Energetics of photosynthesis in Zea mays. I. Studies of the flash-induced electrochromic shift and fluorescence induction in bundle sheath cells

Bundle sheath strands free of mesophyll contamination were isolated from 3–4-week-old leaves of maize (Zea mays L.). Patterns of electron flow in the preparations were studied in the presence of physiological substrates. Relative electron flow rates were estimated from the flash-induced electrochromic band shift changes (P-518) and cytochrome f turnover. Induction of chlorophyll fluorescence was also measured. Little Photosystem II activity was found to be present, the principal pathway of electron flow being Photosystem I-driven cyclic electron transfer. The latter was activated through reductive poising by NADPH, generated via malate decarboxylation or (less efficiently) from dihydroxyacetone phosphate. The actions of these electron donors and of oxygen, nitrite and methyl viologen as electron acceptors in redox poising the Photosystem I-driven cycle were investigated and are discussed in relation to the regulation of photosynthesis in the bundle sheath.     

Effect of NADP+ on light-induced cytochrome changes in membrane fragments from a blue-green alga

The effect of NADP⁺ on light-induced steady-state redox changes of membrane-bound cytochromes was investigated in membrane fragments prepared from the blue-green algae Nostoc muscorum (Strain 7119) that had high rates of electron transport from water to NADP⁺ and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH₂) to NADP⁺. The membrane fragments contained very little phycocyanin and had excellent optical properties for spectrophotometric assays. With DCIPH₂ as the electron donor, NADP⁺ had no effect on the light-induced redox changes of cytochromes: with or without NADP⁺, 715- or 664-nm illumination resulted mainly in the oxidation of cytochrome f and of other component(s) which may include a c-type cytochrome with an α peak at 549 nm. With 664 nm illumination and water as the electron donor, NADP⁺ had a pronounced effect on the redox state of cytochromes, causing a shift toward oxidation of a component with a peak at 549 nm (possibly a c-type cytochrome), cytochrome f, and particularly cytochrome b₅₅₉. Cytochrome b₅₅₉ appeared to be a component of the main noncyclic electron transport chain and was photooxidized at physiological temperatures by Photosystem II. This photooxidation was apparent only in the presence of a terminal acceptor (NADP⁺) for the electron flow from water.     

Functional homogeneity of P-700 in cyclic and non-cyclic electron transport reactions in thylakoids

The photoinduced turnover of P-700 (the reaction center chlorophyll a of photosystem I) in higher plant thylakoids was examined at room temperature by observation of the kinetics and amplitude of the transmission signal at 700 nm. The concentration of P-700 functional in cyclic and non-cyclic electron transfer reactions was compared. For the cyclic reactions mediated by N-methylphenazonium-p-methosulfate, 2,3,5,6-tetramethylphenylenediamine, 2,6-dichlorophenolindophenol and N,N,N′,N′-tetramethylphenylenediamine and non-cyclic reactions utilizing either methylviologen or NADP⁺ as acceptor, the illuminated steady-state concentration of P-700⁺ was shown to be similar. The data support the concept of a homogeneous pool of P-700 that is capable of interaction in both cyclic and non-cyclic electron transfer reactions and are consistent with previous data obtained in vivo.The amplitude and kinetics of the P-700 signal were found to be very dependent upon the composition of the reaction medium and differences were noted for turnover in the cyclic and non-cyclic reactions. Specifically, at white light saturation, the addition of low concentrations of divalent cations, such as Mg²⁺ or Ca²⁺, had no effect on the signal amplitude during the cyclic reactions, but, in confirmation of previous work, caused an attenuation of the signal amplitude during non-cyclic flow. At low light intensities, the divalent cations caused a similar reduction in redox level of P-700 in the steady-state during non-cyclic flow and also reduced the rate of P-700 photooxidation in the cyclic reactions. The concentration of divalent cation that reduced the signal amplitude of P-700⁺ during non-cyclic flow was compared with that required for the stimulation of the variable component of fluorescence, and it was shown to be similar with half maximal effects at 1 mM Mg²⁺. The observations confirm that divalent cations control non-cyclic electron transport by an activation of Photosystem II in addition to regulating the distribution of excitation energy between the two photosystems.     

Relative activities of linear and cyclic electron flows during chloroplast CO2-fixation

Evolution of oxygen and turnover of cytochromes b-563 and ? were measured upon illumination of isolated intact spinach chloroplasts with a series of flashes. The flash yield of cytochrome ? oxidation approximated the sum of the yields of cytochrome b-563 reduction and electron transfer through Photosystem II, regardless of whether HCO^?₃, 3-phosphoglycerate or O₂ served as the terminal electron acceptor. No absorbance contribution from cytochrome b-559 was discerned within the time range studied. Some pseudocyclic electron flow occurred when both HCO^?₃ and 3-phosphoglycerate were omitted, and possibly also during induction of photosynthesis; however, the flash yield data suggest that O₂ is not reduced at a significant rate during steady state photosynthesis. The maximum rate of cytochrome ? turnover (1000 μequiv./mg chlorophyll per h) was adequate to support the highest rates of photosynthesis observed in isolated chloroplasts.These results agree with the concept that cytochrome ? is a component both of the linear and cyclic pathways whereas cytochrome b-563 functions only in the cyclic pathway. NH₄Cl decreased the half time of cytochrome b-563 oxidation from 11.6 to 8.2 ms and decreased the half time of cytochrome ? reduction from 7.2 to 2.8 ms. The cyclic and linear pathways thus seem to be jointly regulated by a transthylakoid H⁺ gradient through a common control point on the reducing side of cytochrome ?. Cyclic turnover also increased during the induction phase of photosynthesis, when linear throughput is limited by the rate of utilization of NADPH. The slow rise in the P-518 transient correlated with increased cyclic activity under the above conditions.It is proposed that flexibility in the utilization of linear and cyclic pathways allows the chloroplast to generate ATP and NADPH in ratios appropriate to varying needs.     

Chlorophyll fluorescence induction in anaerobic Scenedesmus obliquus

Fluorescence time curves (Kautsky effect) were studied in anaerobic Scenedesmus obliquus, with an apparatus capable of simultaneous recording of O₂ exchange, and far-red actinic illumination. Results, as interpreted in terms of electron transport reactions, suggest: In the course of becoming anaerobic, fluorescence induction undergoes a series of changes, indicating at least three different effects of the absence of O₂ on electron transport. (1) Immediately on removal of O₂, once the pool of intermediates between the two photo-systems is reduced by light reaction II, electron flow stops, resulting in high fluorescence yield and a cessation of O₂ evolution. O₂ appears to regulate linear electron flow and cyclic feedback of electrons to the intermediate pool. (2) An endogenous reductant formed anaerobically reduces the System II acceptors in the dark. The time course of this reduction is at least biphasic, indicative of inhomogeneity of the primary acceptor pool. Prolonged dark anaerobic treatment induces maximal initial fluorescence which decays rapidly in light and with a System I action spectrum. (3) Anaerobic treatment eventually results in deactivation of the oxidizing side of System II, limiting System II even when the acceptors are oxidized by System I pre-illumination.     

A major site of bicarbonate effect in system II reaction. Evidence from ESR signal IIvf, fast fluorescence yield changes and delayed light emission

In order to determine the major site of bicarbonate action in the electron transport complex of Photosystem II, the following experimental techniques were used: electron spin resonance measurements of Signal II_vf, measurements of chlorophyll a fluorescence yield rise and decay kinetics, and delayed light emission decay. From data obtained using these experimental techniques the following conclusions were made: (1) absence of bicarbonate causes a reversible inactivation of up to 40% of Photosystem II reaction center activity; (2) there is no significant effect of bicarbonate on electron flow from the charge accumulating S state to Z; (3) there is no significant effect of bicarbonate on electron flow from Z to P-680⁺; (4) electron flow from Q^? to the intersystem electron transport pool is inhibited by from 4- to 6-fold under bicarbonate depletion conditions.     

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.     

Lactoperoxidase-catalyzed iodination of chloroplast membranes. II. Evidence for surface localization of Photosystem II reaction centers

The enzyme lactoperoxidase was used to specifically iodinate the surface-exposed proteins of chloroplast lamellae. This treatment had two effects on Photosystem II activity. The first, occurring at low levels of iodination, resulted in a partial loss of the ability to reduce 2,6-dichlorophenolindophenol (DCIP), even in the presence of an electron donor for Photosystem II. There was a parallel loss of Photosystem II mediated variable yield fluorescence which could not be restored by dithionite treatment under anaerobic conditions. The same pattern of inhibition was observed in either glutaraldehyde-fixed or unfixed membranes. Analysis of the lifetime of fluorescence indicated that iodination changes the rate of deactivation of the excited state chlorophyll. We have concluded that iodination results in the introduction of iodine into the Photosystem II reaction center pigment-protein complex and thereby introduces a new quenching. The data indicate that the reaction center II is surface exposed.At higher levels of iodination, an inhibition of the electron transport reactions on the oxidizing side of Photosystem II was observed. That portion of the total rate of photoreduction of DCIP which was inhibited by this action could be restored by addition of an electron donor to Photosystem II. Loss of activity of the oxidizing side enzymes also resulted in a light-induced bleaching of chlorophyll a₆₈₀ and carotenoid pigments and a dampening of the sequence of O₂ evolution observed during flash irradiation of treated chloroplasts. All effects on electron transport on the oxidizing side of Photosystem II could be eliminated by glutaraldehyde fixation of the chloroplast lamellae prior to lactoperoxidase treatment. It is concluded that the electron carriers on the oxidizing side of Photosystem II are not surface localized; the functioning of these components is impaired by structural disorganization of the membrane occurring at high levels of iodination.Our data are in agreement with previously published schemes which suggest that Photosystem II mediated electron transport traverses the membrane.     

Photooxidation of ferrocyanide and iodide ions and associated phosphorylation in NH2OH-treated chloroplasts

NH₂OH-treated, non-water oxidizing chloroplasts are shown to be capable of oxidizing ferrocyanide and I^? via Photosystem II at appreciable rates (? 200 μequiv/h per mg chlorophyll). Using methylviologen as electron acceptor, ferrocyanide oxidation can be measured as O₂ uptake, as ferricyanide formation, or as H⁺ consumption (2 Fe²⁺ + 2H⁺ + O₂ → 2 Fe³⁺ + H₂O₂). I^? oxidation can be measured as methylviologen-mediated O₂ uptake, or spectrophotometrically, using ferricyanide as electron acceptor. The oxidation product I₂ is re-reduced, as it is formed, by unknown reducing substances in the reaction system.The rate-saturating concentrations of these donors are very high: 30 mM with ferricyanide and 15 mM with I^?. Relatively lipophilic Photosystem II donors such as catechol, benzidine and p-aminophenol saturate the photooxidation rate at much lower concentrations (< 0.5 mM). It thus seems that the oxidation of hydrophilic reductants such as ferricyanide and I^? is limited by permeability barriers. Very likely the site of Photosystem II oxidation is embedded in the thylakoid membrane or is situated on the inner surface of the membrane.The efficiency of phosphorylation (P/e₂) is 0.5 to 0.6 with ferrocyanide and about 0.5 with I^?. In contrast the P/e₂ ratio is 1.0 to 1.2 when water, catechol, p-aminophenol or benzidine serves as electron donor. These differences imply that only one of two phosphorylation sites operate when ferrocyanide and I^? are oxidized. Ferrocyanide and I^? are also chemically distinct from other Photosystem II donors in that their oxidation does not involve proton release. It is suggested that the mechanism of energy conservation associated with Photosystem II may be only operative when the removal of electrons from the donor results in release of protons (i.e. with water, hydroquinones, phenylamines, etc.).     

The effect of magnesium ions on action spectra for reactions mediated by Photosystems I and II in spinach chloroplasts

Action spectra were measured for positive changes in variable fluorescence (emission > 665 nm) excited by a beam of 485 nm chopped at 75 Hz. The action of two further beams was compared, one being variable, the other (reference) constant with respect to wavelength and intensity. Comparison was achieved by alternating the reference and the variable wavelength beams at 0.3 Hz and adjusting the intensity of the latter such as to cancel out any 0.3 Hz component in the 75 Hz fluorescence signal. The relative action then was obtained as the reciprocal of the intensity of the variable wavelength beam. Similarly, action spectra were measured for O₂ evolution with ferricyanide/p-phenylenediamine as electron acceptor, and for O₂ uptake mediated by methyl viologen with ascorbate 3-(p-chlorophenyl)-1,1-dimethylurea as electron donor in the presence of 2,6-dichlorophenolindophenol.Addition of 5 mM MgCl₂ increases the relative action around 480 nm for the change in variable fluorescence and p-phenylenediamine-dependent O₂ evolution, and decreases it for methyl viologen-mediated O₂ uptake with 2,6-dichlorophenolindophenol/ascorbate as electron donor in the presence of 3-(p-chlorophenyl-1,1-dimethylurea. The change in variable fluorescence and O₂ evolution are stimulated by MgCl₂, whereas O₂ uptake is inhibited by it.The results are discussed in terms of a model assuming a tripartite organization. of the photosynthetic pigments (Thornber, J. P. and Highkin, H. R. (1974) Eur. J. Biochem. 41, 109–116; Butler, W. L. and Kitajima, M. (1975) Biochim. Biophys. Acta 396, 72–85). MgCl₂ is thought to promote energy transfer to Photosystem II from a light-harvesting pigment complex serving both photosystems.     

Vanadium in photosynthesis of Chlorella fusca and higher plants

The influence of vanadium compounds (vanadate, vanadyl citrate) on photosynthesis in Chlorella fusca and in algal and spinach chloroplasts has been investigated. It was found that: 1. At moderately high concentrations (at least 0.1 mM) both vanadate and vanadyl citrate enhance photosynthetic O₂ production in intact C. fusca cells. At lower V concentration (about 2 μM) only vanadate stimulates photosynthesis. The increase is dependent on culture conditions and on light intensity. 2. Up to 1 mM V, neither vanadium compound influences PS II activity, either in intact cells or in algal or spinach chloroplasts. 3. The PS I reaction in algal and spinach chloroplasts is maximally enhanced (3-fold) in presence of vanadium (20 μM). The increase is independent of light intensity. 4. Cr(VI), Mo(VI), and W(VI) (1 mM) stimulate photosynthesis in intact C. fusca cells, but do not influence the photosystems of isolated chloroplasts. Vanadium is suggested to act as a redox catalyst in the electron transport from PS II to PS I.     

The kinetics of light-induced changes of C-550, cytochrome b559 and fluorescence yield in chloroplasts at low temperature

The kinetics of the photoreduction of C-550, the photooxidation of cytochrome b₅₅₉ and the fluorescence yield changes during irradiation of chloroplasts at ?196 °C were measured and compared. The photoreduction of C-550 proceeded more rapidly than the photooxidation of cytochrome b₅₅₉ and the fluorescence yield increase followed the cytochrome b₅₅₉ oxidation. These results suggest that fluorescence yield under these conditions indicates the dark reduction of the primary electron donor to Photosystem II, P680⁺, by cytochrome b₅₅₉ rather than the photoreduction of the primary electron acceptor.The photoreduction of C-550 showed little if any temperature dependence over the range of ?196 to ?100 °C. The amount of cytochrome b₅₅₉ photooxidized was sensitive to temperature decreasing from the maximal change at temperatures between ?196 to ?160 °C to no change at ?100 °C. To the extent that the reaction occurred at temperatures between ?160 and ?100 °C the rate was largely independent of temperature. The rate of the fluorescence increase was dependent on temperature over this range being 3–4 times more rapid at ?100 than at ?160 °C. At ?100 °C the light-induced fluorescence increase and the photoreduction of C-550 show similar kinetics. The temperature dependence of the fluorescence induction curve is attributed to the temperature dependence of the dark reduction of P680⁺.The intensity dependence of the photoreduction of C-550 and of the photooxidation of cytochrome b₅₅₉ are linear at low intensities (below 200 μW/cm²) but fall off at higher intensities. The failure of reciprocity in the photoreduction of C-550 at the higher intensities is not explained by the simple model proposed for the Photosystem II reaction centers.     

Oxidation-reduction properties of the electron acceptors of Photosystem II. II. Redox titration at various pH values of the flash-induced formation of C550 in Chlamydomonas Photosystem II particles lacking the secondary quinone electron acceptor

Redox titrations of the flash-induced formation of C550 (a linear indicator of Q^?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (E_m,7.5) = ? 30 mV) for redox couple $\text{Q}\text{Q}{}^{\mathrm{?}}$, which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (E_mm = + 10 mV) for this couple. The elevated pH-independent E_m suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q^?. The midpoint potentials of the centers having a pH-dependent E_m are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the E_m of $\text{Q}\text{Q}{}^{\mathrm{?}}$. An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q^?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q^?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph^?). At ?420 mV Ph^? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680⁺Ph^?.     

H2 metabolism in photosynthetic organisms. II. Light-dependent H2 evolution by preparations from Chlamydomonas, Scenedesmus and spinach.

Light-dependent H₂ evolution from dithiothreitol as electron donor was observed with cell-free preparations of anaerobically adapted $\text{Chlamydomonas reinhardii, Scenedesmus obliquus}$ and from spinach chloroplasts mixed with $\text{Chlamydomonas}$ hydrogenase. NADH substituted for dithiothreitol as electron donor only in the $\text{Chlarmydomonas}$ preparation. Dibromothymoquinone, an antagonist of plastoquinone, selectively inhibited H₂ photoevolution from NADH. These results are interpreted as indicating that 3-(3,4-dichlorophenyl)-1,1-dimethyl urea insensitive H₂ photoevolution by algae containing hydrogenase is due to the capability of NADH to reduce plastoquinone in the electron transport chain, and to evolve H₂ by a low redox potential carrier of photosystem I.     

Studies on the functional organization of Photosystem II. The effect of protein-modifying procedures and of ADRY agents on the reaction pattern of photosystem II in Tris-washed chloroplasts

The role of the protein matrix embedding the functionally active redox components of Photosystem II reaction centers has been studied by investigating the effects of procedures which modify the structure of proteins. In order to reduce the influence of the electron transport involving secondary donor and acceptor components, Triswashed chloroplasts were used which are completely deprived of their oxygen-evolving capacity. The functional activity was detected via absorption changes, reflecting at 334 and 690 or 834 nm the turnover of the primary plastoquinone acceptor, X320, and of the photochemically active chlorophyll a complex, Chl a_II, respectively, and at 520 nm the transient formation of a transmembrane electric potential gradient. Under repetitive flash excitation of Tris-washed chloroplasts it was found that: (a) The relaxation kinetics at 690 nm become significantly accelerated in the presence of external electron donors. (b) Trypsin treatment blocks to a high degree the turnover of Chl a_II and X320 unless exogenous acceptors are present, which directly oxidize X320^?, such as K₃Fe(CN)₆. (c) In the presence of K₃Fe(CN)₆ the recovery kinetics of Chl a_II and X320 are retarded markedly by trypsin, followed by a progressive decline in the extent thereof. (d) 2-(3-Chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene (ANT 2p), known to reduce the lifetime of S₂ and S₃ in normal chloroplasts, significantly accelerates the recovery of Chl a_II. 10 μs kinetics are observed which correspond with the electron-transfer rate from D₁ to Chl a⁺_II. ANT 2p simultaneously retards the decay kinetics of X320^? and of the electrochromic absorption changes. (e) The kinetic pattern of the electrochromic absorption changes is also affected by the salt content of the suspension. Under dark-adapted conditions, the 10 μs relaxation kinetics of the 834 nm absorption change due to the first flash are hardly affected by mild trypsinization of 5–10 min duration, whereas the amplitude decreases by approx. 30%. The data obtained in Tris-washed chloroplasts could consistently be interpreted as a modification of the back reaction between X320^? and Chl a⁺_II which is caused solely by a change in the reactivity of X320 due to trypsin-induced degradation of the native X320-B apoprotein. Furthermore, ADRY agents are inferred to stimulate cyclic electron flow, which leads to reduction of D⁺₁ between the flashes. A simplified scheme is discussed which describes the functional organization of the reaction center complex.     

Aerobic hydrogenase activity in Anacystis Nidulans The oxyhydrogen reaction

1. The oxyhydrogen reaction of Anacystis nidulans was studied manometrically and polarographically in whole cells and in cell-free preparations; the activity was found to be associated with the particulate fraction.2. Besides O₂, the isolated membranes reduced artificial electron acceptors of positive redox potential; the reactions were unaffected by O₂ levels <10–15%; aerobically the artificial acceptors were reduced simultaneously with O₂.3. H₂-supported O₂ uptake was inhibited by CO, KCN and 2-n-heptyl-8-hydroxyquinoline-N-oxide. Inhibition by CO was partly reversed by strong light. Uncouplers stimulated the oxyhydrogen reaction.4. The kinetic properties of O₂ uptake by isolated membranes were the same in presence of H₂ and of other respiratory substrates.5. Low rates of H₂ evolution by the membrane preparations were found in presence of dithionite; methyl viologen stimulated the reaction.6. The results indicate that under certain growth conditions Anacystis synthesizes a membrane-bound hydrogenase which appears to be involved in phosphorylative electron flow from H₂ to O₂ through the respiratory chain.     

Redox and ATP control of photosynthetic cyclic electron flow in Chlamydomonas reinhardtii: (II) Involvement of the PGR5–PGRL1 pathway under anaerobic conditions

In oxygenic photosynthesis, cyclic electron flow around photosystem I denotes the recycling of electrons from stromal electron carriers (reduced nicotinamide adenine dinucleotide phosphate, NADPH, ferredoxin) towards the plastoquinone pool. Whether or not cyclic electron flow operates similarly in Chlamydomonas and plants has been a matter of debate. Here we would like to emphasize that despite the regulatory or metabolic differences that may exist between green algae and plants, the general mechanism of cyclic electron flow seems conserved across species. The most accurate way to describe cyclic electron flow remains to be a redox equilibration model, while the supramolecular reorganization of the thylakoid membrane (state transitions) has little impact on the maximal rate of cyclic electron flow. The maximum capacity of the cyclic pathways is shown to be around 60 electrons transferred per photosystem per second, which is in Chlamydomonas cells treated with 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and placed under anoxic conditions. Part I of this work (aerobic conditions) was published in a previous issue of BBA-Bioenergetics (vol. 1797, pp. 44–51) (Alric et al., 2010).