Photosynthetic O2 Research Articles

Photosynthesis in sediments determined at high spatial resolution by the use of microelectrodes

The present study investigated photosynthetic rates and their regulation by light within the upper 5 mm of sediment in a tidal area of Niida River in Hachinohe, Japan. Steady-state concentration profiles of O2, NH4+, NO2−, H2S, and pH in the sediment were measured with microelectrodes. Microzonation of O2 respiration, denitrification and SO42− reduction was found in the sediment. When light intensities exceeded 1050 μmol photons/m2/s, net photosynthetic activity was detected in the upper 0.5 mm of the microbial mat colonizing on the sediment surface in the tidal area. In contrast, gross photosynthetic activity was detected in the upper 1.0 mm of the microbial mat at 1900 μmol photons/m2/s. As light intensity increased, the net photosynthetic rate and O2 penetration depth increased. The maximal net photosynthetic rate and O2 penetration depth were 6.1 μmol O2/cm3/h and 2.2 mm, respectively, at 1900 μmol photons/m2/s. Net photosynthetic rates in the microbial mat in the tidal area were lower than in the upstream sediment. The analysis of continuous O2 concentration measurements in different layers of the microbial mat during artificial light–dark cycles demonstrated that the photosynthetic activity response to changes in light intensity was extremely fast (a few seconds) and the O2 concentration in the microbial mat became stable within 200 s. The measurement of physical and chemical parameters in river water revealed that the study site was relatively polluted and sunlight intensity significantly fluctuated temporally. These results suggested that the in situ microbial processes occurring in the sediment fluctuated in accordance with periodic fluctuations in sunlight intensity.

Mitochondria Provide the Main Source of Cytosolic ATP for Activation of Outward-rectifying K+ Channels in Mesophyll Protoplast of Chlorophyll-deficient Mutant Rice (OsCHLH) Seedlings

The role of mitochondria in providing intracellular ATP that controls the activity of plasma membrane outward-rectifying K+ channels was evaluated. The OsCHLH rice mutant, which lacks chlorophyll in the thylakoids, was isolated by T-DNA gene trapping (Jung, K.-H., Hur, J., Ryu, C.-H., Choi, Y., Chung, Y.-Y., Miyao, A., Hirochika, H., and An, G. (2003) Plant Cell Physiol. 44, 463-472). The OsCHLH mutant is unable to fix CO2 and exhibits reduced growth. Wild type and mutant plants exhibit similar rates of respiratory O2 uptake in the dark, whereas the rate of photosynthetic O2 evolution by the mutant was negligible during illumination. During dark respiration the wild type and mutant exhibited similar levels of cytoplasmic ATP. In the mutant oligomycin treatment (an inhibitor of mitochondrial F1F0-ATPase) drastically reduced ATP production. The fact that this was reversed by the addition of glucose suggested that the mutant produced ATP exclusively from mitochondria but not from chloroplasts. In whole cell patch clamp experiments, the activity of outward-rectifying K+ channels of rice mesophyll cells showed ATP-dependent currents, which were 1.5-fold greater in wild type than in mutant cells. Channels in both wild type and mutant cells were deactivated by the removal of cytosolic ATP, whereas in the presence of ATP the channels remained active. We conclude that mesophyll cells in the OsCHLH rice mutant derive ATP from mitochondrial respiration, and that this is critical for the normal function of plasma membrane outward-rectifying K+ channels.

Limitations in the use of PAM fluorometry for measuring photosynthetic rates of macroalgae at high irradiances

Pulse amplitude modulated (PAM) fluorometry can be used for measuring photosynthetic electron transport rates (ETR) of marine angiosperms and macroalgae both in the laboratory and in situ. Regarding macroalgae, quantitative values and linear correlations between ETR and rates of photosynthetic O2 evolution have so far been shown only for a few species under low irradiances. As a logical continuation of such work, the aim of the present study was to (a) assess to what degree high irradiances would limit such measurements and (b) evaluate whether PAM fluorometry could be used quantitatively also for other marine macroalgae from different phyla. This was done by comparing ETR with rates of gross O2 evolution (net O2 exhcange corrected for dark respiration) at various irradiances for the green alga Ulva lactuca grown at two irradiances, the brown algae Fucus serratus and Laminaria saccharina and the red algae Palmaria palmata and Porphyra umbilicalis. At low irradiances, there was a clear positive correlation between O2 evolution and fluorescence-based ETR. At high irradiances, however, all algae featured an apparent decrease in ETR while O2 evolution remained relatively constant, and this resulted in markedly increasing O2/ETR ratios. This anomaly could be nicely illustrated in plots of O2/ETR as a function of the effective quantum yield of photosystem II (Y). Such plots showed that the O2/ETR ratio generally started to increase when Y reached a critical low value of c. 0.1. It was further found that the irradiance at which this value was reached varied with species and previous light histories. Thus, it is the Y value, rather than the irradiance per se

Low temperature effects on grapevine photosynthesis: the role of inorganic phosphate.

The photosynthetic response of grapevine leaves (Vitis vinifera L. cv. Riesling) to low temperature was studied to determine the role of end-product limitation and orthophosphate (Pi) recycling to the chloroplast under these conditions. As reported previously, the response of photosynthesis in air to stomatal conductance declined at temperatures below 15°C, suggesting that at low temperatures inhibition of photosynthesis in grapevine has a strong non-stomatal component. Stimulation of carbon assimilation at ambient CO2 by reducing O2 from 21 to 2 kPa, O2 declined to zero below 15°C, a phenomenon often associated with a restriction in photosynthesis due to end-product-synthesis limitation. This stimulation could be restored by feeding Pi. Photosynthesis in leaf disks at both high and low irradiances in non-photorespiratory conditions (1% CO2) was highly sensitive to reductions in temperature. Below 15°C, feeding Pi caused a large stimulation of photosynthetic O2 evolution. Metabolite measurements indicated that despite a decline in Rubisco carbamylation state, ribulose 1,5-bisphosphate (RuBP) levels dropped at low temperature and the ratio of 3-phosphoglycerate (3-PGA) to triose phosphate (TP) remained largely unchanged. These results suggest that grapevine-leaf photosynthesis is severely restricted at low temperature by non-stomatal mechanisms. The return of Pi to the chloroplast plays an important role in this limitation but a coordinated set of regulatory processes maintain a homeostasis of phosphorylated sugar levels.

Antioxidative role of nitric oxide on copper toxicity to a chlorophycean alga, Chlorella

The response of Chlorella vulgaris to copper exposure was investigated under laboratory batch culture conditions. Increased toxicity of Cu with respect to photosynthetic carbon fixation, O2 evolution, chlorophyll fluorescence, and oxidative burst was observed for N–NH4+-grown cultures. The addition of sodium nitroprusside, a nitric oxide (NO) donor, in combination with Cu to N–NH4+-grown Chlorella not only lowered the inhibition levels of carbon fixation, O2 evolution, and maximum quantum yield of PS II, but also significantly reduced the oxidative burst. The protective action of sodium nitroprusside was, however, arrested in cultures in which sodium nitroprusside was supplemented in combination with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a specific scavenger of NO in the experimental system. The N–NO3−-grown Chlorella depicted less sensitivity to Cu compared to its N–NH4+-grown counterpart. The N–NO3−-, N–NH4+-, and N–NH4++sodium nitroprusside-grown Chlorella did not show any significant differences with respect to their Cu uptake potential. The role of NO as an antioxidant is discussed.

Effect of cobalt on growth, pigments and the photosynthetic electron transport in Monoraphidium minutum and Nitzchia perminuta

The unicellular green alga Monoraphidium minutum and the diatom Nitzschia perminuta were cultured under different concentrations of Co2+. Growth and pigment content were slightly increased at low concentrations and inhibited by high Co2+ concentrations. The results concerning the effect of different concentrations of Co2+ on photosynthetic O2 evolution showed a reduction in the amount of O2 evolved by each alga in response to increasing Co2+ concentrations. However, an increase in O2 evolution for both M. minutum and N. perminuta was observed at relatively low Co2+ concentrations. Photosynthetic electron transport in M. minutum was more sensitive to Co2+ toxicity than in N. perminuta. On the other hand, the effect of the heavy metal on respiration showed that higher Co2+ concentrations were inhibitory to O2 uptake by the two algal species. Low Co2+ concentrations stimulated O2 uptake by M. minutum throughout the experimental period. However, in N. perminuta, different concentrations of Co2+ led to a reduction of O2 uptake. To localize the action site of Co2+ in the photosynthetic electron transport chain, the fluorescence induction technique was carried out. According to the results obtained, the inhibitory action of Co2+ is located on the acceptor side of PSII for both M. minutum and N. perminuta.

Energy status and its control on embryogenesis of legumes: ATP distribution within Vicia faba embryos is developmentally regulated and correlated with photosynthetic capacity.

To analyse the energy status of Vicia faba embryos in relation to differentiation processes, we measured ATP concentrations directly in cryosections using a quantitative bioluminescence-based imaging technique. This method provides a quantitative picture of the ATP distribution close to the in vivo situation. ATP concentrations were always highest within the axis. In pre-storage cotyledons, the level was low, but it increased strongly in the course of further development, starting from the abaxial region of cotyledons and moving towards the interior. Greening pattern, chlorophyll distribution and photosynthetic O2 production within embryos temporally and spatially corresponded to the ATP distribution, implicating that the overall increase of the energy state is associated to the greening process. ATP patterns were associated to the photosynthetic capacity of the embryo. The general distribution pattern as well as the steady state levels of ATP were developmentally regulated and did not change upon dark/light conditions. The major storage protein legumin started to accumulate in abaxial regions with high ATP, whereas starch localized in regions with relatively lower ATP levels. This suggests a role of the energy state in the partitioning of assimilates into the different storage-product classes. Highest biosynthetic rates occurred when cotyledons became fully green and contained high ATP levels, implicating that a photoheterotrophic state was required to ensure high fluxes. Based on these data, we propose a model for the role of embryonic photosynthesis to improve the energy status of the embryo.

In vivo changes of the oxidation-reduction state of NADP and of the ATP/ADP cellular ratio linked to the photosynthetic activity in Chlamydomonas reinhardtii.

The ATP/ADP and NADP/NADPH ratios have been measured in whole-cell extract of the green alga Chlamydomonas reinhardtii, to understand their availability for CO(2) assimilation by the Calvin cycle in vivo. Measurements were performed during the dark-light transition of both aerobic and anaerobic cells, under illumination with saturating or low light intensity. Two different patterns of behavior were observed: (a) In anaerobic cells, during the lag preceding O(2) evolution, ATP was synthesized without changes in the NADP/NADPH ratio, consistently with the operation of cyclic electron flow. (b) In aerobiosis, illumination increased the ATP/ADP ratio independently of the intensity used, whereas the amount of NADPH was decreased at limiting photon flux and regained the dark-adapted level under saturating photon flux. Moreover, under these conditions, the addition of low concentrations of uncouplers stimulated photosynthetic O(2) evolution. These observations suggest that the photosynthetic generation of reducing equivalents rather than the rate of ATP formation limits the photosynthetic assimilation of CO(2) in C. reinhardtii cells. This situation is peculiar to C. reinhardtii, because neither NADPH nor ATP limited this process in plant leaves, as shown by their increase upon illumination in barley (Hordeum vulgare) leaves, independent of light intensity. Experiments are presented and were designed to evaluate the contribution of different physiological processes that might increase the photosynthetic ATP/NADPH ratio-the Mehler reaction, respiratory ATP supply following the transfer of reducing equivalents via the malate/oxaloacetate shuttle, and cyclic electron flow around PSI-to this metabolic situation.

Increases of chlorophyll a/b ratios during acclimation of tropical woody seedlings to nitrogen limitation and high light.

According to the theory of optimal nitrogen partitioning within a leaf, the chlorophyll (Chl) a/b ratio is expected to increase when leaf N content decreases. Here, we report the first empirical support for this prediction. The Chl a/b ratio increased while Chl content decreased in response to N limitation in photosynthetic cotyledons and leaves of seedlings of four tropical woody species in the Bignoniaceae. The responses of all four species were in the same direction, but differed in magnitude. For Tabebuia rosea, the species that exhibited the greatest increase in Chl a/b ratios (up to values of 5.9), detailed photosynthetic characteristics were also examined. Light and N availability were positively correlated with the light- and CO2-saturated photosynthetic O2 evolution rate, as well as with leaf carboxylation capacity (Vcmax) and electron transport rate (Vj). Severe N limitation and high light did not cause chronic photo-inhibition (i.e. no change in quantum yield or in dark-acclimated Fv/Fm). The observed change in the ratio of Vcmax to leaf N in response to N availability was consistent with likely functional reasons for change in the Chl a/b ratio. Adjustment of the Chl a/b ratio was apparently an integral feature of acclimation to high light conditions and low N availability.

Inhibitory activity of analogs of AM-toxin, a host-specific phytotoxin from the Alternaria alternata apple pathotype, on photosynthetic O2 evolution in apple leaves.

The effect of the host-specific phytotoxins, AM-toxins, on the photosynthetic activity of leaves from susceptible apple cultivars was investigated by using an oxygen electrode. The photosynthetic O2 evolution was inhibited by AM-toxin I in a host-specific manner. The inhibitory activity of several AM-toxin analogs against photosynthesis was also evaluated and the findings were correlated with their necrosis-inducing activity.

Carbon acquisition of bloom‐forming marine phytoplankton

Carbon acquisition in relation to CO2 supply was investigated in three marine bloom‐forming microalgae, the diatom Skeletonema costatum, the flagellate Phaeocystis globosa, and the coccolithophorid Emiliania huxleyi. In vivo activities of extracellular (eCA) and intracellular (iCA) carbonic anhydrase activity, photosynthetic O2 evolution, CO2 and HCO3− uptake rates were measured by membrane inlet mass spectrometry in cells acclimated to pCO2 levels of 36, 180, 360, and 1,800 ppmv. Large differences were obtained between species both with regard to the efficiency and regulation of carbon acquisition. While eCA activity increased with decreasing CO2 concentration in S. costatum and P. globosa, consistently low values were obtained for E. huxleyi. No clear trends with pCO2 were observed in iCA activity for any of the species tested. Half saturation concentrations (K1/2) for photosynthetic O2 evolution, which were highest for E. huxleyi and lowest for S. costatum, generally decreased with decreasing CO2 concentration. In contrast, K1/2 values for P. globosa remained unaffected by pCO2 of the incubation. CO2 and HCO3− were taken up simultaneously by all species. The relative contribution of HCO3− to total carbon uptake generally increased with decreasing CO2, yet strongly differed between species. Whereas K1/2 for CO2 and HCO3− uptake was lowest at the lowest pCO2 for S. costatum and E. huxleyi, it did not change as a function of pCO2 in P. globosa. The observed taxon‐specific differences in CO2 sensitivity, if representative for the natural environment, suggest that changes in CO2 availability may influence phytoplankton species succession and distribution. By modifying the relative contribution of different functional groups, e.g., diatomaceous versus calcareous phytoplankton, to the overall primary production this could potentially affect marine biogeochemical cycling and air—sea gas exchange.

ABA-induced Increase in Thermostability of Photosynthetic Electron Transport of Thylakoids in Leaves of Cucumber (Cucumis sativus L.).

The effect of ABA on thermostability of the photosynthetic apparatus in leaves of cucumber (Cucumis sativus L.) was investigated. Cucumber plants grown in soil at 25°C (day and night) were administered foliar sprays of 1 mM ABA when the third leaves were fully expanded. Compared to untreated control leaves, ABA-treated leaves had faster rates of photosynthetic O2 evolution and a higher photosystem (PS) II chlorophyll fluorescence yield following exposure to 45°C in the dark for 10 min. These effects of ABA were attenuated over time after ABA treatment. Heating the isolated thylakoids from treated and control leaves at 40 °C in the dark for 5 min revealed that heat-induced loss of PS II- and PS I-mediated electron transport rates was alleviated by the ABA treatment. The PS II reaction center was not heat-damaged, regardless of the source of thylakoids. On the other hand, a reduction in the loss of proteins and manganese during heat treatment occurred in thylakoids from ABA-treated leaves compared to those from control leaves. This reduction indicates that ABA increased the thermostability of the O2 evolving complex. Direct treatment of isolated thylakoids with ABA was ineffective in preventing the heat inactivation of the photosystems. Thus, it seems that high foliar levels of ABA induce a mechanism associated with thermostability of the O2 evolving complex. However, these effects of ABA on isolated thylakoids were smaller than were those on intact leaves, indicating that ABA can also induce thermostability in other components of the photosynthetic apparatus besides the O2 evolving complex.

Effect of Chlorophyll-Protein Complex I Deficiency on the Physiological Character of a Chlamydomonas reinhardtii Mutant

In the mutant CC-1047 of Chlamydomonas reinhardtii, LDS-PAGE showed that the chlorophyll-protein complex I (CPI) is almost absent. The mutant could not grow in a culture medium without organic carbon source while the wild type (WT) C. reinhardtii grew quickly. When an organic carbon source was added into the culture medium, the mutant grew almost as well as WT. The rate of photosystem 1 (PS1) electron transport (DCPIP→MV) and the rate of whole chain electron transport (H2O→MV) of chloroplasts of the CC-1047 mutant were both lower than those of WT. The photophosphorylation activity, photosynthetic O2 evolution rate, and rate of NADP+ photoreduction of CC-1047 were also much lower than the activities of WT. There were some differences in ATPase activity between the mutant and WT. Two different activation ways were used to activate the latent ATPase using methanol and dithiothreitol (DTT) as activation substrate. More methanol and DTT were required for the mutant than WT to obtain the maximum activity. Thus the photosynthetic apparatus could not operate normally when CPI was absent because of the abnormal PS1 electron transport. Meanwhile, the other adjacent complexes of the thylakoid membrane, for example, ATP synthase complex, were slightly affected.

ACQUISITION OF INORGANIC CARBON BY THE MARINE HAPTOPHYTE ISOCHRYSIS GALBANA (PRYMNESIOPHYCEAE)1

Inorganic carbon acquisition has been investigated in the marine haptophyte Isochrysis galbana. External carbonic anhydrase (CA) was present in air‐grown (0.034% CO2) cells but completely repressed in high (3%) CO2‐grown cells. External CA was not inhibited by 1.0 mM acetazolamide. The capacity of cells to take up bicarbonate was examined by comparing the rate of photosynthetic O2 evolution with the calculated rate of spontaneous CO2 supply; at pH 8.2 the rates of O2 evolution exceeded the CO2 supply rate 14‐fold, indicating that this alga was able to take up HCO3−. Monitoring CO2 concentrations by mass spectrometry showed that suspensions of high CO2‐grown cells caused a rapid drop in the extracellular CO2 in the light and addition of bovine CA raised the CO2 concentration by restoring the HCO3−‐CO2 equilibrium, indicating that cells were maintaining the CO2 in the medium below its equilibrium value during photosynthesis. A rapid increase in extracellular CO2 concentration occurred on darkening the cells, indicating that the cells had accumulated an internal pool of unfixed inorganic carbon. Active CO2 uptake was blocked by the photosynthetic electron transport inhibitor 3‐(3′,4′‐dichlorphenyl)‐1,1‐dimethylurea, indicating that CO2 transport was supported by photosynthetic reactions. These results demonstrate that this species has the capacity to take up HCO3− and CO2 actively as sources of substrate for photosynthesis and that inorganic carbon transport is not repressed by growth on high CO2, although external CA expression is regulated by CO2 concentration.

Carbon and nitrogen metabolism in barley plants exposed to UV-B radiation.

The effect of UV-B radiation on FW, leaf and stem length, photosynthetic O2 evolution, levels of carbohydrates and nitrates, and extractable activities of some of the enzymes involved in C and N metabolism was evaluated in barley (Hordeum vulgare L. cv. Express) seedlings during the 9 days following transfer to an UV-B enriched environment. The results show that under our experimental conditions UV-B radiation scarcely affects the photosynthetic competence of barley leaves, expressed as RuBP carboxylase (EC 4.1.1.39) activity, O2 evolution rate and chlorophyll content. Nevertheless, this treatment induced significant alterations of the enzyme activity of nitrate reductase (EC 1.6.6.1) and glutamine synthetase (EC 6.3.1.2), although only after a few days of treatment. The effects were not confined to the exposed tissue, but were detectable also at the root level. In fact, nitrate reductase decreased in response to UV-B in both leaf and root tissue, whereas glutamine synthetase was affected only in the root. In contrast, nitrate content was not influenced by the treatment, neither in root nor in leaf tissue, whilst leaf sucrose diminished in exposed plants only on the last day of treatment.

The Underwater Life of Secondarily Aquatic Plants: Some Problems and Solutions

The freshwater secondarily aquatic plants, most of which are higher plants, are those returned to the water environment after spending a period of time living on land. The readaptation to living underwater has made it necessary for these plants to put in place morphological and functional strategies to cope with some major problems due to features of the aquatic environment, but also deriving from the specialized organization of their “terrestrial” bodies. The poor O2 availability underwater accounted for the evolution of wide aerenchyma tissues throughout the plant organs to improve the photosynthetic O2 flux from the shoot to the roots buried in anoxic sediments and to the neighboring rhizosphere. This favors sediment oxygenation, sustains the aerobic metabolism of roots, and improves the availability and uptake of mineral nutrients, whose delivery to the entire plants, without a transpirational flux, is ensured by an acropetal mass transport depending on root pressure, guttation from hydathodes and channeling by apoplast closure around the vascular tissues. A great expansion of leaf surfaces and an enhanced surface:volume ratio of chloroplast-rich photosynthetic cells help to contact the water medium and to increase the cell/environment exchanges to gain inorganic carbon. Furthermore, different physiological mechanisms operate to cope with the scarce availability of CO2 and the prevalence of HCO3 − as inorganic carbon form in water. Some of them, like cell wall acidification through H+ extrusion by a light-dependent APTase or activation of an apoplastic carbonic anhydrase, operate outside the cells, leading to a conversion of HCO3 − to CO2, which then diffuses into the cells. Others, on the contrary, act inside the cells to load the active site of Rubisco with CO2, thus favoring photosynthesis and lowering photorespiration. Aquatic macrophytes with isoetid life form, moreover, can obtain most ot the fixed CO2 from sediments. In submerged species, in additin to the C3 cycle, the C4 and CAM-like photosynthetic metabolisms can also operate, and are modulated by the environmental inorganic carbon availability and the plant photosynthetic demand. Interestingly, in the aquatic plants the C4 pathway, which can be concomitant with the C3 one, does not depend on the Kranz anatomy of leaves, but relies on the intracellular compartmentation of carboxylative and decarboxylative enzymes. The CAM-like pathway, defined AAM, which also coexists with the C3, allows the submerged plants to fix CO2 in the dark, thus exploiting the higher CO2 availability in the water medium during the night, and extending to 24 h the period of inorganic carbon assimilation. In almost all the aquatic macrophytes the AAM is only expressed in the submersion state, whereas it is quickly inactivated in emerging leaves in a cell by cell way.

Biochemical and morphological characterization of sulfur-deprived and H2-producing Chlamydomonas reinhardtii (green alga).

Sulfur deprivation in green algae causes reversible inhibition of photosynthetic activity. In the absence of S, rates of photosynthetic O2 evolution drop below those of O2 consumption by respiration. As a consequence, sealed cultures of the green alga Chlamydomonas reinhardtii become anaerobic in the light, induce the "Fe-hydrogenase" pathway of electron transport and photosynthetically produce H2 gas. In the course of such H2-gas production cells consume substantial amounts of internal starch and protein. Such catabolic reactions may sustain, directly or in directly, the H2-production process. Profile analysis of selected photosynthetic proteins showed a precipitous decline in the amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) as a function of time in S deprivation, a more gradual decline in the level of photosystem (PS) II and PSI proteins, and a change in the composition of the PSII light-harvesting complex (LHC-II). An increase in the level of the enzyme Fe-hydrogenase was noted during the initial stages of S deprivation (0-72 h) followed by a decline in the level of this enzyme during longer (t >72 h) S-deprivation times. Microscopic observations showed distinct morphological changes in C. reinhardtii during S deprivation and H2 production. Ellipsoid-shaped cells (normal photosynthesis) gave way to larger and spherical cell shapes in the initial stages of S deprivation and H2 production, followed by cell mass reductions after longer S-deprivation and H2-production times. It is suggested that, under S-deprivation conditions, electrons derived from a residual PSII H2O-oxidation activity feed into the hydrogenase pathway, thereby contributing to the H2-production process in Chlamydomonas reinhardtii. Interplay between oxygenic photosynthesis, mitochondrial respiration, catabolism of endogenous substrate, and electron transport via the hydrogenase pathway is essential for this light-mediated H2-production process.

Regulation of Photosynthesis during the Light Period in CAM Plants. Evaluation by a Gas-Phase O2 Electrode and a Compensating Infrared CO2 Analysis System.

The CO2 dependent O2 evolution during the light period, at which the exogenous CO2 uptake was suspended, in 12 CAM plants (including pineapple (Ananas comosus) ) was evaluated with a gas-phase oxygen electrode. At 5% CO2, the rate of photosynthetic O2 evolution in pineapple was saturated at 1 500 μmol m-2 s-1 PPFD and the maximum rate was 60 μmol m-2 s-1, which was 10 times those obtained at ambient CO2 conditions with the CO2 exchange system and significantly higher than the other CAM plants. At the saturated PPFD, the O2 evolution in pineapple substantially increased with increasing CO2 concentration up to 3% and decreased above 4%. However in the other CAM plants, such increment was small. By the use of a novel compensating CO2 gas exchange system, the rate of CO2 exchange in pineapple at very high CO2 was measured and found the enhancement of CO2 uptake in both light (Phases III and IV) and dark (Phase I) periods. Based on the results obtained, possibilities of the further increases in CO2 uptake in CAM plants, especially of pineapple, are discussed in terms of stomatal functions and malate storage capacities.

Oxygen inhibition of dissolved inorganic carbon uptake in unicellular green algae

SUMMARY 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 K0.5(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.

PHOTOAUTOTROPHIC GROWTH OF A RECENTLY ISOLATED N2‐FIXING MARINE NON‐HETEROCYSTOUS FILAMENTOUS CYANOBACTERIUM, SYMPLOCA SP. (CYANOBACTERIA)

Photoautotrophic growth of a marine non‐heterocystous filamentous cyanobacterium, Symploca sp. strain S84, was examined under nitrate‐assimilating and N2‐fixing conditions. Under continuous light, photon flux density of 55 μmol photons·m−2 ·s−1 was at a saturating level for growth, and light did not inhibit the growth rate under N2‐fixing conditions even when the photon flux density was doubled (110 μmol photons·m−2 ·s−1). Doubling times of the N2‐fixing cultures under 55 and 110 μmol photons·m−2 ·s−1 were about 30 and 31 h, respectively. Under 110 μmol photons·m−2 ·s−1 during the light phase of an alternating 12:12‐h light:dark (L:D) cycle, the doubling time of the N2‐fixing culture was also about 30 h. When grown diazotrophically under a 12:12‐h L:D regime, C2H2 reduction activity was observed mainly during darkness. In continuous light, relatively large cyclic fluctuations in C2H2 reduction were observed during growth. The short‐term (<4 h) effect of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU; 5 μM) indicated that C2H2 reduction activity was not influenced by photosynthetic O2 evolution. Long‐term (24 h) effects of DCMU indicated that photosynthesis and C2H2 reduction activity occur simultaneously. These results indicate that strain S84 grows well under diazotrophic conditions when saturating light is supplied either continuously or under a 12:12‐h L:D diel light regime.