Modification of streaming potential by precipitation of calcite in a sand-water system: laboratory measurements pH range from 4 to 12

Spontaneous Potentials associated with volcanic activity are often interpreted by means of the electrokinetic potential, which is usually positive in the flow direction (i.e. Zeta potential of the rock is negative). The water-rock interactions in hydrothermal zones alter the primary minerals leading to the formation of secondary minerals. This work addresses the study of calcite precipitation in a sand composed of 98% quartz and 2% calcite using streaming potential measurements. The precipitation of calcite as a secondary mineral phase, inferred by high calcite saturation indices and by a fall in permeability, has a significant effect on the electrokinetic behaviour, leading to a significant reduction in the Zeta potential (in absolute value) and even a change in sign. The measured decrease in Zeta potential from -16 mV to -27±4 mV takes place as the pH rises from 4 to 7, while it remains constant at -25±1 mV as the pH increases from 8 to 10.5. For pH higher than 10.5, calcite precipitates and is expected to coat the quartz surface. The measured Zeta potential vary from -17 to +8 mV for pH ranging from 10.6 to 11.7 depending on the amount of precipitated calcite indicated by the decrease in permeability. The observed change in sign of the electrical surface potential rules out the usual qualitative interpretation of SP anomalies in order to determine fluid circulations, even at pH lower than 9 if calcite is widely present as a secondary mineral phase, since the electrical surface potential of calcite depends also on CO2 partial pressure and [Ca2+]. Therefore SP anomalies as measured in hydrothermal field, without mineralogical analyses of hydrothermal deposits, and without geochemical fluid survey, should be interpreted with caution.Comment: The definitive version is available at www.blackwell-synergy.com; We acknowledge the Geophysical Journal International, the Royal Astronomical Society and Blackwell Publishing. Full bibliographic reference is : Guichet, X., L. Jouniaux, and N. Catel, Modification of streaming potential by precipitation of calcite in a sand-water system: laboratory measurements in the pH range from 4 to 12, Geophysical Journal International, 166, 445-460, doi:10.1111/j.1365-246X.2006.02922.x, 200

Soil-plant potassium transfer: impact of plant activity on clay minerals as seen from X-ray diffraction

Potassium (K) availability influences many processes in cultivated and natural ecosystems. Several studies suggest that "non-exchangeable" K+ ions fixed in 2:1 clay mineral interlayers contribute to plant nutrition. Although depletion of these K+ ions could be observed by X-ray diffractometry, this technique has never been considered for the observation of short-term changes in illitic 2:1 clay minerals. We established in this study that new treatments of X-ray diffraction patterns allow quantification of short-term 2:1 clay mineral changes through K addition in solution and removal of interlayer K by Lolium multiflorum. Moreover, we obtained a significant relationship (r(2) = 0.95, P < 0.0001) between an indicator calculated from X-ray diffraction patterns and analyzed clay K content. X-ray diffraction should therefore be considered as an appropriate tool to follow qualitatively and quantitatively clay mineral modifications induced by soil K balance. Our results suggest that 2:1 clay minerals behave as a huge, renewable K reservoir whose theoretical capacity in fertile soils could exceed 3 t/ha. Beyond obvious agronomical implications, this new vision of soil K cycle raises ecological questions about plant inter specific competition and soil fertility. Finally, our study clearly shows that soil 2:1 clay minerals could react as quickly as a biological system

Which 2 : 1 clay minerals are involved in the soil potassium reservoir? Insights from potassium addition or removal experiments on three temperate grassland soil clay assemblages

International audienceField and laboratory observations based on X-ray diffraction techniques suggest that 2:1 clay minerals behave as a K reservoir. However, 2:1 soil clay assemblages are most often composed of a mixture of different 2:1 clay mineral populations and the role of these different clay mineral populations in K absorption or K release are not fully understood. This present work addressed this question, through laboratory experiments, by using an X-ray pattern decomposition program to quantify peak area modifications of different clay mineral populations induced by K addition and K removal through plant action. This study was performed on three typical temperate grassland soil clay assemblages containing illite, interstratified illite/smectite and soil vermiculite. Our results revealed that the quantities of well crystallized illite increased at the expense of soil vermiculite as a result of K absorption whereas the opposite trend was observed following K depletion. This particular relationship between well crystallized: illite and soil vermiculite suggests that they constitute a subgroup among 2:1 clay minerals. Poorly crystallized illite and interstratified clay minerals were also significantly affected by K absorption or removal indicating that they are also involved in soil K dynamics. Moreover, we observed that these short term clay mineral modifications (occurring at the day scale) induced by root K extraction are reversible. We also observed that the linear relationship between clay K content and the centre of gravity position of X-ray diffraction patterns in the 2:1 clay mineral range seem to be a general property for soil clay assemblages dominated by illite and interstratified illite/smectite materials. Our results support the view of 2:1 clay minerals as a K reservoir and specify the role of each of the 2:1 clay mineral populations in K absorption and release. All the 2:1 clay minerals commonly found in temperate grassland soil clay assemblages take part in the soil K reservoir

Reduced membrane fluidity of a SigX deficient strain results in altered carbon catabolic repression response in Pseudomonas aeruginosa

International audienceThe extracytoplasmic sigma factor SigX is a master regulator of bacterial adaptation that is involved in fatty acids biosynthesis and membrane homeostasis in P. aeruginosa. The growth of a sigX mutant was strongly affected in LB broth, but not in M9-glucose minimal medium. Through comparative transcriptomics and proteomics conducted in LB medium, we show that the absence of SigX results in strong dysregulation of genes, the products of which are mainly involved in transport, carbon and energy metabolisms. The CbrA/B two components system was strongly activated in a sigX mutant, leading to a higher production of crcZ and a reduced activity of the major translational repressors Hfq and Crc compared to the wildtype strain. Remarkably, growth as well as genes, the expression of which was dysregulated in the mutant strain, were restored when the C189-structurally related detergent Polysorbate 80 was supplemented to LB medium. Anisotropy data further showed that the sigX mutant displayed a less fluid membrane than the WT strain, a phenotype that was restored by adding Polysorbate 80. Altogether, our data show that the altered membrane of the sigX mutant strain was probably the leading cause of the strong metabolic alterations and the reduced carbon catabolic repression response (CCR) encountered by this mutant strain. Thus, by controlling membrane homeostasis, SigX behaves as a master regulator of the nutritional response, in addition to its involvement in P. aeruginosa adaptation, virulence and biofilm formation

Reduced membrane fluidity of a SigX deficient strain results in altered carbon catabolic repression response in Pseudomonas aeruginosa

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