Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces

Relaxed atomic geometries and chemisorption energies have been calculated for the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces. We employ density-functional theory, together with a pseudopotential for Si, and apply the generalized gradient approximation by Perdew and Wang to the exchange-correlation functional. We find the double-atomic-height rebonded D_B step, which is known to be stable on the clean surface, to remain stable on partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si atoms at the rebonded step edge, with a chemisorption energy difference with respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single atomic height S_A and S_B steps gives very similar results. The interaction between H-Si-Si-H mono-hydride units is shown to be unimportant for the calculation of the step-edge hydrogen-occupation. Our results confirm the interpretation and results of the recent H_2 adsorption experiments on vicinal Si surfaces by Raschke and Hoefer described in the preceding paper.Comment: 13 pages, 8 figures, submitted to Phys. Rev. B. Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Implications of Extreme Life Span in Clonal Organisms: Millenary Clones in Meadows of the Threatened Seagrass Posidonia oceanica

The maximum size and age that clonal organisms can reach remains poorly known, although we do know that the largest natural clones can extend over hundreds or thousands of metres and potentially live for centuries. We made a review of findings to date, which reveal that the maximum clone age and size estimates reported in the literature are typically limited by the scale of sampling, and may grossly underestimate the maximum age and size of clonal organisms. A case study presented here shows the occurrence of clones of slow-growing marine angiosperm Posidonia oceanica at spatial scales ranging from metres to hundreds of kilometres, using microsatellites on 1544 sampling units from a total of 40 locations across the Mediterranean Sea. This analysis revealed the presence, with a prevalence of 3.5 to 8.9%, of very large clones spreading over one to several (up to 15) kilometres at the different locations. Using estimates from field studies and models of the clonal growth of P. oceanica, we estimated these large clones to be hundreds to thousands of years old, suggesting the evolution of general purpose genotypes with large phenotypic plasticity in this species. These results, obtained combining genetics, demography and model-based calculations, question present knowledge and understanding of the spreading capacity and life span of plant clones. These findings call for further research on these life history traits associated with clonality, considering their possible ecological and evolutionary implications

Harnessing the NEON data revolution to advance open environmental science with a diverse and data-capable community

It is a critical time to reflect on the National Ecological Observatory Network (NEON) science to date as well as envision what research can be done right now with NEON (and other) data and what training is needed to enable a diverse user community. NEON became fully operational in May 2019 and has pivoted from planning and construction to operation and maintenance. In this overview, the history of and foundational thinking around NEON are discussed. A framework of open science is described with a discussion of how NEON can be situated as part of a larger data constellation—across existing networks and different suites of ecological measurements and sensors. Next, a synthesis of early NEON science, based on >100 existing publications, funded proposal efforts, and emergent science at the very first NEON Science Summit (hosted by Earth Lab at the University of Colorado Boulder in October 2019) is provided. Key questions that the ecology community will address with NEON data in the next 10 yr are outlined, from understanding drivers of biodiversity across spatial and temporal scales to defining complex feedback mechanisms in human–environmental systems. Last, the essential elements needed to engage and support a diverse and inclusive NEON user community are highlighted: training resources and tools that are openly available, funding for broad community engagement initiatives, and a mechanism to share and advertise those opportunities. NEON users require both the skills to work with NEON data and the ecological or environmental science domain knowledge to understand and interpret them. This paper synthesizes early directions in the community’s use of NEON data, and opportunities for the next 10 yr of NEON operations in emergent science themes, open science best practices, education and training, and community building

In-plane/tilted magnetic-field-dependent conductance of 2D electron systems in asymmetric double quantum wells.

The resistance in an asymmetric double-well structure was measured as a function of magnetic fields oriented almost parallel to the plane of the electron layer. It was shown that the shape of the magnetoresistance curves is close to the in-plane magnetic-field-dependent density of states which we obtained by self-consistent numerical calculation. The novel feature is the negative magnetoresistance observed at low magnetic fields

Production of γ-aminobutyric acid (GABA) by lactic acid bacteria strains isolated from traditional, starter-free dairy products made of raw milk

γ-Aminobutyric acid (GABA), an amino acid not used in protein synthesis, intervenes in several physiological functions and has both diuretic and calming effects in humans. Lactic acid bacteria (LAB) strains that produce GABA could be exploited for the manufacture of health-promoting GABA-enriched dairy products. In this study, 262 LAB strains isolated from traditional dairy products made from raw milk without starter cultures were screened for GABA production in culture media supplemented with 1% monosodium glutamate (MSG) using an enzymatic (GABase) method. About half of the strains (123) were found to be GABA producers. Of these, 24, among which were 16 Lactococcus lactis subsp. lactis and three Streptococcus thermophilus strains, produced >1 mM of GABA (range 1.01-2.81 mM) and were selected for further characterisation. GABA production was confirmed in most strains by culturing in 5 mM MSG followed by HPLC quantification. A majority of the strains were confirmed to be GABA producers by this method, although lower production levels were recorded. Using species-specific primers, the gene encoding glutamate decarboxylase (GAD) was PCR-amplified in all but one of the GABA producers analysed. Amplicons sequences were compared to one another and to those held in databases. Except for one Lactobacillus brevis strain, none of the 24 GABA producers investigated produced toxic biogenic amines, such as tyramine, histamine or cadaverine. They were therefore considered safe. Either alone, in mixtures, or in combination with industrial starter or adjunct cultures, these strains might be useful in the development of health-oriented dairy products.This work was partially supported by a project from the Plan for Science, Technology and Innovation of the Asturias Principality, co-funded by FEDER (Ref.335 GRUPIN14-137)

Producción de ácido linoleico conjugado por bacterias ácido-lácticas y bifidobacterias de origen lácteo e intestinal

Trabajo presentado en el VII Congreso Internacional de Ciencia y Tecnología de Alimentos, celebrado en Córdoba (Argentina), del 1 al 3 de octubre de 2018Los ácidos linoleicos conjugados (CLA, conjugated linoleic acid) comprenden una familia de más de veinte isómeros del ácido linoleico (LA, linoleic acid). Los más abundantes en los alimentos son 9Z,11E-18:2 y 10E,12Z-18:2. Entre los alimentos que contienen CLA podemos citar la carne de cordero, ternera, pavo y los productos lácteos grasos (donde >80% corresponde a los isómeros mencionados). Los CLA tiene interés por su actividad antimutagénica y anticancerígena. Además, poseen actividad antioxidante y participan en la modulación de la respuesta inmune y en el metabolismo lipídico (efectos antiaterogénico e hipocolesterolémico). El objetivo de este trabajo fue analizar la producción de CLA por cepas de bacterias ácido-lácticas y bifidobacterias de origen lácteo e intestinal mediante un método rápido y sensible. La producción de CLA se evaluó en medio de cultivo suplementado con 0,5 mg/mL de LA. Se analizaron 265 cepas de los géneros Lactococcus (Lc.), Lactobacillus, Bifidobacterium, Enterococcus, Leuconostoc y Streptococcus (S.), incluyendo una elevada representación de las especies de mayor interés industrial para la formulación de fermentos: Lc. lactis (54%) y S. thermophilus (15%). Los medios de cultivo empleados fueron: MRS (Oxoid) para los bacilos; M17 (Oxoid) para los cocos, con adición de glucosa o lactosa (1%) según requerimientos especie-específicos; y Elliker (Biokar) para Leuconostoc spp. Los microorganismos se cultivaron en condiciones óptimas de temperatura (32, 37, o 40ºC) y en aerobiosis o anaerobiosis según correspondiera. La producción de CLA se detectó y cuantificó según el procedimiento de Barrett y colaboradores (Barret et al., 2007. Appl. Environ. Microbiol. 73(7), 2333-2337) midiendo la absorbancia en la fase hexánica a 233 nm tras una extracción lipídica con isopropanol-hexano. Las curvas de calibrado se obtuvieron utilizando los isómeros comerciales de CLA 10E,12Z y 9Z,11E (Sigma-Aldrich). A partir de la representación de absorbancia vs. concentración se obtuvieron las ecuaciones y=45,58x-0,0442; R2= 0,9958 y y=7,2315x-0,0214; R2= 0,9926, respectivamente. De entre las cepas evaluadas se identificaron 17 aislados capaces de producir más de 25 ppm de CLA. Estos pertenecían a las especies Lc. lactis (14 cepas) y S. thermophilus (3 cepas). Tras su caracterización tecnológica, los microorganismos productores de CLA podrían servir para la elaboración de productos lácteos funcionales enriquecidos en esta sustancia bioactiva.Peer reviewe

Análisis genómico de cepas de Lactococcus lactis subsp. lactis y Streptococcus thermophilus productoras de ácido gamma-aminobutírico (GABA)

Trabajo presentado al XXII Congreso Nacional de Microbiología de los Alimentos, celebrado en Jaén (España), del 12 al 15 de septiembre de 2022El GABA es un aminoácido no proteico ampliamente distribuido en animales y plantas que se sintetiza a partir del ácido glutámico por la enzima glutamato descarboxilasa (GAD), utilizando fosfato de piridoxal (vitamina B6) como cofactor. El GABA actúa como neurotransmisor en el sistema nervioso central de los mamíferos. Su ingesta disminuye la presión arterial y reduce el estrés psicológico, comportándose como un tranquilizante. Debido a sus efectos beneficiosos sobre la salud, la industria alimentaria está muy interesada en la producción de alimentos enriquecidos con GABA. La utilización de bacterias ácido-lácticas (BAL) productoras de GABA permitiría la producción de este compuesto funcional durante la elaboración y maduración de los productos lácteos fermentados. Lactococcus lactis y Streptococcus thermophilus se encuentran entre las bacterias ácido-lácticas (BAL) más utilizadas como fermentos. En un grupo de aislados de estas dos especies procedentes de leche cruda se detectaron varias cepas productoras de GABA a partir del glutamato monosódiso. Con el objetivo de determinar los componentes de la base genética de producción de GABA y su estructura, y contribuir al mismo tiempo a caracterizar la seguridad alimentaria de las cepas productoras, se seleccionaron cuatro cepas de cada especie y se sometieron a secuenciación y análisis genómico. En esta comunicación se presentan los datos genéticos básicos de producción de GABA en las cepas estudiadas, se determinan y se comparan los operones dedicados a la producción de este compuesto funcional en las cepas de L. lactis y S. thermophilus estudiadas y se analiza la presencia de genes involucrados en seguridad alimentaria, incluyendo genes de patogenicidad y virulencia, resistencia a antibióticos y de producción de aminas biógenas. En la mayoría de los casos, los datos genéticos corroboraron el fenotipo de las cepas