Chlorophyll a Fluorescence Yield as a Monitor of Both Active CO(2) and HCO(3) Transport by the Cyanobacterium Synechococcus UTEX 625

Depletion of m-type thioredoxin impairs photosynthesis, carbon fixation, and oxidative stress in cyanobacteria

Thioredoxins (Trxs) are disulfide oxidoreductases that regulate many biological processes. The m-type thioredoxin (TrxA) is the only Trx present in all oxygenic photosynthetic organisms. Extensive biochemical and proteomic analyses have identified many TrxA target proteins in different photosynthetic organisms. However, the precise function of this essential protein in vivo is still poorly known. In this study, we generated a conditional Synechocystis sp. PCC 6803 mutant strain (STXA2) using an on-off promoter that is able to survive with only 2% of the TrxA level of the wild-type (WT) strain. STXA2 characterization revealed that TrxA depletion results in growth arrest and pronounced impairment of photosynthesis and the Calvin-Benson-Bassham (CBB) cycle. Analysis of the in vivo redox state of the bifunctional enzyme fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase showed higher levels of oxidation that affected enzyme activity in STXA2. This result implies that TrxA-mediated redox regulation of the CBB cycle is conserved in both cyanobacteria and chloroplasts, although the targets have different evolutionary origins. The STXA2 strain also accumulated more reactive oxygen species and was more sensitive to oxidative stress than the WT. Analysis of the in vivo redox state of 2-Cys peroxiredoxin revealed full oxidation, corresponding with TrxA depletion. Overall, these results indicate that depletion of TrxA in STXA2 greatly alters the cellular redox state, interfering with essential processes such as photosynthetic machinery operativity, carbon assimilation, and oxidative stress response. The TrxA regulatory role appears to be conserved along the evolution of oxygenic photosynthetic organisms.

Redox changes accompanying inorganic carbon limitation in Synechocystis sp. PCC 6803

Inorganic carbon (Ci) is the major sink for photosynthetic reductant in organisms capable of oxygenic photosynthesis. In the absence of abundant Ci, the cyanobacterium Synechocystis sp. strain PCC6803 expresses a high affinity Ci acquisition system, the CO2-concentrating mechanisms (CCM), controlled by the transcriptional regulator CcmR and the metabolites NADP+ and α-ketoglutarate, which act as co-repressors of CcmR by modulating its DNA binding. The CCM thus responds to internal cellular redox changes during the transition from Ci-replete to Ci-limited conditions. However, the actual changes in the metabolic state of the NADPH/NADP+ system that occur during the transition to Ci-limited conditions remain ill-defined. Analysis of changes in the redox state of cells experiencing Ci limitation reveals systematic changes associated with physiological adjustments and a trend towards the quinone and NADP pools becoming highly reduced. A rapid and persistent increase in F0 was observed in cells reaching the Ci-limited state, as was the induction of photoprotective fluorescence quenching. Systematic changes in the fluorescence induction transients were also observed. As with Chl fluorescence, a transient reduction of the NADPH pool ('M' peak), is assigned to State 2→State 1 transition associated with increased electron flow to NADP+. This was followed by a characteristic decline, which was abolished by Ci limitation or inhibition of the Calvin-Benson-Bassham (CBB) cycle and is thus assigned to the activation of the CBB cycle. The results are consistent with the proposed regulation of the CCM and provide new information on the nature of the Chl and NADPH fluorescence induction curves.

Massive light-dependent cycling of inorganic carbon between oxygenic photosynthetic microorganisms and their surroundings

Membrane inlet mass spectrometry indicated massive light-dependent cycling of inorganic carbon between the medium and the cells of various phytoplankton species representing the main groups of aquatic primary producers. These included diatoms, symbiotic and free living dinoflagellates, a coccolithophorid, a green alga and filamentous and single cell cyanobacteria. These organisms could maintain an ambient CO(2) concentration substantially above or below that expected at chemical equilibrium with HCO(3) (-). The coccolithophorid Emiliania huxleyi shifted from net CO(2) uptake to net CO(2) efflux with rising light intensity. Differing responses of CO(2) uptake and CO(2) fixation to changing light intensity supported the notion that these two processes are not compulsorily linked. Simultaneous measurements of CO(2) and O(2) exchange and of the fluorescence parameters in Synechococcus sp. strain PCC 7942, showed that CO(2) uptake can serve as a sensitive probe of the energy status of the photosynthetic reaction centers. However, during transitions in light intensity, changes in CO(2) uptake did not accord with those expected from fluorescence change. Quantification of the net fluxes of CO(2), HCO(3) (-) and of photosynthesis at steady-state revealed that substantial HCO(3) (-) efflux accompanied CO(2) uptake and fixation in the case of 'CO(2) users'. On the other hand, 'HCO(3) (-) users' were characterized by a rate of net CO(2) uptake below that of CO(2) fixation. The results support the notion that entities associated with the CCM function not only in raising the CO(2) concentration at the site of Rubisco; they may also serve as a means of diminishing photodynamic damage by dissipating excess light energy.

Chlorophyll fluorescence analysis of cyanobacterial photosynthesis and acclimation

Cyanobacteria are ecologically important photosynthetic prokaryotes that also serve as popular model organisms for studies of photosynthesis and gene regulation. Both molecular and ecological studies of cyanobacteria benefit from real-time information on photosynthesis and acclimation. Monitoring in vivo chlorophyll fluorescence can provide noninvasive measures of photosynthetic physiology in a wide range of cyanobacteria and cyanolichens and requires only small samples. Cyanobacterial fluorescence patterns are distinct from those of plants, because of key structural and functional properties of cyanobacteria. These include significant fluorescence emission from the light-harvesting phycobiliproteins; large and rapid changes in fluorescence yield (state transitions) which depend on metabolic and environmental conditions; and flexible, overlapping respiratory and photosynthetic electron transport chains. The fluorescence parameters FV/FM, FV'/FM',qp,qN, NPQ, and phiPS II were originally developed to extract information from the fluorescence signals of higher plants. In this review, we consider how the special properties of cyanobacteria can be accommodated and used to extract biologically useful information from cyanobacterial in vivo chlorophyll fluorescence signals. We describe how the pattern of fluorescence yield versus light intensity can be used to predict the acclimated light level for a cyanobacterial population, giving information valuable for both laboratory and field studies of acclimation processes. The size of the change in fluorescence yield during dark-to-light transitions can provide information on respiration and the iron status of the cyanobacteria. Finally, fluorescence parameters can be used to estimate the electron transport rate at the acclimated growth light intensity.

Energy sources for HCO3- and CO2 transport in air-grown cells of synechococcus UTEX 625

Light-dependent inorganic C (Ci) transport and accumulation in air-grown cells of Synechococcus UTEX 625 were examined with a mass spectrometer in the presence of inhibitors or artificial electron acceptors of photosynthesis in an attempt to drive CO2 or HCO3- uptake separately by the cyclic or linear electron transport chains. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the cells were able to accumulate an intracellular Ci pool of 20 mm, even though CO2 fixation was completely inhibited, indicating that cyclic electron flow was involved in the Ci-concentrating mechanism. When 200 m N,N-dimethyl-p-nitrosoaniline was used to drain electrons from ferredoxin, a similar Ci accumulation was observed, suggesting that linear electron flow could support the transport of Ci. When carbonic anhydrase was not present, initial CO2 uptake was greatly reduced and the extracellular [CO2] eventually increased to a level higher than equilibrium, strongly suggesting that CO2 transport was inhibited and that Ci accumulation was the result of active HCO3- transport. With 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-treated cells, Ci transport and accumulation were inhibited by inhibitors of CO2 transport, such as COS and Na2S, whereas Li+, an HCO3--transport inhibitor, had little effect. In the presence of N,N-dimethyl-p-nitrosoaniline, Ci transport and accumulation were not inhibited by COS and Na2S but were inhibited by Li+. These results suggest that CO2 transport is supported by cyclic electron transport and that HCO3- transport is supported by linear electron transport.

Inorganic Carbon Accumulation Stimulates Linear Electron Flow to Artificial Electron Acceptors of Photosystem I in Air-Grown Cells of the Cyanobacterium Synechococcus UTEX 625

The effect of inorganic carbon (Ci) transport and accumulation on photosynthetic electron transport was studied in air-grown cells of the cyanobacterium Synechococcus UTEX 625. When the cells were depleted of Ci, linear photosynthetic electron flow was almost completely inhibited in the presence of the photosystem I (PSI) acceptor N,N-dimethyl-p-nitrosoaniline (PNDA). The addition of Ci to these cells, in which CO2 fixation was inhibited with glycolaldehyde, greatly stimulated linear electron flow and resulted in increased levels of photochemical quenching and O2 evolution. In aerobic conditions substantial quenching resulted from methyl viologen (MV) addition and further quenching was not observed upon the addition of Ci. In anaerobic conditions MV addition did not result in quenching until Ci was added. Intracellular Ci pools were formed when MV was present in aerobic or anaerobic conditions or PNDA was present in aerobic conditions. There was no inhibitory effect of Ci depletion on electron flow to 2,6-dimethylbenzoquinone and oxidized diaminodurene, which accept electrons from photosystem II. The degree of stimulation of PNDA-dependent O2 evolution varied with the Ci concentration. The extracellular Ci, concentration required for a half-maximum rate (K1/2) was 3.8 [mu]M and the intracellular K1/2 was 1.4 mM for the stimulation of PNDA reduction. These values agreed closely with the K1/2 values of extracellular and intracellular Ci for O2 photoreduction. Linear electron flow to artificial electron acceptors of PSI was enhanced by intracellular Ci, which appeared to exert an effect on PSI or on the intersystem electron transport chain.

Effect of the intracellular inorganic carbon pool on chlorophyll a fluorescence quenching and O2 photoreduction in air-grown cells of the cyanobacterium Synechococcus UTEX 625

Simultaneous measurements were made of O2 exchange, inorganic carbon (Ci) accumulation and assimilation, and chlorophyll a fluorescence of the cyanobacterium Synechococcus UTEX 625. The addition of Ci to cells at the CO2 compensation point resulted in quenching of chlorophyll a fluorescence in the presence or absence of the CO2 fixation inhibitor, iodoacetamide. The magnitude of quenching was related to electron flow to terminal electron acceptors such as CO2 and O2. When photosynthetic CO2 fixation was allowed, the rate of electron transport, as expressed by (F*m – F)/F*m, was highly correlated with the onset of photosynthesis. When CO2 fixation was inhibited by the addition of iodoacetamide, the observed fluorescence quenching was consistent with the enhanced rate of O2 photoreduction that occurred when Ci was added. There was a close correlation (r = 0.98) between the magnitude of O2-dependent fluorescence quenching and the amount of O2 photoreduction. The degree of stimulation of electron flow to O2 photoreduction was dependent on the inorganic carbon concentration. The K1/2 (Ci) for extracellular Ci was 1.36 ± 0.13 μM (mean ± SD, n = 3) and K1/2 (Ci) for the intracellular Ci pool was 1.4 ± 0.18 mM (mean ± SD, n = 3). The reduction of N,N-dimethyl-p-nitrosoaniline was also stimulated by the addition of Ci, whereas the addition of Ci had no effect on the reduction of 2,6-dimethylbenzoquinone and ferricyanide. The results suggest that Ci stimulates electron flow in photosystem I. Key words: cyanobacteria, O2 photoreduction, fluorescence, Ci concentrating mechanism, inorganic carbon pool, linear electron transport, kinetic study.

Oxygen photoreduction and its effect on CO2 accumulation and assimilation in air-grown cells of Synechococcus UTEX 625

Mass spectrometric measurements of 16O2, 18O2, and 13CO2 were used to measure the rates of gross O2 evolution, O2 uptake, and CO2 assimilation in relation to light intensity, temperature, pH, and O2 concentration by air-grown cells of the cyanobacterium Synechococcus UTEX 625. CO2 fixation and O2 photoreduction increased with increased light intensity and, although CO2 fixation was saturated at 250 μmol ∙ m−2 ∙ s−1, O2 photoreduction was not saturated until about 550 μmol ∙ m−2 ∙ s−1. At high light intensity addition of inorganic carbon to the cells stimulated O2 photoreduction 2-fold when CO2, fixation was allowed and 5-fold when CO2, fixation was inhibited with iodoacetamide. The ability of O2, to act as an acceptor of photosynthetically generated reducing power was dependent upon the O2 concentration, and the substrate concentration required for half maximum rate (K½(O2)) was 53.2 ± 4.2 μM (mean ± SD, n = 3). The Q10 for oxygen photoreduction was about 2. A certain amount (10%) of O2 appeared to be required for maximum photosynthesis, as photosynthesis was inhibited under anaerobic conditions, especially at high light intensity. The point of inhibition is unknown but it seemed unlikely to be on CO2 transport or the concentration of intracellular dissolved inorganic carbon (Ci), as the rate of initial CO2 transport was enhanced and the intracellular Q1 pool increased in size under anaerobic conditions. Key words: cyanobacteria, photosynthesis, Ci concentrating mechanism, inorganic carbon pool, O2 photoreduction, electron transport, temperature.

Driving Forces for Bicarbonate Transport in the Cyanobacterium Synechococcus R-2 (PCC 7942)

Air-grown Synechococcus R-2 (PCC 7942) cultures grown in BG-11 medium are very alkaline (outside pH is 10.0) and use HCO3- as their inorganic carbon source. The cells showed a dependence on Na+ for photosynthesis, but low Na+ conditions (1 mol m-3) were sufficient to support saturating photosynthesis. The intracellular dissolved inorganic carbon in the light was greater than 20 mol m-3 in both low-Na+ conditions and in BG-11 medium containing the usual [Na+] (24 mol m-3, designated high-Na+ conditions). The electrochemical potential for HCO3- in the light was in excess of 25 kJ mol-1, even in high-Na+ conditions. The Na+-motive force was greater than -12 kJ mol-1 under both Na+ conditions. On thermodynamic grounds, an Na+-driven co-port process would need to have a stoichiometry of 2 or greater ([greater than or equal to]2Na+ in/HCO3-1 in), but we show that Na+ or K+ fluxes cannot be linked to HCO3- transport. Na+ and K+ fluxes were unaffected by the presence or absence of dissolved inorganic carbon. In low-Na+ conditions, Na+ fluxes are too low to support the observed net 14C-carbon fixation rate. Active transport of HCO3- hyperpolarizes (not depolarizes) the membrane potential.

Effects of inorganic carbon accumulation on photosynthetic oxygen reduction and cyclic electron flow in the cyanobacterium Synechococcus PCC7942

This paper examines the effect of inorganic carbon transport and accumulation in Synechococcus PCC7942 on fluorescence quenching, photosynthetic oxygen reduction and both linear and cyclic electron flow. The data presented support the previous findings of Miller et al. (1991) that the accumulation of Ci by the CO2 concentrating mechanism is able to stimulate oxygen photoreduction, particularly so when CO2 fixation is inhibited by PCR cycle inhibitors such as glycolaldehyde. This effect is found with both high and low-Ci grown cells, but the potential for oxygen photoreduction is about two-fold higher in low-Ci grown cells. This greater potential for O2 photoreduction is also correlated with a higher ability of low-Ci cells to photoreduce H2O2. Experiments with a mutant which transports Ci but does not accumulate it internally, indicates that the stimulation of O2 photoreduction appears to be a direct effect of the internal accumulation of Ci rather than from its participation in the transport process. In the absence of Ci, no specific partial reactions of photosynthetic electron transport appear to be inhibited, and the PS 1 acceptors PNDA and MV as well as the PS 2 acceptor DMQ can all run electron transport at levels approaching those during active CO2 fixation. Measurements of P700(+) show that when the cells are depleted of Ci during photosynthesis, P700 becomes more oxidised. This indicates that the resupply of electrons from the intersystem chain is relatively more restricted under conditions of Ci limitation than is the availability of PS 1 electron acceptors. It is proposed that the accumulated Ci pool can directly stimulate the ability of O2 to act as a PS 1 acceptor and that the ability of PS 1 acceptors, such as O2, to relieve restrictions on intersystem electron transfer is perhaps a result of a reduction in cyclic electron flow and a subsequent increase in the oxidation state of the plastoquinone pool.

Na-Independent HCO(3) Transport and Accumulation in the Cyanobacterium Synechococcus UTEX 625

The active transport and intracellular accumulation of HCO(3) (-) by air-grown cells of the cyanobacterium Synechococcus UTEX 625 (PCC 6301) was strongly promoted by 25 millimolar Na(+).Na(+)-dependent HCO(3) (-) accumulation also resulted in a characteristic enhancement in the rate of photosynthetic O(2) evolution and CO(2) fixation. However, when Synechococcus was grown in standing culture, high rates of HCO(3) (-) transport and photosynthesis were observed in the absence of added Na(+). The internal HCO(3) (-) pool reached levels up to 50 millimolar, and an accumulation ratio as high as 970 was observed. Sodium enhanced HCO(3) (-) transport and accumulation in standing culture cells by about 25 to 30% compared with the five- to eightfold enhancement observed with air-grown cells. The ability of standing culture cells to utilize HCO(3) (-) from the medium in the absence of Na(+) was lost within 16 hours after transfer to air-grown culture and was reacquired during subsequent growth in standing culture. Studies using a mass spectrometer indicated that standing culture cells were also capable of active CO(2) transport involving a high-affinity transport system which was reversibly inhibited by H(2)S, as in the case for air-grown cells. The data are interpreted to indicate that Synechococcus possesses a constitutive CO(2) transport system, whereas Na(+)-dependent and Na(+)-independent HCO(3) (-) transport are inducible, depending upon the conditions of growth. Intracellular accumulation of HCO(3) (-) was always accompanied by a quenching of chlorophyll a fluorescence which was independent of CO(2) fixation. The extent of fluorescence quenching was highly dependent upon the size of the internal pool of HCO(3) (-) + CO(2). The pattern of fluorescence quenching observed in response to added HCO(3) (-) and Na(+) in air-grown and standing culture cells was highly characteristic for Na(+)-dependent and Na(+)-independent HCO(3) (-) accumulation. It was concluded that measurements of fluorescence quenching provide an indirect means for following HCO(3) (-) transport and the dynamics of intracellular HCO(3) (-) accumulation and dissipation.

High Affinity Transport of CO(2) in the Cyanobacterium Synechococcus UTEX 625

The active transport of CO(2) in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO(3) (-) transport. The substrate concentration required to give one half the maximum rate for whole cell CO(2) transport was determined to be 0.4 +/- 0.2 micromolar (mean +/- standard deviation; n = 7) with a range between 0.2 and 0.66 micromolar. The maximum rates of CO(2) transport ranged between 400 and 735 micromoles per milligram of chlorophyll per hour with an average rate of 522 for seven experiments. This rate of transport was about three times greater than the dissolved inorganic carbon saturated rate of photosynthetic O(2) evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO(2) transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO(2) transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO(2) transport was caused by HCO(3) (-) or by Na(+)-dependent HCO(3) (-) transport. However, (12)CO(2) readily interfered with (13)CO(2) transport. CO(2) transport and Na(+)-dependent HCO(3) (-) transport are separate, independent processes and the high affinity CO(2) transporter is not only responsible for the initial transport of CO(2) into the cell but also for scavenging any CO(2) that may leak from the cell during ongoing photosynthesis.

Glycolaldehyde Inhibits CO(2) Fixation in the Cyanobacterium Synechococcus UTEX 625 without Inhibiting the Accumulation of Inorganic Carbon or the Associated Quenching of Chlorophyll a Fluorescence

When studying active CO(2) and HCO(3) (-) transport by cyanobacteria, it is often useful to be able to inhibit concomitant CO(2) fixation. We have found that glycolaldehyde was an efficient inhibitor of photosynthetic CO(2) fixation in Synechococcus UTEX 625. Glycolaldehyde did not inhibit inorganic carbon accumulation due to either active CO(2) or HCO(3) (-) transport. When glycolaldehyde (10 millimolar) was added to rapidly photosynthesizing cells, CO(2) fixation was stopped within 15 seconds. The quenching of chlorophyll a fluorescence remained high (</= 82% control) when CO(2) fixation was completely blocked by glycolaldehyde. This quenching was relieved upon the addition of a glucose oxidase oxygentrap. This is consistent with our previous finding that q-quenching in the absence of CO(2) fixation was due to O(2) photoreduction. Photosynthetic CO(2) fixation was also inhibited by d,l,-glyceraldehyde but a sixfold higher concentration was required. Glycolaldehyde acted much more rapidly than iodoacetamide (15 seconds versus 300 seconds) and did not cause the onset of net O(2) evolution often observed with iodoacetamide. Glycolaldehyde will be a useful inhibitor when it is required to study CO(2) and HCO(3) (-) transport without the complication of concomitant CO(2) fixation.

Selective and Reversible Inhibition of Active CO(2) Transport by Hydrogen Sulfide in a Cyanobacterium

The active transport of CO(2) in the cyanobacterium Synechococcus UTEX 625 was inhibited by H(2)S. Treatment of the cells with up to 150 micromolar H(2)S + HS(-) at pH 8.0 had little effect on Na(+)-dependent HCO(3) (-) transport or photosynthetic O(2) evolution, but CO(2) transport was inhibited by more than 90%. CO(2) transport was restored when H(2)S was removed by flushing with N(2). At constant total H(2)S + HS(-) concentrations, inhibition of CO(2) transport increased as the ratio of H(2)S to HS(-) increased, suggesting a direct role for H(2)S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H(2)S and Na(+) -dependent HCO(3) (-) transport, the extracellular CO(2) concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H(2)S. The inhibition of CO(2) transport, therefore, revealed an ongoing leakage from the cells of CO(2) which was derived from the intracellular dehydration of HCO(3) (-) which itself had been recently transported into the cells. Normally, leaked CO(2) is efficiently transported back into the cell by the CO(2) transport system, thus maintaining the extracellular CO(2) concentration near zero. It is suggested that CO(2) transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO(2) lost from the cell. A schematic model describing the relationship between the CO(2) and HCO(3) (-) transport systems is presented.

Use of Carbon Oxysulfide, a Structural Analog of CO(2), to Study Active CO(2) Transport in the Cyanobacterium Synechococcus UTEX 625

Carbon oxysulfide (carbonyl sulfide, COS) is a close structural analog of CO(2). Although hydrolysis of COS (to CO(2) and H(2)S) does occur at alkaline pH (>9), at pH 8.0 the rate of hydrolysis is slow enough to allow investigation of COS as a possible substrate and inhibitor of the active CO(2) transport system of Synechococcus UTEX 625. A light-dependent uptake of COS was observed that was inhibited by CO(2) and the ATPase inhibitor diethylstilbestrol. The COS taken up by the cells could not be recovered when the lights were turned off or when acid was added. It was concluded that most of the COS taken up was hydrolyzed by intracellular carbonic anhydrase. The production of H(2)S was observed and COS removal from the medium was inhibited by ethoxyzolamide. Bovine erythrocyte carbonic anhydrase catalysed the stoichiometric hydrolysis of COS to H(2)S. The active transport of CO(2) was inhibited by COS in an apparently competitive manner. When Na(+)-dependent HCO(3) (-) transport was allowed in the presence of COS, the extracellular [CO(2)] rose considerably above the equilibrium level. This CO(2) appearing in the medium was derived from the dehydration of transported HCO(3) (-) and was leaked from the cells. In the presence of COS the return to the cells of this leaked CO(2) was inhibited. These results showed that the Na(+)-dependent HCO(3) (-) transport was not inhibited by COS, whereas active CO(2) transport was inhibited. When COS was removed by gassing with N(2), a normal pattern of CO(2) uptake was observed. The silicone fluid centrifugation method showed that COS (100 micromolar) had little effect upon the initial rate of HCO(3) (-) transport or CO(2) fixation. The steady state rate of CO(2) fixation was, however, inhibited about 50% in the presence of COS. This inhibition can be at least partially explained by the significant leakage of CO(2) from the cells that occurred when CO(2) uptake was inhibited by COS. Neither CS(2) nor N(2)O acted like COS. It is concluded that COS is an effective and selective inhibitor of active CO(2) transport.

The Relationship between Ribulose Bisphosphate Concentration, Dissolved Inorganic Carbon (DIC) Transport and DIC-Limited Photosynthesis in the Cyanobacterium Synechococcus leopoliensis Grown at Different Concentrations of Inorganic Carbon

To examine the factors which limit photosynthesis and their role in photosynthetic adaptation to growth at low dissolved inorganic carbon (DIC), Synechococcus leopoliensis was grown at three concentrations (as signified by brackets) of DIC, high (1000-1800 micromolar), intermediate (200-300 micromolar), and low (10-20 micromolar). In all cell types photosynthesis varied from being ribulose bisphosphate (RuBP)-saturated at low external [DIC] to RuBP-limited at high external [DIC]. The maximum rate of photosynthesis (P(max)) was achieved when the internal concentration of RuBP fell below the active site density of RuBP carboxylase/oxygenase (Rubisco). At rates of photosynthesis below P(max), photosynthetic capacity was limited by the ability of the cell to transport inorganic carbon and to supply CO(2) to Rubisco. Adaptation to low DIC was reflected by a decrease in the [DIC] required to half-saturate photosynthesis. Simultaneous mass-spectrometric measurement of rates of photosynthesis and DIC transport showed that the initial slope of the photosynthesis versus [DIC] curve is identical to the initial slope of the DIC transport versus [DIC] curve. This provided evidence that the enhanced capacity for DIC transport which occurs upon adaptation to low [DIC] was responsible for the increase in the initial slope of the photosynthesis versus [DIC] curve and therefore the decrease in the half saturation constant of photosynthesis with respect to DIC. Levels of RuBP and in vitro Rubisco activity varied only slightly between high and intermediate [DIC] grown cells but fell significantly (65-70%) in low [DIC] grown cells. Maximum rates of photosynthesis followed a similar pattern with P(max) only slightly lower in intermediate [DIC] grown cells than in high [DIC] grown cells, but much lower in low [DIC] grown cells. The changing response of photosynthesis to [DIC] during adaptation to low DIC, may be explained by the interaction between DIC-transport limited and [RuBP]-limited photosynthesis.

Characterization of the na-requirement in cyanobacterial photosynthesis

The Na(+) requirement for photosynthesis and its relationship to dissolved inorganic carbon (DIC) concentration and Li(+) concentration was examined in air-grown cells of the cyanobacterium Synechococcus leopoliensis UTEX 625 at pH 8. Analysis of the rate of photosynthesis (O(2) evolution) as a function of Na(+) concentration, at fixed DIC concentration, revealed two distinct regions to the response curve, for which half-saturation values for Na(+) (K((1/2))[Na(+)]) were calculated. The value of both the low and the high K((1/2))(Na(+)) was dependent upon extracellular DIC concentration. The low K((1/2))(Na(+)) decreased from 1000 micromolar at 5 micromolar DIC to 200 micromolar at 140 micromolar DIC whereas over the same DIC concentration range the high K((1/2))(Na(+)) decreased from 10 millimolar to 1 millimolar. The most significant increases in photosynthesis occurred in the 1 to 20 millimolar range. A fraction of total photosynthesis, however, was independent of added Na(+) and this fraction increased with increased DIC concentration. A number of factors were identified as contributing to the complexity of interaction between Na(+) and DIC concentration in the photosynthesis of Synechococcus. First, as revealed by transport studies and mass spectrometry, both CO(2) and HCO(3) (-) transport contributed to the intracellular supply of DIC and hence to photosynthesis. Second, both the CO(2) and HCO(3) (-) transport systems required Na(+), directly or indirectly, for full activity. However, micromolar levels of Na(+) were required for CO(2) transport while millimolar levels were required for HCO(3) (-) transport. These levels corresponded to those found for the low and high K((1/2))(Na(+)) for photosynthesis. Third, the contribution of each transport system to intracellular DIC was dependent on extracellular DIC concentration, where the contribution from CO(2) transport increased with increased DIC concentration relative to HCO(3) (-) transport. This change was reflected in a decrease in the Na(+) concentration required for maximum photosynthesis, in accord with the lower Na(+)-requirement for CO(2) transport. Lithium competitively inhibited Na(+)-stimulated photosynthesis by blocking the cells' ability to form an intracellular DIC pool through Na(+)-dependent HCO(3) (-) transport. Lithium had little effect on CO(2) transport and only a small effect on the size of the pool it generated. Thus, CO(2) transport did not require a functional HCO(3) (-) transport system for full activity. Based on these observations and the differential requirement for Na(+) in the CO(2) and HCO(3) (-) transport system, it was proposed that CO(2) and HCO(3) (-) were transported across the membrane by different transport systems.

Active Transport of Inorganic Carbon Increases the Rate of O(2) Photoreduction by the Cyanobacterium Synechococcus UTEX 625

Chlorophyll a fluorescence of Synechococcus UTEX 625 was quenched during the transport of inorganic carbon, even when CO(2) fixation was inhibited by iodoacetamide. Measurements with a pulse modulation fluorometer showed that at least 75% of the quenching was due to oxidation of Q(a), the primary acceptor of photosystem II. Mass spectrometry revealed that transport of inorganic carbon increased the rate of O(2) photoreduction. Hence, O(2) could serve as an electron acceptor to allow oxidation of Q(a) even in the absence of CO(2) fixation.