Active CO(2) Transport by the Green Alga Chlamydomonas reinhardtii

Water motion and pH jointly impact the availability of dissolved inorganic carbon to macroalgae

The supply of dissolved inorganic carbon to seaweeds is a key factor regulating photosynthesis. Thinner diffusive boundary layers at the seaweed surface or greater seawater carbon dioxide (CO₂) concentrations increase CO₂ supply to the seaweed surface. This may benefit seaweeds by alleviating carbon limitation either via an increased supply of CO₂ that is taken up by passive diffusion, or via the down-regulation of active carbon concentrating mechanisms (CCMs) that enable the utilization of the abundant ion bicarbonate (HCO₃^-). Laboratory experiments showed that a 5 times increase in water motion increases DIC uptake efficiency in both a non-CCM (Hymenena palmata, Rhodophyta) and CCM (Xiphophora gladiata, Phaeophyceae) seaweed. In a field survey, brown and green seaweeds with active-CCMs maintained their CCM activity under diverse conditions of water motion. Whereas red seaweeds exhibited flexible photosynthetic rates depending on CO₂ availability, and species switched from a non-CCM strategy in wave-exposed sites to an active-CCM strategy in sheltered sites where mass transfer of CO₂ would be reduced. 97-99% of the seaweed assemblages at both wave-sheltered and exposed sites consisted of active-CCM species. Variable sensitivities to external CO₂ would drive different responses to increasing CO₂ availability, although dominance of the CCM-strategy suggests this will have minimal impact within shallow seaweed assemblages.

Allophycocyanin A is a carbon dioxide receptor in the cyanobacterial phycobilisome

Light harvesting is fundamental for production of ATP and reducing equivalents for CO₂ fixation during photosynthesis. However, electronic energy transfer (EET) through a photosystem can harm the photosynthetic apparatus when not balanced with CO₂. Here, we show that CO₂ binding to the light-harvesting complex modulates EET in photosynthetic cyanobacteria. More specifically, CO₂ binding to the allophycocyanin alpha subunit of the light-harvesting complex regulates EET and its fluorescence quantum yield in the cyanobacterium Synechocystis sp. PCC 6803. CO₂ binding decreases the inter-chromophore distance in the allophycocyanin trimer. The result is enhanced EET in vitro and in live cells. Our work identifies a direct target for CO₂ in the cyanobacterial light-harvesting apparatus and provides insights into photosynthesis regulation.

The transfer of electronic energy through a photosystem can harm the photosynthetic apparatus when not balanced with CO₂ fixation. Here, the authors show that CO₂ modulates electronic energy transfer in cyanobacteria by binding to and enhancing the activity of the light-harvesting complex.

Membrane Inlet Mass Spectrometry: A Powerful Tool for Algal Research

Since the first great oxygenation event, photosynthetic microorganisms have continuously shaped the Earth's atmosphere. Studying biological mechanisms involved in the interaction between microalgae and cyanobacteria with the Earth's atmosphere requires the monitoring of gas exchange. Membrane inlet mass spectrometry (MIMS) has been developed in the early 1960s to study gas exchange mechanisms of photosynthetic cells. It has since played an important role in investigating various cellular processes that involve gaseous compounds (O2, CO2, NO, or H2) and in characterizing enzymatic activities in vitro or in vivo. With the development of affordable mass spectrometers, MIMS is gaining wide popularity and is now used by an increasing number of laboratories. However, it still requires an important theory and practical considerations to be used. Here, we provide a practical guide describing the current technical basis of a MIMS setup and the general principles of data processing. We further review how MIMS can be used to study various aspects of algal research and discuss how MIMS will be useful in addressing future scientific challenges.

Structure and function of LCI1: a plasma membrane CO2 channel in the Chlamydomonas CO2 concentrating mechanism

Microalgae and cyanobacteria contribute roughly half of the global photosynthetic carbon assimilation. Faced with limited access to CO₂ in aquatic environments, which can vary daily or hourly, these microorganisms have evolved use of an efficient CO₂ concentrating mechanism (CCM) to accumulate high internal concentrations of inorganic carbon (C_i ) to maintain photosynthetic performance. For eukaryotic algae, a combination of molecular, genetic and physiological studies using the model organism Chlamydomonas reinhardtii, have revealed the function and molecular characteristics of many CCM components, including active C_i uptake systems. Fundamental to eukaryotic C_i uptake systems are C_i transporters/channels located in membranes of various cell compartments, which together facilitate the movement of C_i from the environment into the chloroplast, where primary CO₂ assimilation occurs. Two putative plasma membrane C_i transporters, HLA3 and LCI1, are reportedly involved in active C_i uptake. Based on previous studies, HLA3 clearly plays a meaningful role in HCO₃^- transport, but the function of LCI1 has not yet been thoroughly investigated so remains somewhat obscure. Here we report a crystal structure of the full-length LCI1 membrane protein to reveal LCI1 structural characteristics, as well as in vivo physiological studies in an LCI1 loss-of-function mutant to reveal the C_i species preference for LCI1. Together, these new studies demonstrate LCI1 plays an important role in active CO₂ uptake and that LCI1 likely functions as a plasma membrane CO₂ channel, possibly a gated channel.

LCI1, a Chlamydomonas reinhardtii plasma membrane protein, functions in active CO2 uptake under low CO2

In response to high CO₂ environmental variability, green algae, such as Chlamydomonas reinhardtii, have evolved multiple physiological states dictated by external CO₂ concentration. Genetic and physiological studies demonstrated that at least three CO₂ physiological states, a high CO₂ (0.5-5% CO₂ ), a low CO₂ (0.03-0.4% CO₂ ) and a very low CO₂ (< 0.02% CO₂ ) state, exist in Chlamydomonas. To acclimate in the low and very low CO₂ states, Chlamydomonas induces a sophisticated strategy known as a CO₂ -concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO₂ environments. Active uptake of C_i from the environment is a fundamental aspect in the Chlamydomonas CCM, and consists of CO₂ and HCO₃^- uptake systems that play distinct roles in low and very low CO₂ acclimation states. LCI1, a putative plasma membrane C_i transporter, has been linked through conditional overexpression to active C_i uptake. However, both the role of LCI1 in various CO₂ acclimation states and the species of C_i , HCO₃^- or CO₂ , that LCI1 transports remain obscure. Here we report the impact of an LCI1 loss-of-function mutant on growth and photosynthesis in different genetic backgrounds at multiple pH values. These studies show that LCI1 appears to be associated with active CO₂ uptake in low CO₂ , especially above air-level CO₂ , and that any LCI1 role in very low CO₂ is minimal.

Photoprotection mechanisms under different CO2 regimes during photosynthesis in a green alga Chlorella variabilis

Oxygenic photosynthesis converts light energy into chemical energy via electron transport and assimilates CO₂ in the Calvin-Benson cycle with the chemical energy. Thus, high light and low CO₂ conditions induce the accumulation of electrons in the photosynthetic electron transport system, resulting in the formation of reactive oxygen species. To prevent the accumulation of electrons, oxygenic photosynthetic organisms have developed photoprotection mechanisms, including non-photochemical quenching (NPQ) and alternative electron flow (AEF). There are diverse molecular mechanisms underlying NPQ and AEF, and the corresponding molecular actors have been identified and characterized using a model green alga Chlamydomonas reinhardtii. In contrast, detailed information about the photoprotection mechanisms is lacking for other green algal species. In the current study, we examined the photoprotection mechanisms responsive to CO₂ in the green alga Chlorella variabilis by combining the analyses of pulse-amplitude-modulated fluorescence, O₂ evolution, and the steady-state and time-resolved fluorescence spectra. Under the CO₂-limited condition, ΔpH-dependent NPQ occurred in photosystems I and II. Moreover, O₂-dependent AEF was also induced. Under the CO₂-limited condition with carbon supplementation, NPQ was relaxed and light-harvesting chlorophyll-protein complex II was isolated from both photosystems. In C. variabilis, the O₂-dependent AEF and the mechanisms that instantly convert the light-harvesting functions of both photosystems may be important for maintaining efficient photosynthetic activities under various CO₂ conditions.

Light-Harvesting Strategy during CO2-Dependent Photosynthesis in the Green Alga Chlamydomonas reinhardtii

To maximize the efficiency of photosynthesis, photosynthetic organisms must properly balance their light-harvesting ability and CO₂ utilization. However, the molecular mechanisms of light harvesting under various CO₂ conditions remain unclear. To reveal these mechanisms, we performed new analysis on cells of the green alga Chlamydomonas reinhardtii under different CO₂ conditions. The analysis combines three kinds of fluorometries: pulse-amplitude modulated fluorescence, steady-state fluorescence with absolute intensity, and time-resolved fluorescence. Under low CO₂ conditions, the main regulatory mechanism was migration of a light-harvesting chlorophyll-protein complex (LHC) II from photosystem (PS) II to PSI. However, under CO₂-deficient conditions with carbon supplementation, some of the LHCII separated from the PSI and aggregated with quenching. These different light-harvesting abilities of LHCII may play an important role in the regulation of light harvesting in C. reinhardtii under various CO₂ conditions.

Impact of Flue Gas Compounds on Microalgae and Mechanisms for Carbon Assimilation and Utilization

To shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low-carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil-derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon-neutral products, such as biodiesel, the application of microalgae technology to capture CO₂ from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot-scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.

Ecophysiology matters: linking inorganic carbon acquisition to ecological preference in four species of microalgae (Chlorophyceae)

The effect of CO₂ supply is likely to play an important role in algal ecology. Since inorganic carbon (C_i ) acquisition strategies are very diverse among microalgae and C_i availability varies greatly within and among habitats, we hypothesized that C_i acquisition depends on the pH of their preferred natural environment (adaptation) and that the efficiency of C_i uptake is affected by CO₂ availability (acclimation). To test this, four species of green algae originating from different habitats were studied. The pH-drift and C_i uptake kinetic experiments were used to characterize C_i acquisition strategies and their ability to acclimate to high and low CO₂ conditions and high and low pH was evaluated. Results from pH drift experiments revealed that the acidophile and acidotolerant Chlamydomonas species were mainly restricted to CO₂ , whereas the two neutrophiles were efficient bicarbonate users. CO₂ compensation points in low CO₂ -acclimated cultures ranged between 0.6 and 1.4 μM CO₂ and acclimation to different culture pH and CO₂ conditions suggested that CO₂ concentrating mechanisms were present in most species. High CO₂ acclimated cultures adapted rapidly to low CO₂ condition during pH-drifts. C_i uptake kinetics at different pH values showed that the affinity for C_i was largely influenced by external pH, being highest under conditions where CO₂ dominated the C_i pool. In conclusion, C_i acquisition was highly variable among four species of green algae and linked to growth pH preference, suggesting that there is a connection between C_i acquisition and ecological distribution.

Diversity in photosynthetic electron transport under [CO2]-limitation: the cyanobacterium Synechococcus sp. PCC 7002 and green alga Chlamydomonas reinhardtii drive an O2-dependent alternative electron flow and non-photochemical quenching of chlorophyll fluorescence during CO2-limited photosynthesis

Some cyanobacteria, but not all, experience an induction of alternative electron flow (AEF) during CO₂-limited photosynthesis. For example, Synechocystis sp. PCC 6803 (S. 6803) exhibits AEF, but Synechococcus elongatus sp. PCC 7942 does not. This difference is due to the presence of flavodiiron 2 and 4 proteins (FLV2/4) in S. 6803, which catalyze electron donation to O₂. In this study, we observed a low-[CO₂] induced AEF in the marine cyanobacterium Synechococcus sp. PCC 7002 that lacks FLV2/4. The AEF shows high affinity for O₂, compared with AEF mediated by FLV2/4 in S. 6803, and can proceed under extreme low [O₂] (about a few µM O₂). Further, the transition from CO₂-saturated to CO₂-limited photosynthesis leads a preferential excitation of PSI to PSII and increased non-photochemical quenching of chlorophyll fluorescence. We found that the model green alga Chlamydomonas reinhardtii also has an O₂-dependent AEF showing the same affinity for O₂ as that in S. 7002. These data represent the diverse molecular mechanisms to drive AEF in cyanobacteria and green algae. In this paper, we further discuss the diversity, the evolution, and the physiological function of strategy to CO₂-limitation in cyanobacterial and green algal photosynthesis.

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.

Inorganic carbon acquisition in the acid-tolerant alga Chlorella kessleri

The ability of the freshwater alga, Chlorella kessleri, to maintain a carbon concentrating mechanism when grown at acid pH was investigated. The alga grows over the pH range 4.0-9.0 and was found to take up bicarbonate and CO2 actively when grown at pH 6.0. However, when grown at acid pH (below 5.5), it does not have active CO2 uptake. The acidotolerant species maintained an internal pH of 6.1-7.5 over the external pH range 4.5-7.5, thus the pH difference between the cell interior and the external medium was large enough to allow for the diffusive uptake of CO2 at acid external pH. Mass spectrometric monitoring of O2 and CO2 fluxes by suspensions of C. kessleri, grown at acid pH, and maintained at pH 7.5 showed that the rates of O2 evolution did not exceed those of CO2 uptake. The final CO2 compensation concentrations of 14.0-17.7 µM reached by photosynthetic cells were above the CO2 equilibrium concentration in the external medium, indicating a lack of active CO2 uptake at acid pH. Chlorella kessleri accumulated CO2 with internal concentrations that were 9.9, 18.7 and 22.7-fold that of the external medium for cells grown, respectively, at pH 4.5, 5.0 and 5.5. The ability of C. kessleri cells to accumulate high intracellular concentrations of inorganic carbon at acid pH would provide a sufficiently high concentration of CO2 at the active site of Rubisco thus allowing the alga to maintain growth rates similar to those at alkaline pH.

Acclimation to very low CO2: contribution of limiting CO2 inducible proteins, LCIB and LCIA, to inorganic carbon uptake in Chlamydomonas reinhardtii

The limiting-CO2 inducible CO2-concentrating mechanism (CCM) of microalgae represents an effective strategy to capture CO2 when its availability is limited. At least two limiting-CO2 acclimation states, termed low CO2 and very low CO2, have been demonstrated in the model microalga Chlamydomonas reinhardtii, and many questions still remain unanswered regarding both the regulation of these acclimation states and the molecular mechanism underlying operation of the CCM in these two states. This study examines the role of two proteins, Limiting CO2 Inducible A (LCIA; also named NAR1.2) and LCIB, in the CCM of C. reinhardtii. The identification of an LCIA-LCIB double mutant based on its inability to survive in very low CO2 suggests that both LCIA and LCIB are critical for survival in very low CO2. The contrasting effects of individual mutations in LCIB and LCIA compared with the effects of LCIB-LCIA double mutations on growth and inorganic carbon-dependent photosynthetic O2 evolution reveal distinct roles of LCIA and LCIB in the CCM. Although both LCIA and LCIB are essential for very low CO2 acclimation, LCIB appears to function in a CO2 uptake system, whereas LCIA appears to be associated with a HCO3(-) transport system. The contrasting and complementary roles of LCIA and LCIB in acclimation to low CO2 and very low CO2 suggest a possible mechanism of differential regulation of the CCM based on the inhibition of HCO3(-) transporters by moderate to high levels of CO2.

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

The photosynthetic, unicellular green alga, Chlamydomonas reinhardtii, lives in environments that often contain low concentrations of CO2 and HCO3 (-), the utilizable forms of inorganic carbon (Ci). C. reinhardtii possesses a carbon concentrating mechanism (CCM) which can provide suitable amounts of Ci for growth and development. This CCM is induced when the CO2 concentration is at air levels or lower and is comprised of a set of proteins that allow the efficient uptake of Ci into the cell as well as its directed transport to the site where Rubisco fixes CO2 into biomolecules. While several components of the CCM have been identified in recent years, the picture is still far from complete. To further improve our knowledge of the CCM, we undertook a mutagenesis project where an antibiotic resistance cassette was randomly inserted into the C. reinhardtii genome resulting in the generation of 22,000 mutants. The mutant collection was screened using both a published PCR-based approach (Gonzalez-Ballester et al. 2011) and a phenotypic growth screen. The PCR-based screen did not rely on a colony having an altered growth phenotype and was used to identify colonies with disruptions in genes previously identified as being associated with the CCM-related gene. Eleven independent insertional mutations were identified in eight different genes showing the usefulness of this approach in generating mutations in CCM-related genes of interest as well as identifying new CCM components. Further improvements of this method are also discussed.

Regulation of CCM genes in Chlamydomonas reinhardtii during conditions of light-dark cycles in synchronous cultures

We have investigated transcript level changes of CO(2)-concentrating mechanism (CCM) genes during light-dark (12 h:12 h) cycles in synchronized Chlamydomonas reinhardtii at air-level CO(2). CCM gene transcript levels vary at various times of light-dark cycles, even at same air-level CO(2). Transcripts of inorganic carbon transporter genes (HLA3, LCI1, CCP1, CCP2 and LCIA) and mitochondrial carbonic anhydrase genes (CAH4 and CAH5) are up regulated in light, following which their levels decline in dark. Contrastingly, transcripts of chloroplast carbonic anhydrases namely CAH6, CAH3 and LCIB are up regulated in dark. CAH3 and LCIB transcript levels reached maximum by the end of dark, followed by high expression into early light period. In contrast, CAH6 transcript level stayed high in dark, followed by high level even in light. Moreover, the up regulation of transcripts in dark was undone by high CO(2), suggesting that the dark induced CCM transcripts were regulated by CO(2) even in dark when CCM is absent. Thus while the CAH3 transcript level modulations appear not to positively correlate with that of CCM, the protein regulation matched with CCM status: in spite of high transcript levels in dark, CAH3 protein reached peak level only in light and localized entirely to pyrenoid, a site functionally relevant for CCM. Moreover, in dark, CAH3 protein level not only reduced but also the protein localized as a diffused pattern in chloroplast. We propose that transcription of most CCM genes, followed by protein level changes including their intracellular localization of a subset is subject to light-dark cycles.

State-transitions facilitate robust quantum yields and cause an over-estimation of electron transport in Dunaliella tertiolecta cells held at the CO₂ compensation point and re-supplied with DIC

Photosynthetic energy consumption and non-photosynthetic energy quenching processes are inherently linked. Both processes must be controlled by the cell to allow cell maintenance and growth, but also to avoid photodamage. We used the chlorophyte algae Dunaliella tertiolecta to investigate how the interactive regulation of photosynthetic and non-photosynthetic pathways varies along dissolved inorganic carbon (DIC) and photon flux gradients. Specifically, cells were transferred to DIC-deplete media to reach a CO₂ compensation before being re-supplied with DIC at various concentrations and different photon flux levels. Throughout these experiments we monitored and characterized the photophysiological responses using pulse amplitude modulated fluorescence, oxygen evolution, 77 K fluorescence emission spectra, and fast-repetition rate fluorometry. O₂ uptake was not significantly stimulated at DIC depletion, which suggests that O₂ production rates correspond to assimilatory photosynthesis. Fluorescence-based measures of relative electron transport rates (rETRs) over-estimated oxygen-based photosynthetic measures due to a strong state-transitional response that facilitated high effective quantum yields. Adoption of an alternative fluorescence-based rETR calculation that accounts for state-transitions resulted in improved linear oxygen versus rETR correlation. This study shows the extraordinary capacity of D. tertiolecta to maintain stable effective quantum yields by flexible regulation of state-transitions. Uncertainties about the control mechanisms of state-transitions are presented.

Biochemical characterization of the δ-carbonic anhydrase from the marine diatom Thalassiosira weissflogii, TweCA

Diatom genome sequences clearly reveal the presence of different systems for HCO3(-) uptake. Carbon-concentrating mechanisms (CCM) based on HCO3(-) transport and a plastid-localized carbonic anhydrase (CA, EC 4.2.1.1) appear to be more probable than the others because CAs have been identified in the genome of many diatoms. CAs are key enzymes involved in the acquisition of inorganic carbon for photosynthesis in phytoplankton, as they catalyze efficiently the interconversion between carbon dioxide and bicarbonate. Five genetically distinct classes of CAs exist, α-, β-, γ-, δ- and ζ and all of them are metalloenzymes. Recently we investigated for the first time the catalytic activity and inhibition of the δ-class CA from the marine diatom Thalassiosira weissflogii, named TweCA. This enzyme is an efficient catalyst for the CO2 hydration and its inhibition profile with sulfonamide/sulfamate and anions have also been investigated. Here, we report the detailed biochemical characterization and chemico-physical properties of the δ-CA of T. weissflogii. The δ-CA encoding gene was cloned and expressed in Artic Express cells and the recombinant protein purified to homogeneity. Interesting to note that TweCA has no intrinsic esterase activity with 4-nitrophenyl acetate (pNpA) as substrate although the phylogenetic analysis showed that δ-CAs are closer to the α-CAs than to the other classes of such enzymes.

Substrate supply for calcite precipitation in Emiliania huxleyi: assessment of different model approaches

Over the last four decades, different hypotheses of Ca(2+) and dissolved inorganic carbon transport to the intracellular site of calcite precipitation have been put forth for Emiliania huxleyi (Lohmann) Hay & Mohler. The objective of this study was to assess these hypotheses by means of mathematical models. It is shown that a vesicle-based Ca(2+) transport would require very high intravesicular Ca(2+) concentrations, high vesicle fusion frequencies as well as a fast membrane recycling inside the cell. Furthermore, a kinetic model for the calcification compartment is presented that describes the internal chemical environment in terms of carbonate chemistry including calcite precipitation. Substrates for calcite precipitation are transported with different stoichiometries across the compartment membrane. As a result, the carbonate chemistry inside the compartment changes and hence influences the calcification rate. Moreover, the effect of carbonic anhydrase (CA) activity within the compartment is analyzed. One very promising model version is based on a Ca(2+) /H(+) antiport, CO2 diffusion, and a CA inside the calcification compartment. Another promising model version is based on an import of Ca(2+) and HCO3 (-) and an export of H(+) .

Insertional suppressors of Chlamydomonas reinhardtii that restore growth of air-dier lcib mutants in low CO2

Chlamydomonas reinhardtii and other microalgae show adaptive changes to limiting CO(2) conditions by induction of CO(2)-concentrating mechanisms. The limiting-CO(2)-inducible gene, LCIB, encodes a soluble plastid protein and is proposed to play a role in trapping CO(2) released by CAH3 (thylakoid lumen carbonic anhydrase) catalyzed dehydration of accumulated Ci, especially in low CO(2) (L-CO(2); ~0.04% CO(2)) conditions. To gain further insight into the mechanisms of Ci uptake and accumulation in L-CO(2) acclimated C. reinhardtii, we performed an insertional mutagenesis screen to isolate extragenic suppressors that restore the growth of lcib mutants (pmp1 and ad1) in L-CO(2). Four independent suppressors are described here and classified by their photosynthetic affinities for Ci and expression patterns of known limiting-CO(2)-inducible transcripts. Genetic analysis of the four suppressors identified two allelic, dominant suppressors (su4 and su5), and two recessive suppressors (su1 and su8). Consistent with the suppression phenotype, both the relative affinities of photosynthetic O(2) evolution and internal Ci accumulation in all four suppressors were substantially increased relative to pmp1/ad1 in L-CO(2) acclimated cells. The relative affinities of pmp-su1 and ad-su8 for Ci were nearly the same as wild type, but that of pmp-su4/su5 was intermediate between pmp-su1 and pmp1. Also, the interactions between lcib mutations and each of the three suppressors varied over the range of CO(2) acclimation states. Our results suggest complex contributions of LCIB-dependent and independent active Ci uptake/accumulation systems in various CO(2) acclimation states and therefore provide new clues about the roles played by LCIB in limiting Ci acclimation.

Evidence for the occurrence of photorespiration in synurophyte algae

The fluxes of CO(2) and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO(2,) only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa, when O(2) evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO(2), indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO(2) was observed similar to the post-illumination burst characteristic of C(3) higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO(2) burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O(2). This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O(2) evolution rates in all four algae, measured by O(2) electrode, were significantly higher (40-50%) in media at low O(2) (4%) than in air-equilibrated (21% O(2)) media, indicating an O(2) inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.

Carbon dioxide concentrating mechanism in Chlamydomonas reinhardtii: inorganic carbon transport and CO2 recapture

Many microalgae are capable of acclimating to CO(2) limited environments by operating a CO(2) concentrating mechanism (CCM), which is driven by various energy-coupled inorganic carbon (Ci; CO(2) and HCO(3)(-)) uptake systems. Chlamydomonas reinhardtii (hereafter, Chlamydomonas), a versatile genetic model organism, has been used for several decades to exemplify the active Ci transport in eukaryotic algae, but only recently have many molecular details behind these Ci uptake systems emerged. Recent advances in genetic and molecular approaches, combined with the genome sequencing of Chlamydomonas and several other eukaryotic algae have unraveled some unique characteristics associated with the Ci uptake mechanism and the Ci-recapture system in eukaryotic microalgae. Several good candidate genes for Ci transporters in Chlamydomonas have been identified, and a few specific gene products have been linked with the Ci uptake systems associated with the different acclimation states. This review will focus on the latest studies on characterization of functional components involved in the Ci uptake and the Ci-recapture in Chlamydomonas.

The expression of a carbon concentrating mechanism in Chlamydomonas acidophila under variable phosphorus, iron, and CO2 concentrations

The CO(2) acquisition was analyzed in Chlamydomonas acidophila at pH 2.4 in a range of medium P and Fe concentrations and at high and low CO(2) condition. The inorganic carbon concentrating factor (CCF) was related to cellular P quota (Q(p)), maximum CO(2)-uptake rate by photosynthesis (V(max,O2)), half saturation constant for CO(2) uptake (K(0.5)), and medium Fe concentration. There was no effect of the medium Fe concentration on the CCF. The CCF increased with increasing Q(p) in both high and low CO(2) grown algae, but maximum Q(p) was 6-fold higher in the low CO(2) cells. In high CO(2) conditions, the CCF was low, ranging between 0.8 and 3.5. High CCF values up to 9.1 were only observed in CO(2)-limited cells, but P- and CO(2)-colimited cells had a low CCF. High CCF did not relate with a low K(0.5) as all CO(2)-limited cells had a low K(0.5) (<4 μM CO(2)). High C(i)-pools in cells with high Q(p) suggested the presence of an active CO(2)-uptake mechanism. The CCF also increased with increasing V(max,O2) which reflect an adaptation to the nutrient in highest demand (CO(2)) under balanced growth conditions. It is proposed that the size of the CCF in C. acidophila is more strongly related to porter density for CO(2) uptake (reflected in V(max,O2)) and less- to high-affinity CO(2) uptake (low K(0.5)) at balanced growth. In addition, high CCF can only be realized with high Q(p).

PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID-TOLERANT, FREE-LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)(1)

The processes of CO2 acquisition were characterized for the acid-tolerant, free-living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO3 (-) uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO2 formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO2 uptake and O2 evolution by membrane-inlet mass spectrometry demonstrated that air-grown cells reduced the CO2 concentration in the medium to an equilibrium concentration of 15 μM, but AZA-treated cells caused a drop in extracellular CO2 concentration to a compensation concentration of 27 μM at pH 8.0. CO2 -pulsing experiments with cells in the light indicated that the cells do not actively take up CO2 . An internal pool of unfixed inorganic carbon was not detected at the CO2 compensation concentration, probably because of the lack of active CO2 uptake, but was detectable at times before compensation point was reached. These results indicate that this free-living Coccomyxa possesses a CO2 -concentrating mechanism (CCM) due to an active bicarbonate-uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.

Mechanism of CO2 acquisition in an acid-tolerant Chlamydomonas

The acid-tolerant green alga Chlamydomonas (UTCC 121) grows in media ranging in pH from 2.5 to 7.0. Determination of the overall internal pH of the cells, using (14)C-benzoic acid (BA) or [2-(14)C]-5,5-dimethyloxazolidine-2,4-dione (DMO), showed that the cells maintain a neutral pH (6.6 to 7.2) over an external pH range of 3.0-7.0. The cells express an external carbonic anhydrase (CA) when grown in media above pH 5.5, and CA increases to a maximum at pH 7.0. Removal of external CA by trypsin digestion or by acetazolamide (AZA) inhibition indicated that CA was essential for photosynthesis at pH 7.0 and that the cells had no capacity for direct bicarbonate uptake. Monitoring of CO(2) uptake and O(2) evolution by mass spectrometry during photosynthesis did not provide any evidence of active CO(2) uptake. The CO(2) compensation concentration of the cells ranged from 9.4 microM at pH 4.5 to 16.2 microM at pH 7.0. An examination of the kinetics of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), in homogenates of cells grown at pH 7.0, showed that the K(m) (CO(2)) was 16.3 microM. These data indicate that the pH between the cell interior and the external medium was large enough at acid pH to allow the accumulation of inorganic carbon (Ci) by the diffusive uptake of CO(2), and the expression of external CA at neutral pH values would maintain an equilibrium CO(2) concentration at the cell surface. This species does not possess a CO(2)-concentrating mechanism because the whole cell affinity for Ci appears to be determined by the low K(m) (CO(2)) Rubisco of the alga.

Carbon acquisition mechanisms by planktonic desmids and their link to ecological distribution

To test if different inorganic carbon (Ci) uptake mechanisms underlie the ecological distribution pattern of planktonic desmids, we performed pH-drift experiments with 12 strains, belonging to seven species, originating from lakes of different pH. Staurastrum brachiatum Ralfs and Staurodesmus cuspidatus (Ralfs) Teil. var. curvatus (W. West) Teil., species confined to acidic, soft water habitats, showed remarkably different behavior in the pH drift experiments: S. brachiatum appeared to use CO2 only, whereas Staurodesmus cuspidatus appeared to use HCO3– as well. Staurastrum chaetoceras (Schr.) Smith and Staurastrum planctonicum Teil, species well-known for their abundant occurrence in alkaline waters, were the most effective at using HCO3–. Other species, to be encountered in both slightly acidic and slightly alkaline waters, took an intermediate position. Experiments using specific inhibitors suggested that Cosmarium abbreviatum Rac. var. planctonicum W. & G.S. West and S. brachiatum use CO2 by an active CO2 uptake mechanism, whereas S. chaetoceras and Staurodesmus cuspidatus showed an active HCO3– uptake pattern. Most likely, these active uptake mechanisms make use of H+-ATPase, as none of the desmids expressed significant carbonic anhydrase activity. A series of strains of Staurastrum planctonicum isolated from different habitats, all clustered in between the species using HCO3–, but no further differentiation was observed. Therefore, desmids cannot be simply characterized as exclusive CO2 users, and the ecological distribution pattern of a desmid species does not unequivocally link to a certain Ci uptake mechanism. Nevertheless, there does appear to be a general ecological link between a species' Ci uptake mechanism and its ecological distribution.Key words: pH drift, desmids, isolate variation, inorganic carbon acquisition.

Mechanisms of inorganic carbon acquisition in twoEuglenaspecies

The mechanism of inorganic carbon uptake was examined in Euglena gracilis Klebs. and the acidophilic species Euglena mutabilis Schmitz. Both species, whether grown in acidic (pH 3.5) or alkaline (pH 7.5) media lack external carbonic anhydrase. Acid-grown E. gracilis was shown to have no capacity for bicarbonate transport, but transport was induced on transfer to alkaline medium (pH 7.5) in the light over a period of 8 h. In contrast, acid-grown E. mutabilis appears to have no capacity for bicarbonate transport even at neutral pH. The overall internal pH of the cells was determined by equilibration with14C-labelled benzoic acid over the pH range 3.5–5.0 and with14C-labelled 5,5-dimethyloxazolidine-2,4-dione over the range pH 5.5–7.5. The acidophilic species maintains an internal pH range of 6.6–6.8 in an external pH range of 3.5–5.5, whereas the acid-tolerant species E. gracilis maintains a neutral internal pH in an external pH range of 3.5–7.5. Measurement, by mass spectrometry, of the fluxes of CO2and O2in photosynthesizing cells at pH 3.5 demonstrated a rapid uptake of CO2by both species that was completely blocked by iodoacetamide, an inhibitor of CO2fixation. Uptake of CO2by E. gracilis, grown at pH 7.5, was not completely inhibited by iodoacetamide and O2evolution was sustained when the cells reached the CO2compensation concentration, indicating a direct uptake of bicarbonate. These data indicate that the acidophilic species, E. mutabilis, takes up CO2by diffusion, whereas the acid-tolerant species, E. gracilis, takes up CO2by diffusion at acid pH levels but has some capacity for active bicarbonate uptake when grown at alkaline pH levels.Key words: acidophilic alga, acidotolerant alga, bicarbonate uptake, CO2uptake, Euglena gracilis, Euglena mutabilis, internal pH.

Inorganic carbon acquisition by Chlamydomonas acidophila across a pH range

Chlamydomonas acidophila Negoro had a higher maximum growth rate upon aeration with 5% CO2 (v/v) than in nonaerated conditions at an external pH above 2. In medium with a pH of 1.0 or 2.0, a decrease in the maximum growth rate was observed upon CO2 aeration in comparison with nonaerated conditions. At both very low and very high external pH conditions, an induction of external carbonic anhydrase was detected; this being more pronounced in CO2-aerated cells than in nonaerated cells. It is therefore suggested that the induction of carbonic anhydrase is part of a stress response in Chlamydomonas acidophila. Comparison of some physiological characteristics of Chlamydomonas acidophila acclimated at pH 2.65 and at pH 6.0, revealed that CO2 aeration increased gross maximum photosynthesis at both pHs, whereas respiration, light acclimation, and photoinhibition were not effected. At pH 2.65, Chlamydomonas acidophila was found to have a carbon-concentrating mechanism under nonaerated conditions, whereas it did not under CO2-aerated conditions at pH 6. The affinity for CO2 use in O2 production was not dependent on CO2 aeration, but it was much lower at pH 6 than it was at pH 2.65. CO2 kinetic characteristics indicate that the photosynthesis of Chlamydomonas acidophila in its natural environment is not limited by inorganic carbon.Key words: Chlamydomonas acidophila, CCM, external carbonic anhydrase, photosynthesis, growth rates, pH stress, CO2.

Photosynthetic utilisation of inorganic carbon and its regulation in the marine diatom Skeletonema costatum

Photosynthetic uptake of inorganic carbon and regulation of photosynthetic CO₂ affinity were investigated in Skeletonema costatum (Grev.) Cleve. The pH independence of K_1/2(CO₂) values indicated that algae grown at either ambient (12 μmol L^-1) or low (3 μmol L^-1) CO₂ predominantly took up CO₂ from the medium. The lower pH compensation point (9.12) and insensitivity of photosynthetic rate to di-isothiocyanatostilbene disulfonic acid (DIDS) indicated that the alga had poor capacity for direct HCO₃^- utilisation. Photosynthetic CO₂ affinity is regulated by the concentration of CO₂ rather than HCO₃^-, CO₃^2- or total dissolved inorganic carbon (DIC) in the medium. The response of photosynthetic CO₂ affinity to changes in CO₂ concentration was most sensitive within the range 3-48 μmol L^-1 CO₂. Light was required for the induction of photosynthetic CO₂ affinity, but not for its repression, when cells were shifted between high (126 μmol L^-1) and ambient (12 μmol L^-1) CO₂. The time needed for cells grown at high CO₂ (126 μmol L^-1) to fully develop photosynthetic CO₂ affinity at ambient CO₂ was approximately 2 h, but acclimation to low or very low CO₂ levels (3 and 1.3 μmol L^-1, respectively) took more than 10 h. Cells grown at low CO₂ (3 μmol L^-1) required approximately 10 h for repression of all photosynthetic CO₂ affinity when transferred to ambient or high CO₂ (12 or 126 μmol L^-1, respectively), and more than 10 h at very high CO₂ (392 μmol L^-1).

The Chlamydomonas reinhardtii cia3 mutant lacking a thylakoid lumen-localized carbonic anhydrase is limited by CO2 supply to rubisco and not photosystem II function in vivo

The Chlamydomonas reinhardtii cia3 mutant has a phenotype indicating that it requires high-CO(2) levels for effective photosynthesis and growth. It was initially proposed that this mutant was defective in a carbonic anhydrase (CA) that was a key component of the photosynthetic CO(2)-concentrating mechanism (CCM). However, more recent identification of the genetic lesion as a defect in a lumenal CA associated with photosystem II (PSII) has raised questions about the role of this CA in either the CCM or PSII function. To resolve the role of this lumenal CA, we re-examined the physiology of the cia3 mutant. We confirmed and extended previous gas exchange analyses by using membrane-inlet mass spectrometry to monitor(16)O(2),(18)O(2), and CO(2) fluxes in vivo. The results demonstrate that PSII electron transport is not limited in the cia3 mutant at low inorganic carbon (Ci). We also measured metabolite pools sizes and showed that the RuBP pool does not fall to abnormally low levels at low Ci as might be expected by a photosynthetic electron transport or ATP generation limitation. Overall, the results demonstrate that under low Ci conditions, the mutant lacks the ability to supply Rubisco with adequate CO(2) for effective CO(2) fixation and is not limited directly by any aspect of PSII function. We conclude that the thylakoid CA is primarily required for the proper functioning of the CCM at low Ci by providing an ample supply of CO(2) for Rubisco.

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.

Limiting CO2 levels induce a blue light-dependent HCO3- uptake system in Monoraphidium braunii

The in situ photoactivation of an HCO3- uptake system in the green alga Monoraphidium braunii requires the irradiation of the cell suspensions with short wavelength radiation (blue, UVA and/or UVC). Plasma membrane ATPase inhibitors block the uptake of this monovalent anion at pH 9. M. braunii cells grown in high CO2 lack an HCO3- uptake system in their plasma membrane, but those grown in low CO2 can take up this anion at high rates. Cells grown in high CO2, transferred to CO2-limiting conditions in the light, start taking up HCO3- in 30 min, although they take 90 min to reach maximum rates of HCO3- transport. Therefore, this induction process seems to be triggered by low external CO2 concentration. In fact, increasing or decreasing the external HCO3- concentration does not induce the uptake system and only a decrease in CO2 concentration in the medium triggers the induction process. The appearance of the HCO3- transport activity is sensitive to cycloheximide, indicating that cytoplasmic protein biosynthesis is necessary for the induction of the uptake system. Photosynthetically active radiation, but not particularly blue light, is essential for induction of the uptake system to occur and the inhibition of photosynthesis by DCMU blocks it. From these results it can be inferred that when M. braunii cells detect a drop in CO2 concentration, they induce a blue light-dependent HCO3- uptake system.

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.

Models of CO2 concentrating mechanisms in microalgae taking into account cell and chloroplast structure

Detailed mathematical models have been developed for the functioning of CO2 concentration mechanisms in microalgae. The models treat a microalgal cell as several compartments: pyrenoid, chloroplast stroma, cytoplasm and periplasmic space. Cases for both the active bicarbonate transport through the plasmalemma and the passive CO2 diffusion through it with the subsequent concentrating of CO2 inside the chloroplast are analyzed. CO2 evolution from bicarbonate inside the pyrenoid is modeled. The great diffusion resistance for CO2 flux from the pyrenoid is caused by a starch envelope and the concentric thylakoid membranes surrounding it. The role of carbonic anhydrase in the periplasmic space, cytoplasm and inside the chloroplast is evaluated numerically. The models also offer an explanation for the absence of 'short-circuited' inorganic carbon fluxes between the external medium and the cytoplasm under active bicarbonate transport through the plasmalemma and in the presence of carbonic anhydrase in the cytoplasm. If the cytoplasm is driven from the space between a chloroplast envelope and plasmalemma upon the microalgae adaptation to low concentration of the dissolved inorganic carbon, the inorganic carbon leak might be avoided. The models reproduce accurately the majority of known experimental data. The high efficiency of CO2 concentrating mechanisms in microalgae can be explained by a considerable diffusion resistance for CO2 flux from the pyrenoid and by the effective scavenging of CO2 leaking outward from the chloroplast to cytoplasm and from cell to periplasmic space.

Regulation of the mechanism for HCO (3) (-) use by the inorganic carbon level in Porphyra leucosticta Thur. in Le Jolis (Rhodophyta)

The capacity for HCO (3) (-) use by Porphyra leucosticta Thur. in Le Jolis grown at different concentrations of inorganic carbon (C(i)) was investigated. The use of HCO (3) (-) at alkaline pH by P. leucosticta was demonstrated by comparing the O(2) evolution rates measured with the O(2) evolution rates theoretically supported by the CO(2) spontaneously formed from HCO (3) (-) . Both external and internal carbonic anhydrase (CA; EC 4.2.1.1) were implied in HCO (3) (-) use during photosynthesis because O(2) evolution rates and the increasing pH during photosynthesis were inhibited in the presence of azetazolamide and ethoxyzolamide (inhibitors for external and total CA respectively). Both external and internal CA were regulated by the C(i) level at which the algae were grown. A high C(i) level produced a reduction in total CA activity and a low C(i) level produced an increase in total CA activity. In contrast, external CA was increased at low C(i) although it was not affected at high C(i). Parallel to the reduction in total CA activity at high C(i) is a reduction in the affinity for C(i), as estimated from photosynthesis versus C(i) curves, was found. However, there was no evident relationship between external CA activity and the capacity for HCO (3) (-) use because the presence of external CA became redundant when P. leucosticta was cultivated at high C(i). Our results suggest that the system for HCO (3) (-) use in P. leucosticta is composed of different elements that can be activated or inactivated separately. Two complementary hypotheses are postulated: (i) internal CA is an absolute requirement for a functioning C(i)-accumulation mechanism; (ii) there is a CO(2) transporter that works in association with external CA.

Dissolved inorganic carbon utilization and the development of extracellular carbonic anhydrase by the marine diatom Phaeodactylum tricornutum

The presence of extracellular carbonic anhydrase (CA) in relation to medium composition was investigated using cultures of the marine diatom Phaeodactylum tricornutum Bohlin. Large-volume cultures, with low initial cell inocula were grown on ASP-2 (no dissolved inorganic carbon (DIC), 550 μM NO_a ^- ), f/2 (2-0 mM DIC, 880μM NO₃ ^- ) and modified f/2 (2.0 mM DIC, 20 μM NO₃ ^- media. Cells growing on ASP-2 showed extracellular CA in the early stages of growth, whereas extracellular CA was not detected until partial depletion of total DIC in the stationary phase for cultures on f/2 or modified f/2. Both HCO₃ ^- and CO₂ were important in carbon limitation, extracellular CA being present when the free-CO₂ concentration fell below 5 μM, but the HCO₃ ^- concentration needed to be below 1 mM. When carbon-replete cells were transferred to carbon-limited conditions, extracellular CA was recorded within minutes, the process being light-dependent and completely inhibited by 3,3,4-dichlorophenyl-l, 1-dimethylurea (DCMU). The addition of DIC to carbon-limited cells resulted in a rapid decrease in extracellular CA activity. The membrane-impermeable inhibitor of carbonic anhydrase, dextran-bound sulphonamide (DBS) was used to inhibit extracellular CA activity in relation to photosynthetic rate in carbon-replete and carbon-limited cells. At the lowest DIC concentration (O'lOniM), for cells with maximum external CA activity, DBS gave over 80% inhibition of the photosynthetic rate, demonstrating the key role of external CA in maintaining high photosynthetic rate under conditions of carbon limitation. It is proposed that the key factor in the regulation of extracellular CA activity is the total Hux of inorganic carbon (C.) into the cell. This determines the C_i , flux into the chloroplast and when this is inadequate to support the photosynthetic rate attained by a carbon-replete chloroplast at optimum photon flux density, extracellular CA is activated. This mechanism would explain the observed interaction of CO₂ and HCO₃ ^- in the regulation of extracellular CA activity.

A salt-resistant plasma membrane carbonic anhydrase is induced by salt in Dunaliella salina

The mechanisms allowing proliferation of the unicellular green alga Dunaliella salina in up to saturating NaCl concentrations are only partially understood at present. Previously, the level of a plasma membrane Mr 60,000 protein, p60, was found to increase with rising external salinities. Based on cDNA cloning and enzymatic assays, it is now shown that p60 is an internally duplicated carbonic anhydrase, with each repeat homologous to animal and Chlamydomonas reinhardtii carbonic anhydrases, but exceptional in the excess of acidic over basic residues. Increasing salinities, alkaline shift, or removal of bicarbonate induced in D. salina parallel increases in the levels of p60, its mRNA, and external carbonic anhydrase activity. Moreover, purified p60 exhibited carbonic anhydrase activity comparable to other carbonic anhydrases. A p60-enriched soluble preparation showed maximal carbonic anhydrase activity at approximately 1.0 M NaCl and retained considerable activity at higher salt concentrations. In contrast, a similar preparation from C. reinhardtii was approximately 90% inhibited in 0.6 M NaCl. These results identified p60 as a structurally novel carbonic anhydrase transcriptionally regulated by CO2 availability and exhibiting halophilic-like characteristics. This enzyme is potentially suited to optimize CO2 uptake by cells growing in hypersaline media.

Induction of CO2 and Bicarbonate Transport in the Green Alga Chlorella ellipsoidea (II. Evidence for Induction in Response to External CO2 Concentration)

The critical species and concentrations of dissolved inorganic carbon (DIC) required for the induction of DIC transport during adaptation to low CO2 were determined for the green alga Chlorella ellipsoidea. The concentration of dissolved CO2 needed for the induction of both CO2 and HCO3- transport was independent of pH during adaptation, whereas the total DIC concentration required increased at alkaline pH. At pH 7.5, the minimum equilibrium DIC concentration at which high CO2 characteristics were maintained, i.e. transport was repressed, was 2100 [mu]M, whereas the maximum equilibrium DIC concentration below which DIC transport was fully induced (DICIND) was 500 [mu]M. Intracellular DIC concentration during adaptation to DICIND decreased temporarily after 2 h to 60% of the maximum level but recovered after 3 h of adaptation. After 3 h of adaptation to DICIND, cells exhibited maximum O2 evolution rate at DICIND. When cells partially adapted to DICIND were returned to high CO2, there was an immediate halt to the induction of transport and a gradual decrease in transport capacity over 23 h. The capacity for the induction of transport was unaffected by the absence of light. These results indicate that changes in the internal DIC pool during adaptation to low CO2 do not trigger the induction of DIC transport and that the induction is not light dependent. Induction of DIC transport in C. ellipsoidea appears to occur in response to the continuous exposure of cells to a critical CO2 concentration in the external medium.

Induction of CO2 and Bicarbonate Transport in the Green Alga Chlorella ellipsoidea (I. Time Course of Induction of the Two Systems)

Changes in the physiological properties of the green alga Chlorella ellipsoidea (UTEX 20) were determined during adaptation from high CO2 to air. Cells of C. ellipsoidea, grown in high CO2, had an extremely low affinity for dissolved inorganic carbon (DIC). However, high-affinity DIC transport was induced rapidly after switching to air, which caused a massive decrease in the DIC concentration in the medium. Rates of O2 evolution without added carbonic anhydrase (CA) were compared with calculated rates of uncatalyzed CO2 formation in the medium as a measure of active HCO3-uptake. Cells were found to be able to use HCO3- after 5 h of adaptation and this capacity increased during the next 17 h. The stimulation of O2 evolution upon CA addition was used as a measurement of active CO2 transport: such stimulation occurred 2 h after transfer and increased during the next 5 h. Increases in O2 evolution rates were correlated closely with an increasing capacity to accumulate intracellular pools of acid-labile DIC and with decreases in K1/2(CO2) and CO2-compensation point of the cells. Treatment of cells with cycloheximide (5 [mu]g mL-1) during adaptation completely inhibited DIC transport induction, whereas treatment with chloramphenicol (400 [mu]g mL-1) had no effect, indicating the requirement for cytoplasmic protein synthesis in the induction. These results suggest that both CO2 and HCO3- transport are induced upon transfer of cells from high CO2 to air and that there is a temporal separation between the induction of the two systems.

Quantification of the Contribution of CO2, HCO3-, and External Carbonic Anhydrase to Photosynthesis at Low Dissolved Inorganic Carbon in Chlorella saccharophila

An equation has been developed incorporating whole-cell rate constants for CO2 and HCO3- that describes accurately photosynthesis (Phs) in suspensions of unicellular algae at low dissolved inorganic carbon. At pH 8.0 the concentration of CO2 available to the algal cells depends on the rate of supply from, and the loss to, HCO3- and the rate of use by the cells. At elevated cell densities (>30 mg chlorophyll [Chl] L-1), at which CO2 use by the cells is high, the slope of a graph of absolute Phs versus Chl concentration approaches the rate of Phs on a milligram of Chl basis because of HCO3- use alone. The slope of a graph of Phs versus HCO3- will be the rate constant for HCO3-, and for Chlorella saccharophila it was 0.16 L mg-1 Chl h-1. The difference between the constants for dissolved inorganic carbon (measured in cells with external carbonic anhydrase) and HCO3-1 is the constant for CO2, which was 26 L mg-1 Chl h-1. This difference causes the half-saturation constant for Phs to increase 5- to 6-fold at high cell densities. The increase in CO2 use as a result of external carbonic anhydrase is described mathematically as a function of cell density.

Regulation of Periplasmic Carbonic Anhydrase Expression in Chlamydomonas reinhardtii by Acetate and pH

The effects of mixotrophic growth with acetate and growth medium pH on expression of extracellular carbonic anhydrase (CA) in Chlamydomonas reinhardtii were evaluated. Addition of 10 mM acetate to the culture medium resulted in reduction of CA activity that was parallel to the reduction generated by growth of the algae in high external CO2 concentrations. This reduction in activity is a consequence of lower level of the CA protein as determined by western analysis. Transcript abundance of cah-1, the gene encoding the low CO2-induced CA, is also reduced by the addition of acetate as verified by northern analysis. Measurements of photosynthesis and respiration suggest that the acetate-induced reduction of CA expression is not a function of lowered photosynthetic capacity, but may be the result of increased internal CO2 concentration generated by high, acetate-stimulated respiratory rates. Growth medium pH can also influence extracellular CA expression. The induction of CA activity, protein abundance, and transcript levels by exposure to limiting inorganic carbon (Ci) concentrations is much more pronounced at higher than at lower pH values. The relationship between pH regulation of CA expression and its role in the Ci-concentrating mechanism are discussed.

Active uptake of CO2 during photosynthesis in the green alga Eremosphaera viridis is mediated by a CO2-ATPase

Mass spectrometry was used to investigate the uptake of CO2 in Eremosphaera viridis DeBary. Upon illumination, cells preincubated at pH 7.5 with 100 μM dissolved inorganic carbon (DIC) rapidly depleted almost all the free CO2 from the medium. Rapid equilibrium between HCO 3 (-) and CO2 occurred upon addition of bovine carbonic anhydrase (CA) to the medium, showing that CO2 depletion resulted from a selective uptake of CO2 rather than an uptake of all inorganic carbon species. Glycolaldehyde (10 mM) completely inhibited CO2 fixation but had little effect on CO2 transport. Transfer of glycolaldehyde-treated cells to the dark caused a rapid efflux of CO2 from the unfixed intracellular DIC pool which was found to be at least threeto sixfold higher in concentration than that of the external medium. These results indicate that E. viridis actively transports CO2 against a concentration gradient. No external CA was detected in these cells either by potentiometric or mass-spectrometric assay. In the absence of external CA, the rate of photosynthetic O2 evolution in the pH range 7.5 to 8.0 did not exceed the calculated rate of CO2 supply, indicating a limited capacity for HCO2 uptake in these cells. Electrophysiological measurements indicate that CO2 uptake is electrically silent and thus is not a consequence of H(+)-CO2 symport activity. Microsomal membranes isolated from Eremosphaera showed ATPase activity which was enhanced by CO2. These results indicate that active CO2 uptake is mediated by an ATPase.