In silico evolution of diauxic growth

The glucose effect is a well known phenomenon whereby cells, when presented with two different nutrients, show a diauxic growth pattern, i.e. an episode of exponential growth followed by a lag phase of reduced growth followed by a second phase of exponential growth. Diauxic growth is usually thought of as a an adaptation to maximise biomass production in an environment offering two or more carbon sources. While diauxic growth has been studied widely both experimentally and theoretically, the hypothesis that diauxic growth is a strategy to increase overall growth has remained an unconfirmed conjecture. Here, we present a minimal mathematical model of a bacterial nutrient uptake system and metabolism. We subject this model to artificial evolution to test under which conditions diauxic growth evolves. As a result, we find that, indeed, sequential uptake of nutrients emerges if there is competition for nutrients and the metabolism/uptake system is capacity limited. However, we also find that diauxic growth is a secondary effect of this system and that the speed-up of nutrient uptake is a much larger effect. Notably, this speed-up of nutrient uptake coincides with an overall reduction of efficiency. Our two main conclusions are: (i) Cells competing for the same nutrients evolve rapid but inefficient growth dynamics. (ii) In the deterministic models we use here no substantial lag-phase evolves. This suggests that the lag-phase is a consequence of stochastic gene expression

Effect of composition on the structure of lithium- and manganese-rich transition metal oxides

The choice of chemical composition of lithium- and manganese-rich transition metal oxides used as cathode materials in lithium-ion batteries can significantly impact their long-term viability as storage solutions for clean energy automotive applications. Their structure has been widely debated: conflicting conclusions drawn from individual studies often considering different compositions have made it challenging to reach a consensus and inform future research. Here, complementary electron microscopy techniques over a wide range of length scales reveal the effect of lithium-to-transition metal-ratio on the surface and bulk structure of these materials. We found that decreasing the lithium-to-transition metal-ratio resulted in a significant change in terms of order and atomic-level local composition in the bulk of these cathode materials. However, throughout the composition range studied, the materials consisted solely of a monoclinic phase, with lower lithium content materials showing more chemical ordering defects. In contrast, the spinel-structured surface present on specific crystallographic facets exhibited no noticeable structural change when varying the ratio of lithium to transition metal. The structural observations from this study warrant a reexamination of commonly assumed models linking poor electrochemical performance with bulk and surface structure

Structure and lithium transport pathways in Li₂FeSiO₄ cathodes for lithium batteries

The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li(2)FeSiO(4). The structure of Li(2)FeSiO(4) undergoes significant change on cycling, from the as-prepared γ(s) form to an inverse β(II) polymorph; therefore it is important to establish the structure of the cycled material. In γ(s) half the LiO(4), FeO(4), and SiO(4) tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse β(II) polymorph on cycling involves inversion of half the SiO(4), FeO(4), and LiO(4) tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li(+) transport paths and corresponding Li-Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO(4) tetrahedra

The lag-phase during diauxic growth is a trade-off between fast adaptation and high growth rate

Bi-phasic or diauxic growth is often observed when microbes are grown in a chemically defined medium containing two sugars (for example glucose and lactose). Typically, the two growth stages are separated by an often lengthy phase of arrested growth, the so-called lag-phase. Diauxic growth is usually interpreted as an adaptation to maximise population growth in multi-nutrient environments. However, the lag-phase implies a substantial loss of growth during the switch-over. It therefore remains unexplained why the lag-phase is adaptive. Here we show by means of a stochastic simulation model based on the bacterial PTS system that it is not possible to shorten the lag-phase without incurring a permanent growth-penalty. Mechanistically, this is due to the inherent and well established limitations of biological sensors to operate efficiently at a given resource cost. Hence, there is a trade-off between lost growth during the diauxic switch and the long-term growth potential of the cell. Using simulated evolution we predict that the lag-phase will evolve depending on the distribution of conditions experienced during adaptation. In environments where switching is less frequently required, the lag-phase will evolve to be longer whereas, in frequently changing environments, the lag-phase will evolve to be shorter

Le compartiment sauvage de la carotte en France : des ressources génétiques importantes et pourtant méconnues

La France est considérée comme centre secondaire de diversification de la carotte, du fait de l’activité importante de sélection (par des maraîchers ou semenciers), ce qui justifie les actions de conservation et connaissance des variétés anciennes menées par le réseau de ressources génétiques « Carotte et autres Daucus ». Par contre, le compartiment sauvage est méconnu et sous-exploité, alors qu’il s’agit d’une espèce pour laquelle de nombreuses populations existent sur le territoire français, avec une situation très contrastée. Si la carotte sauvage D. carota spp carota n’est pas en danger, d’autres sous espèces sont protégées (ssp gadecaei) ou menacées du fait de la dégradation de leur milieux naturels notamment en zone littorale ou de possibles introgressions avec la sous-espèce carota. Le travail présenté porte donc sur : i/ la sauvegarde et la mise à disposition des ressources génétiques sauvages, à travers l’inventaire de populations in situ et la constitution de collections ex-situ ; ii/ l’approfondissement de la connaissance et de l’identification taxonomique des sous-espèces sauvages ; iii/ la connaissance de la diversité au sein du compartiment sauvage (marqueurs SSR et données écologiques) ; et iv/ l’évaluation des ressources génétiques sauvages de carotte pour permettre leur exploitation (fertilité, croisements avec le compartiment cultivé, tolérance à différents bioagresseurs). Ce programme fait l’objet du soutien d’un contrat de branche du ministère de l’agriculture et implique les membres du réseau « Carotte et autres Daucus ».

Growth dynamics of single bacterial cells

BionanoscienceApplied Science

Mechanochemical synthesis of Li-argyrodite Li 6PS 5X (X = Cl, Br, I) as sulfur-based solid electrolytes for all solid state batteries application

Highly ion-conductive Li 6PS 5X (X = Cl, Br, I) Li-argyrodites were prepared through a high-energy ball milling. Electrical and electrochemical properties were investigated. Ball-milled compounds exhibit a high conductivity between 2 and 7 × 10 - 4 S/cm with an activation energy of 0.3-0.4 eV for conduction. These attractive properties were attributed to the spontaneous formation of crystallized Li-argyrodite during ball-milling. An optimization of milling time led to a conductivity of 1.33 × 10 - 3 S/cm for the Li 6PS 5Cl phase with an electrochemical stability up to 7 V vs. lithium. An all solid state LiCoO 2/Elec./In lithium ion battery using ball-milled Li 6PS 5Cl as electrolyte was successfully assembled, and its room temperature performance is reported. © 2012 Elsevier B.V. All rights reserved

Electrochemical properties of all-solid-state lithium secondary batteries using Li-argyrodite Li6PS5Cl as solid electrolyte

Solid-state electrolytes are being intensively explored as the key to develop all-solid-lithium secondary batteries which present safety advantages over present liquid Li-ion technology. To validate the use of the Li 6PS5Cl-based solid electrolyte obtained by high energy ball milling, all-solid-state batteries using LiCoO2 and Li 4Ti5O12 as active material (AM) have been realized. The optimization of the electrode composition led to a maximum of 46 and 27 mAh per gram of composite for LiCoO2 and Li4Ti 5O12-based half-cells respectively. The assembled all-solid-state LiCoO2/Li6PS5Cl/Li 4Ti5O12 battery presents a sustainable reversible capacity of 27 mAh per gram of AM and a coulombicic efficiency close to 99%. © 2013 Elsevier B.V

New li ion conductors and solid state hydrogen storage materials: LiM(BH 4) 3Cl, M = La, Gd

Multiple reaction mixtures with different composition ratios of MCl 3-LiBH 4 (M = La, Gd) were studied by mechano-chemical synthesis, yielding two new bimetallic borohydride chlorides, LiM(BH 4) 3Cl (M = La, Gd). The Gd-containing phase was obtained only after annealing the ball-milled mixture. Additionally, a solvent extracted sample of Gd(BH 4) 3 was studied to gain insight into the transformation from Gd(BH 4) 3 to LiGd(BH 4) 3Cl. The novel compounds were investigated using in situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectroscopy, Sieverts measurements, Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The two new compounds, LiLa(BH 4) 3Cl and LiGd(BH 4) 3Cl, have high lithium ion conductivities of 2.3 ×10 -4 and 3.5 ×10 -4 S•cm -1 (T = 20 °C) and high hydrogen densities of ρ m = 5.36 and 4.95 wt % H 2, and both compounds crystallize in the cubic crystal system (space group I-43m) with unit cell parameter a = 11.7955(1) and a = 11.5627(1) Å, respectively. The structures contain isolated tetranuclear anionic clusters [M 4Cl 4(BH 4) 12] 4- with distorted cubane M 4Cl 4 cores M = La or Gd. Each lanthanide atom coordinates three chloride ions and three borohydride groups, thus completing the coordination environment to an octahedron. The Li + ions are disordered on 2/3 of the 12d Wyckoff site, which agrees well with the very high lithium ion conductivities. The conductivity is purely ionic, as electronic conductivities were measured to only 1.4 ×10 -8 and 9 ×10 -8 S•cm -1 at T = 20 °C for LiLa(BH 4) 3Cl and LiGd(BH 4) 3Cl, respectively. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) reveals that the decomposition products at 300 °C consist of LaB 6/LaH 2 or GdB 4/GdH 2 and LiCl. The size of the rare-earth metal atom is shown to be crucial for the formation and stability of the borohydride phases in MCl 3-LiBH 4 systems. © 2012 American Chemical Society