Quinone Electron Acceptor Research Articles

The D1:Ser268 residue of Photosystem II contributes to an alternative pathway for QB protonation in the absence of bound bicarbonate.

Photosystem II catalyses the splitting of water and the reduction in plastoquinone in thylakoid membranes of all oxygenic photosynthetic organisms. The final step in quinol formation is protonation of the reduced secondary quinone electron acceptor to give QB H2 . The proton for this step is hypothesized to be provided by a hydrogen-bond network incorporating amino acids from the Photosystem II D1 and D2 reaction center proteins, together with several bound waters and a bicarbonate ion ligated to a non-heme iron found between the primary plastoquinone electron acceptor QA and QB . The aim of this study was to investigate the role of bicarbonate and the D1:Ser268 residue in the formation of QB H2 . Using targeted mutagenesis of the D1 protein in the cyanobacterium Synechocystis sp. PCC 6803, we have created two mutants, S268A and S268T. Our D1:Ser268 mutants exhibited increased sensitivity to formate-induced inhibition of electron transfer between QA and QB and indicate that D1:Ser268 and bicarbonate support the second protonation in the formation of QB H2 via two different pathways that both lead to the protonation of by D1:His215.

Zinc Switch in Pig Heart Lipoamide Dehydrogenase: Steady-State and Transient Kinetic Studies of the Diaphorase Reaction.

Elevation of intracellular Zn2+ following ischemia contributes to cell death by affecting mitochondrial function. Zn2+ is a differential regulator of the mitochondrial enzyme lipoamide dehydrogenase (LADH) at physiological concentrations (Ka = 0.1 µM free zinc), inhibiting lipoamide and accelerating NADH dehydrogenase activities. These differential effects have been attributed to coordination of Zn2+ by LADH active-site cysteines. A detailed kinetic mechanism has now been developed for the diaphorase (NADH-dehydrogenase) reaction catalyzed by pig heart LADH using 2,6-dichlorophenol-indophenol (DCPIP) as a model quinone electron acceptor. Anaerobic stopped-flow experiments show that two-electron reduced LADH is 15-25-fold less active towards DCPIP reduction than four-electron reduced enzyme, or Zn2+-modified reduced LADH (the corresponding values of the rate constants are (6.5 ± 1.5) × 103M-1·s-1, (9 ± 2) × 104M-1·s-1, and (1.6 ± 0.5) × 105M-1·s-1, respectively). Steady-state kinetic studies with different diaphorase substrates show that Zn2+ accelerates reaction rates exclusively for two-electron acceptors (duroquinone, DCPIP), but not for one-electron acceptors (benzoquinone, ubiquinone, ferricyanide). This implies that the two-electron reduced form of LADH, prevalent at low NADH levels, is a poor two-electron donor compared to the four-electron reduced or Zn2+-modified reduced LADH forms. These data suggest that zinc binding to the active-site thiols switches the enzyme from one- to two-electron donor mode. This zinc-activated switch has the potential to alter the ratio of superoxide and H2O2 generated by the LADH oxidase activity.

Rational Redesign of Monoamine Oxidase A into a Dehydrogenase to Probe ROS in Cardiac Aging.

Cardiac senescence is a typical chronic frailty condition in the elderly population, and cellular aging is often associated with oxidative stress. The mitochondrial-membrane flavoenzyme monoamine oxidase A (MAO A) catalyzes the oxidative deamination of neurotransmitters, and its expression increases in aged hearts. We produced recombinant human MAO A variants at Lys305 that play a key role in O2 reactivity leading to H2O2 production. The K305Q variant is as active as the wild-type enzyme, whereas K305M and K305S have 200-fold and 100-fold lower kcat values and similar Km. Under anaerobic conditions, K305M MAO A was normally reduced by substrate, whereas reoxidation by O2 was much slower but could be accomplished by quinone electron acceptors. When overexpressed in cardiomyoblasts by adenoviral vectors, the K305M variant showed enzymatic turnover similar to that of the wild-type but displayed decreased ROS levels and senescence markers. These results might translate into pharmacological treatments as MAO inhibitors may attenuate cardiomyocytes aging.

Special issue in honour of Prof. Reto J. Strasser - Chlorophyll fluorescence of Nicotiana tabacum expressing the green fluorescent protein

Evidence that the green fluorescence protein (GFP) develops a significant toxicity in plants has not been found, but it may represent a source of free radicals as a consequence of its fluorescence. In addition, green light is known to trigger the acclimatisation of the photosynthetic system towards a shady environment. Moreover, the light-harvesting system may acclimate to an increased availability of green light. Each of these effects may be induced by the GFP. Therefore, the hypothesis was tested, whether transformation of Nicotiana tabacum cv. Bursan to express the GFP could affect chlorophyll fluorescence parameters. The analysis revealed a significantly lower absorption of energy per excited cross section in GFP-transformed tobacco, a lower number of active reaction centres per excited cross section, a larger absorption and trapped energy flux leading to the reduction of the primary quinone electron acceptor of PSII per reaction centre, and a lower variable fluorescence.

Does the water-oxidizing Mn4CaO5 cluster regulate the redox potential of the primary quinone electron acceptor QA in photosystem II? A study by Fourier transform infrared spectroelectrochemistry

Redox titration using fluorescence measurements of photosystem II (PSII) has long shown that impairment of the water-oxidizing Mn4CaO5 cluster upshifts the redox potential (Em) of the primary quinone electron acceptor QA by more than 100 mV, which has been proposed as a photoprotection mechanism of PSII. However, the molecular mechanism of this long-distance interaction between the Mn4CaO5 cluster and QA in PSII remains unresolved. In this study, we reinvestigated the effect of depletion of the Mn4CaO5 cluster on Em(QA−/QA) using Fourier transform infrared (FTIR) spectroelectrochemistry, which can directly monitor the redox state of QA at an intended potential. Light-induced FTIR difference measurements at a series of electrode potentials for intact and Mn-depleted PSII preparations from spinach and Thermosynechococcus elongatus showed that depletion of the Mn4CaO5 cluster hardly affected the Em(QA−/QA) values. In contrast, fluorescence spectroelectrochemical measurement using the same PSII sample, electrochemical cell, and redox mediators reproduced a large upshift of apparent Em upon Mn depletion, whereas a smaller shift was observed when weaker visible light was used for fluorescence excitation. Thus, the possibility was suggested that the measuring light for fluorescence disturbed the titration curve in Mn-depleted PSII, in contrast to no interference of infrared light with the PSII reactions in FTIR measurements. From these results, it was concluded that the Mn4CaO5 cluster does not directly regulate Em(QA−/QA) to control the redox reactions on the electron acceptor side of PSII.

Magnesium-Deficiency Effects on Pigments, Photosynthesis and Photosynthetic Electron Transport of Leaves, and Nutrients of Leaf Blades and Veins in Citrus sinensis Seedlings.

Citrus sinensis seedlings were irrigated with nutrient solution at a concentration of 0 (Mg-deficiency) or 2 (Mg-sufficiency) mM Mg (NO3)2 for 16 weeks. Mg-deficiency-induced interveinal chlorosis, vein enlargement and corkiness, and alterations of gas exchange, pigments, chlorophyll a fluorescence (OJIP) transients and related parameters were observed in middle and lower leaves, especially in the latter, but not in upper leaves. Mg-deficiency might impair the whole photosynthetic electron transport, including structural damage to thylakoids, ungrouping of photosystem II (PSII), inactivation of oxygen-evolving complex (OEC) and reaction centers (RCs), increased reduction of primary quinone electron acceptor (QA) and plastoquinone pool at PSII acceptor side and oxidation of PSI end-electron acceptors, thus lowering energy transfer and absorption efficiency and the transfer of electrons to the dark reactions, hence, the rate of CO2 assimilation in Mg-deficiency middle and lower leaves. Although potassium, Mg, manganese and zinc concentration in blades displayed a significant and positive relationship with the corresponding element concentration in veins, respectively, great differences existed in Mg-deficiency-induced alterations of nutrient concentrations between leaf blades and veins. For example, Mg-deficiency increased boron level in the blades of upper leaves, decreased boron level in the blades of lower leaves, but did not affect boron level in the blades of middle leaves and veins of upper, middle and lower leaves. To conclude, Mg-deficiency-induced interveinal chlorosis, vein enlargement, and corkiness, and alterations to photosynthesis and related parameters increased with increasing leaf age. Mg-deficiency-induced enlargement and corkiness of veins were not caused by Mg-deficiency-induced boron-starvation.

Structural Changes in the Acceptor Site of Photosystem II upon Ca2+/Sr2+ Exchange in the Mn4CaO5 Cluster Site and the Possible Long-Range Interactions.

The Mn4CaO5 cluster site in the oxygen-evolving complex (OEC) of photosystem II (PSII) undergoes structural perturbations, such as those induced by Ca2+/Sr2+ exchanges or Ca/Mn removal. These changes have been known to induce long-range positive shifts (between +30 and +150 mV) in the redox potential of the primary quinone electron acceptor plastoquinone A (QA), which is located 40 Å from the OEC. To further investigate these effects, we reanalyzed the crystal structure of Sr-PSII resolved at 2.1 Å and compared it with the native Ca-PSII resolved at 1.9 Å. Here, we focus on the acceptor site and report the possible long-range interactions between the donor, Mn4Ca(Sr)O5 cluster, and acceptor sites.

Accelerated Charge Separation and Stabilization in a Fused Bis Zinc Porphyrin-Quinone Conjugate Via Cation-Quinone Interaction

Accelerated singlet excited state charge separation, improved quantum yields, and ultimate charge stabilization upon cation binding to the electron acceptor quinone in a fused bis zinc porphyrin-quinone (zinc porphyrin located on each side of the pentacenequinone) donor-acceptor conjugate has been accomplished. Selective binding of either Mg2+ or Sc3+ to quinone in the donor-acceptor conjugate led to facile reduction of quinone making the charge separation process thermodynamically more feasible. Consequently, increased population of the charge separated state was witnessed in femtosecond transient absorption studies with a near linear trend in population of the charge separated state versus amount of metal-ion added. GloTarAn software was utilized to process the spectral data which enabled determination of the species associated spectrum and population profiles which thereby allowed deconvolution of the femtosecond spectral data and appropriate designations of each state. The present findings may provide further insight into the role of the interaction between quinones and iron metal ions in the electron transport chain of natural photosynthesis, which may have further implications in other biological systems where metal cations are present in electron transport chains.

Responses of the Photosynthetic Electron Transport Reactions Stimulate the Oxidation of the Reaction Center Chlorophyll of Photosystem I, P700, under Drought and High Temperatures in Rice.

It is of interest how photosynthetic electron transport (PET) reactions respond to excess light energy caused by the combination of drought stress and high temperatures. Since such information is scarcely available for photosystem I (PSI), this question was explored in rice (Oryza sativa L.) plants subjected to drought stress, using culture solutions that contain poly(ethylene glycol) at different concentrations under two day/night temperature regimes. At 27/22 °C (day/night), drought stress led to the oxidation of the reaction center of the chlorophyll of PSI (P700), and also led to decreases in the quantum efficiencies of photosystem II (PSII) and PSI, and a reduction of the primary quinone electron acceptor of PSI. Such drought stress responses were wholly stimulated at 35/30 °C. These parameters were strongly correlated with each other and were minimally affected by temperature. These results indicate that the drought stress responses of the respective PET reactions are closely associated with each other in the oxidization of P700 and that such responses are stimulated at high temperatures. The underlying mechanisms of these phenomena were discussed. While P700 oxidation is thought to suppress reactive oxygen species (ROS) production, PSI photoinhibition was observed under severe stress conditions, implying that P700 oxidation is not sufficient for the protection of PSI under drought stress.

Enhanced NPQ affects long-term acclimation in the spring ephemeral Berteroa incana

The spring ephemeral Berteroa incana is a familial relative of Arabidopsis thaliana and thrives in a diverse range of terrestrial ecosystems. Within this study, the novel chlorophyll fluorescence parameter of photochemical quenching in the dark (qPd) was used to measure the redox state of the primary quinone electron acceptor (QA) in order to estimate the openness of photosystem II (PSII) reaction centres (RC). From this, the early onset of photoinactivation can be sensitively quantified alongside the light tolerance of PSII and the photoprotective efficiency of nonphotochemical quenching (NPQ). This study shows that, with regards to A. thaliana, NPQ is enhanced in B. incana in both low-light (LL) and high-light (HL) acclimation states. Moreover, light tolerance is increased by up to 500%, the rate of photoinactivation is heavily diminished, and the ability to recover from light stress is enhanced in B. incana, relative to A. thaliana. This is due to faster synthesis of zeaxanthin and a larger xanthophyll cycle (XC) pool available for deepoxidation. Moreover, preferential energy transfer via CP47 around the RC further enhances efficient photoprotection. As a result, a high functional cross-section of photosystem II is maintained and is not downregulated when B. incana is acclimated to HL. A greater capacity for protective NPQ allows B. incana to maintain an enhanced light-harvesting capability when acclimated to a range of light conditions. This enhancement of flexible short-term protection saves the metabolic cost of long-term acclimatory changes.

Revisiting the methionine salvage pathway and its paralogues.

SummaryMethionine is essential for life. Its chemistry makes it fragile in the presence of oxygen. Aerobic living organisms have selected a salvage pathway (the MSP) that uses dioxygen to regenerate methionine, associated to a ratchet‐like step that prevents methionine back degradation. Here, we describe the variation on this theme, developed across the tree of life. Oxygen appeared long after life had developed on Earth. The canonical MSP evolved from ancestors that used both predecessors of ribulose bisphosphate carboxylase oxygenase (RuBisCO) and methanethiol in intermediate steps. We document how these likely promiscuous pathways were also used to metabolize the omnipresent by‐products of S‐adenosylmethionine radical enzymes as well as the aromatic and isoprene skeleton of quinone electron acceptors.

PH-Dependent Regulation of the Relaxation Rate of the Radical Anion of the Secondary Quinone Electron Acceptor QB in Photosystem II As Revealed by Fourier Transform Infrared Spectroscopy.

Photosystem II (PSII) is a protein complex that performs water oxidation using light energy during photosynthesis. In PSII, electrons abstracted from water are eventually transferred to the secondary quinone electron acceptor, QB, and upon double reduction, QB is converted to quinol by binding two protons. Thus, excess electron transfer in PSII increases the pH of the stroma. In this study, to investigate the pH-dependent regulation of the electron flow in PSII, we have estimated the relaxation rate of the QB- radical anion in the pH region between 5 and 8 by direct monitoring of its population using light-induced Fourier transform infrared difference spectroscopy. The decay of QB- by charge recombination with the S2 state of the water oxidation center in PSII membranes was shown to be accelerated at higher pH, whereas that of QA- examined in the presence of a herbicide was virtually unaffected at pH ≤7.5 and slightly slowed at pH 8. These observations were consistent with the previous studies that included rather indirect monitoring of the QB- and QA- decays using fluorescence detection. The accelerated relaxation of QB- was explained by the shift of a redox equilibrium between QA- and QB- to the QA- side due to the decrease in the redox potential of QB at higher pH, which is induced by deprotonation of a single amino acid residue near QB. It is proposed that this pH-dependent QB- relaxation is one of the mechanisms of electron flow regulation in PSII for its photoprotection.

Electron-Donating Phenolic and Electron-Accepting Quinone Moieties in Peat Dissolved Organic Matter: Quantities and Redox Transformations in the Context of Peat Biogeochemistry.

Electron-donating phenolic and electron-accepting quinone moieties in peat dissolved organic matter (DOM) are considered to play key roles in processes defining carbon cycling in northern peatlands. This work advances a flow-injection analysis system coupled to chronoamperometric detection to allow for the simultaneous and highly sensitive determination of these moieties in dilute DOM samples. Analysis of anoxic pore water and oxic pool water samples collected across an ombrotrophic bog in Sweden demonstrated the presence of both phenolic and quinone moieties in peat DOM. The pore water DOM had higher quantities of phenolic but not quinone moieties compared with commonly used model aquatic and terrestrial DOM isolates. Significantly lower phenol content in DOM from oxic pools than DOM from anoxic pore waters indicated oxidative DOM processing in the pools. Consistently, treatment of peat DOM with laccase, a phenol-oxidase, under oxic conditions resulted in an irreversible removal of phenols and reversible oxidation of hydroquinones to quinones. Electron transfer to peat DOM was fully reversible over an electrochemical reduction and subsequent O2-reoxidation cycle, supporting that quinones in peat DOM serve as regenerable microbial electron acceptors in peatlands. The results advance our understanding of redox processes involving phenolic and quinone DOM moieties and their roles in northern peatland carbon cycling.

Effects of exogenous phenolic acids on photosystem functions and photosynthetic electron transport rate in strawberry leaves

Our study investigated the physiological and biochemical basis for the effects of exogenous phenolic acids on the function of the photosynthetic apparatus and photosynthetic electron transport rate in strawberry seedlings. Potted seedlings of the strawberry (Fragaria × ananassa Duch.) were used. Syringic acid inhibited net photosynthetic rate and water-use efficiency decreased. Additionally, primary quinone electron acceptor of the PSII reaction centre, the PSII reaction centre and the oxygen evolving complex were also impaired. Both the maximum quantum yield of the PSII primary photochemistry and the performance index on absorption basis were depressed, resulting in reduced function of the photosynthetic electron transport chain. Otherwise, low phthalic acid concentrations enhanced photosynthetic capacity, while high concentrations showed opposite effects. Syringic acid exhibited a higher toxic effect than that of phthalic acid which was more evident at higher concentrations.

Erratum to: Early emergence of the FtsH proteases involved in photosystem II repair

The verb ‚make“ was used instead of ‚form“ in the following sentence. Instead of A phylogenetic analysis of over 6000 FtsH protease sequences revealed that there are three major groups of FtsH proteases originating from gene duplication events in the last common ancestor of bacteria, and that the FtsH proteases involved in PSII repair make a distinct clade branching out before the divergence of FtsH proteases found in all groups of anoxygenic phototrophic bacteria. It should read A phylogenetic analysis of over 6000 FtsH protease sequences revealed that there are three major groups of FtsH proteases originating from gene duplication events in the last common ancestor of bacteria, and that the FtsH proteases involved in PSII repair form a distinct clade branching out before the divergence of FtsH proteases found in all groups of anoxygenic phototrophic bacteria. The word ‚photosynthetic“ was omitted in the following sentence: Instead of We conclude that the phylogeny of FtsH proteases is consistent with an early origin of water oxidation chemistry. It should read We conclude that the phylogeny of FtsH proteases is consistent with an early origin of photosynthetic water oxidation chemistry. The words ‚reactions“ and ‚for“ were used mistakenly in the following paragraph: Instead of Oxygenic photosynthetic electron transport from water to NADP+ requires the participation of two functionally distinct reactions centers (RCs) acting in series: photosystem II (PSII, a Type II RC containing quinone electron acceptors) and photosystem I (PSI, a Type I RC containing redox-active iron-sulphur clusters). Early ideas on for the emergence of oxygenic photosynthesis focused on the evolution of PSI and PSII from pre-existing RCs found in anoxygenic photosynthetic bacteria (Nitschke and Rutherford 1991) and the horizontal transfer of genes encoding chlorophyll biosynthetic enzymes and RC proteins (Hohmann-Marriott and Blankenship 2011, Fischer et al. 2016). It should read Oxygenic photosynthetic electron transport from water to NADP+ requires the participation of two functionally distinct reaction centers (RCs) acting in series: photosystem II (PSII, a Type II RC containing quinone electron acceptors) and photosystem I (PSI, a Type I RC containing redox-active iron-sulphur clusters). Early ideas on the emergence of oxygenic photosynthesis focused on the evolution of PSI and PSII from pre-existing RCs found in anoxygenic photosynthetic bacteria (Nitschke and Rutherford 1991) and the horizontal transfer of genes encoding chlorophyll biosynthetic enzymes and RC proteins (Hohmann-Marriott and Blankenship 2011, Fischer et al. 2016). The word ‚other“ was missing in the following sentence: Instead of Possible evolutionary constraints include the need to maintain protein quality control in the multiple membrane compartments found in cyanobacteria, chloroplasts, and mitochondria and the fact that oxygenic photosynthesis is associated with the production of singlet oxygen and ROS (reactive oxygen species), leading to protein damage, particularly in the thylakoid membrane which houses the photosynthetic apparatus. It should read Possible evolutionary constraints include the need to maintain protein quality control in the multiple membrane compartments found in cyanobacteria, chloroplasts and mitochondria and the fact that oxygenic photosynthesis is associated with the production of singlet oxygen and other ROS (reactive oxygen species), leading to protein damage, particularly in the thylakoid membrane which houses the photosynthetic apparatus. The word ‚of“ was mistakenly used instead of ‚to“ in the following sentence: Instead of Recognition of damaged D1 by cyanobacterial FtsH complexes is thought to be mediated by partial disassembly of damaged PSII (Krynicka et al. 2015) and binding of the N-terminal tail of D1 (Komenda et al. 2007). It should read Recognition of damaged D1 by cyanobacterial FtsH complexes is thought to be mediated by partial disassembly of damaged PSII (Krynicka et al. 2015) and binding to the N-terminal tail of D1 (Komenda et al. 2007).

Plant-type phytoene desaturase: Functional evaluation of structural implications.

Phytoene desaturase (PDS) is an essential plant carotenoid biosynthetic enzyme and a prominent target of certain inhibitors, such as norflurazon, acting as bleaching herbicides. PDS catalyzes the introduction of two double bonds into 15-cis-phytoene, yielding 9,15,9'-tri-cis-ζ-carotene via the intermediate 9,15-di-cis-phytofluene. We present the necessary data to scrutinize functional implications inferred from the recently resolved crystal structure of Oryza sativa PDS in a complex with norflurazon. Using dynamic mathematical modeling of reaction time courses, we support the relevance of homotetrameric assembly of the enzyme observed in crystallo by providing evidence for substrate channeling of the intermediate phytofluene between individual subunits at membrane surfaces. Kinetic investigations are compatible with an ordered ping-pong bi-bi kinetic mechanism in which the carotene and the quinone electron acceptor successively occupy the same catalytic site. The mutagenesis of a conserved arginine that forms a hydrogen bond with norflurazon, the latter competing with plastoquinone, corroborates the possibility of engineering herbicide resistance, however, at the expense of diminished catalytic activity. This mutagenesis also supports a “flavin only” mechanism of carotene desaturation not requiring charged residues in the active site. Evidence for the role of the central 15-cis double bond of phytoene in determining regio-specificity of carotene desaturation is presented.

Effects of modified clay on the physiological and photosynthetic activities of Amphidinium carterae Hulburt

Among the strategies for treating harmful algal blooms, flocculation using modified clay (MC) has been widely applied in the field. This paper studied the mitigation of MC on Amphidinium carterae Hulburt, finding that MC could not only effectively remove A. carterae, but also affect the physiological activities of the residual algae and inhibit their normal growth. The superoxide dismutase (SOD) activity, catalase (CAT) activity and malondialdehyde (MDA) content of the residual algae significantly increased compared with the control, indicating that MC stimulated the accumulation of reactive oxygen species (ROS) in algal cells. In addition, the cell density was significantly correlated with the SOD activity, CAT activity and MDA content in the experiment groups, suggesting that intracellular ROS might be the main internal factor inhibiting cell growth. To reveal the mechanism of ROS generation, this paper further evaluated the effect of MC on photosynthesis in the residual microalgae, and found that compared with the control the absorption flux per photosystem II (PSII) reaction center (ABS/RC), the trapping flux per RC (TR0/RC) and the electron transport flux per RC (ET0/RC) increased, while the TR0/ABS and ET0/ABS decreased after adding 0.10g/L and 0.25g/L MC. These findings indicate that the MC led to an imbalance between photosynthetic light absorption and energy utilization and that the partial RCs became non-primary quinone electron acceptor (QA)-RCs, further inducing the over-excitation of the active RCs. And MC caused the suppression of the electron transport chain (ETC): the ETC from the QA to the secondary quinone electron acceptor (QB) was blocked and the size of plastoquinone pool decreased, which could induce the over-reduction of PSII. The over-excitation of PSII and the damaged ETC likely induce the generation of ROS during photosynthesis. Thus, MC likely induced the accumulation of intracellular ROS due to photosynthesis inhibition, consequently hindering the growth of the residual algae.

Photosynthetic physiology of Scenedesmus sp. (Chlorophyceae) under photoautotrophic and molasses-based heterotrophic and mixotrophic conditions

:Heterotrophically grown cells of a newly isolated strain of Scenedesmus sp. retained their photosynthetic pigment content after prolonged darkness. When these cells had reached an apparent stationary phase and were subsequently exposed to light (mixotrophy), growth rapidly resumed and the biomass increased by 5.5-fold relative to photoautotrophically grown cultures and doubled compared with heterotrophic cultures. Although it is expected, and supported by the majority of reports in the literature, that dark acclimation of algal cultures should lead to eventual loss of photosynthetic pigments, some algae defy these expectations and synthesize and retain their photosynthetic pigments independent of light, despite the high associated maintenance costs. Here we examined the photosynthetic activity of heterotrophically grown cells in an attempt to explain this variance, using Scenedesmus sp. as a model organism. The photosynthetic capacity of heterotrophically grown cells was comparable with that of autotrophically grown cultures, associated with an interesting set of changes to the photosynthetic apparatus that includes lower nonphotochemical quenching, chlorophyll content, absorption cross-sectional area, higher connectivity between reaction centers, higher electron transport flux per reaction center, and probability at t = 0 that a trapped exciton moves an electron into the electron transport chain beyond the primary quinone electron acceptor, and performance index. As a result, when these heterotrophically grown cultures were transferred back to light, they were still able to perform photosynthesis and enhance overall growth, which was otherwise limited in complete darkness.

The Effects of ultraviolet irradiation on hybrid films of photosynthetic reaction centers and quantum dots in various organic matrices

The effects of ultraviolet radiation (up to 0.6 J/cm2) on the absorption spectra and electron transfer in dehydrated films of photosynthetic reaction centers from purple bacteria Rb. sphaeroides and hybrid structures that included reaction centers, quantum dots, and protein structure stabilizers (trehalose, polyvinyl alcohol, and methylcellulose) have been studied. Ultraviolet irradiation led to partial destruction of bacteriochlorophyll molecules (pheophytinization) and the reaction center carotenoid. In this case, ultraviolet irradiation did not exert a significant effect on electron transfer between the photoactive bacteriochlorophyll and quinone electron acceptors. The incorporation of reaction centers into organic matrices reduced pheophytinization. Trehalose was the most efficient in reducing the damage evoked by ultraviolet irradiation of the carotenoid molecule. Hybrid films that contained quantum dots were resistant to pheophytinization upon ultraviolet irradiation, but the presence of quantum dots did not affect the processes of carotenoid destruction upon exposure to ultraviolet radiation. Ultraviolet radiation had an insignificant effect on the characteristics of quantum dots (the fluorescence lifetime).

Increased Electron-Accepting and Decreased Electron-Donating Capacities of Soil Humic Substances in Response to Increasing Temperature.

The electron transfer capacities (ETCs) of soil humic substances (HSs) are linked to the type and abundance of redox-active functional moieties in their structure. Natural temperature can affect the chemical structure of natural organic matter by regulating their oxidative transformation and degradation in soil. However, it is unclear if there is a direct correlation between ETC of soil HS and mean annual temperature. In this study, we assess the response of the electron-accepting and -donating capacities (EAC and EDC) of soil HSs to temperature by analyzing HSs extracted from soil set along glacial-interglacial cycles through loess-palaeosol sequences and along natural temperature gradients through latitude and altitude transects. We show that the EAC and EDC of soil HSs increase and decrease, respectively, with increasing temperature. Increased temperature facilitates the prevalence of oxidative degradation and transformation of HS in soils, thus potentially promoting the preferentially oxidative degradation of phenol moieties of HS or the oxidative transformation of electron-donating phenol moieties to electron-accepting quinone moieties in the HS structure. Consequently, the EAC and EDC of HSs in soil increase and decrease, respectively. The results of this study could help to understand biogeochemical processes, wherein the redox functionality of soil organic matter is involved in the context of increasing temperature.