Regulation of the induction of bicarbonate uptake by dissolved CO2 in the marine diatom, Phaeodactylum tricornutum

Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO₂ to air and related to H₂CO₃ dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis (K_0·5[DIC]) value in high CO₂‐grown cells was 1009 mmol m ^{? 3} but was reduced three‐fold by the addition of bovine carbonic anhydrase (CA), whereas in air‐grown cells K_0·5[DIC] was 71 mmol m ^{? 3}, irrespective of the presence of CA. The maximum rate of photosynthesis (P_max) values varied between 300 and 500 μ mol O₂ mg Chl ^{? 1} h ^{? 1} regardless of growth pCO₂. Bicarbonate dehydration kinetics in artificial seawater were re‐examined to evaluate the direct HCO₃ ^? uptake as a substrate for photosynthesis. The uncatalysed CO₂ formation rate in artificial seawater of 31·65°/_oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m ^{? 3} min ^{? 1} at 100 mmol m ^{? 3} DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air‐ and high CO₂‐grown cells, respectively. These data indicate that even high CO₂‐grown cells of P. tricornutum can take up both CO₂ and HCO₃ ^? as substrates for photosynthesis and HCO₃ ^? use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO₂‐grown cells to air. The development of high‐affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO₂‐grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K_0·5[DIC] values in acclimated cells appeared to be correlated only with [CO_2(aq)] in the medium but not to HCO₃ ^? , CO₃^{2 ?} , total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum, is [CO_2(aq)] in the medium.     

Bicarbonate uptake via an anion exchange protein is the main mechanism of inorganic carbon acquisition by the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae) under variable pH

Macrocystis pyrifera is a widely distributed, highly productive, seaweed. It is known to use bicarbonate (HCO₃^?) from seawater in photosynthesis and the main mechanism of utilization is attributed to the external catalyzed dehydration of HCO₃^? by the surface‐bound enzyme carbonic anhydrase (CA_ext). Here, we examined other putative HCO₃^? uptake mechanisms in M. pyrifera under pH_T 9.00 (HCO₃^?: CO₂ = 940:1) and pH_T 7.65 (HCO₃^?: CO₂ = 51:1). Rates of photosynthesis, and internal CA (CA_int) and CA_ext activity were measured following the application of AZ which inhibits CA_ext, and DIDS which inhibits a different HCO₃^? uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO₃^? uptake by M. pyrifera is via an AE protein, regardless of the HCO₃^?: CO₂ ratio, with CA_ext making little contribution. Inhibiting the AE protein led to a 55%–65% decrease in photosynthetic rates. Inhibiting both the AE protein and CA_ext at pH_T 9.00 led to 80%–100% inhibition of photosynthesis, whereas at pH_T 7.65, passive CO₂ diffusion supported 33% of photosynthesis. CA_int was active at pH_T 7.65 and 9.00, and activity was always higher than CA_ext, because of its role in dehydrating HCO₃^? to supply CO₂ to RuBisCO. Interestingly, the main mechanism of HCO₃^? uptake in M. pyrifera was different than that in other Laminariales studied (CA_ext‐catalyzed reaction) and we suggest that species‐specific knowledge of carbon uptake mechanisms is required in order to elucidate how seaweeds might respond to future changes in HCO₃^?:CO₂ due to ocean acidification.     

EFFECTS OF CO2 ENRICHMENT ON THE BLOOM‐FORMING CYANOBACTERIUM MICROCYSTIS AERUGINOSA (CYANOPHYCEAE): PHYSIOLOGICAL RESPONSES AND RELATIONSHIPS WITH THE AVAILABILITY OF DISSOLVED INORGANIC CARBON1

Microcystis aeruginosa Kütz. 7820 was cultured at 350 and 700 μL·L ^{? 1} CO₂ to assess the impacts of doubled atmospheric CO₂ concentration on this bloom‐forming cyanobacterium. Doubling of CO₂ concentration in the airflow enhanced its growth by 52%–77%, with pH values decreased and dissolved inorganic carbon (DIC) increased in the medium. Photosynthetic efficiencies and dark respiratory rates expressed per unit chl a tended to increase with the doubling of CO₂. However, saturating irradiances for photosynthesis and light‐saturated photosynthetic rates normalized to cell number tended to decrease with the increase of DIC in the medium. Doubling of CO₂ concentration in the airflow had less effect on DIC‐saturated photosynthetic rates and apparent photosynthetic affinities for DIC. In the exponential phase, CO₂ and HCO₃ ^? levels in the medium were higher than those required to saturate photosynthesis. Cultures with surface aeration were DIC limited in the stationary phase. The rate of CO₂ dissolution into the liquid increased proportionally when CO₂ in air was raised from 350 to 700 μL·L ^{? 1}, thus increasing the availability of DIC in the medium and enhancing the rate of photosynthesis. Doubled CO₂ could enhance CO₂ dissolution, lower pH values, and influence the ionization fractions of various DIC species even when the photosynthesis was not DIC limited. Consequently, HCO₃ ^? concentrations in cultures were significantly higher than in controls, and the photosynthetic energy cost for the operation of CO₂ concentrating mechanism might decrease.     

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

Induction 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 (C_i) limited conditions. The CCM components investigated were active HCO₃^? transport, active CO₂ transport and extracellular carbonic anhydrase (CA_ext) activity. The CA_ext 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 CA_ext 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 C_i concentrations effecting the induction of active C_i transport and CA_ext 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 C_i concentration eliciting the induction and repression of active C_i 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 C_i transport and CA_ext activity were 10 and 100 μM , respectively. The induction of CA_ext and active C_i transport are not correlated temporally, but are regulated by the same critical CO₂ concentration in the medium.     

Photosynthesis and inorganic carbon acquisition in the cyanobacterium Chlorogloeopsis sp. ATCC 27193

The ability of the morphologically complex cyanobacterium Chlorogloeopsis sp. ATCC 27193 to actively transport and accumulate inorganic carbon (C₁= CO₂+ HCO₃^?+ CO₃^2?) for photosynthetic CO₂ fixation was investigated. Mass-spectrometric assays revealed that Chlorogloeopsis cells grown under C₁ limitation rapidly took up CO₂ from the medium in a light-dependent reaction which was independent of CO₂ fixation. Ethoxyzolamide, a carbonic anhydrase (CA) inhibitor, inhibited CO₂ transport. Since electrometric and mass-spectrometric assays did not detect the presence of a periplasmic CA, it is suggested that CO₂ transport was mediated by a CA-like activity which converted CO₂ to HCO₃^? during passage across the membrane. Radiochemical assays, using H¹⁴CO₃ as substrate, showed that C₃-limited cells also had a high affinity (K_0.5 HCO₃^?= 37 μM), Na⁺-independent HCO₃^? uptake mechanism. HCO₃^?uptake was light dependent and occurred against its electrochemical potential indicating a carrier-mediated, active transport process. The rate of Na⁺-independent HCO₃^? transport was sufficient to account for the steady state rate of CO₂ fixation. Although not absolutely required. Na⁺ did specifically enhance the rate of HCO₃^? transport by up to 2-fold, but had no effect on the apparent affinity of the transport system for HCO₃^? Combined CO₂ and HCO₃^? transport resulted in C₁ accumulation as high as 25 mM and in excess of 300 times the external concentration. The C₁ pool was the source of CO₂ for photo-synthetic fixation and was generated, presumably, by the dehydration of HCO₃^? catalyzed by an intracellular CA. The collective evidence indicates that Chlorogloeopsis has a physiologically functional CO₂-concentrating mechanism which is essential for photosynthesis.     

The Role of External Carbonic Anhydrase in Inorganic Carbon Acquisition by Chlamydomonas reinhardii at Alkaline pH

The role of external carbonic anhydrase in inorganic carbon acquisition and photosynthesis by Chlamydomonas reinhardii at alkaline pH (8.0) was studied. Acetazolamide (50 micromolar) completely inhibited external carbonic anhydrase (CA) activity as determined from isotopic disequilibrium experiments. Under these conditions, photosynthetic rates at low dissolved inorganic carbon (DIC) were far greater than could be maintained by CO₂ supplied from the spontaneous dehydration of HCO₃⁻ thereby showing that C. reinhardii has the ability to utilize exogenous HCO₃⁻. Acetazolamide increased the concentration of DIC required to half-saturate photosynthesis from 38 to 80 micromolar, while it did not affect the maximum photosynthetic rate. External CA activity was also removed from the cell-wall-less mutant (CW-15) by washing. This had no effect on the photosynthetic kinetics of the algae while the addition of acetazolamide to washed cells (CW-15) increased the K_½^DIC from 38 to 80 micromolar. Acetazolamide also caused a buildup of the inorganic carbon pool upon NaHCO₃ addition, indicating that this compound partially inhibited internal CA activity. The effects of acetazolamide on the photosynthetic kinetics of C. reinhardii are likely due to the inhibition of internal rather than a consequence of the inhibition of external CA. Further analysis of the isotopic disequilibrium experiments at saturating concentration of DIC provided evidence consistent with active CO₂ transport by C. reinhardii. The observation that C. reinhardii has the ability to take up both CO₂ and bicarbonate throws into question the role of external CA in the accumulation of DIC in this alga.     

The utilization of bicarbonate ions by the marine microalga Nannochloropsis oculata (Droop) Hibberd

HCO₃^? utilization by the marine microalga Nannochloropsis oculata was investigated using a pH drift technique in a closed system. Light-dependent alkalization of the medium resulted in a final pH of 10.5, confirming substantial HCO₃^? use by this alga. Alkalinity remained constant throughout the pH drift. Measurement of dissolved inorganic carbon (DIC) or the uptake of H¹⁴CO₃^? showed that nearly 50% of the total DIC remained external to the plasma membrane on completion of a pH drift. The rate of light-driven alkalization was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and thus was dependent on photosynthesis. Light-driven alkalization was not inhibited by a membrane-impermeable inhibitor of carbonic anhydrase (CA), dcxtran-bound sulphonamide (DBS), indicating that external CA was not involved in HCO₃^? utilization. The anion-cxchangc inhibitor 4′,4′-diisothiocyanostilbene-2,2-disulphonic acid (DIDS) completely inhibited light-driven alkalization of the medium and H¹⁴CO₃^? uptake, providing unequivocal support for a direct uptake of H¹⁴CO₃^?. Chloride ions were essential for DIC-dependent photosynthetic oxygen evolution, suggesting that bicarbonate transport occurs by HCO₃^?/CI^? exchange.     

PHOTOSYNTHETIC PROPERTIES OF PROTOPLASTS,AS COMPARED WITH THALLI,OF ULVA FASCIATA (CHLOROPHYTA)1

Protoplasts were prepared from Ulva fasciata Delile, and their photosynthetic performance was measured and compared with that of thalli discs. These protoplasts maintained maximal rates of photosynthesis as high as those of thalli (up to 300 μmol O₂·mg chlorophyll^?1·h^?1) for several hours after preparation and were therefore considered suitable for kinetic studies of inorganic carbon utilization. The photosynthetic K_1/2(inorganic carbon) at pH 6.1 was 3.8 μM and increased to 67, 158, and 1410 μM at the pH values 7.0, 7.9, and 8.9, respectively. Compared with these protoplasts, thalli had a much lower affinity for CO₂ but approximately the same affinity for HCO₃^?. Comparisons between rates of photosynthesis and the spontaneous dehydration of HCO₃^? (at 50 μM inorganic carbon) revealed that photosynthesis of both protoplasts (which lacked apparent activity of extracellular/surface-bound carbonic anhydrase) and thalli (which were only 25% inhibited by the external carbonic anhydrase inhibitor acetazolamide) could not be supported by CO₂ formation in the medium at the higher pH values, indicating HCO₃^? uptake. Since both protoplasts and thalli were sensitive to 4,4′-diisothiocyanostilbene-2,2′-disulfonate, we suggest that HCO₃^? transport was facilitated by the membrane-located anion exchange protein recently reported to function in certain Ulva thalli. These findings suggest that the presence of a cell wall may constitute a diffusion barrier for CO₂, but not for HCO₃^?, utilization under natural seawater conditions.     

Carbonic anhydrase activity and inorganic carbon fluxes in low- and high-C1 cells of Chlamydomonas reinhardtü and Scenedesmus obliquus

Carbonic anhydrase (CA) activity associated with high- and low-dissolved inorganic carbon (C₁) grown cells was examined in whole cells by measuring ¹⁸O exchange from doubly labeled CO₂ (¹³C¹⁸O¹⁸O). Both algal species showed the presence of extracellular (periplasmic) as well as intracellular CA activity, which were both greatly increased in low-C₁ cells. The periplasmic CA activity was at least 40-fold higher in lowcompared to high-C₁ cells in both C. reinhardtii and S. obliquus. while low-C₁ cells of S. obliquus showed the highest activity of internal CA. The CA inhibitor ethoxyzolamide showed a strong inhibition of the C₁ uptake process in both C. reinhardtii and S. obliquus as in cyanobacteria. which may indicate that the nature of the primary uptake process is similar in both green algae and cyanobacteria. By using a mass spectrometnc disequilibrium technique it was possible to separate the C₁ fluxes of net HCO^?₃-uptake and net CO₂-uptake during steady-state photosynthesis in high- and Sow-C₁ grown cells of Chlamydomonas reinhardtii (WT. 2137+) and Scenedesmus obliquus (WT. D3). It was found that both high- and low-C₁ cells of the two algae can utilize both CO₂ and HCO^?₃ for photosynthesis, although low-C₁ cells have a higher affinity for the uptake of both C₁ species. Induction at low-C₁ causes an increase in the affinity of both species for HCO^?₃ and CO₂; changes in net CO₂-uptake were, however, significantly greater.     

Markedly low requirement of added CO2 for photosynthesis by mesophyll protoplasts of pea (Pisum sativum): possible roles of photorespiratory CO2 and carbonic anhydrase

Mesophyll protoplasts of pea required only 74.1 μM CO₂ for maximal photosynthesis, unlike chloroplasts, which required up to 588 μM CO₂. Such a markedly low requirement for CO₂ could be because of an internal carbon source and/or a CO₂ concentrating mechanism in mesophyll protoplasts. Ethoxyzolamide (EZA), an inhibitor of internal carbonic anhydrase (CA) suppressed photosynthesis by mesophyll protoplasts at low CO₂ (7.41 μM) but had no significant effect at high CO₂ (741 μM). However, acetazolamide, another inhibitor of CA, did not exert as much dramatic effect as EZA. Three photorespiratory inhibitors, aminoacetonitrile or glycine hydroxamate (GHA) or aminooxyacetate inhibited markedly photosynthesis at low CO₂ but not at high CO₂. Inhibitors of glycolysis or tricarboxylic acid cycle (NaF, sodium malonate) or phosphoenolpyruvate carboxylase (3,3‐dichloro‐2‐dihydroxy phosphinoyl‐methyl‐2‐propenoate) had no significant effect on photosynthesis. The CO₂ requirement of protoplast photosynthesis and the sensitivity of photosynthesis to EZA were much higher at low oxygen (65 nmol ml^?1) than that at normal oxygen (212 nmol ml^?1). In contrast, the inhibitory effect of photorespiratory inhibitors on protoplast photosynthesis was similar in both normal and low oxygen medium. The marked elevation of glycine/serine ratio at low O₂ or in presence of GHA confirmed the suppression of photorespiratory decarboxylation by GHA. While demonstrating interesting difference between the response of protoplasts and chloroplasts to CO₂, we suggest that photorespiration could be a significant source of CO₂ for photosynthesis in mesophyll protoplasts at limiting CO₂ and at atmospheric levels of oxygen. Obviously, carbonic anhydrase is essential to concentrate or retain CO₂ in mesophyll cells.     

Studies of Elodea nuttallii grown under photorespiratory conditions. I. Photosynthetic characteristics

Abstract. The photosynthetic characteristics of Elodea nuttallii grown in wastewater in continuous flow reactors in a greenhouse were investigated. The diurnal changes in dissolved inorganic carbon (DIC), dissolved oxygen (DO) and pH were monitored. Photosynthesis removed both CO₂(aq) and HCO₃^? from the reactors. A stoichiometry of 1.19:1 was observed between HCO₃^? removal during photosynthesis and OH^? production during photosynthesis, consistent with theories regarding direct bicarbonate utilization. In laboratory experiments, the light compensation points (г_PPFD) were similar (31–35μmol m^?2 s^?1) to reported values for other macrophytes; however, the light saturation level was high (1100μmol m^?2 s^?1) and similar to values reported for aerial portions Of heterophyllous macrophytes. The kinetics of photosynthetic oxygen evolution (K_m (CO₂) = 96mmol m^?3; V_max= 133mmol g^?1 Chl h^?1) and the CO₂ compensation point (г= 44cm³ m^?3) suggested an adaptive, low photorespiratory state in response to low carbon concentrations. Photosynthetic V_max values were slightly, but significantly higher (P 0.001) at pH 8.0 compared to pH 4.5. While CO₂ utilization at pH 8 could account for most of the observed phototsynthetic rates, an HCO₃^? component was present, suggesting two separate transport systems for HCO₃^? and CO₂(aq) in E. nuttallii. The activity of RUBISCO (160.3 mmol g^?1 Chl h^?1 was one of the highest reported values for aquatic macrophytes. Compared to RUBISCO, we observed lower activities of the β-carboxylating enzymes phopho enolpyruvate carboyxlase (PEPcase), 24.1 mmol g^?1 Chl h^?1; phosphor enol pyruvate carboxykinase (PEPCKase), 14 mmol g^?1 Chl h^?1. This suggests that the potential light-independent fixation of carbon in E. nuttallii was much less than RUBISCO-dependent fixation. The RUBISCO/PEPcase ratio was 6.6, indicating that E. nuttallii was similar to Myriophyllum sp. in possessing a physiological adaptation to low CO₂ levels which is hypothesized to include carbonic anhydrase (CA) and an active transport system for HCO₃^?. CA levels were surprisingly low in E. nuttallii (14.2 EUmg Chl^?).     

Acclimation of Photosynthetic Light Reactions during Induction of Inorganic Carbon Accumulation in the Green Alga Chlamydomonas reinhardtii

Cells of the unicellular green algae Chlamydomonas reinhardtii were grown in high dissolved inorganic carbon (DIC) concentrations (supplied with 50 milliliters per liter CO₂[g]) and transferred to low DIC concentrations (supplied with ≤ 100 microliters per liter CO₂[g]). Immediately after transfer from high to low DIC the emission of photosystem II related chlorophyll a fluorescence was substantially quenched. It is hypothesized that the suddenly induced inorganic carbon limitation of photosynthesis resulted in a phosphorylation of LHCII, leading to the subsequent state 1 to state 2 transition. After 2 hours of low-DIC acclimation, 77 K fluorescence measurements revealed an increase in the fluorescence emitted from photosystem I, due to direct excitation, suggesting a change in photosystem II/photosystem I stoichiometry or an increased light harvesting capacity of photosystem I. After 5 to 6 hours of acclimation a considerable increase in spillover from photosystem II to photosystem I was observed. These adjustments of the photosynthetic light reactions reached steady-state after about 12 hours of low DIC treatment. The quencher of fluorescence could be removed by 5 minutes of dark treatment followed by 5 minutes of weak light treatment, of any of four different light qualities. It is hypothesized that this restoration of fluorescence was due to a state 2 to state 1 transition in low-DIC acclimated cells. A decreased ratio of violaxanthin to zeaxanthin was also observed in 12 hour low DIC treated cells, compared with high DIC grown cells. This ratio was not coupled to the level of fluorescence quenching. The role of different processes during the induction of a DIC accumulating mechanism is discussed.     

Detection of a variable intracellular acid‐labile carbon pool in Thalassiosira weissflogii (Heterokontophyta) and Emiliania huxleyi (Haptophyta) in response to changes in the seawater carbon system

Accumulation of an intracellular pool of carbon (C_i pool) is one strategy by which marine algae overcome the low abundance of dissolved CO₂ (CO_2(aq)) in modern seawater. To identify the environmental conditions under which algae accumulate an acid‐labile C_i pool, we applied a ¹⁴C pulse‐chase method, used originally in dinoflagellates, to two new classes of algae, coccolithophorids and diatoms. This method measures the carbon accumulation inside the cells without altering the medium carbon chemistry or culture cell density. We found that the diatom Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle] and a calcifying strain of the coccolithophorid Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler] develop significant acid‐labile C_i pools. Ci pools are measureable in cells cultured in media with 2–30 µmol l^?1 CO_2(aq), corresponding to a medium pH of 8.6–7.9. The absolute C_i pool was greater for the larger celled diatoms. For both algal classes, the C_i pool became a negligible contributor to photosynthesis once CO_2(aq) exceeded 30 µmol l^?1. Combining the ¹⁴C pulse‐chase method and ¹⁴C disequilibrium method enabled us to assess whether E. huxleyi and T. weissflogii exhibited thresholds for foregoing accumulation of DIC or reduced the reliance on bicarbonate uptake with increasing CO_2(aq). We showed that the C_i pool decreases with higher CO₂:HCO₃^? uptake rates.     

The active uptake of carbon dioxide by the marine diatoms Phaeodactylum ticornutum and Cyclotella sp.

Mass spectromelry has been used to investigate the uptake of CO₂ by two marine diatoms, Phaeodactylum tricornutum and Cyclotella sp. The time course of CO₂ formation in the dark after addition of 100 mmol m^?3 dissolved inorganic carbon (DIC) to cell suspensions showed that external carbonic anhydrase (CA) was not present in cells of P. tricornutum but was present in Cyclotella sp. In the absence of external CA, or when it was inhibited by 5 mmol m^?3 acetazolamide, cells of both species preincubated with 100 mmol m^?3 DIG rapidly depleted almost all of the free CO₂ (3·2mmol m^?31 at pH7·5) from the suspending medium within seconds of illumination and prior to the onset of steady-state photosynthesis. Addition of bovine CA quickly restored the HCO₃^?–CO₂ equilibrium in the medium, indicating that the initial depletion of CO₂ resulted from the selective uptake of CO₂ rather than uptake of all DIG species. Transfer of cells to the dark caused a rapid increase in the CO₂ concentration in the medium, largely as a result of the efflux of unfixed inorganic carbon from the cells. The measured CO₂ uptake rates for both species accounted for 50% of the total DIG uptake at HCO₃^?–CO₂ equilibrium, indicating that HCOHCO₃^? was also being taken up. These results indicate that both Phaeodactylum tricornutum and Cyclotella sp. have the capacity to transport CO₂ actively against concentration and pH gradients.     

Characterization of the na-requirement in cyanobacterial photosynthesis

The Na⁺ requirement for photosynthesis and its relationship to dissolved inorganic carbon (DIC) concentration and Li⁺ concentration was examined in air-grown cells of the cyanobacterium Synechococcus leopoliensis UTEX 625 at pH 8. Analysis of the rate of photosynthesis (O₂ evolution) as a function of Na⁺ concentration, at fixed DIC concentration, revealed two distinct regions to the response curve, for which half-saturation values for Na⁺ (K_½[Na⁺]) were calculated. The value of both the low and the high K_½(Na⁺) was dependent upon extracellular DIC concentration. The low K_½(Na⁺) decreased from 1000 micromolar at 5 micromolar DIC to 200 micromolar at 140 micromolar DIC whereas over the same DIC concentration range the high K_½(Na⁺) decreased from 10 millimolar to 1 millimolar. The most significant increases in photosynthesis occurred in the 1 to 20 millimolar range. A fraction of total photosynthesis, however, was independent of added Na⁺ and this fraction increased with increased DIC concentration. A number of factors were identified as contributing to the complexity of interaction between Na⁺ and DIC concentration in the photosynthesis of Synechococcus. First, as revealed by transport studies and mass spectrometry, both CO₂ and HCO₃⁻ transport contributed to the intracellular supply of DIC and hence to photosynthesis. Second, both the CO₂ and HCO₃⁻ transport systems required Na⁺, directly or indirectly, for full activity. However, micromolar levels of Na⁺ were required for CO₂ transport while millimolar levels were required for HCO₃⁻ transport. These levels corresponded to those found for the low and high K_½(Na⁺) for photosynthesis. Third, the contribution of each transport system to intracellular DIC was dependent on extracellular DIC concentration, where the contribution from CO₂ transport increased with increased DIC concentration relative to HCO₃⁻ transport. This change was reflected in a decrease in the Na⁺ concentration required for maximum photosynthesis, in accord with the lower Na⁺-requirement for CO₂ transport. Lithium competitively inhibited Na⁺-stimulated photosynthesis by blocking the cells' ability to form an intracellular DIC pool through Na⁺-dependent HCO₃⁻ transport. Lithium had little effect on CO₂ transport and only a small effect on the size of the pool it generated. Thus, CO₂ transport did not require a functional HCO₃⁻ transport system for full activity. Based on these observations and the differential requirement for Na⁺ in the CO₂ and HCO₃⁻ transport system, it was proposed that CO₂ and HCO₃⁻ were transported across the membrane by different transport systems.     

MORPHOLOGICAL AND PHYSIOLOGICAL EFFECTS IN PROBOSCIA ALATA (BACILLARIOPHYCEAE) GROWN UNDER DIFFERENT LIGHT AND CO2 CONDITIONS OF THE MODERN SOUTHERN OCEAN1

The combined effects of different light and aqueous CO₂ conditions were assessed for the Southern Ocean diatom Proboscia alata (Brightwell) Sundström in laboratory experiments. Selected culture conditions (light and CO_2(aq)) were representative for the natural ranges in the modern Southern Ocean. Light conditions were 40 (low) and 240 (high) μmol photons · m^?2 · s^?1. The three CO_2(aq) conditions ranged from 8 to 34 μmol · kg^?1 CO_2(aq) (equivalent to a pCO₂ from 137 to 598 μatm, respectively). Clear morphological changes were induced by these different CO_2(aq) conditions. Cells in low [CO_2(aq)] formed spirals, while many cells in high [CO_2(aq)] disintegrated. Cell size and volume were significantly affected by the different CO_2(aq) concentrations. Increasing CO_2(aq) concentrations led to an increase in particulate organic carbon concentrations per cell in the high light cultures, with exactly the opposite happening in the low light cultures. However, other parameters measured were not influenced by the range of CO_2(aq) treatments. This included growth rates, chlorophyll a concentration and photosynthetic yield (F_V/F_M). Different light treatments had a large effect on nutrient uptake. High light conditions caused an increased nutrient uptake rate compared to cells grown in low light conditions. Light and CO₂ conditions co‐determined in various ways the response of P. alata to changing environmental conditions. Overall P. alata appeared to be well adapted to the natural variability in light availability and CO_2(aq) concentration of the modern Southern Ocean. Nevertheless, our results showed that P. alata is susceptible to future changes in inorganic carbon concentrations in the Southern Ocean.     

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

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

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.     

Controls of streamwater dissolved inorganic carbon dynamics in a forested watershed

Iinvestigated controls of stream dissolved inorganic carbon (DIC) sources andcycling along a stream size and productivity gradient in a temperate forestedwatershed in northern California. Dissolved CO₂ (CO_2(aq))dynamics in heavily shaded streams contrasted strongly with those of larger,open canopied sites. In streams with canopy cover > 97%, CO_{2 (aq)}was highest during baseflow periods (up to 540 M) and wasnegatively related to discharge. Effects of algal photosynthesis on CO_2(aq) were minimal and stream CO_{2 (aq)} was primarily controlledby groundwater CO_{2 (aq)} inputs and degassing losses to theatmosphere. In contrast to the small streams, CO_{2 (aq)} in larger,open-canopied streams was often below atmospheric levels at midday duringbaseflow and was positively related to discharge. Here, stream CO_2(aq) was strongly influenced by the balance between autotrophic andheterotrophic processes. Dynamics of HCO₃ ^– werelesscomplex. HCO₃ ^– and Ca²⁺ were positivelycorrelated, negatively related to discharge, and showed no pattern with streamsize. Stable carbon isotope ratios of DIC (i.e. ¹³C DIC)increased with stream size and discharge, indicating contrasting sources of DICto streams and rivers. During summer baseflows, ¹³C DIC were¹³C-depleted in the smallest streams (minimum of–17.7) due to the influence of CO_{2 (aq)} derived frommicrobialrespiration and HCO₃ ^– derived from carbonateweathering. ¹³C DIC were higher (up to –6.6)inthe larger streams and rivers due to invasion of atmospheric CO₂enhanced by algal CO_{2 (aq)} uptake. While small streams wereinfluenced by groundwater inputs, patterns in CO_{2 (aq)} and evidencefrom stable isotopes demonstrate the strong influence of stream metabolism andCO₂ exchange with the atmosphere on stream and river carbon cycles.     

Oxygen inhibition of dissolved inorganic carbon uptake in unicellular green algae

Unicellular green algae have a dissolved inorganic carbon (DIC) concentrating mechanism, commonly known as the DIC pump, to concentrate inorganic carbon into cells and chloroplasts. The DIC pump activity is normally measured as the K_0.5(DIC) that equals the CO₂ plus HCO₃‐ concentration at a cited pH at which the rate of DIC‐dependent photosynthetic O₂ evolution is half‐maximal, or by the amount of intra‐cellular DIC accumulation in 15–60 s, using a limited amount of NaH¹⁴CO₃, measured by the silicone oil cen‐trifugation technique. The dissolved oxygen in the assay inhibits or reduces the DIC uptake by the cells of unicellular green algae Chlamydomonas reinhardtii Dangeard, strain 137 and in a cell wall‐less marine algae Dunaliella tertiolecta Butcher. The algal cells concentrated the highest amount of DIC when little or no oxygen was present in the assay medium. The results suggest that the amount of O₂ and DIC must be carefully monitored before DIC‐pump assay.