Active CO2 Transport Research Articles

Scalable Biomass-Derived Hydrogels for Sustainable Carbon Dioxide Capture.

Carbon capture and sequestration are promising emissions mitigation technologies to counteract ongoing climate change. The aqueous amine scrubbing process is industrially mature but suffers from low energy efficiency and inferior stability. Solid sorbent-based carbon capture systems present a potentially advantageous alternative. However, practical implementation remains challenging due to limited CO2 uptake at dilute concentrations and difficulty in regeneration. Here, we develop sustainable carbon-capture hydrogels (SCCH) with an excellent CO2 uptake of 3.6 mmol g-1 (400 ppm) at room temperature. The biomass gel network consists of konjac glucomannan and hydroxypropyl cellulose, facilitating hierarchically porous structures for active CO2 transport and capture. Precaptured moisture significantly enhances CO2 binding by forming water molecule-stabilized zwitterions to improve the amine utilization efficiency. The thermoresponsive SCCH exhibits a notable advantage of low regeneration temperature at 60 °C, enabling solar-powered regeneration and highlighting the potential for sustainable carbon capture to meet global decarbonization targets.

Calcification and ocean acidification: new insights from the coccolithophore Emiliania huxleyi

Calcification and ocean acidification: new insights from the coccolithophore <i>Emiliania huxleyi</i>

Involvement of potassium channel in AQP1-mediated water and gas transport in erythrocytes

Elevated plasma cholesterol affects nearly 100 million Americans, a third of the US population, resulting in devastating health consequences and nearly one million deaths annually. Although the long-term effect of elevated plasma cholesterol is known for its accretion in arterial walls, little was understood of its immediate effects until recently (Jena and Lee, 2010). Cholesterol in blood plasma is known to rapidly enter and incorporate into the red blood cell (RBC) plasma membrane (Tall, 1993; Glomset et al., 1995; Ishizake et al., 1994). As early as the 1970s, it was demonstrated in human studies that blood plasma and RBC membrane cholesterol levels are inversely associated with the transmembrane O2 diffusion rate of RBC (Steinbach et al., 1974): as cholesterol levels in the blood plasma and consequently in the RBC membrane decrease, O2 diffusion in RBC increases (Steinbach et al., 1974; Menchaca et al., 2004). Then in 2010 (Jena and Lee, 2010), we reported for the first time that the immediate effect of elevated plasma cholesterol is dysfunction of active water and CO2 transport via the AQP1 channel through the RBC membrane. The effect of the phospholipase A2 (PLA2) inhibitor ONO-RS-082 (ONO) on ameliorating the detrimental effects of cholesterol was further demonstrated from that study (Jena and Lee, 2010). In the past decade, the tetradecapeptide wasp venom mastoparan, GTP, the GTP-binding heterotrimeric Gαi/ Gαo proteins, vH +-ATPase, PLA2, the K+ and Cl– channels, and aquaporins (Abu-Hamdah et al., 2004), have been implicated in the gating of water into secretory vesicles. A similar mechanism operating at the RBC membrane is therefore hypothesized. Gαi3 has been implicated in various tissues to regulate both K+ and Cl– ion channels at the plasma membrane (Delport et al., 1997; Knepper and Inoue, 1994, 1997). PLA2 has also been found to regulate both K+ and Cl– ion channels. Since Gαi3 and PLA2 are present at the RBC membrane (Mukherjee and Maxfield, 2004; Hillman et al., 2005), I hypothesize the involvement of both Gαi3 and PLA2 in the regulation of AQP1 activity. Furthermore, since K+ and Cl– ion channels are present at the RBC membrane (Hillman et al., 2005), the involvement of Gαi3 in the regulation of K+ and Cl– ion channels at the RBC membrane and its possible implication for AQP1-mediated water entry were investigated. Results from my studies demonstrate that K+ and Cl– ion channels are associated with AQP1 at the RBC membrane, and that the K+ channel inhibitor Glyburide currently used in treating diabetes reverses the detrimental effects of cholesterol on RBC function. Involvement of potassium channel in AQP1-mediated water and gas transport in erythrocytes Siddhartha G. Jena*

Response of cyanobacterial carbon concentrating system to light intensity: a simulated analysis

Cyanobacteria possess a delicate system known as the carbon concentrating mechanism (CCM), which can efficiently elevate the intracellular inorganic carbon (Ci) concentration via active transportation. The system requires energy supplied by photosystems; therefore, the activity of the Ci transporter is closely related to light intensity. However, the relationship between CCM and light intensity has rarely been evaluated. Here, we present an improved quantitative model of CCM in which light is incorporated, and developed a CCM model that modified after Fridlyand et a1. in 1996. Some equations used in this model were inducted to describe the relationship between transport capacity and light intensity, by which the response of the CCM to light change is simulated. Our results indicate that the efficiency of the carbon concentrating system is sensitive to light intensity. When the external Ci concentration was low, CO2 uptake dominated the total Ci uptake with increasing light intensity, while under high external Ci concentrations HCO3− uptake primarily contributed to the total Ci uptake. Variations in the ratio of energy allocated between the transport systems could markedly affect the operation of CCM. Indeed, our simulations suggest that various combinations of Ci fluxes can provide a possible approach to detect the way by which the cell distributes energy produced by the photosystems to the two active Ci transport processes. The proportion of the energy consumed on CCM to the total energy expenditure for the fixation of one CO2 molecule was determined at 18%–40%.

Inorganic carbon acquisition in two species of marine prymnesiophytes

Inorganic carbon acquisition has been investigated in the marine prymnesiophytes Dicrateria inornata and Ochrosphaera neapolitana. The presence of external and internal carbonic anhydrase (CA) was detected in air-grown cells of D. inornata by potentiometric assay and corroborated by mass spectrometry. Growth on 3% CO2 repressed external CA activity. Extracts of air-grown cells of O. neapolitana showed considerable internal CA activity, but no external CA activity was detected by mass spectrometry. Illumination of air-grown cells of D. inornata incubated with 100 μM H13CO3 and treated with the CA inhibitor acetazolamide caused a rapid drop in the extracellular CO2 concentration to a level below its equilibrium concentration indicating that cells were actively taking up CO2 from the medium. The rate of O2 evolution at the CO: compensation concentration was higher than could be supported by the uncatalysed formation of CO2 from HCO3, . demonstrating that the cells were taking up HCO3 −directly. Similar results were obtained with air-grown cells of O. neapolitana. Active CO2 transport in both species was not repressed by growth under high levels of CO2. The photosynthetic electron transport inhibitor 3-(3′ 4′-dichlorophenyl)-l, 1-dimethylurea (DCMU) inhibited CO2 uptake in air-grown cells of both species, demonstrating that the energy to support active CO2 transport was derived from photosynthetic linear electron transport. These results show that the physiological characteristics of the CO2 concentrating mechanism in related species do not appear to be correlated with the availability of CO2 in the surrounding medium.

The energy source for CO 2 transport in the marine microalga Nannochloris atomus

CO2 fluxes in the marine microalga Nannochloris atomus were studied by mass spectrometry using inhibitors and artificial acceptors of photosynthetic electron transport to investigate the energy source for CO2 uptake. This algal species is capable of taking up CO2 from the external medium by active transport but lacks active HCO(3)(-) transport and extracellular carbonic anhydrase. The capacity of cells to take up CO2 was a function of photosynthetic photon flux density. Dark respiration rates were also dependent upon the light intensity during the preceding illumination period, indicating the presence of light-enhanced dark respiration. Addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea to illuminated cell suspensions that had been allowed to concentrate inorganic carbon internally during photosynthesis caused a rapid burst of CO2, demonstrating that active CO2 transport had been abolished. A similar response was obtained when cell suspensions were treated with 2,5-dibromo-6-isopropyl-3methyl-1,4-benzoqinone or hydroxylamine. When methyl viologen was used to drain electrons from ferredoxin, cells were still able to take up CO2 from the external medium, although C-fixation decreased with time. These results demonstrate that active CO2 transport in N. atomus is supported by photosynthetic linear electron transport.

The diversity of inorganic carbon acquisition mechanisms in eukaryotic microalgae.

Eukaryotic microalgae have developed CO2concentrating mechanisms to maximise the concentration of CO2 at the active site of Rubisco in response to the low CO2 concentrations in the external aquatic medium. In these organisms, the modes of inorganic carbon (Ci) uptake are diverse, ranging from diffusive CO2 uptake to the active transport of HCO3 -and CO2 and many have an external carbonic anhydrase to facilitate HCO3- use. There is unequivocal evidence for the mechanisms of Ci uptake in only about 25 species of microalgae of the chlorophyte, haptophyte, rhodophyte, diatom, and eustigmatophyte groups. Most of these species take up both CO2 and HCO3-, but the rates of uptake of each of these substrates varies with the algal species. A few species take up only one of the two forms of Ci, an adaptation that is not necessarily correlated with their ecological distribution. Evidence is presented for the active uptake of HCO3- and CO2 in two marine haptophytes,Isochrysis galbana Parke and Dicrateria inornata Parke, and for active transport of CO2 but lack of HCO3- uptake in two marine dinoflagellates, Amphidinium carteraeHulburt and Heterocapsa oceanica Stein.

The induction of inorganic carbon transport and external carbonic anhydrase in Chlamydomonas reinhardtii is regulated by external CO2 concentration

ABSTRACTInduction of the carbon concentrating mechanism (CCM) has been investigated during the acclimation of 5% CO2‐grown Chlamydomonas reinhardtii 2137 mt + cells to well‐defined dissolved inorganic carbon (Ci) limited conditions. The CCM components investigated were active HCO3− transport, active CO2 transport and extracellular carbonic anhydrase (CAext) activity. The CAext activity increased 10‐fold within 6 h of acclimation to 0·035% CO2 and there was a further slight increase over the next 18 h. The CAext activity also increased substantially after an 8 h lag period during acclimation to air in darkness. Active CO2 and HCO3− uptake by C. reinhardtii cells were induced within 2 h of acclimation to air, but active CO2 transport was induced prior to active HCO3− transport. Similar results were obtained during acclimation to air in darkness. The critical Ci concentrations effecting the induction of active Ci transport and CAext activity were determined by allowing cells to acclimate to various inflow CO2 concentrations in the range 0·035–0·84% at constant pH. The total Ci concentration eliciting the induction and repression of active Ci transport was higher during acclimation at pH 7·5 than at pH 5·5, but the external CO2 concentration was the same at both pHs of acclimation. The concentration of external CO2 required for the full induction and repression of Ci transport and CAext activity were 10 and 100 μM, respectively. The induction of CAext and active Ci transport are not correlated temporally, but are regulated by the same critical CO2 concentration in the medium.

Active transport of CO2 and bicarbonate is induced in response to external CO2 concentration in the green alga Chlorella kessleri

The time-course of induction of CO(2) and HCO(3)- transport has been investigated during the acclimation of high CO(2)-grown Chlorella kessleri cells to dissolved inorganic carbon (DIC)-limited conditions. The rate of photosynthesis of the cells in excess of the uncatalysed supply rate of CO(2) from HCO(3)- was taken as an indicator of HCO(3)- transport, while a stimulation of photosynthesis on the addition of bovine carbonic anhydrase was used as an indicator of CO(2) transport. The maximum rate of photosynthesis (Pmax) was similar for high CO(2)-grown and low CO(2)-grown cells, but the apparent whole cell affinity for DIC and CO(2) of high CO(2)-grown cells was found to be about 30-fold greater than in air-grown cells, which indicates a lower affinity for DIC and CO(2). It was found that HCO(3)- and CO(2) transport were induced in 5.5 h in cells acclimating to air in the light and in the presence and absence of 21% O(2), which indicates that a change in the CO(2)/O(2) ratio in the acclimating medium does not trigger induction of DIC transport. No active DIC transport was detected in high CO(2)-grown cells maintained on high CO(2) for 5.5 h in the presence of 5 mM aminooxyacetate, an aminotransferase inhibitor. These results indicate no involvement of photorespiration in triggering induction. Active DIC transport induction was inhibited in cells treated with 5 microgram ml(-1) cycloheximide, but was unaffected by chloramphenicol treatment, indicating that the induction process requires de novo cytoplasmic protein synthesis. The total DIC concentration eliciting the induction and repression of CO(2) and HCO(3)- transport was higher at pH 7.5 than at pH 6.6. The concentrations of external CO(2) required for the induction and repression of DIC transport were 0 and 120 microM, respectively, and was independent of the pH of the acclimation medium. Prolonged exposure to a critical external CO(2) concentration elicits the induction of DIC transport in C. kessleri.

Active transport of CO2 by three species of marine microalgae

The occurrence of an active CO2 transport system and of carbonic anhydrase (CA) has been investigated by mass spectrometry in the marine, unicellular rhodophyte Porphyridium cruentum (S.F. Gray) Naegeli and two marine chlorophytes Nannochloris atomus Butcher and Nannochloris maculata Butcher. Illumination of darkened cells incubated with 100 μM H13CO3− caused a rapid initial drop, followed by a slower decline in the extracellular CO2 concentration. Addition of bovine CA to the medium raised the CO2 concentration by restoring the HCO3−–CO2 equilibrium, indicating that cells were taking up CO2 and were maintaining the CO2 concentration in the medium below its equilibrium value during photosynthesis. Darkening the cell suspensions caused a rapid increase in the extracellular CO2 concentration in all three species, indicating that the cells had accumulated an internal pool of unfixed inorganic carbon. CA activity was detected by monitoring the rate of exchange of 18O from 13C18O2 into water. Exchange of 18O was rapid in darkened cell suspensions, but was not inhibited by 500 μM acetazolamide, a membrane‐impermeable inhibitor of CA, indicating that external CA activity was not present in any of these species. In all three species, the rate of exchange was completely inhibited by 500 μM ethoxyzolamide, a membrane‐permeable CA‐inhibitor, showing that an intracellular CA was present. These results demonstrate that the three species are capable of CO2 uptake by active transport for use as a carbon source for photosynthesis.

Glycolaldehyde inhibition of CO2 transport in the cyanobacterium Synechococcus UTEX 625

In studies of the inorganic carbon (Ci) concentrating mechanism of cyanobacteria glycolaldehyde is often used to inhibit photosynthetic CO2 fixation. As a partial inhibition of active CO2 transport by glycolaldehyde under steady-state Ci flux conditions has been recently suggested, in this paper, using mass spectrometry, we investigate in detail the relationship between glycolaldehyde concentration ((GLY)) and CO2 transport both during the initial period following illumination of a cyanobacterial cell suspension and during the so-called steady state when an internal Ci pool has been allowed to develop. Carbon dioxide uptake following illumination of a cyanobacterial cell suspension was progressively reduced by increasing (GLY) in the medium, both in the absence or in the presence of carbonic anhydrase. The same features were found when either Ci in the form of bicarbonate was injected to a pre-illuminated cell suspension in the presence of carbonic anhydrase in the medium or a pulse of CO2 was provided to the cells in the light without carbonic anhydrase in the extracellular medium. Although photosynthesis was completely abolished with 10 mM glycolaldehyde, CO2 uptake was only inhibited 20% but was decreased further by higher (GLY). Double reciprocal plots of CO2 uptake versus initial (CO2) determined with a range of (GLY) showed the inhibition to be noncompetitive. To simultaneously study CO2 uptake and CO2 efflux, H 13 C 18 O3 was used with cells in which CO2 transport alone, or both CO2 and HCO3 n transport system were allowed to proceed. Cells were inhibited with either iodoacetamide or glycolaldehyde and the time course of the various CO2 species in the medium demonstrated that CO2 uptake was inhibited partially with glycolaldehyde resulting in a greater CO2 efflux from the cells. When the cell suspension was again illuminated, in the presence of glycolaldehyde, initial CO2 uptake was greatly reduced, and the extracellular (CO2) eventually rose to a level higher than that which would be expected to be in equilibrium with the amount of Ci present in the medium. Results are discussed in terms of glycolaldehyde use in Ci transport mechanism studies in cyanobacteria.

Glycolaldehyde inhibition of CO2transport in the cyanobacteriumSynechococcusUTEX 625

In studies of the inorganic carbon (Ci) concentrating mechanism of cyanobacteria glycolaldehyde is often used to inhibit photosynthetic CO2fixation. As a partial inhibition of active CO2transport by glycolaldehyde under steady-state Ciflux conditions has been recently suggested, in this paper, using mass spectrometry, we investigate in detail the relationship between glycolaldehyde concentration ([GLY]) and CO2transport both during the initial period following illumination of a cyanobacterial cell suspension and during the so-called steady state when an internal Cipool has been allowed to develop. Carbon dioxide uptake following illumination of a cyanobacterial cell suspension was progressively reduced by increasing [GLY] in the medium, both in the absence or in the presence of carbonic anhydrase. The same features were found when either Ciin the form of bicarbonate was injected to a pre-illuminated cell suspension in the presence of carbonic anhydrase in the medium or a pulse of CO2was provided to the cells in the light without carbonic anhydrase in the extracellular medium. Although photosynthesis was completely abolished with 10 mM glycolaldehyde, CO2uptake was only inhibited 20% but was decreased further by higher [GLY]. Double reciprocal plots of CO2uptake versus initial [CO2] determined with a range of [GLY] showed the inhibition to be noncompetitive. To simultaneously study CO2uptake and CO2efflux, H13C18O3was used with cells in which CO2transport alone, or both CO2and HCO3-transport system were allowed to proceed. Cells were inhibited with either iodoacetamide or glycolaldehyde and the time course of the various CO2species in the medium demonstrated that CO2uptake was inhibited partially with glycolaldehyde resulting in a greater CO2efflux from the cells. When the cell suspension was again illuminated, in the presence of glycolaldehyde, initial CO2uptake was greatly reduced, and the extracellular [CO2] eventually rose to a level higher than that which would be expected to be in equilibrium with the amount of Cipresent in the medium. Results are discussed in terms of glycolaldehyde use in Citransport mechanism studies in cyanobacteria. Key words: Synechococcus UTEX 625, blue-green algae, cyanobacteria, CO2transport, HCO3-transport, CO2efflux, inhibitor, Ciconcentrating mechanism.

Simulation of C1fluxes in cyanobacteria: comparison between the carboxysome hypothesis and the equilibrium proposal

Inorganic carbon fluxes were simulated by a mathematical model using an equilibrium hypothesis for a wide range of conditions in a closed system composed of air-grown cells of Synechococcus UTEX 625 in a reaction vessel connected to a mass spectrometer. The metabolic scheme took into account the input fluxes of CO2and HCO3−transport into the cells, the output fluxes of CO2and HCO3−efflux, the diversion of Q toward the formation of the internal C2pool, and photosynthetic CO2fixation. The equations expressed the variation in concentration of each inorganic species outside and inside the cell as a function of time. The input fluxes were previously characterized by their kinetic constants (K1/2and Vm) both during initial uptake occurring upon illumination of the cells and under steady-state photosynthesis conditions. The efflux rates of the various Cispecies from the cells were investigated under a wide variety of experimental conditions. Using these efflux rates, the permeability coefficients of the cell for CO2and HCO3−were calculated previously. Using the kinetic constants for CO2and HCO3−transport, the permeability coefficients of the cell for CO2and HCO3−and the geometrical characteristics of the cells, the model simulated precisely the [HCO3−]/[CO2] ratio and the [CO2] and [O2] changes in the extracellular medium as well as the rate of filling of the internal Cipool under various conditions. Accurate fitting of experimental data with calculated values were possible only when the intracellular Cispecies were assumed to be in equilibrium throughout the entire cell volume. Results are discussed and compared with those given by previous hypotheses. Key words: Synechococcus UTEX 625, blue green algae, cyanobacteria, mathematical model, active CO2transport, active HCO3−transport, steady state, photosynthesis, Ciconcentrating mechanism.

Measurement of the amount and isotopic composition of the CO2released from the cyanobacteriumSynechococcusUTEX 625 after rapid quenching of the active CO2transport system

Air-grown cells of the cyanobacterium Synechococcus UTEX 625 were suspended in a cuvette connected to a mass spectrometer and supplied with H13C18O3−to investigate the intracellular interconversion between CO2and HCO3−as determined from the isotopic composition of CO2appearing in the extracellular medium under a wide variety of experimental conditions. Upon injection of H13C18O3−to the cell suspension in the light, the extracellular [13C16O2] increased. As the CO2species were13C labelled, this demonstrated that the18O-depleted CO2was originating from the added H13C18O3−. A comparison of the rates of13C16O16O appearance in the medium with the formation of13C16O16O from spontaneous dehydration–hydration in the extracellular medium in the presence of cells demonstrated that most of it had to originate from a series of intracellular dehydration–hydration cycles of H13C18O3−that had been recently transported into the cells. During the time course of the experiments both the m/z (mass to charge) = 49 (i.e.,13C18O18O) and 47 (i.e.,13C18O16O) signals decreased constantly, whereas the m/z = 45 signal (i.e.,13C16O2) always increased. Inhibiting CO2fixation enhanced the amount of CO2arising in the medium but did not change its isotopic composition, and the CO2was always fully depleted of18O. When the CO2transport system was inhibited by darkening the cells, adding inhibitors such as Na2S or COS, or quenching the uptake of inorganic13C with an excess of inorganic12C, the magnitude of the extracellular [13C16O2] was increased but the CO2species were still always depleted of18O. Various incubation times of the illuminated cells in the presence of H13C18O3−were used to obtain a variety of internal Cipool sizes. When the inhibitor (COS) was added, the amount of13C16O2arising during the response time of the mass spectrometer was equivalent to the amount of CO2that would have been present in the whole cell if CO2and HCO3−were in equilibrium throughout the entire cell volume, but it was at least 40 times higher than the amount of CO2that would have been present in the cell if the CO2was confined to the carboxysomes. Experiments were also conducted at pH 9.0 where the spontaneous rate of13C16O2production from H13C1803−dehydration–hydration would be negligible, and again the same features were observed. Results show that intracellular HCO3−and CO2are in rapid equilibrium throughout the entire cell volume. Key words: Synechococcus UTEX 625, cyanobacteria, CO2leakage,18O exchange, active CO2transport, carboxysomes, inorganic C concentrating mechanism.

Phytoplanktonic carbon isotope fractionation: equations accounting for CO2-concentrating mechanisms

A model of carbon isotope discrimination by phytoplankton was developed which took into account the occurrence of a carbon-concentrating mechanism (CCM). A simple equation was obtained for the model involving CO2 active transport. In the case of HCO3− active transport, another equation was developed based on a series of approximations. The former equation was used to analyse reported and newly obtained data from culture experiments and field observations in both freshwater and marine environments. In most cases, a linear relationship between a combined parameter, (1−f)Ci, which was made up of the relative contribution of active CO2 uptake to total carbon uptake (f) and the intracellular CO2 concentration (Ci), and CO2 concentration in bulk solution (Ce) was obtained as (1−f)Ci = ace−b, with a high correlation cmfficient (r2>0.9). The slope a is suggested as a measure of the ratio of diffusive to total (diffusive+active) CO2 transport, while bla represents CO2 demand.

Ethoxyzolamide Differentially Inhibits CO2 Uptake and Na+-Independent and Na+-Dependent HCO3- Uptake in the Cyanobacterium Synechococcus sp. UTEX 625.

The effects of ethoxyzolamide (EZ), a carbonic anhydrase inhibitor, on the active CO2 and Na+-independent and Na+-dependent HCO3- transport systems of the unicellular cyanobacterium Synechococcus sp. UTEX 625 were examined. Measurements of transport and accumulation using radiochemical, fluorometric, and mass spectrometric assays indicated that active CO2 transport and active Na+-independent HCO3- transport were inhibited by EZ. However, Na+-independent HCO3- transport was about 1 order of magnitude more sensitive to EZ inhibition than was CO2 transport (50% inhibition = 12 [mu]M versus 80 [mu]M). The data suggest that both the active CO2 (G.D. Price, M.R. Badger [1989] Plant Physiol 89: 37-43) and the Na+ -independent HCO3 - transport systems possessed carbonic anhydrase-like activity as part of their mechanism of action. In contrast, Na+-dependent HCO3- transport was only partially (50% inhibition = 230 [mu]M) and noncompetitively inhibited by EZ. The collective evidence suggested that EZ inhibition of Na+ -dependent HCO3- transport was an indirect consequence of the action of EZ on the CO2 transport system, rather than a direct effect on HCO3- transport. A model is presented in which the core of the inorganic carbon translocating system is formed by Na+-dependent HCO3- transport and the CO2 transport system. It is argued that the Na+-independent HCO3 - utilizing system was not directly involved in translocation, but converted HCO3- to CO2 for use in CO2 transport.

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.

Photosynthetic metabolism of cyanate by the cyanobacterium Synechococcus UTEX 625

Intact cells of the unicellular cyanobacterium Synechococcus UTEX 625 degraded exogenously supplied cyanate (as KOCN) to CO2 and NH3 in a light-dependent reaction. NH3 release to the medium was as high as 80 μmol(mgChl)-1h-1 and increased 1.7-fold in the presence of methionine sulfoximine, a glutamine synthetase inhibitor. Cyanate also supporte photosynthetic O2 evolution to a maximum rate of 188 μmol O2(mgChl)-1h-1 at pH 8 and 30°C. Cyanate decomposition in cell-free extracts, measured by mass spectrometry as 13CO2 production from KO13CN, occurred in the soluble enzyme fraction, but not in the thylakoid/carboxysome fraction, and was enhanced by HCO3 − and inhibited by the dianion oxalate. CO2, rather, than HCO3 −, was a product of cyanate decomposition. The ability to decompose cyanate was not dependent upon pre-exposure of cells to cyanate to induce activity. The collective results indicate that Synechococcus UTEX 625 possesses a constitutive, cytosolic cyanase (EC 4.3.99.1), similar in mechanism to that found in some species of heterotrophic bacteria. The reaction catalyzed was: OCN+HCO3+2H+→2CO2+NH3. In intact cells, the CO2 produced by the action of cyanase on OCN- was either directly fixed by the Calvin cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, leading to O2 evolution, or leaked into the medium where it was returned to the cell by the active CO2/HCO3 − transport systems for fixation. However, leakage of CO2 from air-grown cells was only observed when the active CO2 transport system was inhibited by darkness or the CO2 analogue carbon oxysulfide.

Active HCO3− transport in cyanobacteria

Cyanobacteria possess systems for the active transport of both CO2 and HCO3−. While the active CO2 transport system seems to be present in cells grown on all levels of CO2 or dissolved inorganic carbon, the bicarbonate transport systems are only present in cells grown on low levels of CO2 or dissolved inorganic carbon (air levels or lower). Active bicarbonate transport can be shown to occur when the rate of photosynthesis exceeds that which could be sustained by the production of CO2 from the dehydration of bicarbonate or when CO2 transport is inhibited with carbon oxysulfide or hydrogen sulfide. Two systems for active bicarbonate transport have been identified: one is dependent on the presence of millimolar concentrations of sodium, and the other is independent of the sodium requirement. Cells grown with air bubbling normally possess the first whereas cells grown in standing culture normally possess the second. The sodium-dependent bicarbonate transport can be inhibited by omitting sodium from the reaction medium or competitively with lithium when sodium is present. Monensin and amiloride also inhibit sodium-dependent bicarbonate transport. It does not appear to be inhibited by ethoxyzolamide. The inhibition of sodium-independent bicarbonate transport is not yet established. Bicarbonate transport appears to have no effect on CO2 transport and CO2 transport appears to have no effect on bicarbonate transport. Hence, the transport systems seems to be independent. Although a number of mechanisms have been proposed for bicarbonate transport, the experimental data are not sufficient to clearly distinguish between them. Key words: cyanobacteria, active CO2 transport, active HCO3− transport, photosynthesis, sodium.

Active CO2 transport in cyanobacteria

Cyanobacteria appear to possess an active transport system for molecular CO2. This system, first discovered by Badger and Andrews in 1982 (1982. Plant Physiol. 70: 517–523), is without reported precedence in the bacterial, animal, or plant literature. The transport system operates so efficiently that in dense cell suspensions the extracellular CO2 concentration is pulled far below the equilibrium value. This CO2 drawdown is not due to CO2 fixation but can be accounted for by a transport system that recognizes molecular CO2 and causes it to be transported into the cell. The fact that operation of the system causes a massive disequilibration of the extracellular CO2–HCO3− system means that there must be an expenditure of metabolic energy. The CO2 is actually moved against a considerable CO2 concentration gradient. In this review we discuss methods that can be used to monitor CO2 transport in cyanobacteria. We present evidence that CO2 transport is an active process. It is emphasized that little is known about the concomitant ion fluxes that must occur to ensure charge and pH regulation during CO2 transport. Key words: cyanobacteria, active CO2 transport, metabolic inhibitors, transport models.