Light-dependent Electron Transport Research Articles

Net O2 exchange rates under dark and light conditions across different stem compartments.

Woody plants display some photosynthetic activity in stems, but the biological role of stem photosynthesis and the specific contributions of bark and wood to carbon uptake and oxygen evolution remain poorly understood. We aimed to elucidate the functional characteristics of chloroplasts in stems of different ages in Fraxinus ornus. Our investigation employed diverse experimental approaches, including microsensor technology to assess oxygen production rates in whole stem, bark, and wood separately. Additionally, we utilized fluorescence lifetime imaging microscopy (FLIM) to characterize the relative abundance of photosystems I and II (PSI : PSII chlorophyll ratio) in bark and wood. Our findings revealed light-induced increases in O2 production in whole stem, bark, and wood. We present the radial profile of O2 production in F. ornus stems, demonstrating the capability of stem chloroplasts to perform light-dependent electron transport. Younger stems exhibited higher light-induced O2 production and dark respiration rates than older ones. While bark emerged as the primary contributor to net O2 production under light conditions, our data underscored that wood chloroplasts are also photosynthetically active. The FLIM analysis unveiled a lower PSI abundance in wood than in bark, suggesting stem chloroplasts are not only active but also acclimate to the spectral composition of light reaching inner compartments.

Structure and function of bark and wood chloroplasts in a drought-tolerant tree (Fraxinus ornus L.).

Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level.

Cotton physiological and agronomic response to nitrogen application rate

Recently published information on cotton agronomic and photosynthetic component responses to nitrogen (N) application rate is limited for the southeastern US. This experiment was conducted to 1) evaluate cotton yield, fiber quality, growth, and maturity responses to N application rate and 2) quantify associations between photosynthetic component processes and N-induced variation in net photosynthesis. A two-year field experiment was conducted during the 2015 and 2016 growing seasons at a field site in Tifton, GA and evaluated cotton agronomic and physiological response to six N fertilizer application rates ranging from 0 to 168 kg N ha−1. Cotton lint yield was 42–51% lower in 0 N treatments than all other fertilized treatments in both years of the study, and N rates that substantially limited yield decreased final plant height, reduced mainstem node development, and substantially hastened maturity. Date of cutout ranged from 73 to 91 DAP for all N rates and years of the study. Photosynthetic rates were also responsive to N rate on later sample dates (July and August) for both growing seasons. Although declines in chlorophyll content, specific leaf nitrogen, and stomatal conductance were observed for low N plants in some instances, none of these parameters were strongly associated with net photosynthesis overall. In contrast, light-dependent electron transport rate (ETR) was strongly correlated with N-induced variation in net photosynthesis. Estimates of ETR may need to be included along with SLN estimates in future efforts to scale from leaf to canopy level photosynthetic performance in cotton.

A Genome-Scale Metabolic Model of Anabaena 33047 to Guide Genetic Modifications to Overproduce Nylon Monomers.

Nitrogen fixing-cyanobacteria can significantly improve the economic feasibility of cyanobacterial production processes by eliminating the requirement for reduced nitrogen. Anabaena sp. ATCC 33047 is a marine, heterocyst forming, nitrogen fixing cyanobacteria with a very short doubling time of 3.8 h. We developed a comprehensive genome-scale metabolic (GSM) model, iAnC892, for this organism using annotations and content obtained from multiple databases. iAnC892 describes both the vegetative and heterocyst cell types found in the filaments of Anabaena sp. ATCC 33047. iAnC892 includes 953 unique reactions and accounts for the annotation of 892 genes. Comparison of iAnC892 reaction content with the GSM of Anabaena sp. PCC 7120 revealed that there are 109 reactions including uptake hydrogenase, pyruvate decarboxylase, and pyruvate-formate lyase unique to iAnC892. iAnC892 enabled the analysis of energy production pathways in the heterocyst by allowing the cell specific deactivation of light dependent electron transport chain and glucose-6-phosphate metabolizing pathways. The analysis revealed the importance of light dependent electron transport in generating ATP and NADPH at the required ratio for optimal N2 fixation. When used alongside the strain design algorithm, OptForce, iAnC892 recapitulated several of the experimentally successful genetic intervention strategies that over produced valerolactam and caprolactam precursors.

Diurnal regulation of photosynthetic light absorption, electron transport and carbon fixation in two contrasting oceanic environments

Abstract. Understanding the dynamics of marine phytoplankton productivity requires mechanistic insight into the non-linear coupling of light absorption, photosynthetic electron transport and carbon fixation in response to environmental variability. In the present study, we examined the variability of phytoplankton light absorption characteristics, light-dependent electron transport and 14C-uptake rates over a 48 h period in the coastal subarctic north-east (NE) Pacific. We observed an intricately coordinated response of the different components of the photosynthetic process to diurnal irradiance cycles, which acted to maximize carbon fixation, while simultaneously preventing damage by excess absorbed light energy. In particular, we found diurnal adjustments in pigment ratios, excitation energy transfer to reaction centre II (RCII), the capacity for non-photochemical quenching (NPQ), and the light efficiency (α) and maximum rates (Pmax) of RCII electron transport (ETRRCII) and 14C uptake. Comparison of these results from coastal waters to previous observations in offshore waters of the subarctic NE Pacific provides insight into the effects of iron limitation on the optimization of photosynthesis. Under iron-limited, low-biomass conditions, there was a significant reduction of iron-rich photosynthetic units per chlorophyll a, which was partly offset by higher light absorption and electron transport per photosystem II (PSII). Iron deficiency limited the capacity of phytoplankton to utilize peak midday irradiance for carbon fixation and caused an upregulation of photoprotective mechanisms, including NPQ, and the decoupling of light absorption, electron transport and carbon fixation. Such decoupling resulted in an increased electron requirement (Φe,C) and decreased quantum efficiency (ΦC) of carbon fixation at the iron-limited station. In both coastal and offshore waters, Φe,C and ΦC correlated strongly to NPQ, albeit with a significantly different slope. We discuss the implications of our results for the interpretation of bio-optical data and the parameterization of numerical productivity models, both of which are vital tools in monitoring marine photosynthesis over large temporal and spatial scales.

The electron transfer pathway upon H2 oxidation by the NiFe bidirectional hydrogenase of Synechocystis sp. PCC 6803 in the light shares components with the photosynthetic electron transfer chain in thylakoid membranes

In anaerobic conditions the NiFe hydrogenase in the cyanobacterium Synechocystis sp. PCC 6803 catalyzes transient H2 production upon a darkness-to-light transition, followed by a rapid H2 uptake. We measured H2 uptake in Synechocystis mutants lacking photosystem I, photosystem II or terminal oxidases and in the wild-type strain with and without active cytochrome b6f. Rapid light-induced H2 uptake was dependent on cytochrome b6f and the presence of photosystem I. We propose light-dependent electron transport from H2 to plastoquinone, probably via NAD(P)H dehydrogenase, and on to cytochrome b6f and photosystem I. In darkness H2 uptake is ∼10-fold slower than in the light and is independent of thylakoid redox components. The plastoquinone redox state may be key in determining the ultimate H2 redox partner. H2 uptake and production in darkness likely use the same redox partners. NADH and NADPH, but not reduced ferredoxin, were confirmed as hydrogenase redox donors in vitro.

Oxygen-evolving diatom thylakoid membranes

Two protocols were developed that yielded purified oxygen-evolving thylakoid membranes from the diatom Cylindrotheca fusiformis. One protocol employed sonication, while the second involved French press lysis of protoplasts formed by brief culture of cells in a cation-depleted medium. Regardless of the method of cell breakage, some damage to electron transport components occurred. For preservation of both light-dependent electron transport activity and in vivo fluorescence properties, 2 M sorbitol proved to be more effective than 1 M sorbitol, regardless of the method used for cell lysis. Thylakoids purified in 2 M sorbitol using the protoplast/French press method showed the best preservation of in vivo fluorescence emission signals and Photosystem II activity with ferricyanide was completely inhibited by DCMU. Thylakoids purified in 2 M sorbitol using sonication had higher rates of Photosystem II activity with ferricyanide, but this activity was less sensitive to DCMU. Whole-chain electron transport activity was low in all preparations. This is the first report of O 2 evolution and of long-wavelength fluorescence in purified thylakoids of any chromophytic alga.

Oxygen-dependent electron transport and protection from photoinhibition in leaves of tropical tree species.

The roles of photorespiration and the Mehlerperoxidase pathway in sustaining electron transport and protection from photoinhibition were studied in outer canopy leaves of two species of tropical trees: the drought-deciduous Pseudobombax septenatum (Jacq.) Dug. and the evergreen Ficus insipida Willd. Ficus had a higher photosynthetic capacity than Pseudobombax and also a greater capacity for light-dependent electron transport under photorespiratory conditions (in the absence of CO2). As a consequence, in the absence of CO2, Ficus was able to maintain a largely oxidized electron-transport chain at higher photon flux densities than Pseudobombax. Under the same light conditions, photoinhibition (reduction in Fv/Fm) was always greater in Pseudobombax than Ficus, was increased when leaves were exposed to 2% O2 in nitrogen compared to 21% O2 in CO2-free air, but was not increased by the absence of CO2. Rates of electron transport due to the Mehler-peroxidase pathway (assessed in 2% O2 in nitrogen) ranged between 16-40 μmol · m-2·s-1 in both species. As the dry season approached and Pseudobombax neared leaf senescence there was a decline in the capacity for photorespiratory flux to maintain electron transport in Pseudobombax, but not in Ficus. Ratios of light-dependent electron transport to net CO2 fixation for Pseudobombax, Ficus and two other species in the field, Luehea seemannii Tr. & Planch, and Didymopanax morototoni (Aubl.) Dec. & Planch., ranged from 6.2 (Ficus) to 16.7 (Pseudobombax). High in-situ rates of photorespiration combined with the decreased capacity of Pseudobombax for photorespiratory flux as the dry season approached indicates a decreased capacity to protect against photooxidative damage. This may contribute to the promotion of leaf senescence in Pseudobombax during the transition from wet to dry season.

The Function of Ascorbic Acid in Photosynthetic Phosphorylation.

Ascorbate is oxidized to the free radical monodehydroascorbate by O2.- and by H2O2 (through the action of ascorbate peroxidase) formed in the Mehler reaction by isolated spinach (Spinacia oleracea) thylakoids. Light-dependent electron transport from water to monodehydroascorbate is shown to be coupled to ATP formation with a ratio ATP:O2 of 2. In the presence of ascorbate the net O2 exchange balance of the Mehler reaction is close to zero, and the synthesis of ATP is increased 2 to 3 times due to the extra electron transport to the monodehydroascorbate free radical. A scheme of the electron transport in the presence of ascorbate is discussed.

Evidence for a physiological role of CO 2 in the regulation of photosynthetic electron transport in intact leaves

The effect of CO 2 upon photosynthetic electron transport was examined in wheat and maize leaves in order to establish whether CO 2 had a direct role in electron-transport regulation in vivo. When intercellular CO 2 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 CO 2 fixation rate. However, when intercellular CO 2 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 CO 2 fixation. In very low intercellular CO 2 when no measurable CO 2 fixation was sustained, an appreciable fraction of Q A still remained oxidised, however, maximal reduction of Q A occurred when O 2 was also removed. Q A could then be substantially reoxidised by the readdition of small amounts of CO 2 (20–40 μmol) which only facilitated a very small increase in CO 2 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 CO 2-free atmosphere and Q B was poised in a more oxidised state. Electron transport that was independent of CO 2 fixation was measured in methyl viologen-treated leaf discs. In 1% O 2, but not in 21% O 2, light-dependent electron transport to methyl viologen was decreased significantly by the depletion of CO 2. It is concluded that CO 2 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 CO 2 and O 2 in the regulation of photosynthetic electron transport. The possibility that CO 2 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.

A low molecular weight phytotoxin produced by Phoma tracheiphila, the cause of mal secco disease in citrus

Phoma tracheiphila (Petri) K & G, the causal organism of mal secco disease in citrus secretes a low molecular weight phytotoxin during growth on a synthetic medium. The toxin is capable of inciting interveinal chlorosis and necrosis in lemon leaves. It also induces electrolyte leakage from lemon leaves or carrot discs and causes uncoupling of electron transport in lettuce chloroplasts. A linear relation was established between toxin concentration and the uncoupling activity. The electrolyte leakage activity and uncoupling activity were reduced to varying extents by various organic solvents. The toxin causes reversible inhibition of proton uptake, inhibition of photophosphorylation and stimulation of light-dependent electron transport by lettuce chloroplasts. It also stimulates the initial rate of Mg ++ -ATPase in the dark but the stimulation is followed in time with inhibition. This established the present toxin as an uncoupler of electron transport in chloroplast preparations. The toxin was partially purified by gel filtration on Sepharose 6B, Bio-Gel P-2, Sephadex G-10 and descending paper chromatography. The specific activity of chloroplasts per mg toxin (μmol O 2 mg −1 chl h −1 mg −1 dry wt toxin) was increased approximately 8-fold by purification. The estimated molecular weight for the toxin as judged by gel filtration is between 350 and 700 daltons. The toxin is hydrophilic and heat-stable (100 °C, 15 min) compound. It is inactivated by either acid or base hydrolysis, but is insensitive to periodate oxidation.

Composition and development of the bacterial photosynthetic apparatus.

The order of the Rhodospirillales, commonly known as the photosynthetic or more adequately as the phototrophic bacteria, comprises prokaryotes that are able to cover their energy requirements by coupling to light-dependent electron transport processes. In contrast to the plant-type photosynthesis not only of plants but also of cyanobacteria, the photosynthesis of phototrophic bacteria does not evolve oxygen and is dependent on anaerobiosis or, at least, largely reduced oxygen partial pressures. Apart from these physiological characteristics, the phototrophic bacteria comprise organisms of rather different properties (see Table I for the classification of phototrophic bacteria referred to in this chapter). Based on the distribution of different bacteriochlorophyll derivatives contained in the photosynthetic apparatus, two suborders can be distinguished (Truper and Pfennig, 1978). The first suborder of the Rhodospirillineae, i.e., the purple bacteria, comprises organisms that exhibit either bacteriochlorophyll a or b. This suborder contains two families, the Rhodospirillaceae (formerly Athiorhodaceae) and the Chromatiaceae (formerly Thiorhodaceae), which differ from each other by their inability and ability, respectively, to grow on sulfide (exceptions are known). The second suborder, the Chlorobiineae, comprises all of the phototrophic bacteria that produce bacteriochlorophyll c, d, or e, in addition to minor quantities of bacteriochlorophyll a.

An improved method of measuring photosynthetic electron transport in Euglena under in vivo conditions

A method is described to measure photochemical activity in intact cells of Euglena under in vivo conditions. The method employs a cell wall digesting enzyme (cellulysin) to induce enough permeability in the cell walls and membranes in order to allow dyes, commonly used to investigate light-dependent electron transport reactions to enter, but without inducing a concomittant efflux of metabolites. Between 1 and 2 h of incubation in 5% (w/v) cellulysin provided conditions which allowed measurement of photosystem I-, II- and I+II-dependent electron transport with rates up to 600% higher than in control cells; whereas other cell wall degrading enzymes (cellulase and pectinase) still did not increase the entry of the dyes. Cellulysin up to 2 h of incubation had little or no effect on whole cell respiration, photosynthetic O2 evolution, or the export of potassium and (14C) labeled compounds out of cells; therefore cellulysin obviously did not change the normal habit or physiology of Euglena. Cellulysin (4 h digestion), cellulase and pectinase (2–4 h of incubation) on the other hand led to a lowering of respiration and light-dependent O2 evolution, and increased the efflux of K+, but apparently decreased that of (14C)labeled fixation products.

Light-induced electron transport pathways in membrane preparations from Rhodopseudomonas capsulata

The photosynthetic electron transport chain in Rhodopseudomonas capsulata cells was investigated by studying light-induced noncyclic electron transport from external donors to O 2. Two membrane preparations with opposite membrane polarity, heavy chromatophores and regular chromatophores, were used to characterize this electron transport. It was shown that with lipophylic electron donors such as dichloroindophenol, diaminobenzidine, and phenazine methosulfate the electron transport activities were similar in both types of chromatophores, whereas horse heart cytochrome c, K 4Fe(CN) 6, 3-sulfonic acid phenazine methosulfate, and ascorbate, which cannot penetrate the membrane, were more active in the heavy chromatophores than in the regular chromatophores. Partial depletion of cytochrome c 2 from the heavy chromatophores caused a decrease in the light-induced O 2 uptake from reduced dichloroindophenol or ascorbate. The activity could be restored with higher concentrations of dichloroindophenol or with purified cytochrome c 2 from Rps. capsulata. It is assumed that in the heavy chromatophores the artificial electron donors are oxidized on the cytochrome c 2 level which faces the outside medium. However, cytochrome c 2 is not exposed to the outside medium in the regular chromatophores. Therefore, only lipophylic donors would interact with cytochrome c 2 in this system, while hydrophylic donors would be oxidized by another component of the electron transport chain which is exposed to the external medium. Studies with inhibitors of photophosphorylation show that antimycin A enhances the light-dependent electron transport to O 2 whereas 1:10 phenanthroline inhibited the reaction, but dibromothymoquinone did not affect it. It is assumed that a nonheme iron protein is taking part in this electron transport but not a dibromothymoquinone-sensitive quinone. The terminal oxidase of the light-dependent pathway is different from the two oxidases of the respiratory chain. The ratio between electrons entering the system and molecules of O 2 consumed is 4, which means that the end product of O 2 reduction is H 2O.

The adjustment of photosynthetically grown cells of Rhodospirillum rubrum to aerobic light conditions

The growth of photosynthetically precultured cells of Rhodospirillum rubrum under aerobic condition in light is investigated. Special emphasis is given to the question of whether the photosynthetic electron transport chain is influenced under these conditions. Light-induced absorbance changes under anaerobic conditions show that although in whole cells a variation can be noted, the reactions of isolated membranes decrease only very slowly and parallel to each other. The photophosphorylation activity remains constant on a bacteriochlorophyll basis. On a cell mass basis this activity decreases parallel to the decreasing bacteriochlorophyll content. Light-dependent NAD+ reduction by ascorbate-DCPI remains constant on a bacteriochlorophyll basis, whereas succinate supported NAD+ reduction in light increases. On a cell mass basis the activity of succinate supported NAD+ reduction stays nearly constant, thus showing similar responses to the presence of oxygen in light as the NADH oxidase system. NADH oxidase activity increases on a bacteriochlorophyll basis and does not change on a cell mass basis. Parallel to the NADH-oxidase system, oxygen uptake in the dark by whole cells does not change after aerobiosis in light. Light inhibits respiration even after several generations of growth in the presence of oxygen; however, the inhibition decreases slowly. Light inhibition of respiration can be totally overcome by the addition of the uncoupler CCCP. These results indicate that light-dependent electron transport is not influenced significantly by the presence of oxygen. Although the respiratory system is formed, cells preferentially grow photosynthetically. Respiration takes over when the amount of bacteriochlorophyll reaches very low values.