Effect of oxygen and temperature on the efficiency of photosynthetic carbon assimilation in two microscopic algae

The CO₂ compensation points of Coccochloris peniocystis, a blue-green alga and Chlamydomonas reinhardtii, a green alga, were determined at pH 8.0 in a closed system by a gas chromatographic technique. The compensation point of Chlamydomonas increased markedly with temperature, rising from 0.79 microliter per liter CO₂ at 15 C to 2.5 microliters per liter CO₂ at 35 C. In contrast, the compensation point of Coccochloris at 20 C was 0.71 microliter per liter CO₂ and rose to only 0.95 microliter per liter CO₂ at 40 C.     

The Effect of pH, O(2), and Temperature on the CO(2) Compensation Point of Isolated Asparagus Mesophyll Cells

The effect of pH, O₂ concentration, and temperature on the CO₂ compensation point (Г[CO₂]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO₂ depending upon experimental conditions. The Г(CO₂) increases with increasing temperature. The rate of increase is dependent upon the O₂ concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O₂ concentrations. The differential effect of temperature on Г(CO₂) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O₂-sensitive component of Г(CO₂) remains constant over this range. The Г(CO₂) of Asparagus cells at 25°C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O₂ concentration. It is suggested that the pH-dependence of Г(CO₂) is related to the ability of the cell to take up CO₂ from the aqueous environment. The correlation between high HCO₃⁻ concentrations and low Г(CO₂) at alkaline pH indicates that extracellular HCO₃⁻ facilitates the uptake of CO₂, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O₂− and temperature-dependence of Г(CO₂) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO₂ to a level sufficient to reduce or suppress photorespiration.     

Light-Dependent Oxygen Uptake, Glycolate, and Ammonia Release in l-Methionine Sulfoximine-Treated Chlamydomonas

Glycolate and ammonia excretion plus oxygen exchanges were measured in the light in l-methionine-dl-sulfoximine treated air-grown Chlamydomonas reinhardii. At saturating CO₂ (between 600 and 700 microliters per liter CO₂) neither glycolate nor ammonia were excreted, whereas at the CO₂ compensation concentration (<10 microliters per liter CO₂) treated algae excreted both glycolate and ammonia at rates of 37 and 59 nanomoles per minute per milligram chlorophyll, respectively. From the excretion values we calculate the amount of O₂ consumed through the glycolate pathway. The calculated value was not significantly different from the component of O₂ uptake sensitive to CO₂ obtained from the difference between O₂ uptake of the CO₂ compensation point and at saturating CO₂. This component was about 40% of stationary O₂ uptake measured at the CO₂ compensation point. From these data we conclude that glyoxylate decarboxylation in air-grown Chlamydomonas represents a minor pathway of metabolism even in conditions where amino donors are deficient and that processes other than glycolate pathway are responsible for the O₂ uptake insensitive to CO₂.     

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus

Scenedesmus cells grown on high CO₂, when adapted to air levels of CO₂ for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO₂ from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O₂ evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO₂ pump with K_0.5 for O₂ evolution of less than 5 micromolar CO₂ was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO₃⁻ concentrating process between pH 8 to 10 lowered the K_0.5 (DIC) from 7300 micromolar HCO₃⁻ in high CO₂ grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO₃⁻ pump was inhibited by vanadate (K_i of 150 micromolar), as if it involved an ATPase linked HCO₃⁻ transporter. The CO₂ pump was formed on low CO₂ by high-CO₂ grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO₃⁻ pump was partially activated on low CO₂ within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO₃⁻ pump at pH 9 had requirements similar to the activation of the CO₂ pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO₂-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO₂ pump and in some algae an additional vanadate-sensitive plasmalemma HCO₃⁻ transporter for a HCO₃⁻ pump.     

C(4) Acid Metabolism and Dark CO(2) Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata)

The CO₂ compensation point of the submersed aquatic macrophyte Hydrilla verticillata varied from high (above 50 microliters per liter) to low (10 to 25 microliters per liter) values, depending on the growth conditions. Plants from the lake in winter or after incubation in an 11 C/9-hour photoperiod had high values, whereas summer plants or those incubated in a 27 C/14-hour photoperiod had low values. The plants with low CO₂ compensation points exhibited dark ¹⁴CO₂ fixation rates that were up to 30% of the light fixation rates. This fixation reduced respiratory CO₂ loss, but did not result in a net uptake of CO₂ at night. The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants. However, dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO₂ compensation points.     

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.     

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum: II. Effects of NO(3) and NH(4) Addition to CO(2) Efflux in the Light

The effects of nitrate and ammonium addition on net and gross photosynthesis, CO₂ efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO₂ efflux in the light. The nitrate and ammmonium induced rates of CO₂ efflux were 31.0 and 33.8 micromoles CO₂ per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO₂ was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279).     

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.     

Salicylhydroxamic Acid (SHAM) Inhibition of the Dissolved Inorganic Carbon Concentrating Process in Unicellular Green Algae

Rates of photosynthetic O₂ evolution, for measuring K_0.5(CO₂ + HCO₃⁻) at pH 7, upon addition of 50 micromolar HCO₃⁻ to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K₁(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO₂ uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O₂ evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO₃), and the rate of ¹⁴CO₂ fixation with 100 micromolar [¹⁴C] HCO₃⁻. Salicylhydroxamic acid inhibition of O₂ evolution and ¹⁴CO₂-fixation was reversed by higher levels of NaHCO₃. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO₂ accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.     

Measurement of the photorespiratory activity of the submerged aquatic plant Myriophyllum spicatum L.

Abstract The CO₂ compensation concentrations (points) of leaves of the submerged vascular aquatic plant Myriophyllum spicatum L. were determined in a closed aqueous system at pH 7.0 by a gas chromatographic technique and over the range 10–30deg;C were found to range from 36 to 46 cm³m^?3 in medium equilibrated with 21% O₂ (0.03 kgm^?3), and 25 to 35 cm³m^?3 in medium equilibrated with 2% O₂ (0.03 kgm^?3). The rates of true (TPS) and apparent (APS) photosynthesis of leaves were measured in medium equilibrated with 21% O₂ and buffered at pH 7.0, at subsaturating concentrations (12.8–18.8 mmol m^?3) of dissolved inorganic carbor. (DIC) containing H¹⁴CO₃, by determining the initial rates of uptake by the leaves of DIC and ¹⁴C-activity from the medium. The rate of photorespiration, the difference between TPS and APS, was 7.0–13.3% of TPS over the range of 10–25°C and rose to 29% of TPS at 35°C. The magnitude of the compensation point of this plant is therefore similar to, but is much less O₂-sensitive than, those of C₃ plants, and the photorespiratory rate, at DIC concentrations near the CO₂ compensation point, is very low compared to that of C₃ plants.     

Effects of decreased net carbon exchange on carbohydrate metabolism in sugar beet source leaves

The relationship between CO₂ concentration and starch synthesis and degradation was studied by measuring leaf starch content and disappearance of ¹⁴C-starch. At a concentration of 340 microliters CO₂ per liter, starch accumulated without degradation of previously synthesized starch. Degradation of starch began when CO₂ concentration was lowered, but its synthesis continued. At 120 microliters CO₂ per liter rates of synthesis and degradation were equal. Even at the CO₂ compensation point, synthesis of starch continued. Concomitant starch synthesis and mobilization supported export from the leaf. Changes in starch metabolism that occur when photosynthesis is CO₂-limited provide a means to study regulation of starch metabolism and carbon allocation in translocating leaves.     

Measurement of photorespiration in algae

The rates of true and apparent photosynthesis of two unicellular green algae, one diatom and four blue-green algae were measured in buffer at pH 8.0 at subsaturating concentrations of dissolved inorganic carbon (13-27 micromolar). Initial rates of depletion from the medium of inorganic carbon and ¹⁴C activity caused by the algae in a closed system were measured by gas chromatography and by liquid scintillation counting, respectively. The rate of photorespiration was calculated as the difference between the rates of apparent and true photosynthesis. The three eucaryotic algae and two blue-green algae had photorespiratory rates of 10 to 28% that of true photosynthesis at air levels of O₂. Reduction of the O₂ level to 2% caused a 52 to 91% reduction in photorespiratory rate. Two other blue-green algae displayed low photorespiratory rates, 2.4 to 6.2% that of true photosynthesis at air levels of O₂, and reduction of the O₂ concentration had no effect on these rates.     

Effect of CO(2), O(2), and Light on Photosynthesis and Photorespiration in Wheat

Unidirectional O₂ fluxes were measured with ¹⁸O₂ in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O₂, O₂ uptake was maximum at 60 microliters per liter CO₂. At lower CO₂ concentrations, it was strongly inhibited, as was photosynthetic O₂ evolution. At 2% O₂, there remained a substantial O₂ uptake, even at high CO₂ level; the O₂ evolution was inhibited at CO₂ concentrations under 330 microliters per liter. The O₂ uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO₂. The behavior of the O₂ uptake and the study of the CO₂ compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis.     

The effect of temperature and oxygen on the CO2 compensation point of the marine alga Ulva lactuca

Abstract The CO₂ compensation point of Ulva lactuca frond sections has been measured in artificial seawater using a sensitive gas-chromatographic method. Under nitrogen the compensation point remained relatively constant at 3–6 cm³ m⁻³ at temperatures from 10 to 30°C while in air-saturated medium (0.3 kg m⁻³ O₂) the compensation point rose from 5 cm³ m⁻³ at 10°C to 11 cm³ m⁻³ at 30°C. These responses of the compensation point to temperature and oxygen concentration indicate that there is little photorespiratory CO₂ loss in this marine macroalga, and the low values of these compensation points indicate that inorganic carbon is actively accumulated by the plant.     

Induction of reduced photorespiratory activity in submersed and amphibious aquatic macrophytes

Incubation under water in a 30 C/14-hour or 12 C/10-hour photoperiod caused the CO₂ compensation points of 10 aquatic macrophytes to decrease below 25 or increase above 50 microliters CO₂ per liter, respectively. Submerged and aerial leaves of two amphibious angiosperms (Myriophyllum brasiliense and Proserpinaca palustris) maintained high compensation points when incubated in air but, when the submerged or aerial leaves of Proserpinaca were incubated under water, the compensation points dropped as low as 10. This suggests that, in addition to temperature and photoperiod, some factor associated with submergence regulates the compensation point of aquatic plants. In the high-compensation point plants, photorespiration, as a percentage of net photosynthesis, was equivalent to that in terrestrial C₃ plants. For Hydrilla verticillata, the decreasing CO₂ compensation points (110, 40, and 10) were associated with reduced photorespiration, as indicated by decreased O₂ inhibition, decreased rates of CO₂ evolution into CO₂-free air, and increased net photosynthetic rates.     

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

Mass spectrometric measurements of dissolved free ¹³CO₂ were used to monitor CO₂ uptake by air grown (low CO₂) cells and protoplasts from the green alga Chlamydomonas reinhardtii. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of external carbonic anhydrase reduced the ¹³CO₂ concentration in the medium to close to zero. Similar results were obtained with low CO₂ cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO₂ uptake revealed that the removal of CO₂ from the medium in the light was due to selective and active CO₂ transport rather than uptake of total dissolved inorganic carbon. In the light, low CO₂ cells and protoplasts incubated with carbonic anhydrase took up CO₂ at an apparently low rate which reflected the uptake of total dissolved inorganic carbon. No net CO₂ uptake occurred in the dark. Measurement of chlorophyll a fluorescence yield with low CO₂ cells and washed protoplasts showed that variable fluorescence was mainly influenced by energy quenching which was reciprocally related to photosynthetic activity with its highest value at the CO₂ compensation point. During the linear uptake of CO₂, low CO₂ cells and protoplasts incubated with carbonic anhydrase showed similar rates of net O₂ evolution (102 and 108 micromoles per milligram of chlorophyll per hour, respectively). The rate of net O₂ evolution (83 micromoles per milligram of chlorophyll per hour) with washed protoplasts was 20 to 30% lower during the period of rapid CO₂ uptake and decreased to a still lower value of 46 micromoles per milligram of chlorophyll per hour when most of the free CO₂ had been removed from the medium. The addition of carbonic anhydrase at this point resulted in more than a doubling of the rate of O₂ evolution. These results show low CO₂ cells of Chlamydomonas are able to transport both CO₂ and HCO₃⁻ but CO₂ is preferentially removed from the medium. The external carbonic anhydrase is important in the supply to the cells of free CO₂ from the dehydration of HCO₃⁻.     

Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways: II. A Search for Species with Intermediate Gas Exchange and Anatomical Characteristics

Thirty-three grass species were examined in two experiments in an attempt to locate plants with photosynthetic responses to O₂, CO₂ compensation concentrations, and leaf anatomy intermediate to those of C₃ and C₄ species. Species examined included seven from the Laxa group in the Panicum genus, one of which, P. milioides Nees ex Trin., has been reported earlier to have intermediate characteristics. The species with O₂-sensitive photosynthesis typical of C₃ plants showed more than 37% increase in apparent photosynthesis at 2% O₂ compared to 21% O₂ at 25 C and 335 microliters per liter CO₂, whereas in Panicum milioides, P. schenckii Hack., and P. decipiens Nees ex Trin., members of the Laxa group of Panicum, increases ranged from 25 to 30%. The remainder of the species did not respond to O₂. Species with O₂ responses characteristic of C₃ plants exhibited CO₂ compensation concentrations of 44 microliters per liter or higher at 21% O₂ and 25 to 27.5 C and species characterized as O₂-insensitive had values of microliters per liter or less. The CO₂ compensation concentration (Г) values of P. milioides, P. schenckii, and P. decipiens ranged from 10.3 to 23.3 microliters per liter. Other species of the Laxa group of Panicum exhibited O₂ response and Г values of either C₃ (P. laxum Sw., P. hylaeicum Mez., and P. rivulare Trin.) or C₄ (P. prionitis Griseb.) plants. Leaves of species with O₂ response and CO₂ compensation values typical of C₃ plants had poorly developed or nearly empty bundle sheath cells, and much larger distances and mesophyll cell numbers between veins than did the O₂-insensitive ones. Vein spacings in P. milioides, P. schenckii, and P. decipiens ranged from 0.18 to 0.28 millimeter and mesophyll cell number between veins from 5.2 to 7.8. While these vein spacings are closer than those of most C₃ grasses, two O₂-sensitive species of Dactylis had vein spacings similar to these Panicums and veins in Glyceria striata, another O₂-sensitive plant, were separated by only four mesophyll cells and 0.12 millimeter. Bundle sheath cells of the three intermediate Panicums contained greater quantities of organelles than are typical for C₃ grasses.     

C(3)-C(4) Intermediate Species in Alternanthera (Amaranthaceae) : Leaf Anatomy, CO(2) Compensation Point, Net CO(2) Exchange and Activities of Photosynthetic Enzymes

Two naturally occurring species of the genus Alternanthera, namely A. ficoides and A. tenella, were identified as C₃-C₄ intermediates based on leaf anatomy, photosynthetic CO₂ compensation point (Γ), O₂ response of г, light intensity response of г, and the activities of key enzymes of photosynthesis. A. ficoides and A. tenella exhibited a less distinct Kranz-like leaf anatomy with substantial accumulation of starch both in mesophyll and bundle sheath cells. Photosynthetic CO₂ compensation points of these two intermediate species at 29°C were much lower than in C₃ plants and ranged from 18 to 22 microliters per liter. Although A. ficoides and A. tenella exhibited similar intermediacy in г, the apparent photorespiratory component of O₂ inhibition in A. ficoides is lower than in A. tenella. The г progressively decreases from 35 microliters per liter at lowest light intensity to 18 microliters per liter at highest light intensity in A. tenella. It was, however, constant in A. ficoides at 20 to 25 microliters per liter between light intensities measured. The rates of net photosynthesis at 21% O₂ and 29°C by A. ficoides and A. tenella were 25 to 28 milligrams CO₂ per square decimeter per hour which are intermediate between values obtained for Tridax procumbens and A. pungens, C₃ and C₄ species, respectively. The activities of key enzymes of C₄ photosynthesis, phosphoenolpyruvate carboxylase, pyruvate Pi dikinase, NAD malic enzyme, NADP malic enzyme and phosphoenolpyruvate carboxykinase in the two intermediates, A. ficoides and A. tenella are very low or insignificant. Results indicated that the relatively low apparent photorespiratory component in these two species is presumably the basis for the C₃-C₄ intermediate photosynthesis.     

Photosynthesis and inorganic carbon transport in isolated asparagus mesophyll cells

The possibility of HCO₃⁻ transport into isolated leaf mesophyll cells of Asparagus sprengeri Regel has been investigated. Measurement of the inorganic carbon pool in these cells over an external pH range 6.2 to 8.0, using the silicone-fluid filtration technique, indicated that the pool was larger than predicted by passive ¹⁴CO₂ distribution, suggesting that HCO₃⁻ as well as CO₂ crosses the plasmalemma. Intracellular pH values, calculated from the distribution of ¹⁴CO₂ between the cells and the medium, were found to be higher (except at pH 8.0) than those previously determined by 5,5-dimethyl[2-¹⁴C]oxazolidine-2,4-dione distribution. It is suggested that the inorganic carbon accumulated above predicted concentrations may be bound to proteins and membranes and thus may not represent inorganic carbon actively accumulated by the cells, inasmuch as in a closed system at constant CO₂ concentration, the photosynthetic rates at pH 7.0 and 8.0 were 5 to 8 times lower than the maximum rate which could be supported by CO₂ arising from the spontaneous dehydration of HCO₃⁻. Furthermore, CO₂ compensation points of the cells in liquid media at 21% O₂ at pH 7.0 and 8.0, and the K_½ CO₂ (CO₂ concentration supporting the half maximal rate of O₂ evolution) at 2% O₂ at pH 7.0 and 8.0 are not consistent with HCO₃⁻ transport. These results indicate that the principal inorganic carbon species crossing the plasmalemma in these cells is CO₂.     

Carbon Oxysulfide Inhibition of the CO(2)-Concentrating Process of Unicellular Green Algae

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O₂ evolution, dissolved inorganic carbon accumulation, and photosynthetic CO₂ fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO₂-concentrating process. This COS inhibition can be attributed to inhibition of external HCO₃⁻ conversion to CO₂ and OH⁻ by the carbonic anhydrase component of an active CO₂ pump. At a low pH of 5 to 6, COS stimulated O₂ evolution during photosynthesis by algae with low CO₂ in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO₂. Although COS had less effect on HCO₃⁻ accumulation at pH 9 by a HCO₃⁻ pump in Scenedesmus, COS reduced O₂ evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO₂ and H₂S, its inhibition of the CO₂ pump activity and photosynthesis is not accurate, when measured by O₂ evolution, by NaH¹⁴CO₃ accumulation, or by ¹⁴CO₂ fixation.