), which is known to provide acetyl-CoA for fatty acid synthesis. Analyses of RNAi lines revealed that DLA2 is involved in the synthesis of both D1 and acetyl-CoA. Gel filtration analyses demonstrated an RNP complex containing DLA2 and the chloroplast psbA mRNA specifically in cells metabolizing acetate. An intrinsic RNA binding activity of DLA2 was confirmed by in vitro RNA binding assays. Results of fluorescence microscopy and subcellular fractionation experiments support a role of DLA2 in acetate-dependent localization of the psbA mRNA to a translation zone within the chloroplast. Reciprocally, the activity of the cpPDC was specifically affected by binding of psbA mRNA. Beyond that, in silico analysis and in vitro RNA binding studies using recombinant proteins support the possibility that RNA binding is an ancient feature of dihydrolipoamide acetyltransferases. Our results suggest a regulatory function of DLA2 in response to growth on reduced carbon energy sources. This raises the intriguing possibility that this regulation functions to coordinate the synthesis of lipids and proteins for the biogenesis of photosynthetic membranes.

Metabolic control of gene expression coordinates the levels of specific gene products to meet cellular demand for their activities. This control can be exerted by metabolites acting as regulatory signals on a class of metabolic enzymes with dual functions as regulators of gene expression. However, little is known about how metabolic signals affect the balance between enzymatic and regulatory roles of these proteins. Here, we report an example of a protein with dual functions in gene expression and carbon metabolism. The chloroplast pyruvate dehydrogenase complex is well-known to produce activated di-carbon precursors for fatty acid, which is required for lipid synthesis. Our results show that a subunit of this enzyme forms ribonucleoprotein particles and influences chloroplast mRNA translation. Conversely, RNA binding affects pyruvate dehydrogenase (metabolic) activity. These findings offer insight into how intracellular metabolic signaling and gene expression are reciprocally regulated during membrane biogenesis. In addition, our results suggest that these dual roles of the protein might exist in evolutionary distant organisms ranging from cyanobacteria to humans.

The carbonic anhydrase isoforms of Chlamydomonas reinhardtii: intracellular location, expression, and physiological roles

Aquatic photosynthetic organisms, such as the green alga Chlamydomonas reinhardtii, respond to low CO(2) conditions by inducing a CO(2) concentrating mechanism (CCM). Carbonic anhydrases (CAs) are important components of the CCM. CAs are zinc-containing metalloenzymes that catalyze the reversible interconversion of CO(2) and HCO(3)(-). In C. reinhardtii, there are at least 12 genes that encode CA isoforms, including three alpha, six beta, and three gamma or gamma-like CAs. The expression of the three alpha and six beta genes has been measured from cells grown on elevated CO(2) (having no active CCM) versus cells growing on low levels of CO(2) (with an active CCM) using northern blots, differential hybridization to DNA chips and quantitative RT-PCR. Recent RNA-seq profiles add to our knowledge of the expression of all of the CA genes. In addition, protein content for some of the CA isoforms was estimated using antibodies corresponding to the specific CA isoforms: CAH1/2, CAH3, CAH4/5, CAH6, and CAH7. The intracellular location of each of the CA isoforms was elucidated using immunolocalization and cell fractionation techniques. Combining these results with previous studies using CA mutant strains, we will discuss possible physiological roles of the CA isoforms concentrating on how these CAs might contribute to the acquisition and retention of CO(2) in C. reinhardtii.

A metabolomic approach to study major metabolite changes during acclimation to limiting CO2 in Chlamydomonas reinhardtii

Using a gas chromatography-mass spectrometry-time of flight technique, we determined major metabolite changes during induction of the carbon-concentrating mechanism in the unicellular green alga Chlamydomonas reinhardtii. In total, 128 metabolites with significant differences between high- and low-CO(2)-grown cells were detected, of which 82 were wholly or partially identified, including amino acids, lipids, and carbohydrates. In a 24-h time course experiment, we show that the amino acids serine and phenylalanine increase transiently while aspartate and glutamate decrease after transfer to low CO(2). The biggest differences were typically observed 3 h after transfer to low-CO(2) conditions. Therefore, we made a careful metabolomic examination at the 3-h time point, comparing low-CO(2) treatment to high-CO(2) control. Five metabolites involved in photorespiration, 11 amino acids, and one lipid were increased, while six amino acids and, interestingly, 21 lipids were significantly lower. Our conclusion is that the metabolic pattern during early induction of the carbon-concentrating mechanism fit a model where photorespiration is increasing.

Identification and characterization of two closely related beta-carbonic anhydrases from Chlamydomonas reinhardtii

Aquatic photosynthetic organisms such as the green alga Chlamydomonas reinhardtii respond to low-CO(2) conditions by inducing a CO(2) concentrating mechanism (CCM). Important components of the CCM are the carbonic anhydrases (CAs), zinc metalloenzymes that catalyze the interconversion of CO(2) and HCO(-)(3). Six CAs have previously been identified in C. reinhardtii. Here, we identify and characterize two additional beta-type CAs. These two CAs are closely related beta-type CAs and have been designated as CAH7 and CAH8. Conceptual translation shows that CAH7 and CAH8 encode proteins of 399 and 333 amino acids, respectively, and they contain targeting sequences. An unusual characteristic of these two CAs is that they have carboxy-terminal extensions containing a hydrophobic sequence. Both these CAs are constitutively expressed at the transcript and protein level. The CAH7 and CAH8 open reading frames were cloned in the overexpression vector pMal-c2x and expressed as recombinant proteins. Activity assays showed that CAH7 and CAH8 are both active CAs. Antibodies were raised against both CAH7 and CAH8, and immunolocalization studies showed that CAH8 was localized in the periplasmic space. A possible role for CAH8 in the inorganic carbon acquisition by C. reinhardtii is discussed.

Characterization and expression analysis of genes encoding alpha and beta carbonic anhydrases in Arabidopsis

Carbonic anhydrases (CAs) are Zn-containing metalloenzymes that catalyse the reversible hydration of CO(2). We investigated the alphaCA and betaCA families in Arabidopsis, which contain eight alphaCA (At alphaCA1-8) and six betaCA genes (At betaCA1-6). Analyses of expressed sequence tags (ESTs) from The Arabidopsis Information Resource (TAIR) database indicate that all the betaCA encoding sequences, but only three of the At alphaCA, are expressed. Using semi-quantitative PCR experiments, functional CA genes were more strongly expressed in green tissue, but strong expression was also found in roots for betaCA3, betaCA6 and alphaCA2. Two alphaCA genes were shown to respond to the CO(2) environment, while the others were unresponsive. Using the green fluorescent reporter protein gene fused with cDNA sequences coding for betaCAs, we provided evidence that betaCAs were targeted to specific subcellular compartments: betaCA1 and betaCA5 were targeted to the chloroplast, betaCA2 and betaCA3 to the cytosol, betaCA4 to the plasma membrane and betaCA6 to the mitochondria. The targeting and the pattern of gene expression suggest that CA isoforms play specific roles in subcellular compartments, tissues and organs. The data indicate that other CA isoforms than the well-characterized betaCA1 may contribute to the CO(2) transfer in the cell to the catalytic site of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco).

Disruption of the glycolate dehydrogenase gene in the high-CO2-requiring mutant HCR89 ofChlamydomonas reinhardtii

One of the most notable contrasts between the photorespiratory pathway of higher plants and that of many of the green algae including Chlamydomonas reinhardtii lies in the enzymes that serve for oxidation of glycolate to glyoxylate. The gene disrupted by insertional mutagenesis in a high-CO2-requiring mutant, HCR89, of C. reinhardtii was determined to encode glycolate dehydrogenase (EC 1.1.99.14), which serves as the counterpart of glycolate oxidase (EC 1.1.3.15) in classical higher plant photorespiration. Neither glycolate nor D-lactate oxidation from the membrane fraction of HCR89 was detected. Excretion of over-accumulated glycolate into media due to the absence of glycolate dehydrogenase activity was observed for HCR89 under both high- and low-CO2conditions. Chlamydomonas glycolate dehydrogenase, CrGDH, with a molecular mass of 118 851 Da, comprises a relatively hydrophobic N-terminal region, a FAD-containing domain homologous to the D subunit of the glycolate oxidase complex from Escherischia coli, and an iron–sulfur cluster containing domain homologous to the C subunit of anaerobic glycerol-3-phosphate dehydrogenase complex from Escherichia coli. The second Cys residue in the second iron–sulfur cluster motif of CrGDH is replaced by Asp, as CxxDxxCxxxCP, indicating the second iron–sulfur cluster coordinates most likely 3Fe–4S instead of 4Fe–4S. The membrane association of the glycolate dehydrogenase activity agrees with three predicted transmembrane regions on the iron–sulfur domain.Key words: algae, Chlamydomonas, CO2, glycolate, lactate, mitochondria, photorespiration, photosynthesis.

CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution

The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO2 and the increase in O2. Consequently, the competition between O2 and CO2 for the active sites of RUBISCO became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO2 concentrations (CO2 concentrating mechanisms, CCMs) in the proximity of RUBISCO. A number of CCM variants are now found among the different groups of algae. Modulating the CCMs may be crucial in the energetic and nutritional budgets of a cell, and a multitude of environmental factors can exert regulatory effects on the expression of the CCM components. We discuss the diversity of CCMs, their evolutionary origins, and the role of the environment in CCM modulation.

Lack of the Rhesus protein Rh1 impairs growth of the green alga Chlamydomonas reinhardtii at high CO2

Although Rhesus (Rh) proteins are best known as antigens on human red blood cells, they are not restricted to red cells or to mammals, and hence their primary biochemical functions can be studied in more tractable organisms. We previously established that the Rh1 protein of the green alga Chlamydomonas reinhardtii is highly expressed in cultures bubbled with air containing high CO(2) (3%), conditions under which Chlamydomonas grows rapidly. By RNA interference, we have now obtained Chlamydomonas rh mutants (epigenetic), which are among the first in nonhuman cells. These mutants have essentially no mRNA or protein for RH1 and grow slowly at high CO(2), apparently because they fail to equilibrate this gas rapidly. They grow as well as their parental strain in air and on acetate plus air. However, during growth on acetate, rh1 mutants fail to express three proteins that are known to be down-regulated by high CO(2): periplasmic and mitochondrial carbonic anhydrases and a chloroplast envelope protein. This effect is parsimoniously rationalized if the small amounts of Rh1 protein present in acetate-grown cells of the parental strain facilitate leakage of CO(2) generated internally. Together, these results support our hypothesis that the Rh1 protein is a bidirectional channel for the gas CO(2). Our previous studies in a variety of organisms indicate that the only other members of the Rh superfamily, the ammonium/methylammonium transport proteins, are bidirectional channels for the gas NH(3). Physiologically, both types of gas channels can apparently function in acquisition of nutrients and/or waste disposal.

Stationary diffusion gradients associated with photosynthetic carbon flux-a study of compartmental versus diffusion-reaction models

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 D(j)--> infinity ). A compartmental model m(c) is termed "associated" to the reaction-diffusion model m(rd) if m(c) is the limit of m(rd) for all D(j)--> infinity. From the analytical solutions of a reaction-diffusion model and its associated compartmental model the extent of a diffusion gradient of m(rd) can be estimated by means of parameters from both m(c) and m(rd). 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 T(e), where T(e) 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 s(2)/D(X) and the turnover time X(C)/v, where X(C) 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 HCO(3)(-) and CO(2) are calculated for some flux situations relevant to photosynthetic carbon fixation in green microalgae.

An anaplerotic role for mitochondrial carbonic anhydrase in Chlamydomonas reinhardtii

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low CO(2) concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO(2) concentrations decreases when the external NH(4)(+) concentration decreases, to the point of being undetectable when NH(4)(+) supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO(2) by increasing the NH(4)(+) concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO(3)(-) for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.

Isolation and characterisation of Chlamydomonas reinhardtii mutants with an impaired CO2-concentrating mechanism

In order to identify new genes involved in the carbon-concentrating mechanism of Chlamydomonas reinhardtii (Dangeard), high-CO2-requiring mutants were isolated by insertional mutagenesis after transformation of strain CC1618 with a plasmid carrying Arg7 as a selectable marker gene. Six mutants were classified by their growth behaviour under ambient CO2, the affinity of the CO2-concentrating mechanism for inorganic carbon and the expression of known low-CO2-inducible proteins. The mass-spectrometric measurement of carbonic anhydrase activity and CO2/HCO3- transport revealed that four of the mutants are unable to induce a high-affinity carbon-concentrating mechanism. The expression of various carbonic anhydrases and chloroplast inner envelope polypeptides was examined with Western Blots. While three high-CO2-requiring mutants showed abnormal expression patterns, one matched the wild type. With Southern blots the total number and structure of the insertion events were determined to select possible candidates for plasmid recovery. Abnormal structures of thylakoid lamellae traversing the pyrenoid were detected by electron microscopy in some of the high-CO2-requiring mutants. Our characterisations of the insertionally generated mutants revealed phenotypes that have not been published before and therefore might be useful tools to obtain new insights on the molecular background of the CO2-concentrating mechanism and its regulation.

Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO(2) conditions

Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO(2) conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO(2) and/or HCO(3) (-) which increased the internal DIC concentration. The feature is referred to as the 'CO(2)-concentrating mechanism (CCM)'. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO(2) and/or HCO(3) (-), depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO(3) (-) transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric beta-type of CA composed of two internally repeating structures. An increase in the CO(2) concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO(2) concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO(2) concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H(+)-transport in extremely high-CO(2) conditions. This same state transition has also been observed when high-CO(2) cells were transferred to low CO(2) conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO(2) conditions.

Mitochondrial-driven bicarbonate transport supports photosynthesis in a marine microalga

The CO(2)-concentrating mechanism (CCM) of the marine eustigmatophycean microalga Nannochloropsis gaditana consists of an active HCO(3)(-) transport system and an internal carbonic anhydrase to facilitate accumulation and conversion of HCO(3)(-) to CO(2) for photosynthetic fixation. Aqueous inlet mass spectrometry revealed that a portion of the CO(2) generated within the cells leaked to the medium, resulting in a significant rise in the extracellular CO(2) concentration to a level above its chemical equilibrium that was diagnostic for active HCO(3)(-) transport. The transient rise in extracellular CO(2) occurred in the light and the dark and was resolved from concurrent respiratory CO(2) efflux using H(13)CO(3)(-) stable isotope techniques. H(13)CO(3)(-) pump-(13)CO(2) leak activity of the CCM was unaffected by 10 microM 3(3,4-dichlorophenyl)-1,1-dimethylurea, an inhibitor of chloroplast linear electron transport, although photosynthetic O(2) evolution was reduced by 90%. However, low concentrations of cyanide, azide, and rotenone along with anoxia significantly reduced or abolished (13)CO(2) efflux in the dark and light. These results indicate that H(13)CO(3)(-) transport was supported by mitochondrial energy production in contrast to other algae and cyanobacteria in which it is supported by photosynthetic electron transport. This is the first report of a direct role for mitochondria in the energization and functioning of the CCM in a photosynthetic organism.

Insertional mutants of Chlamydomonas reinhardtii that require elevated CO(2) for survival

Aquatic photosynthetic organisms live in quite variable conditions of CO(2) availability. To survive in limiting CO(2) conditions, Chlamydomonas reinhardtii and other microalgae show adaptive changes, such as induction of a CO(2)-concentrating mechanism, changes in cell organization, increased photorespiratory enzyme activity, induction of periplasmic carbonic anhydrase and specific polypeptides (mitochondrial carbonic anhydrases and putative chloroplast carrier proteins), and transient down-regulation in the synthesis of Rubisco. The signal for acclimation to limiting CO(2) in C. reinhardtii is unidentified, and it is not known how they sense a change of CO(2) level. The limiting CO(2) signals must be transduced into the changes in gene expression observed during acclimation, so mutational analyses should be helpful for investigating the signal transduction pathway for low CO(2) acclimation. Eight independently isolated mutants of C. reinhardtii that require high CO(2) for photoautotrophic growth were tested by complementation group analysis. These mutants are likely to be defective in some aspects of the acclimation to low CO(2) because they differ from wild type in their growth and in the expression patterns of five low CO(2)-inducible genes (Cah1, Mca1, Mca2, Ccp1, and Ccp2). Two of the new mutants formed a single complementation group along with the previously described mutant cia-5, which appears to be defective in the signal transduction pathway for low CO(2) acclimation. The other mutations represent six additional, independent complementation groups.

Insertional mutants of Chlamydomonas reinhardtii that require elevated CO(2) for survival

Aquatic photosynthetic organisms live in quite variable conditions of CO(2) availability. To survive in limiting CO(2) conditions, Chlamydomonas reinhardtii and other microalgae show adaptive changes, such as induction of a CO(2)-concentrating mechanism, changes in cell organization, increased photorespiratory enzyme activity, induction of periplasmic carbonic anhydrase and specific polypeptides (mitochondrial carbonic anhydrases and putative chloroplast carrier proteins), and transient down-regulation in the synthesis of Rubisco. The signal for acclimation to limiting CO(2) in C. reinhardtii is unidentified, and it is not known how they sense a change of CO(2) level. The limiting CO(2) signals must be transduced into the changes in gene expression observed during acclimation, so mutational analyses should be helpful for investigating the signal transduction pathway for low CO(2) acclimation. Eight independently isolated mutants of C. reinhardtii that require high CO(2) for photoautotrophic growth were tested by complementation group analysis. These mutants are likely to be defective in some aspects of the acclimation to low CO(2) because they differ from wild type in their growth and in the expression patterns of five low CO(2)-inducible genes (Cah1, Mca1, Mca2, Ccp1, and Ccp2). Two of the new mutants formed a single complementation group along with the previously described mutant cia-5, which appears to be defective in the signal transduction pathway for low CO(2) acclimation. The other mutations represent six additional, independent complementation groups.

Periplasmic carbonic anhydrase structural gene (Cah1) mutant in chlamydomonas reinhardtii

To survive in various conditions of CO2 availability, Chlamydomonas reinhardtii shows adaptive changes, such as induction of a CO2-concentrating mechanism, changes in cell organization, and induction of several genes, including a periplasmic carbonic anhydrase (pCA1) encoded by Cah1. Among a collection of insertionally generated mutants, a mutant has been isolated that showed no pCA1 protein and no Cah1 mRNA. This mutant strain, designated cah1-1, has been confirmed to have a disruption in the Cah1 gene caused by a single Arg7 insert. The most interesting feature of cah1-1 is its lack of any significant growth phenotype. There is no major difference in growth or photosynthesis between the wild type and cah1-1 over a pH range from 5.0 to 9.0 even though this mutant apparently lacks Cah1 expression in air. Although the presence of pCA1 apparently gives some minor benefit at very low CO2 concentrations, the characteristics of this Cah1 null mutant demonstrate that pCA1 is not essential for function of the CO2-concentrating mechanism or for growth of C. reinhardtii at limiting CO2 concentrations.

Carbonic anhydrase in eukaryotic algae: characterization, regulation, and possible function during photosynthesis

Carbonic anhydrase (CA) speeds up the equilibrium between CO2 and HCO3- at physiological pH values and has been detected in almost every species of the animal and plant kingdoms. Among eucaryotic micro- and macro-algae the enzyme is widely distributed and plays an important role in photosynthetic CO2 fixation. In some cases, different forms of carbonic anhydrases located extracellularly and intracellularly have been found to occur in the same cell. The expression of the genes encoding these CA isoforms are under the control of the inorganic carbon concentration in the medium, as the activities increase with decreasing the inorganic carbon content. Considerable progress has been made in recent years in isolating and characterizing the various forms of carbonic anhydrases on a biochemical and molecular level. Most of the data have been collected for microalgae like Chlamydomonas reinhardtii (Dangeard), while the situation in macroalgae is still descriptive. Therefore, this review summarizes the recent development with an emphasis on microalgae carbonic anhydrases.Key words: carbonic anhydrase, CO2 concentrating mechanism, macroalgae, microalgae, photosynthesis.