42 research outputs found
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 AsÂ(V) toxicity and bioaccumulation, with little
attention to the influence of P on As redox transformations. Our study
revealed that AsÂ(III) oxidation by <i>Synechocystis</i> appeared
to be more effective with increased P levels. We demonstrated that
the higher AsÂ(III) percentage in the medium under P-limited conditions
was due to enhanced AsÂ(V) uptake and the subsequent efflux of intracellularly
reduced AsÂ(III) which in turn contributed to higher AsÂ(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 AsÂ(III) oxidation (either in the periplasm
or near the outer membrane), AsÂ(V) uptake, intracellular AsÂ(V) reduction,
and AsÂ(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 [AsÂ(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 AsÂ(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 AsÂ(III) in vitro. Methylation of AsÂ(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 AsÂ(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 (AsÂ(III)) oxidase gene (<i>aroA</i>-like)
was greater than on the arsenate (AsÂ(V)) reductase gene (<i>arsC</i>), and AsÂ(V) respiratory reductase gene (<i>arrA</i>),
resulting in As oxidation and sequestration in the rhizosphere, particularly
for cultivar Yangdao. The community of AsÂ(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
[MAsÂ(III)] rapidly to arsenite [AsÂ(III)] and also had the ability
to demethylate monomethylarsenate [MAsÂ(V)] to AsÂ(III). The <i>NsarsI</i> encoding a C·As lyase responsible for MAsÂ(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 MAsÂ(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><sub>0.5</sub> = 7.55 ± 0.33 ÎĽM for MAsÂ(III), <i>V</i><sub>max</sub> = 0.79 ± 0.02 ÎĽM min<sup>–1</sup>, 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 MAsÂ(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 (AsH<sub>3</sub>), followed
by monomethylarsine (MeAsH<sub>2</sub>). The predominant As species
in the soil solutions of CT and DT2 were dimethylarsinic acid (DMAA)
and AsÂ(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 <sup>15</sup>N stable isotope probing (SERS–<sup>15</sup>N 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 <sup>15</sup>N content, because
of the substitution of “light” nitrogen by “heavier” <sup>15</sup>N stable isotope. These shifts, especially in 730 cm<sup>–1</sup> band, were highly distinguishable and universal in
different bacteria, providing a robust indicator for nitrogen assimilation
in bacteria. SERS–<sup>15</sup>N SIP was also demonstrated
in important N<sub>2</sub>-fixing bacteria via <sup>15</sup>N<sub>2</sub> incubations. The same prominent shifts as that induced by <sup>15</sup>NH<sub>4</sub>Cl were observed, indicating the applicability
of SERS–<sup>15</sup>N SIP to different nitrogen sources. SERS–<sup>15</sup>N SIP was further applied to environmental microbial community
via <sup>15</sup>NH<sub>4</sub>Cl, <sup>15</sup>NO<sub>3</sub><sup>–</sup>, and <sup>15</sup>N<sub>2</sub> 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–<sup>15</sup>N SIP was obtained. The nondestructive SERS–<sup>15</sup>N 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
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 (AsÂ(V)) > monomethylarsonic
acid
(MMAsÂ(V)) > dimethylarsinic acid (DMAsÂ(V)) > trimethylarsine
oxide
(TMAsÂ(V)ÂO), 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
TMAsÂ(V)O but not with AsÂ(V), DMAsÂ(V), or MMAsÂ(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 DMAsÂ(V), MMAsÂ(V),
and TMAsÂ(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 TMAsÂ(V)O but not able to convert inorganic As, MMAsÂ(V) or DMAsÂ(V)
into TMAs and that the extent of As volatilization from rice plants
was much smaller than that from the flooded soil
Quantifying Nanoplastics in Soil-Cultured Plants Based on a Microcombustion Calorimeter
Nanoplastics have been detected in a variety of plants
and threaten
plant growth. To further investigate the physiological damage of
nanoplastics to plants and their translocation in plants, it is crucial
to quantify the nanoplastics in the plant. However, until now, no
studies have reported on how to quantify nanoplastics in soil-cultured
plants. Here, we proposed to determine the polyethylene (PE) and poly(methyl
methacrylate) (PMMA) contents in soil-cultured plants using a microcombustion
calorimeter (MCC). Since the thermal properties of nanoplastics were
different from those of plants and soil, this method was not affected
by the environmental background. The linear relationships were developed
between the sample total heat release (THR) and nanoplastic proportions.
Generally, the application of MCC to quantify PE and PMMA resulted
in low detection limits (LODs), quantification limits (LOQs), and
high spiked recoveries. The method also achieved a high level of accuracy
(relative standard deviation and A type uncertainty), demonstrating
the feasibility of the proposed method. This is the first report to
quantify nanoplastics in soil-cultured plants based on MCC. It provides
the possibility for rapid quantification of nanoplastics in plants
and thus allows in-depth research of the behavior of nanoplastics
in a soil–plant system