Utilization of inorganic carbon in the edible cyanobacterium Ge-Xian-Mi (Nostoc) and its role in alleviating photo-inhibition

The present work investigated the inorganic carbon (C_i) uptake, fluorescence quenching and photo‐inhibition of the edible cyanobacterium Ge‐Xian‐Mi (Nostoc) to obtain an insight into the role of CO₂ concentrating mechanism (CCM) operation in alleviating photo‐inhibition. Ge‐Xian‐Mi used HCO₃^– in addition to CO₂ for its photosynthesis and oxygen evolution was greater than the theoretical rates of CO₂ production derived from uncatalysed dehydration of HCO₃^–. Multiple transporters for CO₂ and HCO₃^– operated in air‐grown Ge‐Xian‐Mi. Na⁺‐dependent HCO₃^– transport was the primary mode of active C_i uptake and contributed 53–62% of net photosynthetic activity at 250 µmol L^?1 KHCO₃ and pH 8.0. However, the CO₂‐uptake systems and Na⁺‐independent HCO₃^– transport played minor roles in Ge‐Xian‐Mi and supported, respectively, 39 and 8% of net photosynthetic activity. The steady‐state fluorescence decreased and the photochemical quenching increased in response to the transport‐mediated accumulation of intracellular C_i. Inorganic carbon transport was a major factor in facilitating quenching during the initial stage and the initial rate of fluorescence quenching in the presence of iodoacetamide, an inhibitor of CO₂ fixation, was 88% of control. Both the initial rate and extent of fluorescence quenching increased with increasing external dissolved inorganic carbon (DIC) and saturated at higher than 200 µmol L^?1 HCO₃^–. The operation of the CCM in Ge‐Xian‐Mi served as a means of diminishing photodynamic damage by dissipating excess light energy and higher external DIC in the range of 100–10000 µmol L^?1 KHCO₃ was associated with more severe photo‐inhibition under strong irradiance.     

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

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

CYANOBACTERIAL ACCLIMATION TO RAPIDLY FLUCTUATING LIGHT IS CONSTRAINED BY INORGANIC CARBON STATUS1

Acclimation to rapidly fluctuating light, simulating shallow aquatic habitats, is altered depending on inorganic carbon (C_i) availability. Under steady light of 50 μmol photons·m^?2·s^?1, the growth rate of Synechococcus elongatus PCC7942 was similar in cells grown in high C_i (4 mM) and low C_i (0.02 mM), with induced carbon concentrating mechanisms compensating for low C_i. Growth under fluctuating light of a 1‐s period averaging 50 μmol photons·m^?2·s^?1 caused a drop in growth rate of 28%±6% in high C_i cells and 38%±8% in low C_i cells. In high C_i cells under fluctuating light, the PSI/PSII ratio increased, the PSII absorption cross‐section decreased, and the PSII turnover rate increased in a pattern similar to high‐light acclimation. In low C_i cells under fluctuating light, the PSI/PSII ratio decreased, the PSII absorption cross‐section decreased, and the PSII turnover remained slow. Electron transport rate was similar in high and low C_i cells but in both was lower under fluctuating than under steady light. After acclimation to a 1‐s period fluctuating light, electron transport rate decreased under steady or long‐period fluctuating light. We hypothesize that high C_i cells acclimated to exploit the bright phases of the fluctuating light, whereas low C_i cells enlarged their PSII pool to integrate the fluctuating light and dampen the variation of the electron flux into a rate‐restricted C_i pool. Light response curves measured under steady light, widely used to predict photosynthetic rates, do not properly predict photosynthetic rates achieved under fluctuating light, and exploitation of fluctuating light is altered by C_i status.     

Quantification of bound bicarbonate in photosystem II

In this study, we presented a new approach for quantification of bicarbonate (HCO₃^?) molecules bound to PSII. Our method, which is based on a combination of membrane-inlet mass spectrometry (MIMS) and ¹⁸O-labelling, excludes the possibility of “non-accounted” HCO₃^? by avoiding (1) the employment of formate for removal of HCO₃^? from PSII, and (2) the extremely low concentrations of HCO₃^?/CO₂ during online MIMS measurements. By equilibration of PSII sample to ambient CO₂ concentration of dissolved CO₂/HCO₃^?, the method ensures that all physiological binding sites are saturated before analysis. With this approach, we determined that in spinach PSII membrane fragments 1.1 ± 0.1 HCO₃^? are bound per PSII reaction center, while none was bound to isolated PsbO protein. Our present results confirmed that PSII binds one HCO₃^? molecule as ligand to the non-heme iron of PSII, while unbound HCO₃^? optimizes the water-splitting reactions by acting as a mobile proton shuttle.     

Production of reactive oxygen species in decoupled, Ca2+-depleted PSII and their use in assigning a function to chloride on both sides of PSII

Extraction of Ca²⁺ from the oxygen-evolving complex of photosystem II (PSII) in the absence of a chelator inhibits O₂ evolution without significant inhibition of the light-dependent reduction of the exogenous electron acceptor, 2,6-dichlorophenolindophenol (DCPIP) on the reducing side of PSII. The phenomenon is known as “the decoupling effect” (Semin et al. Photosynth Res 98:235–249, 2008). Extraction of Cl^? from Ca²⁺-depleted membranes (PSII[–Ca]) suppresses the reduction of DCPIP. In the current study we investigated the nature of the oxidized substrate and the nature of the product(s) of the substrate oxidation. After elimination of all other possible donors, water was identified as the substrate. Generation of reactive oxygen species HO, H₂O₂, and O ₂ ^·? , as possible products of water oxidation in PSII(–Ca) membranes was examined. During the investigation of O ₂ ^·? production in PSII(–Ca) samples, we found that (i) O ₂ ^·? is formed on the acceptor side of PSII due to the reduction of O₂; (ii) depletion of Cl^? does not inhibit water oxidation, but (iii) Cl^? depletion does decrease the efficiency of the reduction of exogenous electron acceptors. In the absence of Cl^? under aerobic conditions, electron transport is diverted from reducing exogenous acceptors to reducing O₂, thereby increasing the rate of O ₂ ^·? generation. From these observations we conclude that the product of water oxidation is H₂O₂ and that Cl^? anions are not involved in the oxidation of water to H₂O₂ in decoupled PSII(–Ca) membranes. These results also indicate that Cl^? anions are not directly involved in water oxidation by the Mn cluster in the native PSII membranes, but possibly provide access for H₂O molecules to the Mn₄CaO₅ cluster and/or facilitate the release of H⁺ ions into the lumenal space.     

Clara cell damage and inhibition of pulmonary mixed-function oxidase activity by naphthalene

The photosystem II electron acceptor 3,6-dichloro-2,5-dimethoxy-p-benzoquinone [DCDMQ] is suggested to replace the second quinone-type two electron acceptor B (or R); the DCDMQ Hill reaction is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, but is insensitive to dry heptane extraction of thylakoids and other photosystem II inhibitors. Addition of HCO₃^? to CO₂-depleted thylakoids in silicomolybdate, DCDMQ, diaminodurene and ferricyanide Hill reactions brought about 1,3,10 and 10 fold increase in the electron transport rates; these data confirm that HCO₃^? affects both Q^? to B and B^2? to PQ reactions.     

The interaction between bicarbonate and the herbicide ioxynil in the thylakoid membrane and the effects of amino acid modification on bicarbonate action

Bicarbonate depletion of chloroplast thylakoids reduces the affinity of the herbicide, ioxynil, to its binding site in Photosystem (PS) II. This herbicide is found to be a relatively more efficient inhibitor of the Hill reaction when HCO^?₃ is added to CO₂-depleted thylakoids in subsaturating rather than in saturating concentrations. The reason for this dependence of the inhibitor efficiency on the HCO^?₃ concentration is that the inactive HCO^?₃-deficient PS II reaction chains bind less ioxynil than the active PS II electron-transport chains that have bound HCO^?₃, and, thus, after addition of a certain amount of ioxynil the concentration of the free herbicide increases when the HCO^?₃ concentration decreases. Therefore, the inhibition of electron transport by ioxynil increases at decreasing HCO^?₃ levels. Measurements on the effects of modification of lysine and arginine residues on the rate of electron transport are also presented: the rate of modification is faster in the presence than in the absence of HCO^?₃. Therefore, we suggest that surface-exposed lysine or arginine residues are not involved in binding of HCO^?₃ (or CO₂ or CO^2?₃) to its binding protein, but that HCO^?₃ influences the conformation of its binding environment such that the affinity for certain herbicides and the accessibility for amino acid modifiers are changed.     

Ethoxyzolamide Inhibition of CO(2) Uptake in the Cyanobacterium Synechococcus PCC7942 without Apparent Inhibition of Internal Carbonic Anhydrase Activity

In high inorganic carbon grown (1% CO₂ [volume/volume]) cells of the cyanobacterium Synechococcus PCC7942, the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZ), was found to inhibit the rate of CO₂ uptake and to reduce the final internal inorganic carbon (C_i) pool size reached. The relationship between CO₂ fixation rate and internal C_i concentration in high C_i grown cells was little affected by EZ. This suggests that in intact cells internal CA activity was unaffected by EZ. High C_i grown cells readily took up CO₂ but had little or no capacity for HCO₃⁻ uptake. These cells appear to possess a CO₂ utilizing C_i pump that has a CA-like function associated with the transport step such that HCO₃⁻ is the species delivered to the cell interior. This CA-like step may be the site of inhibition by EZ. Low C_i grown cells possess both CO₂ uptake and HCO₃⁻ uptake activities and EZ inhibited both activities to a similar degree, suggesting that a common step in CO₂ and HCO₃⁻ uptake (such as the C_i pump) may have been affected. The inhibitor had no apparent effect on internal CO₂/HCO₃⁻ equilibria (internal CA function) in low C_i grown cells.     

Fluorescence F 0 of photosystems II and I in developing C3 and C4 leaves,and implications on regulation of excitation balance

This work addresses the question of occurrence and function of photosystem II (PSII) in bundle sheath (BS) cells of leaves possessing NADP-malic enzyme-type C₄ photosynthesis (Zea mays). Although no requirement for PSII activity in the BS has been established, several component proteins of PSII have been detected in BS cells of developing maize leaves exhibiting O₂-insensitive photosynthesis. We used the basal fluorescence emissions of PSI (F _0I) and PSII (F _0II) as quantitative indicators of the respective relative photosystem densities. Chl fluorescence induction was measured simultaneously at 680 and 750 nm. In mature leaves, the F _m(680)/F ₀(680) ratio was 10.5 but less in immature leaves. We propose that the lower ratio was caused by the presence of a distinct non-variable component, F _c, emitting at 680 and 750 nm. After F _c was subtracted, the fluorescence of PSI (F _0I) was detected as a non-variable component at 750 nm and was undetectably low at 680 nm. Contents of Chls a and b were measured in addition to Chl fluorescence. The Chl b/(a + b) was relatively stable in developing sunflower leaves (0.25–0.26), but in maize it increased from 0.09 to 0.21 with leaf tissue age. In sunflower, the F _0I/(F _0I + F _0II) was 0.39 ± 0.01 independent of leaf age, but in maize, this parameter was 0.65 in young tissue of very low Chl content (20–50 mg m^?2) falling to a stable level of 0.53 ± 0.01 at Chl contents >100 mg m^?2. The values of F _0I/(F _0I + F _0II) showed that in sunflower, excitation was partitioned between PSII and PSI in a ratio of 2:1, but the same ratio was 1:1 in the C₄ plant. The latter is consistent with a PSII:PSI ratio of 2:1 in maize mesophyll cells and PSI only in BS cells (2:1:1 distribution). We suggest, moreover, that redox mediation of Chl synthesis, rather than protein accumulation, regulates photosystem assembly to ensure optimum excitation balance between functional PSII and PSI. Indeed, the apparent necessity for two Chls (a and b) may reside in their targeted functions in influencing accumulation of PSI and PSII, respectively, as opposed to their spectral differences.     

Photosynthesis of Ectocarpus siliculosus in red light and after pulses of blue light at high pH – evidence for bicarbonate uptake

Pulses of blue light cause stimulation of red light saturated photosynthesis in Ectocarpus siliculosus, because blue light activates the operation of a pathway for inorganic carbon (C_i) acquisition by inducing the mobilization of CO₂ from an intermediate metabolite. In the absence of exogenous C_i, photosynthetic rates roughly equal those of CO₂ release by respiration. In seawater of pH 9·5 (2·3 mol m^–3 total C_i, but concentrations of free CO₂ below 0·2 mmol m^–3), photosynthesis was clearly above these rates, although they were only ≈ 30% of those in normal seawater (≈ pH 8). The degree and the time course of the stimulations of photosynthesis by pulses of blue light were unaltered at high pH. Essentially the same characteristics were found after buffering or in the presence of acetazolamide, an inhibitor of extracellular carbonic anhydrase activity. Therefore, it is concluded that Ectocarpus is able to directly take up HCO₃^– in addition to CO₂ (uptake of CO₃^2– cannot be excluded). The dependence of photosynthesis on C_i at pH 9·5 was biphasic, with C_i below 0·2 mol m^–3 having no effect at all. In C_i-free seawater, the shapes of the stimulations after blue light pulses differed for pH 6, pH 8 and pH 9·5. At low pH, only the fast peak (maximum ≈ 5 min after blue light) was detected, whereas at high pH mainly the slow peak (maximum ≈ 20 min after blue light) was observed. At the intermediate pH 8, both peaks were present. As inhibition of total carbonic anhydrase by ethoxyzolamide brought out the fast peak of the stimulations at pH 9·5 it is concluded that the fast component was due to a transient disequilibrium of an intracellular pool of C_i which, after blue light, was fed by CO₂ released from the postulated storage intermediate.     

Regulation of the mechanism for HCO 3 − use by the inorganic carbon level in Porphyra leucosticta Thur. in Le Jolis (Rhodophyta)

The capacity for HCO₃ ⁻ use by Porphyra leucosticta Thur. in Le Jolis grown at different concentrations of inorganic carbon (C_i) was investigated. The use of HCO₃ ⁻ at alkaline pH by P. leucosticta was␣demonstrated by comparing the O₂ evolution rates measured with the O₂ evolution rates theoretically supported by the CO₂ spontaneously formed from HCO₃ ⁻ . Both external and internal carbonic anhydrase (CA; EC 4.2.1.1) were implied in HCO₃ ⁻ use during photosynthesis because O₂ evolution rates and the increasing pH during photosynthesis were inhibited in the presence of azetazolamide and ethoxyzolamide (inhibitors for external and total CA respectively). Both external and internal CA were regulated by the C_i level at which the algae were grown. A high C_i level produced a reduction in total CA activity and a low C_i level produced an increase in total CA activity. In contrast, external CA was increased at low C_i although it was not affected at high C_i . Parallel to the reduction in total CA activity at high C_i is a reduction in the affinity for C_i, as estimated from photosynthesis versus C_i curves, was found. However, there was no evident relationship between external CA activity and the capacity for HCO₃ ⁻ use because the presence of external CA became redundant when P. leucosticta was cultivated at high C_i. Our results suggest that the system for HCO₃ ⁻ use in P. leucosticta is composed of different elements that can be activated or inactivated separately. Two complementary hypotheses are postulated: (i) internal CA is an absolute requirement for a functioning C_i-accumulation mechanism; (ii) there is a CO₂ transporter that works in association with external CA. Received: 20 April 1996 / Accepted: 5 August 1996     

The Stoichiometry between CO(2) and H Fluxes Involved in the Transport of Inorganic Carbon in Cyanobacteria

The pH of the medium during CO₂ uptake into the intracellular inorganic carbon (C_i) pool of a high CO₂-requiring mutant (E₁) and wild type of Anacystis nidulans R2 was measured. Experiments were performed under conditions where photosynthetic CO₂ fixation is inhibited. There was an acidification of the medium during CO₂ uptake in the light and an alkalization during CO₂ efflux after darkening. A one to one stoichiometry existed between the amounts of H⁺ appearing in the medium and CO₂ taken up into the intracellular C_i pool, regardless of the carbon species transported. The results indicate that (a) CO₂ is taken up simultaneously with an efflux of equimolar H⁺, probably produced as a result of CO₂ hydration during transport and (b) HCO₃⁻ produced by hydration of CO₂ in the medium was transported into the cells without accompanying net flux of H⁺ or OH⁻. The influx and efflux of C_i during C_i transport produced nonequilibrium between CO₂ and HCO₃⁻ in the medium, with the concentration of HCO₃⁻ being higher than that expected under equilibrium conditions. The nonequilibrium was present even under the conditions where the influx of C_i is compensated by its efflux. The direction of this nonequilibrium suggested that efflux of HCO₃⁻ occurs during uptake of C_i.     

A proposed role for inorganic carbon in water oxidation

This is an article on the peroxydicarbonic acid (PODCA) hypothesis of photosynthetic water oxidation, which follows our first article in this general area (Castelfranco et al., Photosynth Res 94:235–246, 2007). In this article I have expanded on the idea of a protein-bound intermediate containing inorganic carbon in some chemically bound form. PODCA is conceived in this article as constituting a bridge between two proteins of the oxygen-evolving complex (OEC) that are essential for the evolution of O₂. Presumably, these are two proteins which have been shown to possess Mn-dependent carbonic anhydrase activity (Lu et al., Plant Cell Physiol 46:1944–1953, 2005; Shitov et al., Biochemistry (Moscow) 74:509–517, 2009). One of these proteins may be the D_I of the OEC core and the other may be the PsbO extrinsic protein. I attempt to relate briefly the PODCA hypothesis to the role of two cofactors for O₂ evolution: Ca²⁺ and inorganic carbon. In this scheme, inorganic carbon (HCO₃ ^?) mediates the oxidation of peroxide to dioxygen, thus avoiding the homolytic cleavage of the peroxide into two free radicals. I visualize the role of Ca²⁺ in the binding of PODCA to two essential photosystem II proteins. I propose that PODCA alternates between two Phases. In Phase 1, PODCA is broken down with the production of O₂. In Phase 2, PODCA is regenerated.     

Nature of the Inorganic Carbon Species Actively Taken Up by the Cyanobacterium Anabaena variabilis

The nature of the inorganic carbon (C_i) species actively taken up by cyanobacteria CO₂ or HCO₃⁻ has been investigated. The kinetics of CO₂ uptake, as well as that of HCO₃⁻ uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO₂ was higher than that for HCO₃⁻. Though the calculated V_max was the same in both cases, the maximum rate of uptake actually observed was higher when HCO₃⁻ was supplied. C_i uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO₂ was the species supplied. Observations of photosynthetic rate as a function of intracellular C_i level (following supply of CO₂ or HCO₃⁻ for 5 seconds) led to the inference that HCO₃⁻ is the species which arrives at the inner membrane surface, regardless of the species supplied. When the two species were supplied simultaneously, mutual inhibition of uptake was observed.

On the basis of these and other results, a model is proposed postulating that a carboic anhydrase-like subunit of the C_i transport apparatus binds CO₂ and releases HCO₃⁻ at or near a membrane porter. The latter transports HCO₃⁻ ions to the cell interior.

     

Inorganic carbon transport across cell compartments of the halotolerant alga Dunaliella salina

The inorganic carbon (C_i) accumulation and the intracellular location of carbonic anhydrase (CA, EC 4.2.1.1) in the halotolerant unicellular alga Dunaliella salina have been investigated. The rate of HCO₃ -dependent O₂ evolution was determined by growth conditions. Algae grown under high CO₂ conditions (5% CO₂ in air, v/v; high C_i cells) had a very low affinity for HCO₃^? at pH 7.0 and 8.2, whereas algae grown under low CO₂ conditions (0.03% CO₂ in air; low C_i cells) showed a high affinity for HCO₃^? at both pH values and were sensitive to Dextran-bound sulfonamide (DBS), an inhibitor of extracellular CA. The photosynthetic rate or HCO₄^? dependent O₂ evolution was always higher at pH 7.0 than at pH 8.2. Ethoxyzolamide (EZ), an inhibitor of total (extacellular plus intracellular) CA activity, strongly inhibited photosynthesis at both pH values. During adaptation from high to low CO₂ conditions CA activity increased in chloroplasts in a process dependent on the novo protein synthesis. Carbonic anhydrase activity was found in the supernatant and pellet fractions of chloroplast homogenates. The rate of photosynthesis of chloroplasts from low C_i cells was higher at pH 7.0 than at pH 8.2. The alkalinization of the growth medium, which took place only in the presence of C_i, was partially inhibited by DBS and completely by EZ. We suggest that in D. salina CO₂ is the general form of C_i transported across the plasma membrane and the chloroplast envelope and that bicarbonate enters the cell mainly, although not entirely, by an ‘indirect’ mechanism after dehydration to CO₂.     

Effects of different levels of CO2 on photosynthesis and cell components of the red alga Porphyra leucosticta

Photosynthesis and cell composition of Porphyraleucosticta discs grown at low (< 0.0001% in air), current (control) and high (1% CO₂ in air)inorganic carbon (C_i) concentrations were analyzed. Carbohydrate content in discs grown at high C_i increased (15.1 mg g^-1 FW) with respect to the control (6.4 mg g FW^-1), whereas soluble protein content decreased to one-third (5.6 to2.1 mg g^-1 FW). Carbohydrate content was unaffected and soluble protein slightly increased in discs grown at low C_i. As a consequence of these changes, a lower C/N molar ratio (8.6) was found in the discs grown at low compared to high C_i(12.4). Nitrate reductase activity increased at high C_i from 0.3 ± 0.2 to 1.7 ± 0.4 μmolNO₂ ^- g^-1 FW h^-1indicating that reduction and assimilation of nitrate were uncoupled. The response of photosynthesis to increasing irradiance, estimated from O₂evolution vs. irradiance curves, was affected by the treatments. Maximum quantum yield (Φ _O2°) and effective quantum yield (Φ _O2) at 150 μmol photon m^-2s^-1 decreased by 20% and 50%, respectively, at low C_i. These differences could be due to changes in photosynthetic electron flow between PSII and PSI. Treatments also produced changes in maximal (F_v/F_m) and effective (ΔF/F_m′)quantum yield for photosystem II charge separation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanism of Photosynthetic Carbon Dioxide Uptake by the Red Macroalga, Chondrus crispus

The aim of this study was to determine how Chondrus crispus, a marine red macroalga, acquires the inorganic carbon (C_i) it utilizes for photosynthetic carbon fixation. Analyses of C_i uptake were done using silicone oil centrifugation (using multicellular fragments of thallus), infrared gas analysis, and gas chromatography. Inhibitors of carbonic anhydrase (CA), the band 3 anion exchange protein and Na⁺/K⁺ exchange were used in the study. It was found that: (a) C. crispus does not accumulate C_i internally above the concentration attainable by diffusion; (b) the initial C_i fixtion rate of C. crispus fragments saturates at approximately 3 to 4 millimolar C_i; (c) CA is involved in carbon uptake; its involvement is greatest at high HCO₃⁻ and low CO₂ concentration, suggesting its participation in the dehydration of HCO₃⁻ to CO₂; (d) C. crispus has an intermediate C_i compensation point; and (e) no evidence of any active or facilitated mechanism for the transport of HCO₃⁻ was detected. These data support the view that photosynthetic C_i uptake does not involve active transport. Rather, CO₂, derived from HCO₃⁻ catalyzed by external CA, passively diffuses across the plasma membrane of C. crispus. Intracellular CA also enhances the fixation of carbon in C. crispus.     

Response of chlorophyll d-containing cyanobacterium Acaryochloris marina to UV and visible irradiations

We have previously investigated the response mechanisms of photosystem II complexes from spinach to strong UV and visible irradiations (Wei et al J Photochem Photobiol B 104:118–125, 2011). In this work, we extend our study to the effects of strong light on the unusual cyanobacterium Acaryochloris marina, which is able to use chlorophyll d (Chl d) to harvest solar energy at a longer wavelength (740 nm). We found that ultraviolet (UV) or high level of visible and near-far red light is harmful to A. marina. Treatment with strong white light (1,200 μmol quanta m^?2 s^?1) caused a parallel decrease in PSII oxygen evolution of intact cells and in extracted pigments Chl d, zeaxanthin, and α-carotene analyzed by high-performance liquid chromatography, with severe loss after 6 h. When cells were irradiated with 700 nm of light (100 μmol quanta m^?2 s^?1) there was also bleaching of Chl d and loss of photosynthetic activity. Interestingly, UVB radiation (138 μmol quanta m^?2 s^?1) caused a loss of photosynthetic activity without reduction in Chl d. Excess absorption of light by Chl d (visible or 700 nm) causes a reduction in photosynthesis and loss of pigments in light harvesting and photoprotection, likely by photoinhibition and inactivation of photosystem II, while inhibition of photosynthesis by UVB radiation may occur by release of Mn ion(s) in Mn₄CaO₅ center in photosystem II.     

The interrelationship between HCO 3 − , NO 3 − , and Cl− as effective anions in the photosynthetic electron transport

Thylakoids were isolated from the leaves of three different plants (Pisum sativium L., Lactuca sativa L., and Raphanus sativus L.). The addition of HCO ₃ ^? to a suspension of salt-and HCO ₃ ^? -epleted thylakoids (suspended in salt-free medium) raised the rate of O₂ evolution up to fourfold. This stimulation could be partially replaced by the addition of chloride or nitrate ions. However, the addition of HCO ₃ ^? in the presence of Cl^? or NO ₃ ^? resulted in a higher stimulation of O₂ evolution (sixfold in the presence of nitrate and sevenfold in the presence of chloride). On the other hand, the addition of HCO ₃ ^? to the thylakoids depleted from salt only raised the rate of O₂ evolution by 10–15%, whereas 40–70% was obtained by the addition of nitrate or chloride ions. The fluorescence induction studies indicated a significant decrease in the yield of the variable fluorescence of the salt- and HCO ₃ ^? -depleted thylakoids. A partial increase in the fluorescence yield was obtained by the addition of HCO ₃ ^? alone. A typical fluorescence induction curves were obtained by the addition of HCO ₃ ^? in the presence of Cl^? or NO ₃ ^? ions. The data obtained suggest a similar role for chloride and nitrate ions in O₂ evolution in the Hill-reaction, which is restricted at the donor side of photosystem II, whereas bicarbonate plays its role at both sides (acceptor and donor sides). The presented data are those obtained for the thylakoids of P. sativium, which were more or less similar to those obtained for L. sativa and R. sativus.