Electric Cables of Living Cells. II. Bacterial Electron Conductors.

The concept of "electric cables" involved in bioenergetic processes of a living cell was proposed half a century ago [Skulachev, V. P. (1971) Curr. Top. Bioenerg., Elsevier, pp. 127-190]. For many decades, only cell membrane structures have been considered as probable pathways for the electric current, namely, for the transfer of transmembrane electrochemical potential. However, the last ten to fifteen years have brought the discovery of bacterial "electric cables" of a new type. In 2005, "nanowires" conducting electric current over distances of tens of micrometers were discovered in metal- and sulphate-reducing bacteria [Reguera, G. et al. (2005) Nature, 435, pp. 1098-1101]. The next five years have witnessed the discovery of microbial electric currents over centimeter distances [Nielsen, L. P. et al. (2010) Nature, 463, 1071-1074]. This new group of bacteria allowing electric currents to flow over macroscopic distances was later called cable bacteria. Nanowires and conductive structures of cable bacteria serve to solve a special problem of membrane bioenergetics: they connect two redox half-reactions. In other words, unlike membrane "cables", their function is electron transfer in the course of oxidative phosphorylation for the generation of membrane energy rather than of the end-product. The most surprising is the protein nature of these cables (at least of some of them) indicated by recent data, since no protein wires for the long-distance electron transport had been previously known in living systems.

Electric Cables of Living Cells. I. Energy Transfer along Coupling Membranes.

The concept of "electric cables" involved in bioenergetic processes in a living cell was proposed half a century ago [Skulachev, V. P. (1971) Curr. Top. Bioenerg., Elsevier, pp. 127-190]. Membrane structures of a cell were considered as probable pathways for transferring transmembrane electrochemical potential. Further studies have shown that coupling membranes (inner mitochondrial membrane or bacterial cell membrane), i.e., those involved in the generation of membrane potential, can also serve for its transfer. A wide range of organisms from almost all major taxa have been discovered to employ the energy-transmitting function of coupling membranes. Macroscopic (millimeter or even centimeter in length) cable-like structures have been found, the most striking examples of which are giant mitochondria of some unicellular organisms (algae, fungi, protozoa) and animal tissues, filamentous mitochondria, mitochondrial reticulum in animal muscle tissue, and trichomes of cyanobacteria. The importance of such "electric cables" in cells or multicellular structures is determined by their ability to provide rapid energy exchange between metabolic counterparts, energy producers and energy consumers, as the diffusive transport of soluble macroergic molecules (ATP, etc.) requires much longer time. However, in the last 10-15 years, a new type of bacterial "electric cables" of presumably proteinaceous nature has been discovered, which serve a quite different purpose in cell bioenergetics. The molecular structure and functions of these cables will be discussed in the second part of the review ("Electric cables of living cells. II. Bacterial electron conductors").

Senescence and entrenchment in evolution of amino acid sites.

Amino acid propensities at a site change in the course of protein evolution. This may happen for two reasons. Changes may be triggered by substitutions at epistatically interacting sites elsewhere in the genome. Alternatively, they may arise due to environmental changes that are external to the genome. Here, we design a framework for distinguishing between these alternatives. Using analytical modelling and simulations, we show that they cause opposite dynamics of the fitness of the allele currently occupying the site: it tends to increase with the time since its origin due to epistasis ("entrenchment"), but to decrease due to random environmental fluctuations ("senescence"). By analysing the genomes of vertebrates and insects, we show that the amino acids originating at negatively selected sites experience strong entrenchment. By contrast, the amino acids originating at positively selected sites experience senescence. We propose that senescence of the current allele is a cause of adaptive evolution.

Analysis of Photoprotection and Apparent Non-photochemical Quenching of Chlorophyll Fluorescence in Tradescantia Leaves Based on the Rate of Irradiance-Induced Changes in Optical Transparence.

The kinetics of irradiation-induced changes in leaf optical transparence (ΔT) and non-photochemical quenching (NPQ) of chlorophyll fluorescence in Tradescantia fluminensis and T. sillamontana leaves adapted to different irradiance in nature was analyzed. Characteristic times of a photoinduced increase and a dark decline of ΔT in these species were 12 and 20 min, respectively. The ΔT was not confirmed to be the main contributor to the observed middle phase of NPQ relaxation kinetics (τ = 10-28 min). Comparison of rate of photoinduced increase in ΔT and photosystem II quantum yield recovery showed that the former did not affect the tolerance of the photosynthetic apparatus (PSA) to irradiances up to 150 µmol PAR·m-2·s-1. Irradiance tolerance correlated with the rate of "apparent NPQ" induction. Considering that the induction of apparent NPQ involves processes significantly faster than ΔT, we suggest that the photoprotective mechanism induction rate is crucial for tolerance of the PSA to moderate irradiance during the initial stage of light acclimation (first several minutes upon the onset of illumination).

Tolerance of the Photosynthetic Apparatus to Acidification of the Growth Medium as a Possible Determinant of CO2-Tolerance of the Symbiotic Microalga Desmodesmus sp. IPPAS-2014.

The symbiotic unicellular chlorophyte Desmodesmus sp. IPPAS-2014 capable of growth at extremely high CO2 levels prohibitive for most other microalgae is an interesting model for studies of CO2 tolerance mechanisms and a promising organism for CO2 biocapture. We studied the initial (0-60 min) phase of acclimation of this microalga to an abrupt decrease in pH of the medium sparged with air/20% CO2 mixture. Acclimation of the culture to these conditions was accompanied by a sharp decrease in photochemical activity of the chloroplast followed by its recovery with a characteristic time of 10-50 min. We hypothesize that acidification of the cultivation medium by dissolving CO2 plays a key role in the observed decrease in the photochemical activity. The possible role of photosynthetic apparatus tolerance to abrupt acidification in overall high tolerance of symbiotic microalgae to extremely high CO2 levels is discussed.

Influence of extreme ambient temperatures and anaerobic conditions on Peltigera aphthosa (L.) Willd. viability.

Lichen are symbiotic systems constituted by heterotrophic fungi (mycobionts) and photosynthetic microorganism (photobionts). These organisms can survive under extreme stress conditions. The aim of this work was to study the influence of low (-70 °C) or high (+70 °C) temperatures, temperature fluctuations from +70 °C to -70 °C, and anaerobic conditions on P. aphthosa (L.) Willd. viability. None of the studied stress factors affected significantly photosynthetic and respiratory activity of the thalli. No changes in morphology or ultrastructure of the cells were revealed for both photobiont and mycobiont components after extreme temperature treatment of P. aphthosa thalli. The data show the extreme tolerance of P. aphthosa to some stress factors inherent to the space flight conditions.

Mechanism of primary and secondary ion-radical pair formation in photosystem I complexes.

The mechanisms of the ultrafast charge separation in reaction centers of photosystem I (PS I) complexes are discussed. A kinetic model of the primary reactions in PS I complexes is presented. The model takes into account previously calculated values of redox potentials of cofactors, reorganization energies of the primary P700(+)A0(-) and secondary P700(+)A1(-) ion-radical pairs formation, and the possibility of electron transfer via both symmetric branches A and B of redox-cofactors. The model assumes that the primary electron acceptor A0 in PS I is represented by a dimer of chlorophyll molecules Chl2A/Chl3A and Chl2B/Chl3B in branches A and B of the cofactors. The characteristic times of formation of P700(+)A0(-) and P700(+)A1(-) calculated on the basis of the model are close to the experimental values obtained by pump-probe femtosecond absorption spectroscopy. It is demonstrated that a small difference in the values of redox potentials between the primary electron acceptors A0A and A0B in branches A and B leads to asymmetry of the electron transfer in a ratio of 70 : 30 in favor of branch A. The secondary charge separation is thermodynamically irreversible in the submicrosecond range and is accompanied by additional increase in asymmetry between the branches of cofactors of PS I.

Chlorophyll fluorescence induction, chlorophyll content, and chromaticity characteristics of leaves as indicators of photosynthetic apparatus senescence in arboreous plants.

Parameters of chlorophyll fluorescence induction (CFI) are widely used for assessment of the physiological state of higher plant leaves in biochemical, physiological, and ecological studies and in agricultural applications. In this work we have analyzed data on variability of some CFI parameters - ΦPSII(max) = Fv/Fm (relative value of variable fluorescence), qNPQ (non-photochemical quenching coefficient), RFd ("vitality index") - in autumnal leaves of ten arboreous plant species of the temperate climatic zone. The correlation between the chlorophyll content in the leaves and fluorescence parameters characterizing photosynthetic activity is shown for two representative species, the small-leaved linden Tilia cordata and the rowan tree Sorbus aucuparia. During the period of mass yellowing of the leaves, the ΦPSII(max) value can be used as an adequate characteristic of their photochemical activity, while in summer the qNPQ or RFd values are more informative. We have established a correlation between the ΦPSII(max) value, which characterizes the maximal photochemical activity of the photosystem II, and "chromaticity coordinates" of a leaf characterizing its color features. The chromaticity coordinates determined from the optical reflection spectra of the leaves serve as a quantitative measure of their hues, and this creates certain prerequisites for a visual expert assessment of the physiological state of the leaves.

[Biophysical methods of ecological monitoring. Photosynthetic characteristics of tree plants growing in Moscow city].

In this work, we studied an influence of ecological factors (a distance from the highway) on photosynthetic characteristics of leaves of four species of tree plants growing in Moscow city. Photosynthetic activity of leaves was assayed by instrumental methods of probing the functional state of photosynthetic apparatus, using electron paramagnetic resonance method for measuring the kinetics of photooxidation of P700 centers, thermoluminescence, and slow induction of chlorophyll fluorescence. It has been shown that kinetic parameters of the induction curves, as measured from the kinetics of photooxidation of P700 and slow induction of chlorophyll fluorescence in dark-adapted leaves, are sensible to variations of plant growing conditions. These parameters can be used as informative characteristics for ecological monitoring of the environment.

P680 (P(D1)P(D2)) and Chl(D1) as alternative electron donors in photosystem II core complexes and isolated reaction centers.

Low temperature (77-90 K) measurements of absorption spectral changes induced by red light illumination in isolated photosystem II (PSII) reaction centers (RCs, D1/D2/Cyt b559 complex) with different external acceptors and in PSII core complexes have shown that two different electron donors can alternatively function in PSII: chlorophyll (Chl) dimer P(680) absorbing at 684 nm and Chl monomer Chl(D1) absorbing at 674 nm. Under physiological conditions (278 K) transient absorption difference spectroscopy with 20-fs resolution was applied to study primary charge separation in spinach PSII core complexes excited at 710 nm. It was shown that the initial electron transfer reaction takes place with a time constant of ~0.9 ps. This kinetics was ascribed to charge separation between P(680)* and Chl(D1) absorbing at 670 nm accompanied by the formation of the primary charge-separated state P(680)(+)Chl(DI)(-), as indicated by 0.9-ps transient bleaching at 670 nm. The subsequent electron transfer from Chl(D1)(-) occurred within 13-14 ps and was accompanied by relaxation of the 670-nm band, bleaching of the Pheo(D1) Q(x) absorption band at 545 nm, and development of the anion-radical band of Pheo(D1)(-) at 450-460 nm, the latter two attributable to formation of the secondary radical pair P(680)(+)Pheo(D1)(-). The 14-ps relaxation of the 670-nm band was previously assigned to the Chl(D1) absorption in isolated PSII RCs [Shelaev, Gostev, Nadtochenko, Shkuropatov, Zabelin, Mamedov, Semenov, Sarkisov and Shuvalov, Photosynth. Res. 98 (2008) 95-103]. We suggest that the longer wavelength position of P(680) (near 680 nm) as a primary electron donor and the shorter wavelength position of Chl(D1) (near 670 nm) as a primary acceptor within the Q(y) transitions in RC allow an effective competition with an energy transfer and stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as the primary electron donor and Pheo(D1) as the primary acceptor cannot be ruled out, the 20-fs excitation at the far-red tail of the PSII core complex absorption spectrum at 710 nm appears to induce a transition to a low-energy state P(680)* with charge-transfer character (probably P(D1)(δ+)P(D2)(δ-)) which results in an effective electron transfer from P(680)* (the primary electron donor) to Chl(D1) as the intermediary acceptor.

Effects of light environment on the induction of chlorophyll fluorescence in leaves: a comparative study of Tradescantia species of different ecotypes.

In this work, using a PAM-fluorimetry technique, we have compared effects of plant adaptation to the light or dark conditions on the kinetics of chlorophyll a fluorescence yield in Tradecantia leaves of several species (Tradescantia albiflora, Tradescantia fluminensis, Tradescantia navicularis, and Tradescantia sillamontana), which represent plants of different ecotypes. Two fluorescence parameters were used to assess photosynthetic performance in vivo: non-photochemical quenching (NPQ) of chlorophyll fluorescence (q(NPQ)) determined by energy losses in the light-harvesting antenna of photosystem 2 (PS2), and PS2 operating efficiency (Φ(PSII)). Comparative study of light-induced changes in q(NPQ) and Φ(PSII) has demonstrated that shade-tolerant Tradecantia species (T. albiflora Kunth, T. fluminensis Vell.) reveal higher capacities for NPQ and demonstrate slower transitions between the 'light-adapted' and 'dark-adapted' states than succulent species T. navicularis and T. sillamontana, which are typical habitats of semi-deserts. We analyze the photosynthetic performance of Tradescantia species in the context of their adaptabilities to variable environment conditions. The ability of shade-tolerant plants to retain a relatively long-term (∼40-60 min) 'memory' for illumination history may be associated with the regulatory mechanisms that provide the flexibility of photosynthetic apparatus in response to fluctuations of light intensity.

[Slow fluorescence induction and CO2-exchange in bean leaves treated with Reynoutria sachalinensis extract].

It has been shown that the treatment of bean seedlings with a water extract of Reynoutria sachalinensis increases visible photosynthesis and the activity of dark respiration of plants. The increase in CO2-uptake under relatively high light illumination (10000 1x) correlates with an increase in the fluorescence parameter (F(M) - F(T))/F(T).

Oxygen as an alternative electron acceptor in the photosynthetic electron transport chain of C3 plants.

This study deals with effects of oxygen on the kinetics of P(700) photoinduced redox transitions and on induction transients of chlorophyll fluorescence in leaves of C(3) plants Hibiscus rosa-sinensis and Vicia faba. It is shown that the removal of oxygen from the leaf environment has a conspicuous effect on photosynthetic electron transport. Under anaerobic conditions, the concentration of oxidized P700 centers in continuous white light was substantially lower than under aerobic conditions. The deficiency of oxygen released non-photochemical quenching of chlorophyll fluorescence, thus indicating a decrease in the trans-thylakoid pH gradient (DeltapH). Quantitative analysis of experimental data within the framework of an original mathematical model has shown that the steady-state electron flux toward oxygen in Chinese hibiscus leaves makes up to approximately 40% of the total electron flow passing through photosystem 1 (PS1). The decrease in P700+ content under anaerobic conditions can be due to two causes: i) the retardation of electron outflow from PS1, and ii) the release of photosynthetic control (acceleration of electron flow from PS2 to P700+) owing to lower acidification of the intra-thylakoid space. At the same time, cyclic electron transport around PS1 was not stimulated in the oxygen-free medium, although such stimulation seemed likely in view of possible rearrangement of electron flows on the acceptor side of PS1. This conclusion stems from observations that the rates of P700+ reduction in DCMU-poisoned samples, both under aerobic and anaerobic conditions, were negligibly small compared to rates of electron flow from PS2 toward P700+ in untreated samples.

Biological and polymeric self-assembled hybrid systems: structure and properties of thylakoid/polyelectrolyte complexes.

A novel hybrid system composed of biological components and synthetic polymer, thylakoid/polycation complex, has been formed and studied. Effects of complex formation on the structure, electrostatics and functioning of thylakoid membranes have been examined. Thylakoids from bean leaves were used to form complexes with polycation polyallylamine hydrochloride (PAAH) in two systems: (i) thylakoid/polycation complexes formed in an aqueous bulk phase, and (ii) immobilized thylakoid/polycation planar complexes. Immobilized on a solid substrate surface, thylakoid/polycation complexes were prepared using layer-by-layer stepwise alternate adsorption technique, i.e., via the sequential alternate adsorption of thylakoids and polycation molecules. The morphology of built up structures was investigated by scanning electron microscopy. Light-induced electron transport in chloroplasts was studied by the electron paramagnetic resonance (EPR) method. Spin probe technique was employed to study the structural and electrostatic characteristics of thylakoid membranes. We have found that efficiency of light-induced electron transport in thylakoid membranes and membrane structure were not changed noticeably by PAAH binding to thylakoids in a wide range of PAAH concentrations. The data obtained indicate the physiologically-soft character of polycation interactions with thylakoid membranes and demonstrate effectiveness of interfacial self-assembly approach to fabrication of complex planar functional nanostructures from biological components and synthetic polymers.

[Changes in induction of slow fluorescence in leaves of maple located near urban highways]. / Izmenenie medlennoi induktsii fluorestsentsii list'ev klena vblizi transportnykh magistralei.

The slow fluorescence induction parameter FM/FT was measured for green leaves from maple trees grown in the park, depending on the distance (5-65 m) between the tree and the highway cross. We concluded that the decrease in the value of FM/FT for trees grown in the vicinity of the road cross was caused by exhaust gases.