Showing papers by "Bulgarian Academy of Sciences published in 2009"

Quantifying cross-correlations using local and global detrending approaches

Abstract: In order to quantify the long-range cross-correlations between two time series qualitatively, we introduce a new cross-correlations test QCC(m), where m is the number of degrees of freedom. If there are no cross-correlations between two time series, the cross-correlation test agrees well with the χ2(m) distribution. If the cross-correlations test exceeds the critical value of the χ2(m) distribution, then we say that the cross-correlations are significant. We show that if a Fourier phase-randomization procedure is carried out on a power-law cross-correlated time series, the cross-correlations test is substantially reduced compared to the case before Fourier phase randomization. We also study the effect of periodic trends on systems with power-law cross-correlations. We find that periodic trends can severely affect the quantitative analysis of long-range correlations, leading to crossovers and other spurious deviations from power laws, implying both local and global detrending approaches should be applied to properly uncover long-range power-law auto-correlations and cross-correlations in the random part of the underlying stochastic process.

Radio Imaging of the Very-High-Energy gamma-Ray Emission Region in the Central Engine of a Radio Galaxy

Abstract: The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.

Carotenoids from Rhodotorula and Phaffia: yeasts of biotechnological importance.

Abstract: Carotenoids represent a group of valuable molecules for the pharmaceutical, chemical, food and feed industries, not only because they can act as vitamin A precursors, but also for their coloring, antioxidant and possible tumor-inhibiting activity. Animals cannot synthesize carotenoids, and these pigments must therefore be added to the feeds of farmed species. The synthesis of different natural commercially important carotenoids (β-carotene, torulene, torularhodin and astaxanthin) by several yeast species belonging to the genera Rhodotorula and Phaffia has led to consider these microorganisms as a potential pigment sources. In this review, we discuss the biosynthesis, factors affecting carotenogenesis in Rhodotorula and Phaffia strains, strategies for improving the production properties of the strains and directions for potential utility of carotenoid-synthesizing yeast as a alternative source of natural carotenoid pigments.

Isoprene synthesis protects transgenic tobacco plants from oxidative stress.

Abstract: Isoprene emission represents a significant loss of carbon to those plant species that synthesize this highly volatile and reactive compound. As a tool for studying the role of isoprene in plant physiology and biochemistry, we developed transgenic tobacco plants capable of emitting isoprene in a similar manner to and at rates comparable to a naturally emitting species. Thermotolerance of photosynthesis against transient high-temperature episodes could only be observed in lines emitting high levels of isoprene; the effect was very mild and could only be identified over repetitive stress events. However, isoprene-emitting plants were highly resistant to ozone-induced oxidative damage compared with their non-emitting azygous controls. In ozone-treated plants, accumulation of toxic reactive oxygen species (ROS) was inhibited, and antioxidant levels were higher. Isoprene-emitting plants showed remarkably decreased foliar damage and higher rates of photosynthesis compared to non-emitting plants immediately following oxidative stress events. An inhibition of hydrogen peroxide accumulation in isoprene-emitting plants may stall the programmed cell death response which would otherwise lead to foliar necrosis. These results demonstrate that endogenously produced isoprene provides protection from oxidative damage.

Androgen action via testicular peritubular myoid cells is essential for male fertility

Abstract: Androgens are essential for normal spermatogenesis and male fertility, but how androgens exert this effect remains uncertain. Androgen receptors (ARs) are expressed in several testicular cell types, but continuing uncertainty exists over which cell type mediates androgen control of spermatogenesis. Androgen signaling via Sertoli cells (SCs) is essential for complete spermatogenesis, but the role for androgen signaling via peritubular myoid (PTM) cells is contentious. To address this controversy, we generated PTM-specific AR-knockout (PTM-ARKO) mice in which gross reproductive development was normal, but all PTM-ARKO males were azoospermic and infertile. Testis weight was reduced beyond puberty, and in adulthood there was an 86% reduction in germ cells, compared with wild-type littermates. These changes were not explained by any deficits in testosterone, luteinizing hormone, or follicle-stimulating hormone concentrations. SC function was impaired in PTM-ARKO males, indicated by reduced seminiferous tubule fluid production and reduced expression of some androgen-dependent SC genes. Androgen action via PTM cells is therefore essential for normal testis function, spermatogenesis, and fertility in males. This study also provides the first direct evidence for the importance of androgen-driven stromal-epithelial interactions underpinning the regulation of spermatogenesis; PTM-ARKO mice will enable identification of the new molecular pathways involved.—Welsh, M., Saunders, P. T. K., Atanassova, N., Sharpe, R. M., Smith, L. B. Androgen action via testicular peritubular myoid cells is essential for male fertility.

The effect of CeO2 on the surface and catalytic properties of Pt/CeO2–ZrO2 catalysts for methane dry reforming

Abstract: The CO2 reforming of CH4 over Pt catalysts supported on nanocrystalline mesoporous ZrO2 and CeO2–ZrO2 carriers was investigated at atmospheric pressure. The effect of CeO2 content (1–12 wt%) on the surface and catalytic properties of the catalysts was studied. It was found that the pre-treatment temperature and the concentration of CeO2 influence on the morphology of Pt particles. The calcination temperature as high as 1073 K leads to sintering of Pt particles deposited over zirconia- and CeO2-loaded zirconia substrates. Temperature-programmed reduction (TPR) results showed good reductive properties for Pt/CeO2–ZrO2 catalysts due to both, the high surface shell reduction of zirconia and the synergetic effect between Pt and CeO2. X-ray photoelectron spectroscopy (XPS) of reduced catalysts revealed the presence of different Pt oxidation state depending on the catalyst composition. Stabilization of partially oxidized platinum species by ceria was detected for the reduced Pt/CeO2–ZrO2 samples. An activation period was required for the stabilization of the activity of Pt/CeO2–ZrO2 catalysts. The high stability of Pt/CeO2–ZrO2 catalysts was related to the close contact between Pt and CeO2.

Bioprocessing of plant cell cultures for mass production of targeted compounds.

Abstract: More than a century has passed since the first attempt to cultivate plant cells in vitro. During this time, plant cell cultures have become increasingly attractive and cost-effective alternatives to classical approaches for the mass production of plant-derived metabolites. Furthermore, plant cell culture is the only economically feasible way of producing some high-value metabolites (e.g., paclitaxel) from rare and/or threatened plants. This review summarizes recent advances in bioprocessing aspects of plant cell cultures, from callus culture to product formation, with particular emphasis on the development of suitable bioreactor configurations (e.g., disposable reactors) for plant cell culture-based processes; the optimization of bioreactor culture environments as a powerful means to improve yields; bioreactor operational modes (fed-batch, continuous, and perfusion); and biomonitoring approaches. Recent trends in downstream processing are also considered.

The Impact of Rapid Climate Change on Prehistoric Societies during the Holocene in the Eastern Mediterranean

Abstract: In this paper we explore the impact of Rapid Climate Change (RCC) on prehistoric communities in the Eastern Mediterranean during the Early and Middle Holocene. Our focus is on the social implications of the four major climate cold anomalies that have recently been identified as key time-windows for global RCC (Mayewski et al. 2004). These cooling anomalies are well-dated, with Greenland ice-core resolution, due to synchronicity between warm/cold foraminifera ratios in Mediterranean core LC21 as a proxy for surface water temperature, and Greenland GISP2 non sea-salt (nss) [K+] ions as a proxy for the intensification of the Siberian High and for polar air outbreaks in the northeast Mediterranean (Rohling et al. 2002). Building on these synchronisms, the GISP2 agemodel supplies the following precise time-intervals for archaeological RCC research: (i) 8.6–8.0 ka, (ii) 6.0–5.2 ka, (iii) 4.2–4.0 ka and (iv) 3.1–2.9 ka calBP. For each of these RCC time intervals, based on detailed 14C-based chronological studies, we investigate contemporaneous cultural developments. From our studies it follows that RCC-related climatic deterioration is a major factor underlying social change, although always at work within a wide spectrum of social, cultural, economic and religious factors.

Electromagnetically induced transparency and slow light in an array of metallic nanoparticles

Abstract: We present a classical analog of electromagnetically induced transparency occurring when light is absorbed by a two-dimensional lattice of metallic spheres mounted on an asymmetric dielectric waveguide. The transparency is manifested as a spectral hole within the surface-plasmon absorption peak of the metallic spheres and is a result of destructive interference of the waveguide modes with incident radiation. The presence of transparency windows is accompanied by slow light effect wherein the group velocity is reduced by a factor of 6000. At the same time, the minimum length for storing a bit of information is of the order of 100 nm. The proposed setup is a much easier means to achieve transparency and slow light compared to existing atomic, solid-state, and photonic systems and allows for the realization of ultracompact optical delay lines and buffers.

Electrospun non-woven nanofibrous hybrid mats based on chitosan and PLA for wound-dressing applications.

Abstract: Continuous defect-free nanofibers containing chitosan (Ch) or quaternized chitosan (QCh) were successfully prepared by one-step electrospinning of Ch or QCh solutions mixed with poly[(L-lactide)-co-(D,L-lactide)] in common solvent. XPS revealed the surface chemical composition of the bicomponent electrospun mats. Crosslinked Ch- and QCh-containing nanofibers exhibited higher kill rates against bacteria S. aureus and E. coli than the corresponding solvent-cast films. SEM observations showed that hybrid mats were very effective in suppressing the adhesion of pathogenic bacteria S. aureus. The hybrid nanofibers are promising for wound-healing applications.

Red List of Bulgarian vascular plants

Abstract: The national threat status of 898 species of vascular plants from the Bulgarian flora has been evaluated using Version 3.1 of the IUCN Red List Categories and Criteria. The resulting Red List of the Bulgarian vascular plants (threat categories) comprises 801 species (20.5 % of the total flora), of which one Extinct (EX), 12 Regionally Extinct (RE), 208 Critically Endangered (CR), 297 Endangered (EN), 204 Vulnerable (VU), and 79 Near Threatened (NT). The list of other evaluated species comprises 96 taxa, of which, 53 Data Deficient (DD) and 43 Least Concern (LC).

MCM-41 supported PdNi catalysts for dry reforming of methane

Abstract: A series of bimetallic PdNi catalysts supported on mesoporous MCM-41 with different Ni content (Ni/Si ratio of 0.2–0.4) was synthesized. The effect of Pd addition to Ni-containing catalysts as well as the effect of the Ni content on the surface and catalytic properties of the catalysts was studied. The samples were characterized using various techniques, such as energy-dispersive X-ray spectroscopy, N2 adsorption–desorption isotherms, X-ray diffraction, thermogravimetric and differential analyses, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and temperature-programmed reduction. Reforming of methane with carbon dioxide was used as a test reaction. The results indicated that the addition of a small amount of Pd (0.5%) to Ni-containing catalysts leads to formation of small nano-sized, easy reducible NiO particles. Agglomeration of NiO as well as of metallic nickel phase over PdNi samples increased with increasing the Ni content. Formation of filamentous carbon over surface of spent monometallic Ni and bimetallic PdNi catalyst was observed. In spite of filamentous carbon deposition, the catalytic activity and stability of bimetallic PdNi catalysts are higher than those of monometallic Ni one. Within bimetallic system, the PdNi catalyst with Ni/Si ratio of 0.3 revealed the best performance and stability caused by presence of small nickel particles well dispersed on the catalyst surface.

High production of 2,3-butanediol from glycerol by Klebsiella pneumoniae G31

Abstract: The microbial production of high amounts of 2,3-butanediol (2,3-BD) from glycerol as a sole carbon source by the Bulgarian isolate Klebsiella pneumoniae G31 was studied in a series of fed-batch processes. The following conditions were evaluated as optimal: micro-aerobic cultivation in modified media, without pH control. Beginning at pH 8, 49.2 g/l of 2,3-BD was produced as negligible concentrations of by-products were received. The pH is the most important factor ruling the 2,3-BD production. Spontaneous pH changes and products formation in time were investigated, performing fermentations with non-controlled pH starting at different initial pH. In lack of external maintenance, the microorganism attempted to control the pH using acetate/2,3-BD alternations of the oxidative pathway of glycerol catabolism, which resulted in pH fluctuations. Thus, the culture secreted 2,3-BD at unequal portions, either allowing or detaining the acetate synthesis. More alkaline initial pH led to enhanced 2,3-BD accumulation as a response to the increased amplitudes of the pH variations. When the pH was maintained constant, the yield of 2,3-BD was very poor. These cultures remained viable only 72 h; whereas, the pH self-controlling cells lived and produced 2,3-BD up to 280 h. In conclusion, the formation of 2,3-BD is a result of an adaptive mechanism of pH self-control, responding to spontaneous pH drops during glycerol fermentation.

Liquid-crystalline nematic phase in aqueous suspensions of a disk-shaped natural beidellite clay.

Abstract: After size-selection and osmotic pressure measurements at fixed ionic strength, the behavior of aqueous colloidal suspensions of anisotropic disklike beidellite clay particles has been investigated by combining optical observations under polarized light, rheological, and small angle X-ray scattering (SAXS) experiments. The obtained phase diagrams (volume fraction/ionic strength) reveal, for ionic strength below 10−3 M/L, a first-order isotropic/nematic (I/N) phase transition before gel formation at low volume fractions, typically around 0.5%. This I/N transition line displays a positive slope for increasing ionic strength and shifts toward lower volume fraction with increasing particle size, confirming that the system is controlled by repulsive interactions. The swelling laws, derived from the interparticle distances obtained by SAXS, display a transition from isotropic swelling at low volume fractions to lamellar swelling at higher volume fractions. The liquid-crystal properties have then been investigat...

Drought stress effects on Rubisco in wheat: changes in the Rubisco large subunit

Abstract: The leaf protein pattern from drought-tolerant and drought-sensitive wheat varieties subjected to severe soil drought but with the possibility for recover from stress was studied by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). The spots representing Rubisco large subunit (RLS) were identified using polyclonal antibodies against Rubisco and immunoblotting. Some qualitative and quantitative differences in the 2D-PAGE protein map of wheat varieties were revealed under drought conditions. Three days recovery of wheat plants were not enough for restoring RLS quantity to the level of controls after 7 days drought, especially in the drought-sensitive variety Miziya. There are contradictory data in the literature concerning increased or diminished RLS level in drought stressed plants. A comparison of RLS after SDS-PAGE and 2D-PAGE was made. The revealed protein pattern depended on the presence or absence of protease inhibitors in the extraction buffer, on the procedure of extraction, and on the degree of stress.

Ecological thresholds in European alpine lakes

Abstract: SUMMARY 1. Species assemblages of diatoms, rotifers, chydorids, planktonic crustaceans and chironomids were studied in 235 alpine lakes in the Alps, Pyrenees, Tatras (Western Carpathians), Retezat (Southern Carpathians) and Rila Mountains (Balkans). 2. For all taxonomic groups we found a hierarchical structure in the community assemblage using distinct scales of lake clustering (number of k-means groups) based on species composition similarity (Hellinger distance). We determined the optimal partition in assemblage types (i.e. number of lake clusters) for each taxonomic group by maximising the sum of the taxon indicative value (IndVal) and performed discriminant analyses, using environmental variables not conditioned by geographical patterns. Relevant environmental variables differed among and within taxonomic groups. Therefore the assemblages respond to a complex environmental mosaic, with the exception of diatom assemblages, which followed an acid–base gradient. 3. The significant environmental variables could be grouped into four general factors: lake size, tropho-dynamic status, acid–base balance and ice-cover duration (i.e., altitudinal gradient). Lake size was significant for the highest number of assemblage types; however, the most significant factor differed among taxonomic groups: acid–base balance for diatoms, lake size for rotifers, ice-cover duration for chydorids and planktonic crustaceans and tropho-dynamic status for chironomids. No single environmental typology accounted for the assemblage structure of all taxonomic groups. 4. However, defining ecological thresholds as values within environmental gradients at which the rate of change in assemblages is accelerated relative to points distant from that

Levels of complexity in scale-invariant neural signals.

Abstract: Many physical and physiological signals exhibit complex scale-invariant features characterized by 1/f scaling and long-range power-law correlations, indicating a possibly common control mechanism. Specifically, it has been suggested that dynamical processes, influenced by inputs and feedback on multiple time scales, may be sufficient to give rise to 1/f scaling and scale invariance. Two examples of physiologic signals that are the output of hierarchical multiscale physiologic systems under neural control are the human heartbeat and human gait. Here we show that while both cardiac interbeat interval and gait interstride interval time series under healthy conditions have comparable 1/f scaling, they still may belong to different complexity classes. Our analysis of the multifractal scaling exponents of the fluctuations in these two signals demonstrates that in contrast to the multifractal behavior found in healthy heartbeat dynamics, gait time series exhibit less complex, close to monofractal behavior. Further, we find strong anticorrelations in the sign and close to random behavior for the magnitude of gait fluctuations at short and intermediate time scales, in contrast to weak anticorrelations in the sign and strong positive correlation for the magnitude of heartbeat interval fluctuations-suggesting that the neural mechanisms of cardiac and gait control exhibit different linear and nonlinear features. These findings are of interest because they underscore the limitations of traditional two-point correlation methods in fully characterizing physiological and physical dynamics. In addition, these results suggest that different mechanisms of control may be responsible for varying levels of complexity observed in physiological systems under neural regulation and in physical systems that possess similar 1/f scaling.