Why Gratzel’s cell works so well

In Gratzel’s cell, the electrons injected by the photo-excitation of dye molecules, anchored to a mesoporous TiO₂ film, efficiently diffuse to the back contact achieving solar energy conversion at efficiencies exceeding 10%. The mesoporous TiO₂ surface constituted of randomly arranged nanocrystallites with a roughness factor of the order 1000 is heavily populated with traps, defects and adsorbed species which act as recombination centers. Nevertheless, the cell functions, mitigating recombination expected to occur via the interaction electrons at the surface. Evidence based mainly on 1/f noise measurements is presented to show that dye bonded to the TiO₂ surface passivates recombination centers. Furthermore the suppression of trapping-detrapping events at the surface increases the diffusion coefficient of the electrons through the nanocrystalline matrix facilitating electron transport to the back contact. The Gratzel cell is also unique, none of the high bandgap oxide materials other than TiO₂ yield energy conversion and quantum efficiencies as high as that of the cells based on TiO₂. 1/f noise measurements also reveal a distinct difference between TiO₂ and ZnO mesoporous films suggesting that the films made from the latter material are more intensely populated with surface states that mediate recombination.     

p‐Type CuI Islands on TiO2 Electron Transport Layer for a Highly Efficient Planar‐Perovskite Solar Cell with Negligible Hysteresis

Compact TiO₂ is widely used as an electron transport material in planar‐perovskite solar cells. However, TiO₂‐based planar‐perovskite solar cells exhibit low efficiencies due to intrinsic problems such as the unsuitable conduction band energy and low electron extraction ability of TiO₂. Herein, the planar TiO₂ electron transport layer (ETL) of perovskite solar cells is modified with ionic salt CuI via a simple one‐step spin‐coating process. The p‐type nature of the CuI islands on the TiO₂ surface leads to modification of the TiO₂ band alignment, resulting in barrier‐free contacts and increased open‐circuit voltage. It is found that the polarity of the CuI‐modified TiO₂ surface can pull electrons to the interface between the perovskite and the TiO₂, which improves electron extraction and reduces nonradiative recombination. The CuI solution concentration is varied to control the electron extraction of the modified TiO₂ ETL, and the optimized device shows a high efficiency of 19.0%. In addition, the optimized device shows negligible hysteresis, which is believed to be due to the removal of trap sites and effective electron extraction by CuI‐modified TiO₂. These results demonstrate the hitherto unknown effect of p‐type ionic salts on electron transport material.     

Anomalous Charge Transport: A New ��Time Domain�� Generalization of the Drude Model

We propose a new generalization of the Drude model for describing the conductors in nanostructured form. The new feature that appeared is the assumption of a collective mode at a finite frequency. This leads to the occurrence of anomalous charge transport. With appropriate scattering times it is possible to mimic the infrared properties of oxides and poor conductors in the form of nanoparticles and nanowires. The electron current in these cases reverses its direction before decaying to zero. Specific examples considered are ZnO, TiO₂, GaAs, Si, SWCN. The model also predicts that the current will have a damped oscillation in time, a possible new effect detectable with femtosecond time resolved techniques. Results are presented for the velocity correlation functions, the mean square deviation of position and the diffusion coefficient.     

The steady-state transport of oxygen through hemoglobin solutions

The steady-state transport of oxygen through hemoglobin solutions was studied to identify the mechanism of the diffusion augmentation observed at low oxygen tensions. A novel technique employing a platinum-silver oxygen electrode was developed to measure the effective diffusion coefficient of oxygen in steady-state transport. The measurements were made over a wider range of hemoglobin and oxygen concentrations than previously reported. Values of the Brownian motion diffusion coefficient of oxygen in hemoglobin solution were obtained as well as measurements of facilitated transport at low oxygen tensions. Transport rates up to ten times greater than ordinary diffusion rates were found. Predictions of oxygen flux were made assuming that the oxyhemoglobin transport coefficient was equal to the Brownian motion diffusivity which was measured in a separate set of experiments. The close correlation between prediction and experiment indicates that the diffusion of oxyhemoglobin is the mechanism by which steady-state oxygen transport is facilitated.     

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.     

Time dependent theory for vesicular transport across vascular endothelium

The steady-state hydro-electrodynamic model of Weinbaum & Caro (1976) for the vesicular transport of macromolecules across endothelial cell layers is extended to unsteady quasi one-dimensional vesicle diffusion and applied to existing time-dependent vesicle labelling studies. The model accounts for the spatially varying hydrodynamic and van der Waals force interaction between the vesicle and the boundary plasmalemma membranes of the endothelial cell. The present study also shows how Rubin's (1977) combined diffusion-reaction kinetics model for steady state permeability can be generalized to include an arbitrary spatial variation of the diffusion coefficient for a variable hydrodynamic resistance law in the cell interior. Approximate time-dependent solutions, valid for all times, are obtained using a novel integral solution technique for a multilayered media with variable diffusivity. Dimensionless time-dependent concentration profiles and flux ratios and dimensionless steady state permeability coefficients are generated independent of the cytoplasmic viscosity. When the time-dependent tracer data of Casley-Smith and Chin (1971) is used, the cytoplasmic viscosity is estimated to be 8·95 P.     

Osmosis in Cortical Collecting Tubules : A Theoretical and Experimental Analysis of the Osmotic Transient Phenomenon

This paper reports a theoretical analysis of osmotic transients and an experimental evaluation both of rapid time resolution of lumen to bath osmosis and of bidirectional steady-state osmosis in isolated rabbit cortical collecting tubules exposed to antidiuretic hormone (ADH). For the case of a membrane in series with unstirred layers, there may be considerable differences between initial and steady-state osmotic flows (i.e., the osmotic transient phenomenon), because the solute concentrations at the interfaces between membrane and unstirred layers may vary with time. A numerical solution of the equation of continuity provided a means for computing these time-dependent values, and, accordingly, the variation of osmotic flow with time for a given set of parameters including: P_f (cm s^–1), the osmotic water permeability coefficient, the bulk phase solute concentrations, the unstirred layer thickness on either side of the membrane, and the fractional areas available for volume flow in the unstirred layers. The analyses provide a quantitative frame of reference for evaluating osmotic transients observed in epithelia in series with asymmetrical unstirred layers and indicate that, for such epithelia, P_f determinations from steady-state osmotic flows may result in gross underestimates of osmotic water permeability. In earlier studies, we suggested that the discrepancy between the ADH-dependent values of P_f and P_DDw (cm s^–1, diffusional water permeability coefficient) was the consequence of cellular constraints to diffusion. In the present experiments, no transients were detectable 20–30 s after initiating ADH-dependent lumen to bath osmosis; and steady-state ADH-dependent osmotic flows from bath to lumen and lumen to bath were linear and symmetrical. An evaluation of these data in terms of the analytical model indicates: First, cellular constraints to diffusion in cortical collecting tubules could be rationalized in terms of a 25-fold reduction in the area of the cell layer available for water transport, possibly due in part to transcellular shunting of osmotic flow; and second, such cellular constraints resulted in relatively small, approximately 15%, underestimates of P_f.     

Dark-interval relaxation kinetics (DIRK) of absorbance changes as a quantitative probe of steady-state electron transfer

We introduce a new, non-invasive technique to measure linear electron transfer in intact leaves under steady-state illumination. Dark-interval relaxation kinetic or ‘DIRK’ analysis is based on measurements of the initial rates of relaxation of steady-state absorbance signals upon a rapid light-dark transition. We show that estimates of electron flux by DIRK analysis of absorbance signals, reflecting redox changes in the photosynthetic electron transfer chain, can yield quantitative information about photosynthetic flux when the light-dependent partitioning of electrons among redox components of the electron transfer chain are considered. This concept is modeled in computer simulations and then demonstrated in vivo with tobacco plants under non-photorespiratory conditions resulting in linear relationships between DIRK analysis and gross carbon assimilation (A_G). Estimation based on DIRK analysis of the number of electrons transferred through the photosynthetic apparatus for each CO₂ fixed was within 20% of the theoretical value. Possible errors and future improvements are discussed. We conclude that the DIRK method represents a useful tool to address issues such as plant stress and photosynthetic regulation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of Nanoanatase TiO2 on Photosynthesis of Spinach Chloroplasts Under Different Light Illumination

With a photocatalyzed characteristic, nanoanatase TiO₂ under light could cause an oxidation–reduction reaction. Our studies had proved that nano-TiO₂ could promote photosynthesis and greatly improve spinach growth. However, the mechanism of nano-TiO₂ on promoting conversion from light energy to electron energy and from electron energy to active chemistry energy remains largely unclear. In this study, we report that the electron transfer, oxygen evolution, and photophosphorylation of chloroplast (Chl) from nanoanatase-TiO₂-treated spinach were greatly increased under visible light and ultraviolet light illumination. It was demonstrated that nanoanatase TiO₂ could greatly improve whole chain electron transport, photoreduction activity of photosystem II, O₂-evolving and photophosphorylation activity of spinach Chl not only under visible light, but also energy-enriched electron from nanoanatase TiO₂, which entered Chl under ultraviolet light and was transferred in photosynthetic electron transport chain and made NADP⁺ be reduced into NADPH, and coupled to photophosphorylation and made electron energy be transformed to ATP. Moreover, nanoanatase h⁺, which photogenerated electron holes, captured an electron from water, which accelerated water photolysis and O₂ evolution.     

Atomic‐Layer‐Deposited Aluminum and Zirconium Oxides for Surface Passivation of TiO2 in High‐Efficiency Organic Photovoltaics

The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high‐efficiency solar cells. Here a strategy to passivate surface trap states of TiO₂ films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al₂O₃) or zirconia (ZrO₂) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO₂ are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open‐circuit voltage and the short‐circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO₂ surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO₂ electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO₂/ALD metal oxide/organic interface.     

In-vivo quantification of electron flow through photosystem I – Cyclic electron transport makes up about 35% in a cyanobacterium

Photosynthetic electron flow, driven by photosystem I and II, provides chemical energy for carbon fixation. In addition to a linear mode a second cyclic route exists, which only involves photosystem I. The exact contributions of linear and cyclic transport are still a matter of debate. Here, we describe the development of a method that allows quantification of electron flow in absolute terms through photosystem I in a photosynthetic organism for the first time. Specific in-vivo protocols allowed to discern the redox states of plastocyanin, P700 and the FeS-clusters including ferredoxin at the acceptor site of PSI in the cyanobacterium Synechocystis sp. PCC 6803 with the near-infrared spectrometer Dual-KLAS/NIR. P700 absorbance changes determined with the Dual-KLAS/NIR correlated linearly with direct determinations of PSI concentrations using EPR. Dark-interval relaxation kinetics measurements (DIRK_PSI) were applied to determine electron flow through PSI. Counting electrons from hydrogen oxidation as electron donor to photosystem I in parallel to DIRK_PSI measurements confirmed the validity of the method. Electron flow determination by classical PSI yield measurements overestimates electron flow at low light intensities and saturates earlier compared to DIRK_PSI. Combination of DIRK_PSI with oxygen evolution measurements yielded a proportion of 35% of surplus electrons passing PSI compared to PSII. We attribute these electrons to cyclic electron transport, which is twice as high as assumed for plants. Counting electrons flowing through the photosystems allowed determination of the number of quanta required for photosynthesis to 11 per oxygen produced, which is close to published values.     

Microscopic diffusion and hydrodynamic interactions of hemoglobin in red blood cells

The cytoplasm of red blood cells is congested with the oxygen storage protein hemoglobin occupying a quarter of the cell volume. The high protein concentration leads to a reduced mobility; the self-diffusion coefficient of hemoglobin in blood cells is six times lower than in dilute solution. This effect is generally assigned to excluded volume effects in crowded media. However, the collective or gradient diffusion coefficient of hemoglobin is only weakly dependent on concentration, suggesting the compensation of osmotic and friction forces. This would exclude hydrodynamic interactions, which are of dynamic origin and do not contribute to the osmotic pressure. Hydrodynamic coupling between protein molecules is dominant at short time- and length scales before direct interactions are fully established. Employing neutron spin-echo-spectroscopy, we study hemoglobin diffusion on a nanosecond timescale and protein displacements on the scale of a few nanometers. A time- and wave-vector dependent diffusion coefficient is found, suggesting the crossover of self- and collective diffusion. Moreover, a wave-vector dependent friction function is derived, which is a characteristic feature of hydrodynamic interactions. The wave-vector and concentration dependence of the long-time self-diffusion coefficient of hemoglobin agree qualitatively with theoretical results on hydrodynamics in hard spheres suspensions. Quantitative agreement requires us to adjust the volume fraction by including part of the hydration shell: Proteins exhibit a larger surface/volume ratio compared to standard colloids of much larger size. It is concluded that hydrodynamic and not direct interactions dominate long-range molecular transport at high concentration.     

Promotion of Energy Transfer and Oxygen Evolution in Spinach Photosystem II by Nano-anatase TiO<Subscript>2</Subscript>

Being a proven photocatalyst, nano-anatase is capable of undergoing electron transfer reactions under light. In previous studies we had proven that nano-anatase improved photosynthesis and greatly promoted spinach growth. The mechanisms by which nano-anatase promotes energy transfer and the conversion efficiency of the process are still not clearly understood. In the present paper, we report the results obtained with the photosystem II (PSII) isolated from spinach and treated by nano-anatase TiO₂ and studied the effect of nano-anatase TiO₂ on energy transfer in PSII by spectroscopy and on oxygen evolution. The results showed that nano-anatase TiO₂ treatment at a suitable concentration could significantly change PSII microenvironment and increase absorbance for visible light, improve energy transfer among amino acids within PSII protein complex, and accelerate energy transport from tyrosine residue to chlorophyll a. The photochemical activity of PSII (fluorescence quantum yield) and its oxygen-evolving rate were enhanced by nano-anatase TiO₂. This is viewed as evidence that nano-anatase TiO₂ can promote energy transfer and oxygen evolution in PSII of spinach.     

Brownian Dynamics Simulations of Colloidal Liquids: Hydrodynamics and Stress Relaxation

Abstract

We describe the statistical mechanics background and additional algorithmic features of a recently proposed simple mean-field Brownian Dynamics algorithm formulated to include many-body hydrodynamics, using a local density approximation for the friction coefficient. We show that the equations of motion satisfy the incompressibility of phase space. We make further developments to the model, computing the hydrodynamic effects on the shear stress relaxation function. We show that stress relaxation takes place over two well-defined regimes, in both cases with and without mean field hydrodynamics, MFH. At short times ta ²/D ₀ < 10^?3, where a is the radius of the colloidal particle and D ₀ is the self-diffusion coefficient at infinite dilution, decay of the stress autocorrelation function, C_s(t) is essentially independent of volume fraction and does not fit to a simple analytic form. At longer times than ta ²/D ₀ < 10^?2 the decay has the fractional exponential form ～exp(-t ^β) with β ? 1. The transition between these two regimes coincides with a rapid fall in the time-dependent diffusion coefficient from the so-called short-time to long-time values. We do not find any evidence for power law decay in the C_s(t) as predicted by recent mode-coupling based analytical expansions.     

Electron and nuclear dynamics in many-electron atoms, molecules and chlorophyll-protein complexes: A review

It has been shown [V.A. Shuvalov, Quantum dynamics of electrons in many-electron atoms of biologically important compounds, Biochemistry (Mosc.) 68 (2003) 1333-1354; V.A. Shuvalov, Quantum dynamics of electrons in atoms of biologically important molecules, Uspekhi biologicheskoi khimii, (Pushchino) 44 (2004) 79-108] that the orbit angular momentum L of each electron in many-electron atoms is L = mVr = n? and similar to L for one-electron atom suggested by N. Bohr. It has been found that for an atom with N electrons the total electron energy equation E = −(Z^eff)²e⁴m/(2n²?²N) is more appropriate for energy calculation than standard quantum mechanical expressions. It means that the value of L of each electron is independent of the presence of other electrons in an atom and correlates well to the properties of virtual photons emitted by the nucleus and creating a trap for electrons. The energies for elements of the 1st up to the 5th rows and their ions (total amount 240) of Mendeleev' Periodical table were calculated consistent with the experimental data (deviations in average were 5 × 10^− 3). The obtained equations can be used for electron dynamics calculations in molecules. For H₂ and H₂⁺ the interference of electron-photon orbits between the atoms determines the distances between the nuclei which are in agreement with the experimental values. The formation of resonance electron-photon orbit in molecules with the conjugated bonds, including chlorophyll-like molecules, appears to form a resonance trap for an electron with E values close to experimental data. Two mechanisms were suggested for non-barrier primary charge separation in reaction centers (RCs) of photosynthetic bacteria and green plants by using the idea of electron-photon orbit interference between the two molecules. Both mechanisms are connected to formation of the exciplexes of chlorophyll-like molecules. The first one includes some nuclear motion before exciplex formation, the second one is related to the optical transition to a charge transfer state.     

The phosphorylation site associated with the oxidation of exogenous donors of electrons to Photosystem I

1. The Photosystem I-mediated transfer of electrons from diaminodurene, diaminotoluene and reduced 2,6-dichlorophenolindophenol to methylviologen is optimal at pH 8–8.5, where phosphorylation is also maximal. In the presence of superoxide dismutase, the efficiency of phosphorylation rises from ? 0.1 at pH 6.5 to 0.6–0.7 at pH 8–8.5, regardless of the exogenous electron donor used.2. The apparent K_m (at pH 8.1) for diaminodurene is 6·10^?4 M and for diaminotoluene is 1.2·10^?3 M. The concentrations of diaminodurene and diaminotoluene required to saturate the electron transport processes are > 2 mM and > 5 mM, respectively. At these higher electron donor concentrations the rates of electron transport are markedly increased by phosphorylation (1.5-fold) or by uncoupling conditions (2-fold).3. Kinetic analysis of the transfer of electrons from reduced 2,6-dichlorophenolindophenol (DCIPH₂) to methylviologen indicates that two reactions with very different apparent K_m values for DCIPH₂ are involved. The rates of electron flux through both pathways are increased by phosphorylation or uncoupling conditions although only one of the pathways is coupled to ATP formation. No similar complications are observed when diaminodurene or diaminotoluene serves as the electron donor.4. In the diaminodurene → methylviologen reaction, ATP formation and that part of the electron transport dependent upon ATP formation are partially inhibited by the energy transfer inhibitor HgCl₂. This partial inhibition of ATP formation rises to about 50% at less than 1 atom of mercury per 20 molecules of chlorophyll, then does not further increase until very much higher levels of mercury are added.5. It is suggested that exogenous electron donors such as diaminodurene, diaminotoluene and DCIPH₂ can substitute for an endogenous electron carrier in donating electrons to cytochrome f via the mercury-sensitive coupling site (Site I) located on the main electron-transporting chain. If this is so, there would seem to be no reason for postulating yet another coupling site on a side branch of the electron transport chain in order to account for cyclic photophosphorylation.     

Regulation of photosynthetic electron transport and photophosphorylation in intact chloroplasts and leaves of Spinacia oleracea L.

Oxygen ist reduced by the electron transport chain of chloroplasts during CO₂ reduction. The rate of electron flow to oxygen is low. Since antimycin A inhibited CO₂-dependent oxygen evolution, it is concluded that cyclic photophosphorylation contributes ATP to photosynthesis in chloroplasts which cannot satisfy the ATP requirement of CO₂ reduction by electron flow to NADP and to oxygen. Inhibition of photosynthesis by antimycin A was more significant at high than at low light intensities suggesting that cyclic photophosphorylation contributes to photosynthesis particularly at high intensities. Cyclic electron flow in intact chloroplasts is under the control of electron acceptors. At low light intensities or under far-red illumination it is decreased by substrates which accept electrons from photosystem I such as oxaloacetate, nitrite or oxygen. Obviously, the cyclic electron transport pathway is sensitive to electron drainage. In the absence of electron acceptors, cyclic electron flow is supported by far-red illumination and inhibited by red light. The inhibition by light exciting photosystem II demonstrated that the cyclic electron transport pathway is accessible to electrons from photosystem II. Inhibition can be relieved by oxygen which appears to prevent over-reduction of electron carriers of the cyclic pathway and thus has an important regulatory function. The data show that cyclic electron transport is under delicate redox control. Inhibition is caused both by excessive oxidation and by over-reduction of electron carriers of the pathway.     

Measurements of mesophyll conductance,photosynthetic electron transport and alternative electron sinks of field grown wheat leaves

Photosynthetic electron transport drives the carbon reduction cycle, the carbon oxidation cycle, and any alternative electron sinks such as nitrogen reduction. A chlorophyll fluorescence— based method allows estimation of the total electron transport rate while a gas-exchange-based method can provide estimates of the electron transport needed for the carbon reduction cycle and, if the CO₂ partial pressure inside the chloroplast is accurately known, for the carbon oxidation cycle. The gas-exchange method cannot provide estimates of alternative electron sinks. Photosynthetic electron transport in flag leaves of wheat was estimated by the fluorescence method and gasexchange method to determine the possible magnitude of alternative electron sinks. Under non-photorespiratory conditions the two measures of electron transport were the same, ruling out substantial alternative electron sinks. Under photorespiratory conditions the fluorescence-based electron transport rate could be accounted for by the carbon reduction and carbon oxidation cycle only if we assumed the CO₂ partial pressure inside the chloroplasts to be lower than that in the intercellular spaces of the leaves. To further test for the presence of alternative electron sinks, carbon metabolism was inhibited by feeding glyceraldehyde. As carbon metabolism was inhibited, the electron transport was inhibited to the same degree. A small residual rate of electron transport was measured when carbon metabolism was completely inhibited which we take to be the maximum capacity of alternative electron sinks. Since the alternative sinks were small enough to ignore, the comparison of fluorescence and gas-exchange based methods for measuring the rate of electron transport could be used to estimate the mesophyll conductance to CO₂ diffusion. The mesophyll conductance estimated this way fell as wheat flag leaves senesced. The age-related decline in photosynthesis may be attributed in part to the reduction of mesophyll conductance to CO₂ diffusion and in part to the estimated decline of ribulose 1,5-bisphosphate carboxylase amount.     

Alternative photosynthetic electron flow to oxygen in marine Synechococcus

Cyanobacteria dominate the world's oceans where iron is often barely detectable. One manifestation of low iron adaptation in the oligotrophic marine environment is a decrease in levels of iron-rich photosynthetic components, including the reaction center of photosystem I and the cytochrome b₆f complex [R.F. Strzepek and P.J. Harrison, Photosynthetic architecture differs in coastal and oceanic diatoms, Nature 431 (2004) 689-692.]. These thylakoid membrane components have well characterised roles in linear and cyclic photosynthetic electron transport and their low abundance creates potential impediments to photosynthetic function. Here we show that the marine cyanobacterium Synechococcus WH8102 exhibits significant alternative electron flow to O₂, a potential adaptation to the low iron environment in oligotrophic oceans. This alternative electron flow appears to extract electrons from the intersystem electron transport chain, prior to photosystem I. Inhibitor studies demonstrate that a propyl gallate-sensitive oxidase mediates this flow of electrons to oxygen, which in turn alleviates excessive photosystem II excitation pressure that can often occur even at relatively low irradiance. These findings are also discussed in the context of satisfying the energetic requirements of the cell when photosystem I abundance is low.     

Salicylhydroxamic acid (SHAM) inhibits O<Subscript>2</Subscript> photoreduction which protects nitrogenase activity in the cyanobacterium <Emphasis Type="Italic">Synechococcus</Emphasis> sp. RF-1

Synechococcus sp. RF-1, a unicellular N₂-fixing cyanobacterium, can grow photosynthetically and diazotrophically in continuous light. How the organism protects its nitrogenase from damage by oxygen is unclear. In cyanobacerial cells, electron transport carriers associated with photosynthesis and respiration are all on the thylakoid membranes and share some common components, including plastoquinone pool and cytochrome b ₆ f complex, and the pathways are interacting with each other. In this work, a pulse amplitude modulation (PAM) fluorometer (PAM-101) and an O₂ electrode are used simultaneously to study the chlorophyll a fluorescence and to monitor O₂ exchanges in Synechococcus sp. RF-1 cells. At the CO₂ compensation point, the photochemical quenching activity remained high unless the O₂ was exhausted by the glucose oxidase system (GOS). It indicates that in addition to CO₂, O₂ can also act as electron acceptor to receive electrons derived from Q_A. Studies with various inhibitors of the electron transport chain demonstrated that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and salicylhydroxamic acid (SHAM) inhibited the photoreduction of O₂, while glycolaldehyde, disalicylidenepropanediamine (DSPD), methyl viologen (MV) and KCN did not. These results imply that a KCN-resistant and SHAM-sensitive oxidase transfers electrons generated from Photosystem II to O₂ between cytochrome b ₆ f complex and ferredoxin. When SHAM blocked this alternative electron transport pathway, the dinitrogen-fixing activity decreased significantly. The results indicate that a novel oxidase may function as an intracellular O₂-scavenger in Synechococcus sp. RF-1 cells. This revised version was published online in June 2006 with corrections to the Cover Date.