Light-driven Uptake of Oxygen, Carbon Dioxide, and Bicarbonate by the Green Alga Scenedesmus

Development and validation of an in situ and real-time quantification method for bicarbonate, carbonate and orthophosphate ions by ATR FT-IR spectroscopy in aqueous solutions

Bicarbonate salts are used in various industrial processes and could even serve as an alternative source of carbon in bioprocesses involving photosynthetic organisms. Industrial productions require efficient monitoring and control to ensure that their output will meet target specifications. To this end, a simple and rapid in situ quantification method was developed for bicarbonate, carbonate and phosphate ions using Attenuated Total Reflectance-Fourier Transform Infrared (ATR FT-IR) spectroscopy combined with partial least squares (PLS). The resulting multivariate approach allows the simultaneous determination of inorganic carbon and orthophosphate ions concentrations in aqueous solutions (R2 > 0.98, root-mean-square-errors of the cross validation RMSECV < 3.3%). Validation of the method was achieved through replicability and repeatability tests. Univariate calibration graphs are linear over a concentration range of 150 mM (R2 > 0.9990). Quantification limits for those ions were in the 6.9-17.2 mM range, as determined from univariate models. The multivariate model was successfully applied to a microalgal culture of Scenedesmus obliquus using bicarbonate as the carbon source and a phosphate buffer to maintain the pH. This analytical technique did not require extraction or chemical treatment and no sample preparation is needed. The results demonstrate the potential of ATR FT-IR method to study inorganic carbon and phosphate species during a bioprocess.

Measurement of Gross Photosynthesis, Respiration in the Light, and Mesophyll Conductance Using H218O Labeling

A fundamental challenge in plant physiology is independently determining the rates of gross O₂ production by photosynthesis and O₂ consumption by respiration, photorespiration, and other processes. Previous studies on isolated chloroplasts or leaves have separately constrained net and gross O₂ production (NOP and GOP, respectively) by labeling ambient O₂ with ¹⁸O while leaf water was unlabeled. Here, we describe a method to accurately measure GOP and NOP of whole detached leaves in a cuvette as a routine gas-exchange measurement. The petiole is immersed in water enriched to a δ¹⁸O of ∼9,000‰, and leaf water is labeled through the transpiration stream. Photosynthesis transfers ¹⁸O from H₂O to O₂ GOP is calculated from the increase in δ¹⁸O of O₂ as air passes through the cuvette. NOP is determined from the increase in O₂/N₂ Both terms are measured by isotope ratio mass spectrometry. CO₂ assimilation and other standard gas-exchange parameters also were measured. Reproducible measurements are made on a single leaf for more than 15 h. We used this method to measure the light response curve of NOP and GOP in French bean (Phaseolus vulgaris) at 21% and 2% O₂ We then used these data to examine the O₂/CO₂ ratio of net photosynthesis, the light response curve of mesophyll conductance, and the apparent inhibition of respiration in the light (Kok effect) at both oxygen levels. The results are discussed in the context of evaluating the technique as a tool to study and understand leaf physiological traits.

The Mechanisms of Oxygen Reduction in the Terminal Reducing Segment of the Chloroplast Photosynthetic Electron Transport Chain

The review is dedicated to ascertainment of the roles of the electron transfer cofactors of the pigment-protein complex of PSI, ferredoxin (Fd) and ferredoxin-NADP reductase in oxygen reduction in the photosynthetic electron transport chain (PETC) in the light. The data regarding oxygen reduction in other segments of the PETC are briefly analyzed, and it is concluded that their participation in the overall process in the PETC under unstressful conditions should be insignificant. Data concerning the contribution of Fd to the oxygen reduction in the PETC are examined. A set of collateral evidence as well as results of direct measurements of the involvement of Fd in this process in the presence of isolated thylakoids led to the inference that this contribution in vivo is negligible. The increase in oxygen reduction rate in the isolated thylakoids in the presence of either Fd or Fd plus NADP⁺ under increasing light intensity was attributed to the increase in oxygen reduction executed by the membrane-bound oxygen reductants. Data are presented which imply that a main reductant of the O₂ molecule in the terminal reducing segment of the PETC is the electron transfer cofactor of PSI, phylloquinone. The physiological significance of characteristic properties of oxygen reductants in this segment of the PETC is discussed.

Optimized inorganic carbon regime for enhanced growth and lipid accumulation in Chlorella vulgaris

BACKGROUND

Large-scale algal biofuel production has been limited, among other factors, by the availability of inorganic carbon in the culture medium at concentrations higher than achievable with atmospheric CO2. Life cycle analyses have concluded that costs associated with supplying CO2 to algal cultures are significant contributors to the overall energy consumption.

RESULTS

A two-phase optimal growth and lipid accumulation scenario is presented, which (1) enhances the growth rate and (2) the triacylglyceride (TAG) accumulation rate in the oleaginous Chlorophyte Chlorella vulgaris strain UTEX 395, by growing the organism in the presence of low concentrations of NaHCO3 (5 mM) and controlling the pH of the system with a periodic gas sparge of 5 % CO2 (v/v). Once cultures reached the desired cell densities, which can be "fine-tuned" based on initial nutrient concentrations, cultures were switched to a lipid accumulation metabolism through the addition of 50 mM NaHCO3. This two-phase approach increased the specific growth rate of C. vulgaris by 69 % compared to cultures sparged continuously with 5 % CO2 (v/v); further, biomass productivity (g L(-1) day(-1)) was increased by 27 %. Total biodiesel potential [assessed as total fatty acid methyl ester (FAME) produced] was increased from 53.3 to 61 % (FAME biomass(-1)) under the optimized conditions; biodiesel productivity (g FAME L(-1) day(-1)) was increased by 7.7 %. A bicarbonate salt screen revealed that American Chemical Society (ACS) and industrial grade NaHCO3 induced the highest TAG accumulation (% w/w), whereas Na2CO3 did not induce significant TAG accumulation. NH4HCO3 had a negative effect on cell health presumably due to ammonia toxicity. The raw, unrefined form of trona, NaHCO3∙Na2CO3 (sodium sesquicarbonate) induced TAG accumulation, albeit to a slightly lower extent than the more refined forms of sodium bicarbonate.

CONCLUSIONS

The strategic addition of sodium bicarbonate was found to enhance growth and lipid accumulation rates in cultures of C. vulgaris, when compared to traditional culturing strategies, which rely on continuously sparging algal cultures with elevated concentrations of CO2(g). This work presents a two-phased, improved photoautotrophic growth and lipid accumulation approach, which may result in an overall increase in algal biofuel productivity.

Revised scheme for the mechanism of photoinhibition and its application to enhance the abiotic stress tolerance of the photosynthetic machinery

When photosynthetic organisms are exposed to abiotic stress, their photosynthetic activity is significantly depressed. In particular, photosystem II (PSII) in the photosynthetic machinery is readily inactivated under strong light and this phenomenon is referred to as photoinhibition of PSII. Other types of abiotic stress act synergistically with light stress to accelerate photoinhibition. Recent studies of photoinhibition have revealed that light stress damages PSII directly, whereas other abiotic stresses act exclusively to inhibit the repair of PSII after light-induced damage (photodamage). Such inhibition of repair is associated with suppression, by reactive oxygen species (ROS), of the synthesis of proteins de novo and, in particular, of the D1 protein, and also with the reduced efficiency of repair under stress conditions. Gene-technological improvements in the tolerance of photosynthetic organisms to various abiotic stresses have been achieved via protection of the repair system from ROS and, also, by enhancing the efficiency of repair via facilitation of the turnover of the D1 protein in PSII. In this review, we summarize the current status of research on photoinhibition as it relates to the effects of abiotic stress and we discuss successful strategies that enhance the activity of the repair machinery. In addition, we propose several potential methods for activating the repair system by gene-technological methods.

Carbonic anhydrase: a key regulatory and detoxifying enzyme for Karst plants

Karstification is a rapid process during which calcidic stones/limestones undergo dissolution with the consequence of a desertification of karst regions. A slow-down of those dissolution processes of Ca-carbonate can be approached by a reforestation program using karst-resistant plants that can resist alkaline pH and higher bicarbonate (HCO₃⁻) concentrations in the soil. Carbonic anhydrases (CA) are enzymes that mediate a rapid and reversible interconversion of CO₂ and HCO₃⁻. In the present study, the steady-state expression of a CA gene, encoding for the plant carbonic anhydrase from the parsley Petroselinum crispum, is monitored. The studies were primarily been performed during germination of the seeds up to the 12/14-day-old embryos. The CA cDNA was cloned. Quantitative polymerase chain reaction (qPCR) analysis revealed that the gene expression level of the P. crispum CA is strongly and significantly affected at more alkaline pH in the growth medium (pH 8.3). This abolishing effect is counteracted both by addition of HCO₃⁻ and by addition of polyphosphate (polyP) to the culture medium. In response to polyP, the increased pH in the vacuoles of the growing plants is normalized. The effect of polyP let us to propose that this polymer acts as a buffer system that facilitates the adjustment of the pH in the cytoplasm. In addition, it is proposed that polyP has the potential to act, especially in the karst, as a fertilizer that allows the karstic plants to cope with the adverse pH and HCO₃⁻ condition in the soil.

Effect of pH on Inorganic Carbon Uptake in Algal Cultures

Biomass production by the green algae Scenedesmus obliquus and Chlorella vulgaris in intensive laboratory continuous cultures was considerably affected by the pH at which the cultures were maintained. Carbon photoassimilation experiments revealed that pH values in the range of 8 to 9 were important for determining the free CO(2) concentrations in the medium. With higher pH values, additional pH effects were observed involving a decrease in the relative high affinity of low CO(2)-adapted algae to free CO(2). The carbon uptake rate by high CO(2)-adapted algae after transfer to low free CO(2) medium was characterized by a lag period of about 30 min, after which the affinity of the algae to CO(2) increased considerably. Both continuous growth and carbon uptake experiments indicated that artificially maintained high free CO(2) concentrations are recommended for maximal production in intensive outdoor algal cultures.

On-line mass spectrometry: membrane inlet sampling

Significant insights into plant photosynthesis and respiration have been achieved using membrane inlet mass spectrometry (MIMS) for the analysis of stable isotope distribution of gases. The MIMS approach is based on using a gas permeable membrane to enable the entry of gas molecules into the mass spectrometer source. This is a simple yet durable approach for the analysis of volatile gases, particularly atmospheric gases. The MIMS technique strongly lends itself to the study of reaction flux where isotopic labeling is employed to differentiate two competing processes; i.e., O(2) evolution versus O(2) uptake reactions from PSII or terminal oxidase/rubisco reactions. Such investigations have been used for in vitro studies of whole leaves and isolated cells. The MIMS approach is also able to follow rates of isotopic exchange, which is useful for obtaining chemical exchange rates. These types of measurements have been employed for oxygen ligand exchange in PSII and to discern reaction rates of the carbonic anhydrase reactions. Recent developments have also engaged MIMS for online isotopic fractionation and for the study of reactions in inorganic systems that are capable of water splitting or H(2) generation. The simplicity of the sampling approach coupled to the high sensitivity of modern instrumentation is a reason for the growing applicability of this technique for a range of problems in plant photosynthesis and respiration. This review offers some insights into the sampling approaches and and the experiments that have been conducted with MIMS.

How do environmental stresses accelerate photoinhibition?

Environmental stress enhances the extent of photoinhibition, a process that is determined by the balance between the rate of photodamage to photosystem II (PSII) and the rate of its repair. Recent investigations suggest that exposure to environmental stresses, such as salt, cold, moderate heat and oxidative stress, do not affect photodamage but inhibit the repair of PSII through suppression of the synthesis of PSII proteins. In particular, production of D1 protein is downregulated at the translation step by the direct inactivation of the translation machinery and/or by primarily interrupting the fixation of CO2. The latter results in the creation of reactive oxygen species (ROS), which in turn block the synthesis of PSII proteins in chloroplasts.

Photoinhibition of photosystem II under environmental stress

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to an imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. In the "classical" scheme for the mechanism of photoinhibition, strong light induces the production of reactive oxygen species (ROS), which directly inactivate the photochemical reaction center of PSII. By contrast, in a new scheme, we propose that photodamage is initiated by the direct effect of light on the oxygen-evolving complex and that ROS inhibit the repair of photodamaged PSII by suppressing primarily the synthesis of proteins de novo. The activity of PSII is restricted by a variety of environmental stresses. The effects of environmental stress on damage to and repair of PSII can be examined separately and it appears that environmental stresses, with the exception of strong light, act primarily by inhibiting the repair of PSII. Studies have demonstrated that repair-inhibitory stresses include CO(2) limitation, moderate heat, high concentrations of NaCl, and low temperature, each of which suppresses the synthesis of proteins de novo, which is required for the repair of PSII. We postulate that most types of environmental stress inhibit the fixation of CO(2) with the resultant generation of ROS, which, in turn, inhibit protein synthesis.

Glycerate-3-phosphate, produced by CO2 fixation in the Calvin cycle, is critical for the synthesis of the D1 protein of photosystem II

We demonstrated recently that, in intact cells of Chlamydomonas reinhardtii, interruption of CO2 fixation via the Calvin cycle inhibits the synthesis of proteins in photosystem II (PSII), in particular, synthesis of the D1 protein, during the repair of PSII after photodamage. In the present study, we investigated the mechanism responsible for this phenomenon using intact chloroplasts isolated from spinach leaves. When CO2 fixation was inhibited by exogenous glycolaldehyde, which inhibits the phosphoribulokinase that synthesizes ribulose-1,5-bisphosphate, the synthesis de novo of the D1 protein was inhibited. However, when glycerate-3-phosphate (3-PGA), which is a product of CO2 fixation in the Calvin cycle, was supplied exogenously, the inhibitory effect of glycolaldehyde was abolished. A reduced supply of CO2 also suppressed the synthesis of the D1 protein, and this inhibitory effect was also abolished by exogenous 3-PGA. These findings suggest that the supply of 3-PGA, generated by CO2 fixation, is important for the synthesis of the D1 Protein. It is likely that 3-PGA accepts electrons from NADPH and decreases the level of reactive oxygen species, which inhibit the synthesis of proteins, such as the D1 protein.

Interruption of the Calvin cycle inhibits the repair of Photosystem II from photodamage

In photosynthetic organisms, impairment of the activities of enzymes in the Calvin cycle enhances the extent of photoinactivation of Photosystem II (PSII). We investigated the molecular mechanism responsible for this phenomenon in the unicellular green alga Chlamydomonas reinhardtii. When the Calvin cycle was interrupted by glycolaldehyde, which is known to inhibit phosphoribulokinase, the extent of photoinactivation of PSII was enhanced. The effect of glycolaldehyde was very similar to that of chloramphenicol, which inhibits protein synthesis de novo in chloroplasts. The interruption of the Calvin cycle by the introduction of a missense mutation into the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) also enhanced the extent of photoinactivation of PSII. In such mutant 10-6C cells, neither glycolaldehyde nor chloramphenicol has any additional effect on photoinactivation. When wild-type cells were incubated under weak light after photodamage to PSII, the activity of PSII recovered gradually and reached a level close to the initial level. However, recovery was inhibited in wild-type cells by glycolaldehyde and was also inhibited in 10-6C cells. Radioactive labelling and Northern blotting demonstrated that the interruption of the Calvin cycle suppressed the synthesis de novo of chloroplast proteins, such as the D1 and D2 proteins, but did not affect the levels of psbA and psbD mRNAs. Our results suggest that the photoinactivation of PSII that is associated with the interruption of the Calvin cycle is attributable primarily to the inhibition of the protein synthesis-dependent repair of PSII at the level of translation in chloroplasts.

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

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

Direct interface of chemistry to microbiological systems: membrane inlet mass spectrometry

Direct measurement of dissolved gases and low molecular weight volatiles through permeable membranes (e.g. 50-microm-thick silicone rubber), provides an invaluable tool for the investigation of the activities of microorganisms in the laboratory and in their natural environments. Multiple molecular species are monitored at a single point. Fast response times (t(90%)<1 min) and long-term stability, (<1% week(-1)); high specificity and high sensitivity (e.g. 0.2 microM for O(2), <0.5 mM for ethanol), provides a technique that can provide information on the kinetics of processes over many decades (10(0)-10(6)) of minutes. Spatial resolution of <1 mm enables 3D mapping of gases in complex ecosystems (sediments, peat, soils, biofilms, foodstuffs). Results with membrane inlet mass spectrometry (MIMS) when used in conjunction with confocal scanning laser microscopy, provides a powerful approach to the analysis of kinetic and spatial aspects of natural environments. Examples discussed are peat cores and cheese.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

The diversity and coevolution of Rubisco, plastids, pyrenoids, and chloroplast-based CO2-concentrating mechanisms in algae

Algae have adopted two primary strategies to maximize the performance of Rubisco in photosynthetic CO2 fixation. This has included either the development of a CO2-concentrating mechanism (CCM), based at the level of the chloroplast, or the evolution of the kinetic properties of Rubisco. This review examines the potential diversity of both Rubisco and chloroplast-based CCMs across algal divisions, including both green and nongreen algae, and seeks to highlight recent advances in our understanding of the area and future areas for research. Overall, the available data show that Rubisco enzymes from algae have evolved a higher affinity for CO2 when the algae have adopted a strategy for CO2 fixation that does not utilise a CCM. This appears to be true of both Green and Red Form I Rubisco enzymes found in green and nongreen algae, respectively. However, the Red Form I Rubisco enzymes present in nongreen algae appear to have reduced oxygenase potential at air level of O2. This has resulted in a photosynthetic physiology with a reduced potential to be inhibited by O2 and a reduced need to deal with photorespiration. In the limited number of microalgae that have been examined, there is a strong correlation between the existence of a high-affinity CCM physiology and the presence of pyrenoids in all algae, highlighting the potential importance of these chloroplast Rubisco-containing bodies. However, in macroalgae, there is greater diversity in the apparent relationships between pyrenoids and chloroplast features and the CCM physiology that the species shows. There are many examples of microalgae and macroalgae with variations in the presence and absence of pyrenoids as well as single and multiple chloroplasts per cell. This occurs in both green and nongreen algae and should provide ample material for extending studies in this area. Future research into the function of the pyrenoid and other chloroplast features, such as thylakoids, in the operation of a chloroplast-based CCM needs to be addressed in a diverse range of algal species. This should be approached together with assessment of the coevolution of Rubisco, particularly the evolution of Red Form I Rubisco enzymes, which appear to achieve superior kinetic characteristics when compared with the Rubisco of C3 higher plants, which are derived from green algal ancestors.Key words: Rubisco, CO2-concentrating mechanism, carbonic anhydrase, aquatic photosynthesis, algae, pyrenoids, inorganic carbon.

Hill Reaction, Hydrogen Peroxide Scavenging, and Ascorbate Peroxidase Activity of Mesophyll and Bundle Sheath Chloroplasts of NADP-Malic Enzyme Type C(4) Species

Intact mesophyll and bundle sheath chloroplasts wee isolated from the NADP-malic enzyme type C(4) plants maize, sorghum (monocots), and Flaveria trinervia (dicot) using enzymic digestion and mechanical isolation techniques. Bundle sheath chloroplasts of this C(4) subgroup tend to be agranal and were previously reported to be deficient in photosystem II activity. However, following injection of intact bundle sheath chloroplasts into hypotonic medium, thylakoids had high Hill reaction activity, similar to that of mesophyll chloroplasts with the Hill oxidants dichlorophenolindophenol, p-benzoquinone, and ferricyanide (approximately 200 to 300 micromoles O(2) evolved per mg chlorophyll per hour). In comparison to that of mesophyll chloroplasts, the Hill reaction activity of bundle sheath chloroplasts of maize and sorghum was labile and lost activity during assay. Bundle sheath chloroplasts of maize also exhibited some capacity for 3-phosphoglycerate dependent O(2) evolution (29 to 58 micromoles O(2) evolved per milligram chlorophyll per hour). Both the mesophyll and bundle sheath chloroplasts were equally effective in light dependent scavenging of hydrogen peroxide. The results suggest that both chloroplast types have noncyclic electron transport and the enzymology to reduce hydrogen peroxide to water. The activities of ascorbate peroxidase from these chloroplast types was consistent with their capacity to scavenge hydrogen peroxide.

On the nature of the oxygen uptake in the light by Chondrus crispus. Effects of inhibitors, temperature and light intensity

The nature of the different processes of O2 uptake involved in the light in the red macroalga Chondrus crispus Stackhouse (Rhodophyta, Gigartinales) was investigated. At limiting CO2, INH (2.5 mM) did not alter the O2 uptake rate. Glycolate was not excreted and did not accumulate within the cells. KCN reduced the rate of O2 uptake in the light by 76% at limiting CO2 and by 43% at saturating CO2, but caused > 95% inhibition of O2 evolution. DCMU (5 μM) totally blocked the photosynthetic electron transport chain, but allowed a residual O2 uptake of 3.0±0.6 μmol O2 .h(-1).g(-1) FW, irrespective of the CO2 concentration. In saturating CO2, a high light intensity pretreatment significantly stimulated the rate of O2 uptake compared to net O2 evolution, suggesting the persistence, in the light, of mitochondrial respiration. Irrespective of the CO2 concentration, the optimum temperature for O2 evolution was 17°C whereas dark O2 uptake increased linearly with temperature. In contrast, O2 uptake in the light showed an optimum at 17°C in limiting CO2, and 21-25° C in saturating CO2; its Q10 was 2.4 at limiting CO2, a value close to that of RuBP oxygenase, and 3.1 at saturating CO2, a value close to that of dark respiration. It is concluded that: 1) mitochondrial respiration and Mehler reaction are both involved at all CO2 concentrations, 2) RuBP oxygenase activity cannot account for more than 45%, and Mehler reaction for less than 20%, of the total O2 uptake observed in the light at limiting CO2.

Effect of dissolved inorganic carbon on oxygen evolution and uptake by Chlamydomonas reinhardtii suspensions adapted to ambient and CO2-enriched air

Mass spectrometric measurements of (16)O2 and (18)O2 isotopes were used to compare the rates of gross O2 evolution (E0), O2 uptake (U0) and net O2 evolution (NET) in relation to different concentrations of dissolved inorganic carbon (DIC) by Chlamydomonas reinhardtii cells grown in air (air-grown), in air enriched with 5% CO2 (CO2-grown) and by cells grown in 5% CO2 and then adapted to air for 6h (air-adapted).At a photon fluence rate (PFR) saturating for photosynthesis (700 μmol photons m(-2) s(-1)), pH=7.0 and 28°C, U0 equalled E0 at the DIC compensation point which was 10μM DIC for CO2-grown and zero for air-grown cells. Both E0 and U0 were strongly dependent on DIC and reached DIC saturation at 480 μM and 70 μM for CO2-grown and air-grown algae respectively. U0 increased from DIC compensation to DIC saturation. The U0 values were about 40 (CO2-grown), 165 (air-adapted) and 60 μmol O2 mg Chl(-1) h(-1) (air-grown). Above DIC compensation the U0/E0 ratios of air-adapted and air-grown algae were always higher than those of CO2-grown cells. These differences in O2 exchange between CO2- and air-grown algae seem to be inducable since air-adapted algae respond similarly to air-grown cells.For all algae, the rates of dark respiratory O2 uptake measured 5 min after darkening were considerably lower than the rates of O2 uptake just before darkening. The contribution of dark respiration, photorespiration and the Mehler reaction to U0 is discussed and the energy requirement of the inducable CO2/HCO3 (-) concentrating mechanism present in air-adapted and air-grown C. reinhardtii cells is considered.

Photorespiration and Internal CO(2) Accumulation in Chara corallina as Inferred from the Influence of DIC and O(2) on Photosynthesis

An O(2) electrode system with a specially designed chamber for ;whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O(2) concentrations on photosynthetic O(2) evolution. At pH = 5.5 and 20% O(2), cells grown in HCO(3) (-) medium (low CO(2), pH >/= 9.0) exhibited a higher affinity for external CO(2) (K((1/2))(CO(2)) = 40 +/- 6 micromolar) than the cells grown for at least 24 hours in high-CO(2) medium (pH = 6.5), (K((1/2))(CO(2)) = 94 +/- 16 micromolar). With O(2) </= 2% in contrast, both types of cells showed a high apparent affinity (K((1/2))(CO(2)) = 50 - 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO(2) medium (pH = 6.5). The high-pH, HCO(3) (-)-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO(2) which exceeded the CO(2) externally supplied, and the reverse situation has been observed in high-CO(2)-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO(3) (-) (K((1/2))[HCO(3) (-)]) was 0.6 +/- 0.2 millimolar, at 20% O(2). But under low O(2) concentrations (</=2%), surprisingly, an inhibition of net O(2) evolution was elicited, which was maximal at low HCO(3) (-) concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO(2) medium, (b) Chara cells are able to accumulate CO(2) internally by means of a process apparently independent of the plasmalemma HCO(3) (-) transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO(3) (-): pseudocyclic electron flow, sustained by O(2) photoreduction, may produce the additional ATP needed for the HCO(3) (-) transport.

Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater: Effects of Oxygen Concentration

With an experimental system developed for aquatic plants using the mass spectrometry technique and infrared gas analysis of CO(2), we studied the responses to various O(2) concentrations of gas exchanges with the red macroalga Chondrus crispus S. The results were as follows. (a) Irrespective of the CO(2) concentration, net photosynthesis was O(2) sensitive with a 45 to 70% stimulation at 2% O(2). Even with high CO(2), a significant Warburg effect was detected. (b) Although photosynthesis was CO(2) sensitive, O(2) photoconsumption was only weakly affected by CO(2) even at high CO(2) where it was still photodependent. (c) O(2) photoconsumption was always sensitive to O(2) concentration whatever the CO(2) concentration, but with O(2) exceeding 20% the kinetics disagreed with the Michaelis-Menten model, with saturation being reached more rapidly. With various CO(2) concentrations, the apparent K(m) (O(2)) ranged from 4 to 16% O(2) with a relatively constant V(max) (O(2)) of about one-third the V(max) (CO(2)). (d) Dark respiration seemed to be O(2) insensitive. These results are discussed in relation to the nature of the processes able to consume O(2) in the light, and seem to be consistent with a significant involvement of a Mehlertype reaction.

Incorporation of Oxygen into Glycolate, Glycine, and Serine during Photorespiration in Maize Leaves

Glycolate, glycine, and serine extracted from excised Zea mays L. leaves which had been allowed to photosynthesize in the presence of (18)O(2) were analyzed by gas chromatography-mass spectrometry. In each case, only one of the oxygen atoms of the carboxyl group had become labeled. The maximum enrichment observed in glycine and serine was attained after 5 minutes and 15 minutes of exposure to (18)O(2) at the CO(2) compensation point; the labeling was very high, reaching 70 to 73% of that in the applied O(2). Thus, it appears that all or nearly all of the glycine and serine are synthesized in maize leaves via fixation of O(2). In the presence of CO(2) (380 or 800 microliters per liter), (18)O-labeling was markedly slower.Glycolate enrichment was variable and much lower than that in glycine and serine. It is possible that there are additional pathways of glycolate synthesis which do not result in the incorporation of (18)O from molecular oxygen. An estimation of the metabolic flow of O(2) through the photorespiratory cycle was made. It appeared that less than 75% of the O(2) taken up by maize leaves is involved in this pathway. Therefore, other processes of O(2) metabolism must occur in the light.

Aerobic hydrogen production by the heterocystous cyanobacteria Anabaena spp. strains CA and 1F

Aerobic photoproduction of H2 was demonstrated in Anabaena spp. strains CA and 1F when cells were growing under nitrogen-fixing conditions. The rates of production, measured either by the hydrogen electrode or in a flow system by gas chromatography, were 10 to 15% of the rate of photosynthetic O2 evolution or 50 to 80% of the rates of acetylene reduction. Strains CA and 1F differed in several respects. In strain CA, H2 production was immediately partially sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, whereas strain 1F was not immediately affected. Strain CA also showed a consistently higher rate of H2 production than did strain 1F. H2 production in strain CA was also markedly influenced by the light intensity used for growth, although the growth rates indicated that the light intensities used were essentially saturating.

Mechanism of photoactivation of electron transport in intact bryopsis chloroplasts

The mechanism of photoactivation of photosystem I electron transport was studied in intact Bryopsis corticulans chloroplasts. The evidence from chemical and photochemical studies suggests that photoactivation is a consequence of a reduction of an electron transport component, presumably ferredoxin-NADP(+) reductase. O(2) does not act as a mediator of the process but rather acts as an electron acceptor after photoactivation has occurred. We suggest that the initial function of the chloroplasts in a transition from dark to light is to initiate pseudocyclic electron flow.

Hydrogen peroxide synthesis in isolated spinach chloroplast lamellae : an analysis of the mehler reaction in the presence of NADP reduction and ATP formation

Light-dependent O(2) reduction concomitant with O(2) evolution, ATP formation, and NADP reduction were determined in isolated spinach (Spinacia oleracea L. var. America) chloroplast lamellae fortified with NADP and ferredoxin. These reactions were investigated in the presence or absence of catalase, providing a tool to estimate the reduction of O(2) to H(2)O(2) (Mehler reaction) concomitant with NADP reduction. In the presence of 250 micromolar O(2), O(2) photoreduction, simultaneous with NADP photoreduction, was dependent upon light intensity, ferredoxin, Mn(2+), NADP, and the extent of coupling of phosphorylation to electron flow.In the presence of an uncoupling concentration of NH(4) (+), saturating light intensity (>500 watts/square meter), saturating ferredoxin (10 micromolarity) rate-limiting to saturating NADP (0.2-0.9 millimolarity), and Mn(2+) (50-1000 micromolarity), the maxium rates of O(2) reduction were 13-25 micromoles/milligram chlorophyll per hour, while concomitant rates of O(2) evolution and NADP reduction were 69 to 96 and 134 to 192 micromoles/milligram chlorophyll per hour, respectively. Catalase did not affect the rate of NADPH or ATP formation but decreased the NADPH:O(2) ratios from 2.3-2.8 to 1.9-2.1 in the presence of rate-limiting as well as saturating concentrations of NADP.Photosynthetic electron flow at a rate of 31 micromoles O(2) evolved/milligram chlorophyll per hour was coupled to the synthesis of 91 micromoles ATP/milligram chlorophyll per hour, while the concomitant rate of O(2) reduction was 0.6 micromoles/milligram chlorophyll per hour and was calculated to be associated with an apparent ATP formation of only 2 micromoles/milligram chlorophyll per hour. Thus, electron flow from H(2)O to O(2) did not result in ATP formation significantly above that produced during NADP reduction.

Photosynthesis and Inorganic Carbon Usage by the Marine Cyanobacterium, Synechococcus sp

The marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1) changes its affinity for external inorganic carbon used in photosynthesis, depending on the concentration of CO(2) provided during growth. The high affinity for CO(2) + HCO(3) (-) of air-grown cells (K((1/2)) < 80 nanomoles [pH 8.2]) would seem to be the result of the presence of an inducible mechanism which concentrates inorganic carbon (and thus CO(2)) within the cells. Silicone-oil centrifugation experiments indicate that the inorganic carbon concentration inside suitably induced cells may be in excess of 1,000-fold greater than that in the surrounding medium, and that this accumulation is dependent upon light energy. The quantum requirements for O(2) evolution appear to be some 2-fold greater for low CO(2)-grown cells, compared with high CO(2)-grown cells. This presumably is due to the diversion of greater amounts of light energy into inorganic carbon transport in these cells.A number of experimental approaches to the question of whether CO(2) or HCO(3) (-) is primarily utilized by the inorganic carbon transport system in these cells show that in fact both species are capable of acting as substrate. CO(2), however, is more readily taken up when provided at an equivalent concentration to HCO(3) (-). This discovery suggests that the mechanistic basis for the inorganic carbon concentrating system may not be a simple HCO(3) (-) pump as has been suggested. It is clear, however, that during steady-state photosynthesis in seawater equilibrated with air, HCO(3) (-) uptake into the cell is the primary source of internal inorganic carbon.

Photosynthetic o(2) exchange kinetics in isolated soybean cells

Light-dependent O(2) exchange was measured in intact, isolated soybean (Glycine max. var. Williams) cells using isotopically labeled O(2) and a mass spectrometer. The dependence of O(2) exchange on O(2) and CO(2) was investigated at high light in coupled and uncoupled cells. With coupled cells at high O(2), O(2) evolution followed similar kinetics at high and low CO(2). Steady-state rates of O(2) uptake were insignificant at high CO(2), but progressively increased with decreasing CO(2). At low CO(2), steady-state rates of O(2) uptake were 50% to 70% of the maximum CO(2)-supported rates of O(2) evolution. These high rates of O(2) uptake exceeded the maximum rate of O(2) reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O(2)-uptake process (i.e. photorespiration).Rates of O(2) exchange in uncoupled cells were half-saturated at 7% to 8% O(2). Initial rates (during induction) of O(2) exchange in uninhibited cells were also half-saturated at 7% to 8% O(2). In contrast, steady-state rates of O(2) evolution and O(2) uptake (at low CO(2)) were half-saturated at 18% to 20% O(2). O(2) uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O(2) for photoreductant.These results suggest that two mechanisms (O(2) reduction and photorespiration) are responsible for the light-dependent O(2) uptake observed in uninhibited cells under CO(2)-limiting conditions. The relative contribution of each process to the rate of O(2) uptake appears to be dependent on the O(2) level. At high O(2) concentrations (>/=40%), photorespiration is the major O(2)-consuming process. At lower (ambient) O(2) concentrations (</=20%), O(2) reduction accounts for a significant portion of the total light-dependent O(2) uptake.