Backreaction in Axion Monodromy, 4-forms and the Swampland

Axion monodromy models can always be described in terms of an axion coupled to 3-form gauge fields with non-canonical kinetic terms. The presence of the saxions parametrising the kinetic metrics of the 3-form fields leads to backreaction effects in the inflationary dynamics. We review the case in which saxions backreact on the K\"ahler metric of the inflaton leading to a logarithmic scaling of the proper field distance at large field. This behaviour is universal in Type II string flux compactifications and consistent with a refinement of the Swampland Conjecture. The critical point at which this behaviour appears depends on the mass hierarchy between the inflaton and the saxions. However, in tractable compactifications, such a hierarchy cannot be realised without leaving the regime of validity of the effective theory, disfavouring transplanckian excursions in string theory.Comment: Proceedings prepared for the "Workshop on Geometry and Physics", November 2016, Ringberg Castl

Cyanobacteria-Mediated Arsenic Redox Dynamics Is Regulated by Phosphate in Aquatic Environments

Studies of cyanobacteria in environments where arsenic (As) and phosphate (P) both occur in significant concentrations have so far only focused on the effect of P on As­(V) toxicity and bioaccumulation, with little attention to the influence of P on As redox transformations. Our study revealed that As­(III) oxidation by <i>Synechocystis</i> appeared to be more effective with increased P levels. We demonstrated that the higher As­(III) percentage in the medium under P-limited conditions was due to enhanced As­(V) uptake and the subsequent efflux of intracellularly reduced As­(III) which in turn contributed to higher As­(III) concentrations in the medium. Arsenic redox changes by <i>Synechocystis</i> under P-limited conditions is a dynamic cyclic process that includes the following: surface As­(III) oxidation (either in the periplasm or near the outer membrane), As­(V) uptake, intracellular As­(V) reduction, and As­(III) efflux. These observations not only expand our understanding of how P influences microbial As redox metabolisms but also provide insights into the biogeochemical coupling between As and P in As contaminated eutrophic aquatic environments and artificial wetland-paddy fields

Identification and Characterization of Arsenite Methyltransferase from an Archaeon, <i>Methanosarcina acetivorans</i> C2A

Arsenic is a ubiquitous toxic contaminant in the environment. The methylation of arsenic can affect its toxicity and is primarily mediated by biological processes. Few studies have focused on the mechanism of arsenic methylation in archaea although archaea are widespread in the environment. Here, an arsenite [As­(III)] methyltransferase (ArsM) was identified and characterized from an archaeon <i>Methanosarcina acetivorans</i> C2A. Heterologous expression of <i>MaarsM</i> was shown to confer As­(III) resistance to an arsenic-sensitive strain of <i>E. coli</i> through arsenic methylation and subsequent volatilization. Purified MaArsM protein was further identified the function in catalyzing the formation of various methylated products from As­(III) in vitro. Methylation of As­(III) by MaArsM is highly dependent on the characteristics of the thiol cofactors used, with some of them (coenzyme M, homocysteine, and dithiothreitol) more efficient than GSH. Site-directed mutagenesis demonstrated that three conserved cysteine (Cys) residues (Cys62, Cys150, and Cys200) in MaArsM were necessary for As­(III) methylation, of which only Cys150 and Cys200 were required for the methylation of monomethylarsenic. These results present a molecular pathway for arsenic methylation in archaea and provide some insight into the role of archaea in As biogeochemistry

Arsenic Uptake by Rice Is Influenced by Microbe-Mediated Arsenic Redox Changes in the Rhizosphere

Arsenic (As) uptake by rice is largely determined by As speciation, which is strongly influenced by microbial activities. However, little is known about interactions between root and rhizosphere microbes, particularly on arsenic oxidation and reduction. In this study, two rice cultivars with different radial oxygen loss (ROL) ability were used to investigate the impact of microbially mediated As redox changes in the rhizosphere on As uptake. Results showed that the cultivar with higher ROL (Yangdao) had lower As uptake than that with lower ROL (Nongken). The enhancement of the rhizospheric effect on the abundance of the arsenite (As­(III)) oxidase gene (<i>aroA</i>-like) was greater than on the arsenate (As­(V)) reductase gene (<i>arsC</i>), and As­(V) respiratory reductase gene (<i>arrA</i>), resulting in As oxidation and sequestration in the rhizosphere, particularly for cultivar Yangdao. The community of As­(III)-oxidizing bacteria in the rhizosphere was dominated by α-Proteobacteria and β-Proteobacteria and was influenced by rhizospheric effects, rice straw application, growth stage, and cultivar. Application of rice straw into the soil increased As release and accumulation into rice plants. These results highlighted that uptake of As by rice is influenced by microbial processes, especially As oxidation in the rhizosphere, and these processes are influenced by root ROL and organic matter application

Arsenic Demethylation by a C·As Lyase in Cyanobacterium <i>Nostoc</i> sp. PCC 7120

Arsenic, a ubiquitous toxic substance, exists mainly as inorganic forms in the environment. It is perceived that organoarsenicals can be demethylated and degraded into inorganic arsenic by microorganisms. Few studies have focused on the mechanism of arsenic demethylation in bacteria. Here, we investigated arsenic demethylation in a typical freshwater cyanobacterium <i>Nostoc</i> sp. PCC 7120. This bacterium was able to demethylate monomethylarsenite [MAs­(III)] rapidly to arsenite [As­(III)] and also had the ability to demethylate monomethylarsenate [MAs­(V)] to As­(III). The <i>NsarsI</i> encoding a C·As lyase responsible for MAs­(III) demethylation was cloned from <i>Nostoc</i> sp. PCC 7120 and heterologously expressed in an As-hypersensitive strain Escherichia coli AW3110 (Δ<i>arsRBC</i>). Expression of <i>NsarsI</i> was shown to confer MAs­(III) resistance through arsenic demethylation. The purified NsArsI was further identified and functionally characterized in vitro. NsArsI existed mainly as the trimeric state, and the kinetic data were well-fit to the Hill equation with <i>K</i>_0.5 = 7.55 ± 0.33 μM for MAs­(III), <i>V</i>_max = 0.79 ± 0.02 μM min^–1, and <i>h</i> = 2.7. Both of the NsArsI truncated derivatives lacking the C-terminal 10 residues (ArsI10) or 23 residues (ArsI23) had a reduced ability of MAs­(III) demethylation. These results provide new insights for understanding the important role of cyanobacteria in arsenic biogeochemical cycling in the environment

Arsenic Speciation and Volatilization from Flooded Paddy Soils Amended with Different Organic Matters

Arsenic (As) methylation and volatilization in soil can be increased after organic matter (OM) amendment, though the factors influencing this are poorly understood. Herein we investigate how amended OM influences As speciation as well as how it alters microbial processes in soil and soil solution during As volatilization. Microcosm experiments were conducted on predried and fresh As contaminated paddy soils to investigate microbial mediated As speciation and volatilization under different OM amendment conditions. These experiments indicated that the microbes attached to OM did not significantly influence As volatilization. The arsine flux from the treatment amended with 10% clover (clover-amended treatment, CT) and dried distillers grain (DDG) (DDG-amended treatment, DT2) were significantly higher than the control. Trimethylarsine (TMAs) was the dominant species in arsine derived from CT, whereas the primary arsine species from DT2 was TMAs and arsine (AsH₃), followed by monomethylarsine (MeAsH₂). The predominant As species in the soil solutions of CT and DT2 were dimethylarsinic acid (DMAA) and As­(V), respectively. OM addition increased the activities of arsenite-oxidizing bacteria (harboring <i>aroA</i>-like genes), though they did not increase or even decrease the abundance of arsenite oxidizers. In contrast, the abundance of arsenate reducers (carrying the <i>arsC</i> gene) was increased by OM amendment; however, significant enhancement of activity of arsenate reducers was observed only in CT. Our results demonstrate that OM addition significantly increased As methylation and volatilization from the investigated paddy soil. The physiologically active bacteria capable of oxidization, reduction, and methylation of As coexisted and mediated the As speciation in soil and soil solution

Anthropogenic Cycles of Arsenic in Mainland China: 1990–2010

Arsenic (As) is a trace element in the global environment with toxicity to both humans and ecosystem. This study characterizes China’s historical anthropogenic arsenic cycles (AACs) from 1990 to 2010. Key findings include the following: (1) the scale of China’s AACs grew significantly during the studied period, making China the biggest miner, producer, and user of arsenic today; (2) the majority of arsenic flows into China’s anthroposphere are the impurity of domestically mined nonferrous metal ores, which far exceeds domestic intentional demands; (3) China has been a net exporter of arsenic trioxide and arsenic metalloid, thus suffering from the environmental burdens of producing arsenic products for other economies; (4) the growth of arsenic use in China is driven by simultaneous increases in many applications including glass making, wood preservatives, batteries, semiconductors, and alloys, implying the challenge for regulating arsenic uses in multiple applications/industries at the same time; (5) the dissipative arsenic emissions resulting from intentional applications are at the same order of magnitude as atmospheric emissions from coal combustion, and their threats to human and ecosystem health can spread widely and last years to decades. Our results demonstrate that the characterization of AACs is indispensable for developing a complete arsenic emission inventory

Surface-Enhanced Raman Spectroscopy Combined with Stable Isotope Probing to Monitor Nitrogen Assimilation at Both Bulk and Single-Cell Level

Microbe-mediated biogeochemical cycle of nitrogen is a critical process in the environment. In this study, surface-enhanced Raman spectroscopy combined with ¹⁵N stable isotope probing (SERS–¹⁵N SIP) was developed as a new, nondestructive, and robust approach to probe nitrogen assimilation by bacteria at both bulk and single-cell level, and from pure culture to environmental microbial community. Multiple distinguishable SERS band shifts were observed and displayed a linear relationship with ¹⁵N content, because of the substitution of “light” nitrogen by “heavier” ¹⁵N stable isotope. These shifts, especially in 730 cm^–1 band, were highly distinguishable and universal in different bacteria, providing a robust indicator for nitrogen assimilation in bacteria. SERS–¹⁵N SIP was also demonstrated in important N₂-fixing bacteria via ¹⁵N₂ incubations. The same prominent shifts as that induced by ¹⁵NH₄Cl were observed, indicating the applicability of SERS–¹⁵N SIP to different nitrogen sources. SERS–¹⁵N SIP was further applied to environmental microbial community via ¹⁵NH₄Cl, ¹⁵NO₃^–, and ¹⁵N₂ incubation. Bacteria- and nitrogen source-dependent activity in nitrogen assimilation were revealed in environmental microbial community, pointing to the bacterial diversity and necessity of single-cell level investigation. Finally, by mixing optimized ratio of bacteria with Ag NPs, explicit single-cell SERS–¹⁵N SIP was obtained. The nondestructive SERS–¹⁵N SIP approach will be useful not only to identify active nitrogen-assimilating cells, but also enable Raman activated cell sorting and downstream genomic analysis, which will bring in deep insights into nitrogen metabolism of environmental microorganisms

Effect of Long-Term Wastewater Irrigation on Potential Denitrification and Denitrifying Communities in Soils at the Watershed Scale

Wastewater irrigation mitigates the problem of water shortage but leads to the potential accumulation of pollutants and causes corresponding changes in denitrifying communities and denitrification, hence the potential ecological risk of long-term wastewater irrigation should not be overlooked. We investigated the relative contributions of different environmental factors to the abundance and diversity of denitrifying communities harboring <i>nirK</i>, <i>nirS</i>, and <i>nosZ</i> genes and the relative importance of these biotic and abiotic variables in potential denitrification activity (PDA) in soils with wastewater irrigation for around 25 years at a large watershed scale. Results showed that soil physicochemical properties, pollutants, including heavy metals and PAHs, and vegetation are the major factor groups influencing the abundance and structure of the three denitrifying communities and PDA. <i>NirK</i>-, <i>nirS</i>-, or <i>nosZ</i>-harboring denitrifiers responded in different manners to environmental changes, and were mainly influenced by substrate concentration, carbon source, or pollutants, respectively. The structure of the three denitrifying communities was more relevant to the environmental changes than their abundance. Conversely, the abundance, rather than diversity, was correlated with PDA. Pollutants and vegetation could affect PDA by both direct and indirect paths through soil physicochemical properties including pH, carbon and nitrogen sources, or through the abundance of denitrifying functional genes. The abundance of denitrifying functional genes is a valuable index that integrates potential activity and various environmental factors, and is therefore a good predictor of denitrification in the presence of environmental changes

Pathways and Relative Contributions to Arsenic Volatilization from Rice Plants and Paddy Soil

Recent studies have shown that higher plants are unable to methylate arsenic (As), but it is not known whether methylated As species taken up by plants can be volatilized. Rice (<i>Oryza sativa</i> L.) plants were grown axenically or in a nonsterile soil using a two-chamber system. Arsenic transformation and volatilization were investigated. In the axenic system, uptake of As species into rice roots was in the order of arsenate (As­(V)) > monomethylarsonic acid (MMAs­(V)) > dimethylarsinic acid (DMAs­(V)) > trimethylarsine oxide (TMAs­(V)­O), but the order of the root-to-shoot transport index (T<i>i</i>) was reverse. Also, volatilization of trimethylarsine (TMAs) from rice plants was detected when plants were treated with TMAs­(V)O but not with As­(V), DMAs­(V), or MMAs­(V). In the soil culture, As was volatilized mainly from the soil. Small amounts of TMAs were also volatilized from the rice plants, which took up DMAs­(V), MMAs­(V), and TMAs­(V)O from the soil solution. The addition of dried distillers grain (DDG) to the soil enhanced As mobilization into the soil solution, As methylation and volatilization from the soil, as well as uptake of different As species and As volatilization from the rice plants. Results show that rice is able to volatilize TMAs after the uptake of TMAs­(V)O but not able to convert inorganic As, MMAs­(V) or DMAs­(V) into TMAs and that the extent of As volatilization from rice plants was much smaller than that from the flooded soil