Inorganic Carbon Transport Research Articles

Slr0643, an S2P homologue, is essential for acid acclimation in the cyanobacterium Synechocystis sp. PCC 6803

Every cyanobacterial species contains genes encoding site-2-protease (S2P) homologues. The studied prokaryotic S2P homologues play essential roles in regulating stress responses through intramembrane proteolysis of membrane-bound anti-sigma factors. Here, the gene encoding Slr0643, one of four S2P homologues in Synechocystis sp. PCC 6803, was insertionally disrupted to explore its physiological role. Only a partially segregated mutant was obtained, indicating the essentiality of the gene product for growth. A pivotal role of fully functional Slr0643 in acid acclimation was demonstrated by defective acid acclimation to pH 6.5 in the mutant and transient induction of slr0643 in the wild-type after transfer from pH 7.5 to 6.5. DNA microarray and quantitative RT-PCR analyses of mutant and wild-type strains at pH 7.5 versus pH 6.5 identified genes involved in early acid acclimation and revealed genes expressed differentially due to slr0643 disruption. Early acid acclimation to pH 6.5 in the wild-type strain included upregulation of sigH, hik16 and hik35 and downregulation of pcrR and sigG, as well as downregulation of porins and upregulation of inorganic carbon and nitrogen transporters. The inability of the mutant strain to survive at pH 6.5 was found to be related to defective photosynthesis and excess expression of NADH dehydrogenase, together with excessive upregulation of carbon transporter and repression of nitrogen transporter and metabolism genes. Most interestingly, analysis of microarray data revealed the close relationship between slr0643 disruption and expression of the sigH operon. Thus it is suggested that Slr0643/Sll0857/SigH might act through an S2P/anti-Sigma factor/Sigma factor mechanism to play a role in acid acclimation.

Transcriptome-wide changes in Chlamydomonas reinhardtii gene expression regulated by carbon dioxide and the CO2-concentrating mechanism regulator CIA5/CCM1.

We used RNA sequencing to query the Chlamydomonas reinhardtii transcriptome for regulation by CO(2) and by the transcription regulator CIA5 (CCM1). Both CO(2) and CIA5 are known to play roles in acclimation to low CO(2) and in induction of an essential CO(2)-concentrating mechanism (CCM), but less is known about their interaction and impact on the whole transcriptome. Our comparison of the transcriptome of a wild type versus a cia5 mutant strain under three different CO(2) conditions, high CO(2) (5%), low CO(2) (0.03 to 0.05%), and very low CO(2) (<0.02%), provided an entry into global changes in the gene expression patterns occurring in response to the interaction between CO(2) and CIA5. We observed a massive impact of CIA5 and CO(2) on the transcriptome, affecting almost 25% of all Chlamydomonas genes, and we discovered an array of gene clusters with distinctive expression patterns that provide insight into the regulatory interaction between CIA5 and CO(2). Several individual clusters respond primarily to either CIA5 or CO(2), providing access to genes regulated by one factor but decoupled from the other. Three distinct clusters clearly associated with CCM-related genes may represent a rich source of candidates for new CCM components, including a small cluster of genes encoding putative inorganic carbon transporters.

Low-carbon acclimation in carboxysome-less and photorespiratory mutants of the cyanobacterium Synechocystis sp. strain PCC 6803

Using metabolic and transcriptomic phenotyping, we studied acclimation of cyanobacteria to low inorganic carbon (LC) conditions and the requirements for coordinated alteration of metabolism and gene expression. To analyse possible metabolic signals for LC sensing and compensating reactions, the carboxysome-less mutant ΔccmM and the photorespiratory mutant ΔglcD1/D2 were compared with wild-type (WT) Synechocystis. Metabolic phenotyping revealed accumulation of 2-phosphoglycolate (2PG) in ΔccmM and of glycolate in ΔglcD1/D2 in LC- but also in high inorganic carbon (HC)-grown mutant cells. The accumulation of photorespiratory metabolites provided evidence for the oxygenase activity of RubisCO at HC. The global gene expression patterns of HC-grown ΔccmM and ΔglcD1/D2 showed differential expression of many genes involved in photosynthesis, high-light stress and N assimilation. In contrast, the transcripts of LC-specific genes, such as those for inorganic carbon transporters and components of the carbon-concentrating mechanism (CCM), remained unchanged in HC cells. After a shift to LC, ΔglcD1/D2 and WT cells displayed induction of many of the LC-inducible genes, whereas ΔccmM lacked similar changes in expression. From the coincidence of the presence of 2PG in ΔccmM without CCM induction and of glycolate in ΔglcD1/D2 with CCM induction, we regard a direct role for 2PG as a metabolic signal for the induction of CCM during LC acclimation as less likely. Instead, our data suggest a potential role for glycolate as a signal molecule for enhanced expression of CCM genes.

Molecular Cloning and Characterization Analysis of Low CO&lt;sub&gt;2&lt;/sub&gt;–Inducible Protein Gene from &lt;i&gt;Dunaliella parva&lt;/i&gt;

Microalgae could modulate their photosynthesis to acclimate to CO2-limiting stress through inducing a carbon-concentrating mechanism (CCM) which included carbonic anhydrases and inorganic carbon (Ci) transporters. However, Ci-specific transporters have not been well described in algae so far. The former studies indicated that low CO2-inducible protein (Lci) played a significant role in the CCM, could increase Ci uptake and intracellular Ci accumulation under low CO2 conditions. The full-length cDNA encoding Lci was obtained from oleaginous microalgae Dunaliella parva, which include 1302 bp open reading frame (ORF), 255 bp 5′-untranslated sequence and 447 bp 3′-untranslated sequence. Similarity analysis revealed that the highest identity was found among Dunaliella parva, Chlamydomonas reinhardtii and Volvox carteri. The Dunaliella parva Lci also showed wide similarity with other species.

Expression of biomineralization‐related ion transport genes in Emiliania huxleyi

Biomineralization in the marine phytoplankton Emiliania huxleyi is a stringently controlled intracellular process. The molecular basis of coccolith production is still relatively unknown although its importance in global biogeochemical cycles and varying sensitivity to increased pCO₂ levels has been well documented. This study looks into the role of several candidate Ca²⁺, H⁺ and inorganic carbon transport genes in E. huxleyi, using quantitative reverse transcriptase PCR. Differential gene expression analysis was investigated in two isogenic pairs of calcifying and non-calcifying strains of E. huxleyi and cultures grown at various Ca²⁺ concentrations to alter calcite production. We show that calcification correlated to the consistent upregulation of a putative HCO₃⁻ transporter belonging to the solute carrier 4 (SLC4) family, a Ca²⁺/H⁺ exchanger belonging to the CAX family of exchangers and a vacuolar H⁺-ATPase. We also show that the coccolith-associated protein, GPA is downregulated in calcifying cells. The data provide strong evidence that these genes play key roles in E. huxleyi biomineralization. Based on the gene expression data and the current literature a working model for biomineralization-related ion transport in coccolithophores is presented.

Inorganic carbon transporters of the cyanobacterial CO2 concentrating mechanism

Cyanobacteria possess an environmental adaptation known as a CO(2) concentrating mechanism (CCM) that evolved to improve photosynthetic performance, particularly under CO(2)-limiting conditions. The CCM functions to actively transport dissolved inorganic carbon species (Ci; HCO(3)(-) and CO(2)) resulting in accumulation of a pool of HCO(3)(-) within the cell that is then utilised to provide an elevated CO(2) concentration around the primary CO(2) fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). Rubisco is encapsulated in unique micro-compartments known as carboxysomes and also provides the location for elevated CO(2) levels in the cell. Five distinct transport systems for active Ci uptake are known, including two types of Na(+)-dependent HCO(3)(-) transporters (BicA and SbtA), one traffic ATPase (BCT1) for HCO(3)(-) uptake and two CO(2) uptake systems based on modified NADPH dehydrogenase complexes (NDH-I(3) and NDH-I(4)). The genes for a number of these transporters are genetically induced under Ci limitation via transcriptional regulatory processes. The in-membrane topology structures of the BicA and SbtA HCO(3)(-) transporters are now known and this may aid in determining processes related to transporter activation during dark to light transitions or under severe Ci limitation.

Metabolic and Transcriptomic Phenotyping of Inorganic Carbon Acclimation in the Cyanobacterium Synechococcus elongatus PCC 7942

The amount of inorganic carbon is one of the main limiting environmental factors for photosynthetic organisms such as cyanobacteria. Using Synechococcus elongatus PCC 7942, we characterized metabolic and transcriptomic changes in cells that had been shifted from high to low CO(2) levels. Metabolic phenotyping indicated an activation of glycolysis, the oxidative pentose phosphate cycle, and glycolate metabolism at lowered CO(2) levels. The metabolic changes coincided with a general reprogramming of gene expression, which included not only increased transcription of inorganic carbon transporter genes but also genes for enzymes involved in glycolytic and photorespiratory metabolism. In contrast, the mRNA content for genes from nitrogen assimilatory pathways decreased. These observations indicated that cyanobacteria control the homeostasis of the carbon-nitrogen ratio. Therefore, results obtained from the wild type were compared with the MP2 mutant of Synechococcus 7942, which is defective for the carbon-nitrogen ratio-regulating PII protein. Metabolites and genes linked to nitrogen assimilation were differentially regulated, whereas the changes in metabolite concentrations and gene expression for processes related to central carbon metabolism were mostly similar in mutant and wild-type cells after shifts to low-CO(2) conditions. The PII signaling appears to down-regulate the nitrogen metabolism at lowered CO(2), whereas the specific shortage of inorganic carbon is recognized by different mechanisms.

Expression of a Low CO2–Inducible Protein, LCI1, Increases Inorganic Carbon Uptake in the Green Alga Chlamydomonas reinhardtii

Aquatic photosynthetic organisms can modulate their photosynthesis to acclimate to CO₂-limiting stress by inducing a carbon-concentrating mechanism (CCM) that includes carbonic anhydrases and inorganic carbon (Ci) transporters. However, to date, Ci-specific transporters have not been well characterized in eukaryotic algae. Previously, a Chlamydomonas reinhardtii mutant (lcr1) was identified that was missing a Myb transcription factor. This mutant had reduced light-dependent CO₂ gas exchange (LCE) activity when grown under CO₂-limiting conditions and did not induce the CAH1 gene encoding a periplasmic carbonic anhydrase, as well as two as yet uncharacterized genes, LCI1 and LCI6. In this study, LCI1 was placed under the control of the nitrate reductase promoter, allowing for the induction of LCI1 expression by nitrate in the absence of other CCM components. When the expression of LCI1 was induced in the lcr1 mutant under CO₂-enriched conditions, the cells showed an increase in LCE activity, internal Ci accumulation, and photosynthetic affinity for Ci. From experiments using indirect immunofluorescence, LCI1-green fluorescent protein fusions, and cell fractionation procedures, it appears that LCI1 is mainly localized to the plasma membrane. These results provide strong evidence that LCI1 may contribute to the CCM as a component of the Ci transport machinery in the plasma membrane.

Light and Low-CO2-Dependent LCIB–LCIC Complex Localization in the Chloroplast Supports the Carbon-Concentrating Mechanism in Chlamydomonas reinhardtii

The carbon-concentrating mechanism (CCM) is essential to support photosynthesis under CO2-limiting conditions in aquatic photosynthetic organisms, including the green alga Chlamydomonas reinhardtii. The CCM is assumed to be comprised of inorganic carbon transport systems that, in conjunction with carbonic anhydrases, maintain high levels of CO2 around ribulose-1, 5-bisphosphate carboxylase/oxygenase in a specific compartment called the pyrenoid. A set of transcripts up-regulated during the induction of the CCM was identified previously and designated as low-CO2 (LC)-inducible genes. Although the functional importance of one of these LC-inducible genes, LciB, has been shown recently, the biochemical properties and detailed subcellular localization of its product LCIB remain to be elucidated. Here, using yeast two-hybrid, immunoprecipitation and mass spectrometry analyses we provide evidence to demonstrate that LCIB interacts with the LCIB homologous protein LCIC in yeast and in vivo. We also show that LCIB and LCIC are co-localized in the vicinity of the pyrenoid under LC conditions in the light, forming a hexamer complex of approximately 350 kDa, as estimated by gel filtration chromatography. LCIB localization around the pyrenoid was dependent on light illumination and LC conditions during active operation of the CCM. In contrast, in the dark or under high-CO2 conditions when the CCM was inactive, LCIB immediately diffused away from the pyrenoid. Based on these observations, we discuss possible functions of the LCIB-LCIC complex in the CCM.

Imaging intracellular pH in a reef coral and symbiotic anemone

The challenges corals and symbiotic cnidarians face from global environmental change brings new urgency to understanding fundamental elements of their physiology. Intracellular pH (pHi) influences almost all aspects of cellular physiology but has never been described in anthozoans or symbiotic cnidarians, despite its pivotal role in carbon concentration for photosynthesis and calcification. Using confocal microscopy and the pH sensitive probe carboxy SNARF-1, we mapped pHi in short-term light and dark-incubated cells of the reef coral Stylophora pistillata and the symbiotic anemone Anemonia viridis. In all cells isolated from both species, pHi was markedly lower than the surrounding seawater pH of 8.1. In cells that contained symbiotic algae, mean values of pHi were significantly higher in light treated cells than dark treated cells (7.41 +/- 0.22 versus 7.13 +/- 0.24 for S. pistillata; and 7.29 +/- 0.15 versus 7.01 +/- 0.27 for A. viridis). In contrast, there was no significant difference in pHi in light and dark treated cells without algal symbionts. Close inspection of the interface between host cytoplasm and algal symbionts revealed a distinct area of lower pH adjacent to the symbionts in both light and dark treated cells, possibly associated with the symbiosome membrane complex. These findings are significant developments for the elucidation of models of inorganic carbon transport for photosynthesis and calcification and also provide a cell imaging procedure for future investigations into how pHi and other fundamental intracellular parameters in corals respond to changes in the external environment such as reductions in seawater pH.

Chemical tracer transport in the eastern boundary current system of the North Atlantic

The MedBox region, comprising the Strait of Gibraltar to 22ºW and from 24ºN to 41ºN, has been defined using thermohaline and chemical data from three WOCE cruises conducted in 1997/98. The water mass structure of the region was objectively solved by an extended optimum multiparameter (OMP) analysis. Volume transports were estimated from field data and an inverse box model that ensures volume conservation and no deep water formation within the region. The combination of the volume transports, the OMP water mass analysis, and the distributions of chemical parameters allowed for the assessment of the oxygen, inorganic carbon, nitrate, phosphate, and silicate transport mechanisms, their vertical and lateral variability, and the relative contribution of the different water masses in the mid-latitudes of the eastern North Atlantic to these transports. La región MedBox comprende desde el Estrecho de Gibraltar hasta 22ºW y entre 24ºN y 41ºN.

Influence of CO2 concentrating mechanism on photoinhibition in Synechococcus sp. PCC7942 (Cyanophyceae)

H. Cheng, G. Dai, L. Yu, X. Zhong, P. Juneau and B. Qiu. 2008. Influence of CO2 concentrating mechanism on photoinhibition in Synechococcus sp. PCC7942 (Cyanophyceae). Phycologia 47: 588–598. DOI: 10.2216/07-44.1.In this study the effects of inorganic carbon transport in the cyanobacterium Synechococcus sp. PCC7942 on its photoinhibition were investigated. The photosynthetic oxygen evolution of samples under a strong irradiance treatment at 100 µM KHCO3 was much higher than that at 10,000 µM KHCO3. After strong irradiance treatment, the abundance of the sbtA transcript encoding a Na+-dependent transporter was significantly higher under treatment at 100 µM KHCO3 than at 10,000 µM KHCO3. Lincomycin had little effect on the difference of photosystem II (PSII) activity decline under these two treatments, which suggests that CCM activity reduces photodamage but does not boost the rate of PSII repair. The maximal photosynthetic efficiency was decreased significantly during the strong irradiance treatment, and its decrease was enhanced when the external DIC concentration was raised from 100 to 10,000 µM KHCO3. The JIP test showed that there were more active reaction centres under low Ci conditions after high light treatment. Studies on 77-K fluorescence emission spectra showed that the fluorescence peaks F692 nm and F710 nm in 10,000-µM KHCO3 treated samples both decreased after strong irradiance treatment and were significantly smaller than those of 100-µM KHCO3 treated samples. Samples treated with 100 µM KHCO3, compared to 10,000 µM KHCO3 had a higher possibility of transmitting an exciton to a closed reaction centre rather than to an open reaction centre and thus to reduce the overexcitation of the reaction centres. These results indicated that higher external inorganic carbon concentration was associated with more severe photoinhibition under high irradiance and that the operation of CO2 concentrating mechanism might reduce the overexcitation of reaction centres to reduce photodamage.

Distribution of lipid biomarkers and carbon isotope fractionation in contrasting trophic environments of the South East Pacific

Abstract. The distribution of lipid biomarkers and their stable carbon isotope composition was investigated on suspended particles from different contrasting trophic environments at six sites in the South East Pacific. High algal biomass with diatom-related lipids (24-methylcholesta-5,24(28)-dien-3β-ol, C25 HBI alkenes, C16:4 FA, C20:5 FA) was characteristic in the upwelling zone, whereas haptophyte lipids (long-chain (C37-C39) unsaturated ketones) were proportionally most abundant in the nutrient-poor settings of the centre of the South Pacific Gyre and on its easter edge. The dinoflagellate–sterol, 4α-23,24-trimethylcholest-22(E)-en-3β-ol, was a minor contributor in all of the studied area and the cyanobacteria-hydrocarbon, C17n-alkane, was at maximum in the high nutrient low chlorophyll regime of the subequatorial waters near the Marquesas archipelago. The taxonomic and spatial variability of the relationships between carbon photosynthetic fractionation and environmental conditions for four specific algal taxa (diatoms, haptophytes, dinoflagellates and cyanobacteria) was also investigated. The carbon isotope fractionation factor (εp) of the 24-methylcholesta-5,24(28)-dien-3β-ol diatom marker, varied over a range of 16% along the different trophic systems. In contrast, εp of dinoflagellate, cyanobacteria and alkenone markers varied only by 7–10‰. The low fractionation factors and small variations between the different phytoplankton markers measured in the upwelling area likely reveals uniformly high specific growth rates within the four phytoplankton taxa, and/or that transport of inorganic carbon into phytoplankton cells may not only occur by diffusion but also by other carbon concentrating mechanisms (CCM). In contrast, in the oligotrophic zone, i.e. gyre and eastgyre, relatively high εp values, especially for the diatom marker, indicate diffusive CO2 uptake by the eukaryotic phytoplankton. At these nutrient-poor sites, the lower εp values for haptophytes, dinoflagellates and cyanobacteria indicate higher growth rates or major differences on the carbon uptake mechanisms compared to diatoms.

Identification and characterisation of a novel inorganic carbon acquisition gene, CIA7, from an insertional mutant of Chlamydomonas reinhardtii.

Chlamydomonas reinhardtii is a unicellular eukaryotic alga which possesses a CO2-concentrating mechanism (CCM) that enables it to grow at low CO2 concentrations. Previously, insertional mutants were generated to enable isolation of inorganic carbon transporters and other proteins that might be essential for a functional CCM. These mutants have an antibiotic resistance gene that encodes a protein that binds to Zeocin inhibiting Zeocin's DNA strand cleavage activity. The DNA flanking the BleR insert of one of the high CO2 requiring strains, named cia7, was cloned with inverse-PCR and sequenced. Sequence analysis showed homology to conserved bacterial proteins of unknown function, but there were no ESTs in this region of the genome. However, the presence of a gene was established by PCR and RLM-RACE. CIA7 was found to have four exons and the BleR insert was in the fourth exon. CIA7 encodes a protein of 104 amino acids with a calculated molecular mass of 11.3 kDa. Based on the ChloroP prediction program, the protein is predicted to have a chloroplast targeting signal. Complementation analyses results showed possible partially rescued mutants, and RNAi showed several transformants with a sick on low CO2 phenotype with reduced expression of CIA7. These results suggest that CIA7 is a gene that facilitates growth in C. reinhardtii under low CO2 conditions. One possible role of CIA7 would be in the delivery or storage of a metal ion. It may play a potential role as either a domain of a metal transporter or as a metallochaperone.

The CO2‐concentrating mechanism in the bloom‐forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) and effects of UVB radiation on its operation1

The bloom‐forming cyanobacterium Microcystis aeruginosa (Kütz.) Kütz. 854 was cultured with 1.05 W · m−2 ultraviolet‐B radiation (UVBR) for 3 h every day, and the CO2‐concentrating mechanism (CCM) within this species as well as effects of UVBR on its operation were investigated. Microcystis aeruginosa 854 possessed at least three inorganic carbon transport systems and could utilize external HCO3− and CO2 for its photosynthesis. The maximum photosynthetic rate was approximately the same, but the apparent affinity for dissolved inorganic carbon was significantly decreased from 74.7 μmol · L−1 in the control to 34.7 μmol · L−1 in UVBR‐treated cells. At 150 μmol · L−1 KHCO3 and pH 8.0, Na+‐dependent HCO3− transport contributed 43.4%–40.2% to the photosynthesis in the control and 34.5%–31.9% in UVBR‐treated cells. However, the contribution of Na+‐independent HCO3− transport increased from 8.7% in the control to 18.3% in UVBR‐treated cells. The contribution of CO2‐uptake systems showed little difference: 47.9%–51.0% in the control and 49.8%–47.2% in UVBR‐treated cells. Thus, the rate of total inorganic carbon uptake was only marginally affected, although UVBR had a differential effect on various inorganic carbon transporters. However, the number of carboxysomes in UVBR‐treated cells was significantly decreased compared to that in the control.

Osmoregulation in Dunaliella, Part I: Effects of osmotic stress on photosynthesis, dark respiration and glycerol metabolism in Dunaliella tertiolecta and its salt-sensitive mutant (HL 25/8)

The photosynthetic oxygen evolution increased by about 30% over control when Dunaliella tertiolecta and its salt-sensitive mutant (HL 25/8) were stressed by raising NaCl concentration from 0.17 to 0.4 M, however, during the dilution stress the photosynthetic oxygen evolution was progressively decreased with increasing dilution (decreasing the salinity). The photosynthetic oxygen evolution is affected by the water potential of the medium rather than by the ionic strength. Mitochondrial dark respiration was unaffected by salt stress, however, it was increased by about 50% (parent strain) and 35% (the mutant) upon dilution as if reduced pyridine nucleotide generated during glycerol dissimilation reaction were continuously oxidized by the mitochondria. The salt stress-induced changes in photosynthetic 14CO 2 fixation were consistent with the observed rates of photosynthetic oxygen evolution. The mutant strain showed about one-half the capability for photosynthesis, and glycerol synthesis compared to the parent strain. However, the proportion of photosynthetically newly fixed carbon during salt stress in glycerol was similar in both strains. The glycerol dissimilation capabilities of both strains were also similar. It is suggested that the salt sensitivity of the mutant is probably due to its reduced dissolved inorganic carbon transport, photosynthetic and starch metabolism capabilities to provide carbon for glycerol synthesis in the time frame of adaptation process.

Inorganic carbon transport during sea ice growth and decay: A carbon pump in polar seas

During sea ice formation in polar areas, brine rejection increases the density in the underlying water column and thereby contributes to the formation of deep and intermediate water masses in the world ocean. Here we present evidence that dissolved inorganic carbon (TCO2) is rejected together with brine from growing sea ice and that low temperatures may result in a significant change in the ratio of TCO2 and alkalinity in Arctic sea ice compared with surface waters. Model calculations show that this sea ice–driven carbon pump affects surface water partial pressure of CO2 significantly in polar seas and potentially sequesters large amounts of CO2 to the deep ocean.

Carbon isotopic fractionation by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions

A theoretical model was developed to explain the characteristics of carbon isotopic fractionation ( ε P) by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions. The model takes into consideration active transport and diffusion of inorganic carbon through the cell membrane and chloroplast membrane and the energetic tradeoff between production of Rubisco and operation of a carbon-concentrating mechanism to achieve a given growth rate. The model is able to explain 88% of the variance in experimental ε p data reported in this study and in previous work and is able to account for the observed pattern of Rubisco activity in nitrate-limited chemostats. Two important implications of the model include the fact that ε p is not a unique function of the ratio of growth rate to external CO 2 concentration (as opposed to the predictions of several previous models) and that changes in light-limited and nutrient-limited growth rates have opposite effects on the fraction of CO 2 taken up by the chloroplast that is lost to diffusion and hence on certain patterns of carbon isotopic fractionation.

An inorganic carbon transport system responsible for acclimation specific to air levels of CO2 in Chlamydomonas reinhardtii.

Many photosynthetic microorganisms acclimate to CO(2) limited environments by induction and operation of CO(2)-concentrating mechanisms (CCMs). Despite their central role in CCM function, inorganic carbon (Ci) transport systems never have been identified in eukaryotic photosynthetic organisms. In the green alga Chlamydomonas reinhardtii, a mutant, pmp1, was described in 1983 with deficiencies in Ci transport, and a Pmp1 protein-associated Ci uptake system has been proposed to be responsible for Ci uptake in low CO(2) (air level)-acclimated cells. However, even though pmp1 represents the only clear genetic link to Ci transport in microalgae and is one of only a very few mutants directly affecting the CCM itself, the identity of Pmp1 has remained unknown. Physiological analyses indicate that C. reinhardtii possesses multiple Ci transport systems responsible for acclimation to different levels of limiting CO(2) and that the Pmp1-associated transport system is required specifically for low (air level) CO(2) acclimation. In the current study, we identified and characterized a pmp1 allelic mutant, air dier 1 (ad1) that, like pmp1, cannot grow in low CO(2) (350 ppm) but can grow either in high CO(2) (5% CO(2)) or in very low CO(2) (<200 ppm). Molecular analyses revealed that the Ad1/Pmp1 protein is encoded by LciB, a gene previously identified as a CO(2)-responsive gene. LciB and three related genes in C. reinhardtii compose a unique gene family that encode four closely related, apparently soluble plastid proteins with no clearly identifiable conserved motifs.

δ13C tracing of dissolved inorganic carbon sources in Patagonian rivers (Argentina)

AbstractThe main Patagonian rivers (Colorado, Negro, Chubut, Deseado, Coyle, Chico, Santa Cruz and Gallegos) were sampled between September 1995 and November 1998 to determine their chemical and isotopic compositions, the origins of the suspended and dissolved river loads and their inputs to the South Atlantic Ocean. This paper focuses on the dissolved inorganic carbon (DIC) transport and its δ13C isotopic signature. The δ13CDIC values vary between −12·8 and −1·8‰ and allow one to distinguish two river groups: (i) the Colorado, Negro, Chubut and Santa Cruz, which display the highest values and the lowest seasonal variations; (ii) the Deseado, Coyle, Chico and Gallegos, which show the lowest values and the highest seasonal variations. For the first group, δ13CDIC is mainly controlled by important exchanges between the river waters and atmospheric CO2, due to the presence of lakes and dams. For the second group, δ13CDIC also appears to be controlled by the oxidation of organic carbon, showing a negative relationship between δ13CDIC and the dissolved organic carbon. These biogeochemical processes interfere with the contribution of carbonate and silicate weathering to the riverine DIC and do not allow use of δ13CDIC alone to distinguish these contributions.The annual DIC flux exported by Patagonian Rivers to the South Atlantic Ocean averages 621 × 109 g. of C, i.e. a specific yield of 2·7 g m−2 year−1. The mean δ13CDIC can be estimated to − 4·9‰, which is high compared with other rivers of the world. Copyright © 2005 John Wiley &amp; Sons, Ltd.