Nitrate Transport and Not Photoinhibition Limits Growth of the Freshwater Cyanobacterium Synechococcus Species PCC 6301 at Low Temperature1

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20°C and 38°C, the growth rate decreased with decreasing temperature, and growth ceased at 15°C. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38°C remained at 15°C. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m−2 s−1) at 15°C. Little or no loss of oxygen evolution was observed under the normal light intensity (250 μE m−2 s−1) for growth at 15°C. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15°C, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.

Interaction of Osmotic Stress, Temperature, and Abscisic Acid in the Regulation of Gene Expression in Arabidopsis

The impact of simultaneous environmental stresses on plants and how they respond to combined stresses compared with single stresses is largely unclear. By using a transgene (RD29A-LUC) consisting of the firefly luciferase coding sequence (LUC) driven by the stress-responsive RD29A promoter, we investigated the interactive effects of temperature, osmotic stress, and the phytohormone abscisic acid (ABA) in the regulation of gene expression in Arabidopsis seedlings. Results indicated that both positive and negative interactions exist among the studied stress factors in regulating gene expression. At a normal growth temperature (22°C), osmotic stress and ABA act synergistically to induce the transgene expression. Low temperature inhibits the response to osmotic stress or to combined treatment of osmotic stress and ABA, whereas low temperature and ABA treatments are additive in inducing transgene expression. Although high temperature alone does not activate the transgene, it significantly amplifies the effects of ABA and osmotic stress. The effect of multiple stresses in the regulation of RD29A-LUC expression in signal transduction mutants was also studied. The results are discussed in the context of cold and osmotic stress signal transduction pathways.

Expression of Small Heat-Shock Proteins at Low Temperatures1 : A Possible Role in Protecting against Chilling Injuries

We previously reported that short exposure of tomato (Lycopersicon esculentum L.) fruits to high temperature protects them from chilling injury. To study the involvement of heat-shock proteins (HSPs) in the acquisition of low-temperature tolerance, we cloned two heat-shock-induced genes that are also expressed at low temperatures. The cloned cDNAs belong to the small HSP group. Sequence analyses of the clones showed perfect homology to the tomato-ripening gene tom66 and to the tomato chloroplastic HSP21 gene tom111. The expression of both genes was induced by high temperature in fruits, flowers, leaves, and stems, but not by low or ambient temperatures or by other stresses such as drought and anaerobic conditions. When the heated fruits were transferred to low temperature, tom66 and tom111 mRNA levels first decreased but were then reinduced. Induction was not observed in nonheated fruits at low temperature. Immunodetection of tom111-encoded protein indicated that this protein is present at low temperatures in the heated fruits. The results of this study show that the expression of tom66 and tom111 is correlated with protection against some, but not all, symptoms of chilling injury.

Bathorhodopsin structure in the room-temperature rhodopsin photosequence: picosecond time-resolved coherent anti-Stokes Raman scattering.

Structural changes in the retinal chromophore during the formation of the bathorhodopsin intermediate (bathoRT) in the room-temperature rhodopsin (RhRT) photosequence (i.e., vision) are examined using picosecond time-resolved coherent anti-Stokes Raman scattering. Specifically, the retinal structure assignable to bathoRT following 8-ps excitation of RhRT is measured via vibrational Raman spectroscopy at a 200-ps time delay where the only intermediate present is bathoRT. Significant differences are observed between the C=C stretching frequencies of the retinal chromophore at low temperature where bathorhodopsin is stabilized and at room temperature where bathorhodopsin is a transient species in the RhRT photosequence. These vibrational data are discussed in terms of the formation of bathoRT, an important step in the energy storage/transduction mechanism of RhRT.

Photocatalytic oxidation of propene in gas phase at low concentration by optimized TiO2 nanoparticles

In the present study, nanocrystalline titanium dioxide (TiO2) was prepared by sol–gel method at low temperature from titanium tetraisopropoxide (TTIP) and characterized by different techniques (gas adsorption, XRD, TEM and FTIR). Variables of the synthesis, such as the hydrolyzing agent (acetic acid or isopropanol) and calcination temperatures (300–800 °C), were analyzed to get uniform size TiO2 nanoparticles. The effect that these two variables have on the structure of the resultant TiO2 nanoparticles and on their photocatalytic activity is investigated. The photocatalytic activities of TiO2 nanoparticles were evaluated for propene oxidation at low concentration (100 ppmv) under two different kinds of UV light (UV-A ∼ 365 nm and UV-C ∼ 257.7 nm) and compared with Degussa TiO2 P-25, used as reference sample. The results show that both hydrolyzing agents allow to prepare TiO2 nanoparticles and that the hydrolyzing agent influences the crystalline structure and its change with the thermal treatments. Interestingly, the prepared TiO2 nanoparticles possess anatase phase with small crystalline size, high surface area and higher photocatalytic activity for propene oxidation than commercial TiO2 (Degussa P-25) under UV-light. Curiously, these prepared TiO2 nanoparticles are more active with the 365 nm source than with the 257.7 nm UV-light, which is a remarkable advantage from an application point of view. Additionally, the obtained results are particularly good when acetic acid is the hydrolyzing agent at both wavelengths used, possibly due to the high crystallinity, low anatase phase size and high surface oxygen groups’ content in the nanoparticles prepared with it, in comparison to those prepared using isopropanol.

Computational modeling of intermediate temperature proton exchange membrane (PEM) fuel cells

A two-phase three-dimensional computational model of an intermediate temperature (120--190°C) proton exchange membrane (PEM) fuel cell is presented. This represents the first attempt to model PEM fuel cells employing intermediate temperature membranes, in this case, phosphoric acid doped polybenzimidazole (PBI). To date, mathematical modeling of PEM fuel cells has been restricted to low temperature operation, especially to those employing Nafion ® membranes; while research on PBI as an intermediate temperature membrane has been solely at the experimental level. This work is an advancement in the state of the art of both these fields of research. With a growing trend toward higher temperature operation of PEM fuel cells, mathematical modeling of such systems is necessary to help hasten the development of the technology and highlight areas where research should be focused.^ This mathematical model accounted for all the major transport and polarization processes occurring inside the fuel cell, including the two phase phenomenon of gas dissolution in the polymer electrolyte. Results were presented for polarization performance, flux distributions, concentration variations in both the gaseous and aqueous phases, and temperature variations for various heat management strategies. The model predictions matched well with published experimental data, and were self-consistent.^ The major finding of this research was that, due to the transport limitations imposed by the use of phosphoric acid as a doping agent, namely low solubility and diffusivity of dissolved gases and anion adsorption onto catalyst sites, the catalyst utilization is very low (∼1--2%). Significant cost savings were predicted with the use of advanced catalyst deposition techniques that would greatly reduce the eventual thickness of the catalyst layer, and subsequently improve catalyst utilization. The model also predicted that an increase in power output in the order of 50% is expected if alternative doping agents to phosphoric acid can be found, which afford better transport properties of dissolved gases, reduced anion adsorption onto catalyst sites, and which maintain stability and conductive properties at elevated temperatures.^

Design of a low power - high temperature heated ceramic sensor to detect halogen gases

The design, construction and optimization of a low power-high temperature heated ceramic sensor to detect leaking of halogen gases in refrigeration systems are presented. The manufacturing process was done with microelectronic assembly and the Low Temperature Cofire Ceramic (LTCC) technique. Four basic sensor materials were fabricated and tested: Li2SiO3, Na2SiO3, K2SiO3, and CaSiO 3. The evaluation of the sensor material, sensor size, operating temperature, bias voltage, electrodes size, firing temperature, gas flow, and sensor life was done. All sensors responded to the gas showing stability and reproducibility. Before exposing the sensor to the gas, the sensor was modeled like a resistor in series and the calculations obtained were in agreement with the experimental values. The sensor response to the gas was divided in surface diffusion and bulk diffusion; both were analyzed showing agreement between the calculations and the experimental values. The sensor with 51.5%CaSiO3 + 48.5%Li 2SiO3 shows the best results, including a stable current and response to the gas. ^

Design of a Low Power – High Temperature Heated Ceramic Sensor to Detect Halogen Gases

The design, construction and optimization of a low power-high temperature heated ceramic sensor to detect leaking of halogen gases in refrigeration systems are presented. The manufacturing process was done with microelectronic assembly and the Low Temperature Cofire Ceramic (LTCC) technique. Four basic sensor materials were fabricated and tested: Li2SiO3, Na2SiO3, K2SiO3, and CaSiO3. The evaluation of the sensor material, sensor size, operating temperature, bias voltage, electrodes size, firing temperature, gas flow, and sensor life was done. All sensors responded to the gas showing stability and reproducibility. Before exposing the sensor to the gas, the sensor was modeled like a resistor in series and the calculations obtained were in agreement with the experimental values. The sensor response to the gas was divided in surface diffusion and bulk diffusion; both were analyzed showing agreement between the calculations and the experimental values. The sensor with 51.5%CaSiO3 + 48.5%Li2SiO3 shows the best results, including a stable current and response to the gas.

Functional genomic and molecular analyses of transcripts related to glycerol synthesis and trafficking in rainbow smelt (Osmerus mordax) using cold temperature-induced models of glycerol production

The rainbow smelt (Osmerus mordax) is an anadromous teleost that produces type II antifreeze protein (AFP) and accumulates modest urea and high glycerol levels in plasma and tissues as adaptive cryoprotectant mechanisms in sub-zero temperatures. It is known that glyceroneogenesis occurs in liver via a branch in glycolysis and gluconeogenesis and is activated by low temperature; however, the precise mechanisms of glycerol synthesis and trafficking in smelt remain to be elucidated. The objective of this thesis was to provide further insight using functional genomic techniques [e.g. suppression subtractive hybridization (SSH) cDNA library construction, microarray analyses] and molecular analyses [e.g. cloning, quantitative reverse transcription - polymerase chain reaction (QPCR)]. Novel molecular mechanisms related to glyceroneogenesis were deciphered by comparing the transcript expression profiles of glycerol (cold temperature) and non-glycerol (warm temperature) accumulating hepatocytes (Chapter 2) and livers from intact smelt (Chapter 3). Briefly, glycerol synthesis can be initiated from both amino acids and carbohydrate; however carbohydrate appears to be the preferred source when it is readily available. In glycerol accumulating hepatocytes, levels of the hepatic glucose transporter (GLUT2) plummeted and transcript levels of a suite of genes (PEPCK, MDH2, AAT2, GDH and AQP9) associated with the mobilization of amino acids to fuel glycerol synthesis were all transiently higher. In contrast, in glycerol accumulating livers from intact smelt, glycerol synthesis was primarily fuelled by glycogen degradation with higher PGM and PFK (glycolysis) transcript levels. Whether initiated from amino acids or carbohydrate, there were common metabolic underpinnings. Increased PDK2 (an inhibitor of PDH) transcript levels would direct pyruvate derived from amino acids and / or DHAP derived from G6P to glycerol as opposed to oxidation via the citric acid cycle. Robust LIPL (triglyceride catabolism) transcript levels would provide free fatty acids that could be oxidized to fuel ATP synthesis. Increased cGPDH (glyceroneogenesis) transcript levels were not required for increased glycerol production, suggesting that regulation is more likely by post-translational modification. Finally, levels of a transcript potentially encoding glycerol-3-phosphatase, an enzyme not yet characterized in any vertebrate species, were transiently higher. These comparisons also led to the novel discoveries that increased G6Pase (glucose synthesis) and increased GS (glutamine synthesis) transcript levels were part of the low temperature response in smelt. Glucose may provide increased colligative protection against freezing; whereas glutamine could serve to store nitrogen released from amino acid catabolism in a non-toxic form and / or be used to synthesize urea via purine synthesis-uricolysis. Novel key aspects of cryoprotectant osmolyte (glycerol and urea) trafficking were elucidated by cloning and characterizing three aquaglyceroporin (GLP)-encoding genes from smelt at the gene and cDNA levels in Chapter 4. GLPs are integral membrane proteins that facilitate passive movement of water, glycerol and urea across cellular membranes. The highlight was the discovery that AQP10ba transcript levels always increase in posterior kidney only at low temperature. This AQP10b gene paralogue may have evolved to aid in the reabsorption of urea from the proximal tubule. This research has contributed significantly to a general understanding of the cold adaptation response in smelt, and more specifically to the development of a working scenario for the mechanisms involved in glycerol synthesis and trafficking in this species.

Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)

Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.

Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature gradient, with application to “Snowball Earth"

When NaCl precipitates out of a saturated solution, it forms anhydrous crystals of halite at temperatures above +0.11?C, but at temperatures below this threshold it instead precipitates as the dihydrate ‘‘hydrohalite,’’ NaCl * 2H2O. When sea ice is cooled, hydrohalite begins to precipitate within brine inclusions at about -23C. In this work, hydrohalite crystals are examined in laboratory experiments: their formation, their shape, and their response to warming and desiccation. Sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite, which has the character of a fine powder. The precipitation of hydrohalite in brine inclusions raises the albedo of sea ice, and the subsequent formation of a surface accumulation further raises the albedo. Although these processes have limited climatic importance on the modern Earth, they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a Snowball Earth event. However, brine inclusions in sea ice migrate downward to warmer ice, so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration. The migration rates are measured in a laboratory experiment at temperatures from -2C to -32C; the migration appears to be too slow to prevent formation of a salt crust on Snowball Earth.

Modelling of ORC components and systems for low-medium temperature heat recovery applications with reduced environmental impact

Although its great potential as low to medium temperature waste heat recovery (WHR) solution, the ORC technology presents open challenges that still prevent its diffusion in the market, which are different depending on the application and the size at stake. Focusing on the micro range power size and low temperature heat sources, the ORC technology is still not mature due to the lack of appropriate machines and working fluids. Considering instead the medium to large size, the technology is already available but the investment is still risky. The intention of this thesis is to address some of the topical themes in the ORC field, paying special attention in the development of reliable models based on realistic data and accounting for the off-design performance of the ORC system and of each of its components. Concerning the “Micro-generation” application, this work: i) explores the modelling methodology, the performance and the optimal parameters of reciprocating piston expanders; ii) investigates the performance of such expander and of the whole micro-ORC system when using Hydrofluorocarbons as working fluid or their new low GWP alternatives and mixtures; iii) analyzes the innovative ORC reversible architecture (conceived for the energy storage), its optimal regulation strategy and its potential when inserted in typical small industrial frameworks. Regarding the “Industrial WHR” sector, this thesis examines the WHR opportunity of ORCs, with a focus on the natural gas compressor stations application. This work provides information about all the possible parameters that can influence the optimal sizing, the performance and thus the feasibility of installing an ORC system. New WHR configurations are explored: i) a first one, relying on the replacement of a compressor prime mover with an ORC; ii) a second one, which consists in the use of a supercritical CO2 cycle as heat recovery system.

Optimization and development of low environmental impact propulsion systems

The aim of the Ph.D. research project was to explore Dual Fuel combustion and hybridization. Natural gas-diesel Dual Fuel combustion was experimentally investigated on a 4-Stroke, 2.8 L, turbocharged, light-duty Diesel engine, considering four operating points in the range between low to medium-high loads at 3000 rpm. Then, a numerical analysis was carried out using a customized version of the KIVA-3V code, in order to optimize the diesel injection strategy of the highest investigated load. A second KIVA-3V model was used to analyse the interchangeability between natural gas and biogas on an intermediate operating point. Since natural gas-diesel Dual Fuel combustion suffers from poor combustion efficiency at low loads, the effects of hydrogen enriched natural gas on Dual Fuel combustion were investigated using a validated Ansys Forte model, followed by an optimization of the diesel injection strategy and a sensitivity analysis to the swirl ratio, on the lowest investigated load. Since one of the main issues of Low Temperature Combustion engines is the low power density, 2-Stroke engines, thanks to the double frequency compared to 4-Stroke engines, may be more suitable to operate in Dual Fuel mode. Therefore, the application of gasoline-diesel Dual Fuel combustion to a modern 2-Stroke Diesel engine was analysed, starting from the investigation of gasoline injection and mixture formation. As far as hybridization is concerned, a MATLAB-Simulink model was built to compare a conventional (combustion) and a parallel-hybrid powertrain applied to a Formula SAE race car.

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In(0.4)Ga(0.6) As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Influence of temperature and excitation procedure on the athermal behavior of Nd(3+)-doped phosphate glass: Thermal lens, interferometric, and calorimetric measurements

In this work, thermal and optical properties of the commercial Q-98 neodymium-doped phosphate glass have been measured at low temperature, from 50 to 300 K. The time-resolved thermal lens spectrometry together with the optical interferometry and the thermal relaxation calorimetry methods were used to investigate the glass athermal characteristics described by the temperature coefficient of the optical path length change, ds/dT. The thermal diffusivity was also determined, and the temperature coefficients of electronic polarizability, linear thermal expansion, and refractive index were calculated and used to explain ds/dT behavior. ds/dT measured via thermal lens method was found to be zero at 225 K. The results provided a complete characterization of the thermo-optical properties of the Q-98 glass, which may be useful for those using this material for diode-pumped solid-state lasers. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3234396]