Regulation of Photosynthetic Electron Transport in Intact Spinach Chloroplasts: I. INFLUENCE OF EXOGENOUS SALTS ON OXALOACETATE REDUCTION

Relatively high concentrations of monovalent salts (150 millimolar) stimulated light-saturated uncoupled rates of O₂ evolution linked to oxaloacetic acid (OAA) reduction by intact chloroplasts 2-to 3-fold. In contrast, monovalent salts partially inhibited light-saturated rates of O₂ evolution coupled to CO₂ fixation and uncoupled rates of nitrite reduction. In the presence of high salt concentration, light-saturated rates of electron transport were about equivalent for all three terminal electron acceptors. It is inferred that exogenous monovalent salts have at least two effects on photosynthetic electron transport, independent of photophosphorylation and CO₂ metabolism: a partial inhibitory effect common to OAA, NO₂⁻ and CO₂ reduction and a marked stimulatory effect unique to the photoreduction of OAA.     

A test of the binary chloroplast membrane hypothesis by using a nonpenetrating chemical probe,p-(diazonium)-benzenesulfonic acid

Summary Spinach chloroplasts were exposed to³⁵S-labeledp-(diazonium)-benzenesulfonic acid (DABS), a water soluble compound which does not penetrate lipophilie regions of membranes, and which is highly reactive toward amino acid functionagroups such as -amino, sulfhydryl, histidine, and tyrosine groups. Amino groups inl lipids can also form similar, stable covalent bonds by diazo coupling. Both chloroplast lipids and proteins were labeled with DABS, the total binding being about 1 DABS per 10 chlorophylls, depending on the reaction conditions.After diazo coupling and subsequent digitonin fractionation into photosystems I and II enriched fractions, it was observed that PS-I was more highly labeled than PS-III usually by a factor of 10 to 24 times (on a per chlorophyll basis). After digitonin isolation, however, the PS-II portion bound an amount of DABS similar to the PS-I binding, We interpret these data as consistent with the binary membrane hypothesis (Arntzen. Dilley and Crane (1969),J. Cell Biol. 43:16), which visualizes PS-I on the externa, half of a 90 Å grana membrane, and PS-II occurring on the interior half of thel membrane. The alternative explanation that PS-II and PS-I are arranged as a mosaic, and that the low DABS binding in PS-II is caused by burial of the diazo reactive groups in the interior of the proteins (and only exposed through the denaturing effect of digitonin) is not directly ruled out. However, this alternative is not consistent with the facts that: (a) most of the membrane proteins in PS-I and PS-II are identical in electrophoretic properties and therefore probably have similar overall structures; and (b) digitonin does not lead to appreciable denaturation of proteins, evidenced by the retention of PS-II electron transport activity.     

Evolution of photosynthetic reaction centers

The common ancestor of all photosynthetic prokaryotes and organelles contained chlorophyll (Chl) a. All green and purple photosynthetic bacteria descended from a common bacteriochlorophyll (Bchl) a-containing ancestor which diverged from the Chl a line. Separate PS-I and PS-II reaction centers may have evolved before the appearance of Bchl a. When the transition to Bchl a occurred, the resultant organism contained two types of reaction center, “PS-I” and “PS-II.” One line of development eliminated “PS-II” and evolved into the green bacteria. The other line eliminated “PS-I” and became the purple bacteria. In the Chl a-containing organisms the evolution of PS-II continued until oxygen evolution was achieved.     

Amitrole Absorption by Bean (Phaseolus vulgaris L. cv ;Red Kidney') Roots : Mechanism of Absorption

The mechanism of transport of the herbicide 3-amino-1,2,4-triazole (amitrole) into Phaseolus vulgaris roots appears to be passive, as judged by the effect of temperature (Q₁₀ = 1.3 between 15 and 25°C) and the lack of sensitivity to metabolic inhibition afforded by 2,4-dinitrophenol and NaN₃. Amitrole absorption is a linear function of external concentration over several orders of magnitude and, thus, is not facilitated by a carrier mechanism. The absorption of amitrole is sensitive to external pH, being stimulated under acid conditions. This stimulation of amitrole absorption is seen at low (≤1 millimolar) amitrole concentrations, but not at high (50 millimolar) amitrole levels. While the apparent octanol-water partition coefficient varies with the pH of the aqueous phase, there is no clear correspondence between absorption and the apparent partition coefficient. Roots do not accumulate amitrole above concentration equilibrium; however, at a time when the net amitrole content of the root tissue begins to saturate, amitrole can be detected in the xylem stream. On a fresh-weight basis, amitrole absorption by roots is equal to that accomplished by trifoliate-leaf tissue. An estimate of the permeability coefficient (according to the analysis of Tyree et al. 1979 Plant Physiol 63: 367-374) suggests that amitrole possesses near-optimal permeability for an ambimobile solute, on the order of 2.12 (± 0.47) × 10⁻⁹ meters per second.     

Isolation and characterization of polysaccharides of a hybrid mushroom (backcross mating between PfloVv12 and Volvariella volvacea)

Three polysaccharide fractions (PS-I, PS-II, and PS-III) were isolated from the aqueous extract of a hybrid mushroom obtained through backcross mating of a somatic hybrid mushroom PfloVv12 (Sterile line) with Volvariellavolvacea. PfloVv12 was obtained through protoplast fusion of Pleurotusflorida and V. volvacea. PS-I was identified as 1,6-β glucan. PS-II and PS-III were identified as mannoglucogalactan but differing in molecular weights only. On the basis of total acid hydrolysis, methylation analysis, periodate oxidation, and NMR experiment (¹H, ¹³C, DEPT-135, DQF-COSY, TOCSY, NOESY, ROESY, HMQC, and HMBC) the structures of these polysaccharides were established as;     

14CO2 fixation and glycolate metabolism in the dark in isolated maize (Zea mays L.) bundle sheath strands

Bundle sheath strands capable of assimilating up to 68 μmoles CO₂ per mg chlorophyll per hr in the dark have been isolated from fully expanded leaves of Zea mays L. This dark CO₂-fixing system is dependent on exogenous ribose-5-phosphate, ADP or ATP, and Mg²⁺ for maximum activity. The principal product of dark fixation in this system is 3-phosphoglycerate, indicating that the CO₂-fixing reaction is mediated by ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). The rate of dark CO₂ uptake in the strands in the presence of saturating levels of ribose-5-phosphate plus ADP is inhibited by oxygen. The inhibitory effect of oxygen is rapidly and completely reversible, and is relieved by increased levels of CO₂. Glycolate is synthesized in this dark system in the presence of [U-¹⁴C]ribose-5-phosphate, ADP, oxygen, and an inhibitor of glycolate oxidase (EC 1.1.3.1). Glycolate formation is completely abolished by heating the strands, and the rate of glycolate synthesis is markedly reduced by either lowering the oxygen tension or increasing the level of CO₂.These results, obtained with intact cells in the absence of light, indicate that the direct inhibitory effect of oxygen on photosynthesis is associated with photosynthetic carbon metabolism, probably at the level of ribulose-1,5-bisphosphate carboxylase, and not with photophosphorylation or photosynthetic electron transport. Furthermore, the findings indicate that the synthesis of glycolate from exogenous substrate can readily occur in the absence of photosynthetic electron transport, an observation consistent with the ribulose-1, 5-bisphosphate “oxygenase” scheme for glycolate formation during photosynthesis.     

Proton efflux through the chloroplast ATP synthase (CF0 · CF1) in the presence of sulfhydryl-modifying agents

The rate of photosynthetic electron transport measured in the absence of ADP and P_i is stimulated by low levels of Hg²⁺ or Ag⁺ (50% stimulation ≈ 3 Hg²⁺ or 6 Ag⁺/100 chlorophyll) to a plateau equal to the transport rate under normal phosphorylating conditions (i.e. +ADP, +P_i). Chloroplasts pretreated in the light under energizing conditions with N-ethylmaleimide show a similar stimulation of non-phosphorylating electron transport. The stimulations of non-phosphorylating electron transport by Hg²⁺, Ag⁺ and N-ethylmaleimide are reversed by the CF₁ inhibitor phlorizin, the CF₀ inhibitor triphenyltin chloride, and can be further stimulated by uncouplers such as methylamine. The Hg²⁺ and N-ethylmaleimide stimulations, but not the Ag⁺ stimulation, are completely reversed by low levels of ADP (2 μM), ATP (2 μM), and P_i (400 μM). Ag⁺, which is a potent inhibitor of ATP synthesis, has little or no effect upon phosphorylating electron transport (+ADP, +P_i). Concomitant with the stimulations of non-phosphorylating electron transport by Hg²⁺, Ag⁺ and ADP + P_i, there is a decrease in the level of membrane energization (as measured by atebrin fluorescence quenching) which is reversed when the CF₀ channel is blocked by triphenyltin. These results suggest that modification of critical CF₁ sulfhydryl residues by Hg²⁺, Ag⁺ or N-ethylmaleimide leads to the loss of intra-enzyme coupling between the transmembrane protontransferring and the ATP synthesis activities of the CF₀-CF₁ ATP synthase complex.     

Rates and properties of endogenous cyclic photophosphorylation of isolated intact chloroplasts measured by CO2 fixation in the presence of dihydroxyacetone phosphate

1. Dihydroxyacetone phosphate in concentrations ? 2.5 mM completely inhibits CO₂-dependent O₂ evolution in isolated intact spinach chloroplasts. This inhibition is reversed by the addition of equimolar concentrations of P_i, but not by addition of 3-phosphoglycerate. In the absence of P_i, 3-phosphoglycerate and dihydroxyacetone phosphate, only about 20% of the ¹⁴C-labelled intermediates are found in the supernatant, whereas in the presence of each of these substances the percentage of labelled intermediates in the supernatant is increased up to 70–95%. Based on these results the mechanism of the inhibition of O₂ evolution by dihydroxyacetone phosphate is discussed with respect to the function of the known phosphate translocator in the envelope of intact chloroplasts.2. Although O₂ evolution is completely suppressed by dihydroxyacetone phosphate, CO₂ fixation takes place in air with rates of up to 65μ mol · mg^?1 chlorophyll · h^?1. As non-cyclic electron transport apparently does not occur under these conditions, these rates must be due to endogenous pseudocyclic and/or cyclic photophosphorylation.3. Under anaerobic conditions, the rates of CO₂ fixation in presence of dihydroxyacetone phosphate are low (2.5–7 μmol · mg^?1 chlorophyll · h^?1), but they are strongly stimulated by addition of dichlorophenyl-dimethylurea (e.g. 2 · 10^?7 M) reaching values of up to 60 μmol · mg^?1 chlorophyll · h^?1. As under these conditions the ATP necessary for CO₂ fixation can be formed by an endogenous cyclic photophosphorylation, the capacity of this process seems to be relatively high, so it might contribute significantly to the energy supply of the chloroplast. As dichlorophenyl-dimethylurea stimulates CO₂ fixation in presence of dihydroxyacetone phosphate under anaerobic but not under aerobic conditions, it is concluded that only under anaerobic conditions an “overreduction” of the cyclic electron transport system takes place, which is removed by dichlorophenyl-dimethylurea in suitable concentrations. At concentrations above 5 · 10^?7 M dichlorophenyl-dimethylurea inhibits dihydroxyacetone phosphate-dependent CO₂ fixation under anaerobic as well as under aerobic conditions in a similar way as normal CO₂ fixation. Therefore, we assume that a properly poised redox state of the electron transport chain is necessary for an optimal occurrence of endogenous cyclic photophosphorylation.4. The inhibition of dichlorophenyl-dimethylurea-stimulated CO₂ fixation in presence of dihydroxyacetone phosphate by dibromothymoquinone under anaerobic conditions indicates that plastoquinone is an indispensible component of the endogenous cyclic electron pathway.     

Inhibition of photorespiration and increase of net photosynthesis in isolated maize bundle sheath cells treated with glutamate or aspartate

Net photosynthetic ¹⁴CO₂ fixation by isolated maize (Zea mays) bundle sheath strands was stimulated 20 to 35% by the inclusion of l-glutamate or l-aspartate in the reaction mixture. Maximal stimulation occurred at a 7.5 mm concentration of either amino acid. Since the photosynthetic rate and the glutamate-dependent stimulation in the rate were equally sensitive to a photosynthetic electron transport inhibitor, 3-(p-chlorophenyl)-1,1-dimethylurea, it was concluded that glutamate did not stimulate CO₂ fixation by supplying needed NADPH (NADH) through glutamate dehydrogenase. Treatment of the bundle sheath strands with glutamate inhibited glycolate synthesis by 59%. Photorespiration in this tissue, measured as the O₂ inhibition of CO₂ fixation (the Warburg effect), was inhibited by treatment with glutamate. The stimulation in net photosynthetic CO₂ fixation probably results from the decrease in photorespiratory CO₂ loss. This metabolic regulation of the rate of glycolate synthesis and photorespiration observed with isolated bundle sheath strands could account for the inability to detect rapid photorespiration in the mature intact maize leaf.     

Active Transport of CO(2) by the Cyanobacterium Synechococcus UTEX 625 : Measurement by Mass Spectrometry

Mass spectrometry has been used to confirm the presence of an active transport system for CO₂ in Synechococcus UTEX 625. Cells were incubated at pH 8.0 in 100 micromolar KHCO₃ in the absence of Na⁺ (to prevent HCO₃⁻ transport). Upon illumination the cells rapidly removed almost all the free CO₂ from the medium. Addition of carbonic anhydrase revealed that the CO₂ depletion resulted from a selective uptake of CO₂, rather than a total uptake of all inorganic carbon species. CO₂ transport stopped rapidly (<3 seconds) when the light was turned off. Iodoacetamide (3.3 millimolar) completely inhibited CO₂ fixation but had little effect on CO₂ transport. In iodoacetamide poisoned cells, transport of CO₂ occurred against a concentration gradient of about 18,000 to 1. Transport of CO₂ was completely inhibited by 10 micromolar diethylstilbestrol, a membrane-bound ATPase inhibitor. Studies with DCMU and PSI light indicated that CO₂ transport was driven by ATP produced by cyclic or pseudocyclic photophosphorylation. Low concentrations of Na⁺ (<100 microequivalents per liter), but not of K⁺, stimulated CO₂ transport as much as 2.4-fold. Unlike Na⁺-dependent HCO₃⁻ transport, the transport of CO₂ was not inhibited by high concentrations (30 milliequivalents per liter) of Li⁺. During illumination, the CO₂ concentration in the medium remained far below its equilibrium value for periods up to 15 minutes. This could only happen if CO₂ transport was continuously occurring at a rapid rate, since the continuing dehydration of HCO₃⁻ to CO₂ would rapidly raise the CO₂ concentration to its equilibrium value if transport ceased. Measurement of the rate of dissolved inorganic carbon accumulation under these conditions indicated that at least part of the continuing CO₂ transport was balanced by HCO₃⁻ efflux.     

Cold-acclimation protects photosystem II against freezing damage in the halotolerant alga Dunaliella salina

Cold-acclimation (CA) of the halotolerant alga Dunaliella was inhibited by light and by high salt. CA was associated with enhanced resistance to freezing in saline growth solutions, as manifested by protection of photosynthetic oxygen evolution and by reduced permeabilisation of the plasma membrane. Oxygen evolution activity in isolated chloroplasts was not affected by freezing, but was inhibited by high salt and the inhibition could be reversed or protected by glycerol. The activity of chloroplasts from cold-acclimated cells was more resistant to salt than of non-acclimated cells. Electron transport measurements in chloroplasts indicated that high salt inhibited PS-II, but not PS-I electron transport. High salt also inhibited PS-II thermoluminescence (TL) activity in chloroplasts. Similar inhibition of PS-II TL was observed by freezing intact cells in saline solutions. Chloroplasts from cold-acclimated cells had enhanced resistance to inhibition of PS-II electron transport and of PS-II TL by high salt. These results suggest that inhibition of oxygen evolution upon freezing Dunaliella cells may result from inactivation of PS-II due to massive influx of salt and loss of glycerol. The enhanced freeze-resistance of cold-acclimated cells to inhibition of oxygen evolution can be accounted for partly by protection of PS-II against high salt.     

Evidence for a physiological role of CO2 in the regulation of photosynthetic electron transport in intact leaves

The effect of CO₂ upon photosynthetic electron transport was examined in wheat and maize leaves in order to establish whether CO₂ had a direct role in electron-transport regulation in vivo. When intercellular CO₂ was depleted a transient rise in chlorophyll fluorescence occurred which correlated with an increase in the reduction of the Photosystem II primary quinone acceptor, Q_A, and a decrease in CO₂ fixation rate. However, when intercellular CO₂ was reduced from an already low level (50 μmol·mol⁻¹) towards zero a substantial further reduction in Q_A occurred with little change in fluorescence or CO₂ fixation. In very low intercellular CO₂ when no measurable CO₂ fixation was sustained, an appreciable fraction of Q_A still remained oxidised, however, maximal reduction of Q_A occurred when O₂ was also removed. Q_A could then be substantially reoxidised by the readdition of small amounts of CO₂ (20–40 μmol) which only facilitated a very small increase in CO₂ fixation. Changes in the kinetics of the fast rise in fluorescence emission indicated that Q_A-to-Q_B electron transfer was decreased in a CO₂-free atmosphere and Q_B was poised in a more oxidised state. Electron transport that was independent of CO₂ fixation was measured in methyl viologen-treated leaf discs. In 1% O₂, but not in 21% O₂, light-dependent electron transport to methyl viologen was decreased significantly by the depletion of CO₂. It is concluded that CO₂ can modify the redox state of Photosystem II electron transport acceptors in vivo independently of carbon assimilation and that there is a complex interaction between CO₂ and O₂ in the regulation of photosynthetic electron transport. The possibility that CO₂ operates via the reversible binding to PS II and thereby acts as a cofactor for efficient PS II electron transport in the leaf is discussed.     

Light-saturated photosynthesis — Limitation by electron transport or carbon fixation?

The minimal turnover time, τ, for in vivo electron transport from water to CO₂, was calculated from oxygen flash yields and steady-state light-saturated photosynthetic rates in the marine chlorophyte, Dunaliella tertiolecta, cultured at different growth irradiance levels. As cells adapted to lower growth irradiance levels, τ increased from 3.5 to 14.5 ms, in parallel with increases in the contents of chlorophyll a, Photosystem II, PQ, cytochrome b₆f, Photosystem I and thylakoid surface density. Thus, at all growth irradiance levels examined, the relative proportion of these membrane-bound electron-transport components remained constant. However, the cellular pool size of ribulose-1,5-bisphosphate carboxylase/oxygenase, determined by radioimmunoassay, was independent of growth irradiance. Hence the ratio of the enzyme to electron-transport chain components varied between 4.8 and 1.2 as a function of growth irradiance levels. The change in this ratio was related quantitatively to the minimal turnover time of electron transport from water to carbon dioxide. Taking into account thylakoid surface density, cellular contents of electron-transport components and diffusion coefficient of plastoquinol, a diffusion time of 2.3 ms was calculated for transport of PQH₂ from Photosystem II to cytochrome b₆f. This rate is 1.5- to 13-times faster than τ. The data strongly suggest that under nutrient saturated conditions the absolute rate of light-saturated photosynthesis is limited by carbon fixation rather than electron transport. It is predicted, however, that in cells grown above 3000 μmol quanta per m² per s, electron transport rather than carbon fixation would become the rate-limiting step of light saturated photosynthesis.     

The stoichiometry (ATP/2e− ratio) of non-cyclic photophosphorylation in isolated spinach chloroplasts

1. The stoichiometry of non-cyclic photophosphorylation and electron transport in isolated chloroplasts has been re-investigated. Variations in the isolation and assay techniques were studied in detail in order to obtain optimum conditions necessary for reproducibly higher ADP/O (equivalent to ATP/2e^?) and photosynthetic control ratios.2. Studies which we carried out on the possible contribution of cyclic phosphorylation to non-cyclic phosphorylation suggested that not more than 10% of the total phosphorylation found could be due to cyclic phosphorylation.3. Photosynthetic control, and the uncoupling of electron transport in the presence of NH₄Cl, were demonstrated using oxidised diaminodurene as the electron acceptor. A halving of the ADP/O ratio was found, suggesting that electrons were being accepted between two sites of energy conservation, one of which is associated with Photosystem I and the other associated with Photosystem II.4. ATP was shown to inhibit State 2 and State 3 of electron transport, but not State 4 electron transport or the overall ADP/O ratio, thus confirming its activity as an energy transfer inhibitor. It is suggested that part of the non-phosphorylating electron transport rate (State 2) which is not inhibited by ATP is incapable of being coupled to subsequent phosphorylation triggered by the addition of ADP (State 3). If the ATP-insensitive State 2 electron transport is deducted from the State 3 electron transport when calculating the ADP/O ratio, a value of 2.0 is obtained.5. The experiments reported demonstrate that there are two sites of energy conservation in the non-cyclic electron transfer pathway: one associated with Photosystem II and the other with Photosystem I. Thus, non-cyclic photophosphorylation can probably produce sufficient ATP and NADPH “in vivo” to allow CO₂ fixation to proceed.     

Retention of xenobiotics along the phloem path

Detached leaves of Cyclamen persicum Mill. can be used as a simple source-sink system. Phloem transport in the excised material was monitored by the noninvasive ¹¹C-technique. Assimilate movement stopped immediately when the petiole was cut off. However, within 20 min a recovery of transport was observed. The translocation rate in the detached leaf was only 13% of that in the intact plant. ¹⁴C-Xenobiotics and [³H]sucrose were injected into the upper petiole parenchyma (source). They moved downstream by a symplastic route. The stump of the petiole was inserted into a buffer solution containing ethylenediaminetetraacetic acid (sink). After 3 h, the distribution of sucrose and xenobiotics was determined in five subsequent segments of the petiole (path). The retention coefficient (r) was calculated from the ratio of radioactivity in the vascular bundle to that in the petiole parenchyma. The distribution along the vascular path was given by a geometric progression, whereas its constant was the transport coefficient (q). Values of r and q corresponded with the degree of phloem mobility and ambimobility. Four groups of compounds were classified: (i) acidic substances with log K_ow = — 2 to — 2.4 (K_ow is the partition coefficient octanol/water) at pH 8 (pH of sieve tube sap), retained by ion trapping and exhibiting small lateral efflux (q0.7; maleic hydrazide, dalapon); (ii) acidic substances with log K_ow = — 0.7 to — 0.8 at pH 8, retained by ion trapping and subjected to a moderate lateral efflux (0.7>q> 0.5; 2,4-dichlorophenoxyacetic acid, 2-methyl-4-chlorophenoxyacetic acid, bromoxynil); (iii) nonionised substances retained by optimum permeability, exhibiting a considerable lateral leakage (q<0.5; glyphosate, amitrole); (iv) substances without basipetal transport in the phloem (atrazine, diuron). Retention of sucrose corresponded quantitatively with that shown in group (i). This classification was also supported by results of uptake and efflux experiments using the isolated conducting tissue. Theoretical translocation profiles were calculated from the determined transport coefficients (q).Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - K_ow partition coefficient octanol/water - MCPA 2-methyl-4-chloro-phenoxyacetic acid - q transport coefficient in the vascular bundle - r retention coefficient in the vascular bundle The authors gratefully acknowledge the assistance of H. Fiedler and M. Neugebauer. We are particularly grateful to K. Dutschka, G. Hudepokl, and Dr. J. Knust for producing ¹¹CO₂.     

High Light Intensity Augments Mercury Toxicity in Cyanobacterium Nostoc muscorum

The present study is aimed at investigating the role of growth irradiance in determining the extent of mercury (Hg) toxicity on various physiological parameters viz. growth, pigment contents, photosynthesis, respiration, ¹⁴CO₂ fixation, photosynthetic electron transport, photorespiration and enzyme activity of cyanobacterium Nostoc muscorum. A general decline was observed in all these parameters with increasing concentration of Hg except for carotenoids content and respiratory activity which exhibited significant enhancement. This effect was more pronounced in high light (130???mol photon m^?2?s^?1) exposed cells as compared to normal (70???mol photon m^?2?s^?1) and low (10???mol photon m^?2?s^?1) light exposed cells. Among the photosynthetic electron transport activities, whole chain was found to be more sensitive than photosystem II (PSII) and photosystem I (PSI). ¹⁴CO₂ fixation was more affected as compared to O₂ evolution when exposed to Hg and different light intensities. Photorespiratory activity, which is an index of protecting organisms from light-induced damage, also showed a similar declining trend. Enzyme assay revealed that among the carboxylating enzymes, activity of RUBISCO was more severely inhibited than PEPCase. Thus, these results suggest that Hg itself was toxic at all tested concentrations and high light intensity augmented its toxicity in N. muscorum inhibiting the growth, pigment contents and photosynthetic activity of the organism.     

Relationships among the three light-induced absorbance changes related to the reaction-center of photosystem II

The relationships among X₅₉₁, Cyt-b₅₅₉ and C-550 in the primaryphotoact of PS-II were analysed by examining the effects ofvarious inhibitory substances and treatments on the light-inducedabsorbance changes of these components. The results were fully explainable by the scheme previouslypresented by Huzisige, in which two photoreactions are involvedin PS-II. Our conclusion is that X₅₉₁ acts as the electron acceptorfor one of the photoreactions in PS-II. (Received October 23, 1978; )     

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 CO₂ and HCO₃⁻ transport by cyanobacteria, it is often useful to be able to inhibit concomitant CO₂ fixation. We have found that glycolaldehyde was an efficient inhibitor of photosynthetic CO₂ fixation in Synechococcus UTEX 625. Glycolaldehyde did not inhibit inorganic carbon accumulation due to either active CO₂ or HCO₃⁻ transport. When glycolaldehyde (10 millimolar) was added to rapidly photosynthesizing cells, CO₂ fixation was stopped within 15 seconds. The quenching of chlorophyll a fluorescence remained high (≤ 82% control) when CO₂ 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 CO₂ fixation was due to O₂ photoreduction. Photosynthetic CO₂ 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 O₂ evolution often observed with iodoacetamide. Glycolaldehyde will be a useful inhibitor when it is required to study CO₂ and HCO₃⁻ transport without the complication of concomitant CO₂ fixation.     

Effect of Sulfide on Nitrogen Fixation in a Stream Sediment-Water System

Nitrogen fixation (C₂H₂ reduction) in a sediment-water system was studied under anaerobic incubation conditions. Sodium sulfide at low concentrations stimulated activity, with a twofold increase in C₂H₄ production occurring in the presence of 8 μmol of S²⁻ per ml of stream water. Sodium sulfide at concentrations of 16 μmol of S²⁻ per ml or greater inhibited nitrogen fixation, with 64 μmol of S²⁻ per ml being completely inhibitory. Sulfide at levels of 16 μmol/ml or above inhibited CO₂ production, and the degree of inhibition increased with increasing concentration of sulfide. Titanium (III) citrate (used to modify Eh levels) stimulated both nitrogen fixation and CO₂ production, but could not duplicate, at any concentration tested, the twofold increase in nitrogen fixation caused by 8 μmol of S²⁻ per ml. Sulfide additions caused pH changes in the sediment, and when the sediment was adjusted and maintained at pH 7.0 all concentrations of sulfide inhibited nitrogen fixation activity. From considerations of the redox equilibria of H₂, H₂S, and other sulfur species at various pH values, it appeared that H₂S was the toxic entity and that HS⁻ was less toxic. The observed stimulation of activity was apparently due to a pH change coupled with the concurrent production of HS⁻ from H₂S.