Electrodic voltages in the presence of dissolved sulfide: Implications for monitoring natural microbial activity

Abstract There is growing interest in the development of new monitoring strategies for obtaining spatially extensive data diagnostic of microbial processes occurring in the earth. Open-circuit potentials arising from variable redox conditions in the fluid local-to-electrode surfaces (electrodic potentials) were recorded for a pair of silver-silver chloride electrodes in a column experiment, whereby a natural wetland soil containing a known community of sulfate reducers was continuously fed with a sulfate-rich nutrient medium. Measurements were made between five electrodes equally spaced along the column and a reference electrode placed on the column inflow. The presence of a sulfate reducing microbial population, coupled with observations of decreasing sulfate levels, formation of black precipitate (likely iron sulfide),elevated solid phase sulfide, and a characteristic sulfurous smell, suggest microbial-driven sulfate reduction (sulfide generation) in our column. Based on the known sensitivity of a silver electrode to dissolved sulfide concentration, we interpret the electrodic potentials approaching 700mV recorded in this experiment as an indicator of the bisulfide (HS−) concentration gradients in the column. The measurement of the spatial and temporal variation in these electrodic potentials provides a simple and rapid method for monitoring patterns of relative HS− concentration that are indicative of the activity of sulfate-reducing bacteria. Our measurements have implications both for the autonomous monitoring of anaerobic microbial processes in the subsurface and the performance of self-potential electrodes, where it is critical to isolate, and perhaps quantify, electrochemical interfaces contributing to observed potentials

Phenyl radical + propene: a prototypical reaction surface for aromatic-catalyzed 1,2-hydrogen-migration and subsequent resonance-stabilized radical formation

The C[subscript 9]H[subscript 11] potential energy surface (PES) was experimentally and theoretically explored because it is a relatively simple, prototypical alkylaromatic radical system. Although the C[subscript 9]H[subscript 11] PES has already been extensively studied both experimentally (under single-collision and thermal conditions) and theoretically, new insights were made in this work by taking a new experimental approach: flash photolysis combined with time-resolved molecular beam mass spectrometry (MBMS) and visible laser absorbance. The C[subscript 9]H[subscript 11] PES was experimentally accessed by photolytic generation of the phenyl radical and subsequent reaction with excess propene (C[subscript 6]H[subscript 5] + C[subscript 3]H[subscript 6]). The overall kinetics of C[subscript 6]H[subscript 5] + C[subscript 3]H[subscript 6] was measured using laser absorbance with high time-resolution from 300 to 700 K and was found to be in agreement with earlier measurements over a lower temperature range. Five major product channels of C[subscript 6]H[subscript 5] + C[subscript 3]H[subscript 6] were observed with MBMS at 600 and 700 K, four of which were expected: hydrogen (H)-abstraction (measured by the stable benzene, C[subscript 6]H[subscript 6], product), methyl radical (CH[subscript 3])-loss (styrene detected), H-loss (phenylpropene isomers detected) and radical adduct stabilization. The fifth, unexpected product observed was the benzyl radical, which was rationalized by the inclusion of a previously unreported pathway on the C[subscript 9]H[subscript 11] PES: aromatic-catalysed 1,2-H-migration and subsequent resonance stabilized radical (RSR, benzyl radical in this case) formation. The current theoretical understanding of the C[subscript 9]H[subscript 11] PES was supported (including the aromatic-catalyzed pathway) by quantitative comparisons between modelled and experimental MBMS results. At 700 K, the branching to styrene + CH[subscript 3] was 2-4 times greater than that of any other product channel, while benzyl radical + C[subscript 2]H[subscript 4] from the aromatic-catalyzed pathway accounted for ∼10% of the branching. Single-collision conditions were also simulated on the updated PES to explain why previous crossed molecular beam experiments did not see evidence of the aromatic-catalyzed pathway. This experimentally validated knowledge of the C[subscript 9]H[subscript 11] PES was added to the database of the open-source Reaction Mechanism Generator (RMG), which was then used to generalize the findings on the C[subscript 9]H[subscript 11] PES to a slightly more complicated alkylaromatic system.Think Global Education Trus

此時彼刻文化研究 = Cultural studies still in the making

時光荏苒，MCS15年了！ 今年MCS年度研討會與過去有點不同，主題是「展演CROSSOVER：香港文化研究的變奏與另類新選擇」。在舉步維艱的社會政治泥沼中，我們如何理解和面對這境地？透過這次研討會，MCS表達對這個時代的關切，並在這關口中提出新概念，加入「表演研究」元素──不但帶來學術上的協同效應，而且使未來教育的路更闊、與大家走得更遠，發揮更大的潛質。 研討會分為兩部分。第一部分以MCSian的論文為引旨，透過對話去思考在當前處境如何尋找出路，由梁旭明主持，分別由吳紹奇主講〈作為歷史哲學家的班雅明：論歷史哲學兼評〈歷史哲學論綱〉〉，Kong Yee主講〈The Identity of Cheung Chau Kai-fong : The Cultural Disparity of The Northern and Southern Cheung Chau〉，古卓嵐主講〈Modern Educayshun 的啟示──從恐懼女性主義心理 到課室政治的省思〉，並由羅冠聰評論及與講者進行討論

Molecular and Functional Mapping of EED Motifs Required for PRC2-Dependent Histone Methylation

Polycomb Group (PcG) proteins represent a conserved family of developmental regulators that mediate heritable transcriptional silencing by modifying chromatin states. One PcG complex, the PRC2 complex, is composed of several proteins, including the histone H3 lysine 27 (H3K27) methyltransferase EZH2 and the WD-repeat protein EED. Histone H3K27 can be mono- (H3K27me1), di- (H3K27me2), or trimethylated (H3K27me3). However, it remains unclear what regulates the number of methyl groups added to H3K27 in a particular nucleosome. In mammalian cells, EED is present as four distinct isoforms, which are believed to be produced by utilizing four distinct, in-frame translation start sites in a common Eed mRNA. A mutation that disables all four EED isoforms produces defects in H3K27 methylation.1 To assess the roles of individual EED isoforms in H3K27 methylation, we first characterized three of the four EED isoform start sites and then demonstrated that individual isoforms are not necessary for H3K27me1, H3K27me2, or H3K27me3. Instead, we show that the core WD-40 motifs and the histone binding region of EED alone are sufficient for the generation of all three marks, demonstrating that EED isoforms do not control the number of methyl groups added to H3K27

Abundant phosphorus expected for possible life in Enceladus’s ocean

Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the ocean–seafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO42−), and total dissolved inorganic P could reach 10−7 to 10−2 mol/kg H2O, generally increasing with lower pH and higher dissolved CO2, but also depending upon dissolved ammonia and silica. Levels are much higher than <10−10 mol/kg H2O from previous estimates and close to or higher than ∼10−6 mol/kg H2O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H2O within 105 y, much less than the likely age of Enceladus’s ocean (108 to 109 y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability