Dynamics of Dimethylated Monothioarsenate (DMMTA) in Paddy Soils and Its Accumulation in Rice Grains

Linking Chitin Degradation to Arsenic Methylation under Oxic Conditions: A Critical Driver of Arsenic Cycling in Paddy Soils

Aerobic microbial arsenic (As) methylation exhibits significant efficiency, potentially influencing As fluxes and their biogeochemistry in paddy soils. However, the role of primary degraders initiating decomposition of complex biopolymers in As methylation and the underlying driving mechanisms remain largely unexplored. This study uncovers a direct metabolic connection between chitin, a major component of particulate organic matter in the soil, and As methylation facilitated by specialized aerobic chitin-degrading bacteria. Chitin markedly enhanced As methylation and its volatilization in paddy soils under aerobic conditions, resulting in 2.9- and 25-fold higher increases compared with the treatments with lignocellulosic residue and components, respectively. Chitinophagaceae members were enriched in the presence of chitin, with bacteria related to Chitinophaga likely serving as the key contributors. The direct coupling between As methylation and chitin hydrolysis was further confirmed using a representative Chitinophaga strain isolated from a paddy soil, which produced dimethylarsenate and trimethylarsenate as the major MeAs products. Comparative genomic analysis revealed that a considerable proportion of Chitinophaga found diverse soil environments possess complete functional genes for As methylation and chitin utilization, highlighting their potential for regulating As cycling broadly. These findings emphasize the chitin degradation-associated As methylation as a previously overlooked contributor to MeAs fluxes, enhancing our understanding of the interconnected biogeochemistry of As and carbon in soils.

Interface mechanism of Cd bioavailability by porewater prediction in paddy field system

Cadmium (Cd) contamination poses a significant threat to human health. Predicting the risk of Cd in rice grains is challenging due to the heterogeneity and complexity of bioavailable Cd in paddy soils. We proposed that porewater during the grain-filling period can effectively predict Cd bioavailability in rice (R2 > 0.5, p < 0.05). The prediction mechanism was elucidated through soil-porewater interface characterization analysis and DFT calculations. Key factors determining Cd bioavailability included Cd2+ and SO42- concentration, pH, and ORP of porewater, with pH showing the highest correlation. As porewater pH increased from 5 to 9, typical mineral surfaces gradually deprotonated and formed complexation bonds {SOCd+} instead of {SOH} or {SOH2+}. Additionally, the complexation energy between montmorillonite and Cd was 1.57-1.78 eV higher than that between goethite and Cd, while the protonation activation energy barrier on montmorillonite was 1.33-1.52 eV lower than on goethite. Therefore, {H+} concentration in porewater had the potential to quantify {SOCd+} content and binding capacity, aiding Cd bioavailability prediction. This study outlines the mechanism of predicting heavy metal health risks in rice grains through porewater and offers a potential regulation approach for agricultural product safety.

Pristine/magnesium-loaded biochar and ZVI affect rice grain arsenic speciation and cadmium accumulation through different pathways in an alkaline paddy soil

Cadmium (Cd) and arsenic (As) co-contamination has threatened rice production and food safety. It is challenging to mitigate Cd and As contamination in rice simultaneously due to their opposite geochemical behaviors. Mg-loaded biochar with outstanding adsorption capacity for As and Cd was used for the first time to remediate Cd/As contaminated paddy soils. In addition, the effect of zero-valent iron (ZVI) on grain As speciation accumulation in alkaline paddy soils was first investigated. The effect of rice straw biochar (SC), magnesium-loaded rice straw biochar (Mg/SC), and ZVI on concentrations of Cd and As speciation in soil porewater and their accumulation in rice tissues was investigated in a pot experiment. Addition of SC, Mg/SC and ZVI to soil reduced Cd concentrations in rice grain by 46.1%, 90.3% and 100%, and inorganic As (iAs) by 35.4%, 33.1% and 29.1%, respectively, and reduced Cd concentrations in porewater by 74.3%, 96.5% and 96.2%, respectively. Reductions of 51.6% and 87.7% in porewater iAs concentrations were observed with Mg/SC and ZVI amendments, but not with SC. Dimethylarsinic acid (DMA) concentrations in porewater and grain increased by a factor of 4.9 and 3.3, respectively, with ZVI amendment. The three amendments affected grain concentrations of iAs, DMA and Cd mainly by modulating their translocation within plant and the levels of As(III), silicon, dissolved organic carbon, iron or Cd in porewater. All three amendments (SC, Mg/SC and ZVI) have the potential to simultaneously mitigate Cd and iAs accumulation in rice grain, although the pathways are different.

Growth substrate limitation enhances anaerobic arsenic methylation by Paraclostridium bifermentans strain EML

Microbial arsenic methylation is established as a detoxification process under aerobic conditions (converting arsenite to monomethylated arsenate) but is proposed to be a microbial warfare strategy under anoxic conditions due to the toxicity of its main product, monomethylarsonous acid (MMAs(III)). Here we leveraged a paddy soil-derived anaerobic arsenic methylator, Paraclostridium bifermentans strain EML, to gain insights into this process. Strain EML was inoculated into a series of media involving systematic dilutions of Reinforced Clostridial Broth (RCB) with 25 µM arsenite to assess the impact of growth substrate concentration on arsenic methylation. Growth curves evidenced the sensitivity of strain EML to arsenite, and arsenic speciation analysis revealed the production of MMAs(III). Concentrations of MMAs(III) and arsenic methylation gene (arsM) transcription were found to be positively correlated with RCB dilution, suggesting that substrate limitation enhances arsM gene expression and associated anaerobic arsenic methylation. We propose that growth substrate competition among microorganisms may also contribute to an increase in anaerobic arsenic methylation. This hypothesis was further evaluated in an anaerobic co-culture system involving strain EML and either wild-type Escherichia coli K-12 MG1655 (WT) or E. coli expressing the MMAs(III)-resistance gene (arsP) (ArsP E. coli). We observed increased MMAs(III) production in the presence of E. coli than its absence and growth inhibition of WT E. coli to a greater extent than ArsP E. coli, presumably due to the MMAs(III) produced by strain EML. Collectively, our findings suggest an ecological role for anaerobic arsenic methylation, highlighting the significance of microbe-microbe competition and interaction in this process.IMPORTANCEMicrobial arsenic methylation is highly active in rice paddy fields under flooded conditions, leading to increased accumulation of methylated arsenic in rice grains. In contrast to the known detoxification process for aerobic arsenic methylation, the ecological role of anaerobic arsenic methylation remains elusive and is proposed to be an antibiotic-producing process involved in microbial warfare. In this study, we interrogated a rice paddy soil-derived anaerobic arsenic-methylating bacterium, Paraclostridium bifermentans strain EML, to explore the effect of growth substrate limitation on arsenic methylation in the context of the microbial warfare hypothesis. We provide direct evidence for the role of growth substrate competition in anaerobic arsenic methylation via anaerobic prey-predator co-culture experiments. Moreover, we demonstrate a feedback loop, in which a bacterium resistant to MMAs(III) enhances its production, presumably through enhanced expression of arsM resulting from substrate limitation. Our work uncovers the complex interactions between an anaerobic arsenic methylator and its potential competitors.

Organic matter in geothermal springs and its association with the microbial community

Organic matter (OM) plays an important role in the biogeochemical cycles of carbon, nitrogen, and other elements, shaping the structure of the microbiome and vice versa. However, the molecular composition of OM and its impact on the microbial community in terrestrial geothermal environments remain unclear. In this study, we characterized the OM in water and sediment from a typical geothermal field using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. By combining high-throughput amplicon sequencing and multivariate analyses, we deciphered the association between OM components and microbial community. A surprisingly high chemodiversity of OM was observed in the waters (11,088 compounds) and sediments (7772 compounds) in geothermal springs. Sulfur-containing organic compounds, a characteristic molecular signature of geothermal springs, accounted for 21 % ± 5 % in waters and 33 % ± 4 % in sediments. Multivariate analyses revealed that both labile and recalcitrant fractions of OM (e.g., carbohydrates intensity and tannins chemodiversity) influenced the structure and function of the microbial community. Co-occurrence networks showed that Proteobacteria and Crenarchaeota accounted for most of the connections with OM in waters (33 % and 15 %, respectively) and sediments (15 % and 12 %, respectively), highlighting their key roles in carbon cycling. This study expands our understanding of the molecular compositions of OM in geothermal springs and highlights its potentially important role in global climate change through microbial carbon cycling.

Toxic Metals and Metalloids in Food: Current Status, Health Risks, and Mitigation Strategies

PURPOSE OF REVIEW

Exposure to toxic metals/metalloids, such as arsenic (As), cadmium (Cd), and lead (Pb), through food consumption is a global public health concern. This review examines the contamination status of these metals/metalloids in food, assesses dietary intake across different populations, and proposes strategies to reduce metal/metalloid exposures throughout the food chain.

RECENT FINDINGS

For the general population, dietary intake of metals/metalloids is generally lower than health-based guidance values. However, for vulnerable populations, such as infants, children, and pregnant women, their dietary intake levels are close to or even higher than the guidance values. Among different food categories, seafood shows higher total As, but largely present as organic species. Rice accumulates higher As concentration than other cereals, with inorganic As (iAs) and dimethylarsinic acid (DMA) being the main As species. Methylated thioarsenate species, such as dimethylmonothioarsenate, have also been detected in rice. The distribution of iAs and DMA in rice shows geographical variation. Additionally, seafood and cocoa products generally contain more Cd than other food, but seafood consumption does not significantly increase in adverse health effects due to its high zinc and iron content. Compared to As and Cd, Pb concentrations in food are generally lower. To minimize the health risks of metal/metalloid exposure, several strategies are proposed. Food contamination with toxic metals/metalloids poses significant concerns for human health, particularly for vulnerable populations. This review provides scientific evidence and suggestions for policy makers to reduce human exposure of metals/metalloids via dietary intake.

Arsenic Methylation by a Sulfate-Reducing Bacterium from Paddy Soil Harboring a Novel ArsSM Fusion Protein

Microbial arsenic (As) methylation is an important process of As biogeochemistry. Only a few As-methylating microorganisms have been isolated from paddy soil, hindering the mechanistic understanding of the process involved. We isolated 54 anaerobic and 32 aerobic bacteria from paddy soil with a high As methylation potential. Among the 86 isolates, 14 anaerobes, including 7 sulfate-reducing bacteria (SRB), but none of the aerobes were able to methylate arsenite [As(III)] or monomethylarsenite [MMA(III)] or both, suggesting that the As-methylating ability is much more prevalent in anaerobes than in aerobes. We performed a detailed investigation on As methylation by a SRB isolate, Solidesulfovibrio sp. TC1, and identified a novel bifunctional enzyme consisting of a fusion of As(III) S-adenosylmethionine (SAM) methyltransferase (ArsM) and a radical SAM protein. The enzyme (ArsSM) can catalyze As(III) methylation to MMA and DMA and subsequent adenosylation of DMA to form 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), which is a key intermediate in the biosynthesis of arsenosugars. High concentrations of sulfide produced by SRB did not affect As(III) methylation to MMA but inhibited MMA methylation to DMA. Genes encoding ArsSM fusion proteins are widespread in anaerobes, particularly SRB, suggesting that ArsSM-carrying anaerobes may play an important role in As methylation in an anoxic environment.

Microbial arsenic methylation in soil-water systems and its environmental significance

In this review, we have summarized the current knowledge about the environmental importance, relevance, and consequences of microbial arsenic (As) methylation in various ecosystems. In this regard, we have presented As biomethylation in terrestrial and aquatic ecosystems particularly in rice paddy soils and wetlands. The functions of As biomethylation by microbial consortia in anaerobic and aerobic conditions are extensively discussed. In addition, we have tried to explain the interconnections between As transformation and carbon (C), such as microbial degradation of organic compounds and methane (CH4) emission. These processes can cause As release because of the reduction of arsenate (As(V)) to the more mobile arsenite (As(III)) as well as As methylation and the formation of toxic trivalent methylated As species in anaerobic conditions. Furthermore, the sulfur (S) transformation can form highly toxic thiolated As species owing to its interference with As biomethylation. Besides, we have focused on many other mutual interlinks that remain elusive between As and C, including As biomethylation, thiolation, and CH4 emission, in the soil-water systems. Recent developments have clarified the significant and complex interactions between the coupled microbial process in anoxic and submerged soils. These processes, performed by little-known/unknown microbial taxa or well-known members of microbial communities with unrecognized metabolic pathways, conducted several concurrent reactions that contributed to global warming on our planet and have unfavorable impacts on water quality and human food resources. Finally, some environmental implications in rice production and arsenic removal from soil-water systems are discussed. Generally, our understanding of the ecological and metabolic evidence for the coupling and synchronous processes of As, C, and S are involved in environmental contamination-caused toxicity in human food, including high As content in rice grain, water resources, and global warming through methanogenesis elucidate combating global rice safety, drinking water, and climate changes.

Determining soil conservation strategies: Ecological risk thresholds of arsenic and the influence of soil properties

The establishment of ecological risk thresholds for arsenic (As) plays a pivotal role in developing soil conservation strategies. However, despite many studies regarding the toxicological profile of As, such thresholds varying by diverse soil properties have rarely been established. This study aims to address this gap by compiling and critically examining an extensive dataset of As toxicity data sourced from existing literature. Furthermore, to augment the existing information, experimental studies on As toxicity focusing on barley-root elongation were carried out across various soil types. The As concentrations varied from 12.01 to 437.25 mg/kg for the effective concentrations that inhibited 10% of barley-root growth (EC10). The present study applied a machine-learning approach to investigate the complex associations between the toxicity thresholds of As and diverse soil properties. The results revealed that Mn-/Fe-ox and clay content emerged as the most influential factors in predicting the EC10 contribution. Additionally, by using a species sensitivity distribution model and toxicity data from 21 different species, the hazardous concentration for x% of species (HCx) was calculated for four representative soil scenarios. The HC5 values for acidic, neutral, alkaline, and alkaline calcareous soils were 80, 47, 40, and 28 mg/kg, respectively. This study establishes an evidence-based methodology for deriving soil-specific guidance concerning As toxicity thresholds.

Simultaneously decreasing arsenic and cadmium in rice by soil sulfate and limestone amendment under intermittent flooding

Water management in paddy soils can effectively reduce the soil-to-rice grain transfer of either As or Cd, but not of both elements simultaneously due to the higher mobility of As under reducing and Cd under oxidizing soil conditions. Limestone amendment, the common form of liming, is well known for decreasing Cd accumulation in rice grown on acidic soils. Sulfate amendment was suggested to effectively decrease As accumulation in rice, especially under intermittent soil flooding. To study the unknown effects of combined sulfate and limestone amendment under intermittent flooding for simultaneously decreasing As and Cd in rice, we performed a pot experiment using an acidic sandy loam paddy soil. We also included a clay loam paddy soil to study the role of soil texture in low-As rice production under intermittent flooding. We found that liming not only decreased rice Cd concentrations but also greatly decreased dimethylarsenate (DMA) accumulation in rice. We hypothesize that this is due to suppressed sulfate reduction, As methylation, and As thiolation by liming in the sulfate-amended soil and a higher share of deprotonated DMA at higher pH which is taken up less readily than protonated DMA. Decreased gene abundance of potential soil sulfate-reducers by liming further supported our hypothesis. Combined sulfate and limestone amendment to the acidic sandy loam soil produced rice with 43% lower inorganic As, 72% lower DMA, and 68% lower Cd compared to the control soil without amendment. A tradeoff between soil aeration and water availability was observed for the clay loam soil, suggesting difficulties to decrease As in rice while avoiding plant water stress under intermittent flooding in fine-textured soils. Our results suggest that combining sulfate amendment, liming, and intermittent flooding can help to secure rice safety when the presence of both As and Cd in coarse-textured soils is of concern.

Temporal development of arsenic speciation and extractability in acidified and non-acidified paddy soil amended with silicon-rich fly ash and manganese- or zinc-oxides under flooded and drainage conditions

Oxides of silicon (Si), manganese (Mn), and zinc (Zn) have been used as soil amendments to reduce As mobility and uptake in paddy soil systems. However, these amendments are hypothesized to be affected differently depending on the soil pH and their effect on As speciation in rice paddy systems is not fully understood. Herein, we used a microcosm experiment to investigate the effects of natural Si-rich fly ash and synthetic Mn and Zn oxides on the temporal development of porewater chemistry, including aqueous As speciation (As(III), As(V), MMA, DMA, and DMMTA) and solid-phase As solubility, in a naturally calcareous soil with or without soil acidification (with sulfuric acid) during 28 days of flooding and subsequent 14 days of drainage. We found that soil acidification to pH 4.5 considerably increased the solubility of Si, Fe, Mn, and Zn compared to the non-acidified soil. Additions of Mn and Zn oxides decreased the concentrations of dissolved arsenite and arsenate in the non-acidified soil whereas additions of Zn oxide and combined Si-Zn oxides increased them in the acidified soil. The Si-rich fly ash did not increase dissolved Si and As in the acidified and non-acidified soils. Dimethylated monothioarsenate (DMMTA) was mainly observed in the acidified soil during the later stage of soil flooding. The initial 28 days of soil flooding decreased the levels of soluble and exchangeable As and increased As associated with Mn oxides, whereas the subsequent 14 days of soil drainage reversed the trend. This study highlighted that soil acidification considerably controlled the solubilization of Ca and Fe, thus influencing the soil pH-Eh buffering capacity, the solubility of Si, Mn, and Zn oxides, and the mobility of different As species in carbonate-rich and acidic soils under redox fluctuations.

Seasonal Formation of Low-Sorbing Methylthiolated Arsenates Induces Arsenic Mobilization in a Minerotrophic Peatland

Peatlands are known sinks for arsenic (As). In the present study, seasonal As mobilization was observed in an acidic, minerotrophic peatland (called Lehstenbach) in late summer, accompanied by a peak in dissolved sulfide (S(-II)). Arsenic speciation revealed the lowest seasonal porewater concentrations of arsenite and arsenate, likely due to As(III)-S-bridging to natural organic matter. Arsenic mobilization was driven by the formation of arsenite-S(-II) colloids and formation of methylthiolated arsenates (up to 59% of the sum of As species) and to a minor extent also of inorganic thioarsenates (6%-30%) and oxymethylated arsenates (5%-24%). Sorption experiments using a purified model peat, the Lehstenbach peat, natural (to mimic winter conditions) and reacted with S(-II) (to mimic late summer conditions) at acidic and neutral pH confirmed low sorption of methylthiolated arsenates. At acidic pH and in the presence of S(-II), oxymethylated arsenates were completely thiolated. This methylthiolation decreased As sorption up to 10 and 20 times compared with oxymethylated arsenates and arsenite, respectively. At neutral pH, thiolation of monomethylated arsenates was incomplete, and As could be partially retained as oxymethylated arsenates. Dimethylated arsenate was still fully thiolated and highly mobile. Misidentification of methylthiolated arsenates as oxymethylated arsenates might explain previous contradictory reports of methylation decreasing or increasing As mobility.

Soil redox status governs within-field spatial variation in microbial arsenic methylation and rice straighthead disease

Microbial arsenic (As) methylation in paddy soil produces mainly dimethylarsenate (DMA), which can cause physiological straighthead disease in rice. The disease is often highly patchy in the field, but the reasons remain unknown. We investigated within-field spatial variations in straighthead disease severity, As species in rice husks and in soil porewater, microbial composition and abundance of arsM gene encoding arsenite S-adenosylmethionine methyltransferase in two paddy fields. The spatial pattern of disease severity matched those of soil redox potential, arsM gene abundance, porewater DMA concentration, and husk DMA concentration in both fields. Structural equation modelling identified soil redox potential as the key factor affecting arsM gene abundance, consequently impacting porewater DMA and husk DMA concentrations. Core amplicon variants that correlated positively with husk DMA concentration belonged mainly to the phyla of Chloroflexi, Bacillota, Acidobacteriota, Actinobacteriota, and Myxococcota. Meta-omics analyses of soil samples from the disease and non-disease patches identified 5129 arsM gene sequences, with 71% being transcribed. The arsM-carrying hosts were diverse and dominated by anaerobic bacteria. Between 96 and 115 arsM sequences were significantly more expressed in the soil samples from the disease than from the non-disease patch, which were distributed across 18 phyla, especially Acidobacteriota, Bacteroidota, Verrucomicrobiota, Chloroflexota, Pseudomonadota, and Actinomycetota. This study demonstrates that even a small variation in soil redox potential within the anoxic range can cause a large variation in the abundance of As-methylating microorganisms, thus resulting in within-field variation in rice straighthead disease. Raising soil redox potential could be an effective way to prevent straighthead disease.

Potential hazards of typical small molecular organic matters in shale gas wastewater for wheat irrigation: 2-butoxyethanol and dimethylbenzylamine

2-butoxyethanol (BE) and dimethylbenzylamine (DMBA) are small molecular organic compounds commonly found in shale gas wastewater (SGW) and environmental samples, yet their environmental risks in exposure and irrigation reuse have not been thoroughly studied. From the perspectives of physicochemical properties and toxicity, seven groups of irrigation treatment were designed for wheat irrigation according to the concentration gradient. Overall, wheat growth was normal, but higher DMBA concentrations resulted in more severe growth inhibition. The absorption of BE by various tissues of wheat was positively correlated with its concentration, while the absorption of DMBA by wheat stems showed the same trend. Interestingly, there was no significant difference in the absorption of DMBA by wheat grains in different groups. The detection results of nutritional and heavy metal elements in wheat tissues showed that the presence of organic compounds changed the relative sensitivity of wheat leaves and grains to some elements (such as Mg, Mn, Mo, etc.) enrichment. The Cd and Pb contents of wheat grains in all groups complied with national safety standards, but the As or Cr concentration in wheat grains treated with BE or DMBA exceeded the limits in some cases. Transcriptome sequencing, GO annotation, and KEGG enrichment analysis revealed similar gene functions and metabolic pathways enriched by BE and DMBA. The safe and sustainable agricultural reuse of SGW still has great potential as a promising water resources management strategy.

In Planta Arsenic Thiolation in Rice and Arabidopsis thaliana

Inorganic and methylated thioarsenates have recently been reported to form in paddy soil pore waters and accumulate in rice grains. Among them, dimethylmonothioarsenate (DMMTA) is particularly relevant because of its high cytotoxicity and potential misidentification as nonregulated dimethylarsenate (DMA). Studying DMMTA uptake and flag leaf, grain, and husk accumulation in rice plants during grain filling, substantial dethiolation to DMA was observed with only 8.0 ± 0.1, 9.1 ± 0.6, and 1.4 ± 0.2% DMMTA remaining, respectively. More surprisingly, similar shares of DMMTA were observed in control experiments with DMA, indicating in planta DMA thiolation. Exposure of different rice seedling varieties to not only DMA but also to arsenite and monomethylarsenate (MMA) revealed in planta thiolation as a common process in rice. Up to 35 ± 7% DMA thiolation was further observed in the shoots and roots of the model plant Arabidopsis thaliana. Parameters determining the ratio and kinetics of thiolation versus dethiolation are unknown, yet, but less DMA thiolation in glutathione-deficient mutants compared to wild-type plants suggested glutathione concentration as one potential parameter. Our results demonstrate that pore water is not the only source for thioarsenates in rice grains and that especially the currently nonregulated DMA needs to be monitored as a potential precursor of DMMTA formation inside rice plants.

Two-tiered mutualism improves survival and competitiveness of cross-feeding soil bacteria

Metabolic cross-feeding is a pervasive microbial interaction type that affects community stability and functioning and directs carbon and energy flows. The mechanisms that underlie these interactions and their association with metal/metalloid biogeochemistry, however, remain poorly understood. Here, we identified two soil bacteria, Bacillus sp. BP-3 and Delftia sp. DT-2, that engage in a two-tiered mutualism. Strain BP-3 has low utilization ability of pyruvic acid while strain DT-2 lacks hexokinase, lacks a phosphotransferase system, and is defective in glucose utilization. When strain BP-3 is grown in isolation with glucose, it releases pyruvic acid to the environment resulting in acidification and eventual self-killing. However, when strain BP-3 is grown together with strain DT-2, strain DT-2 utilizes the released pyruvic acid to meet its energy requirements, consequently rescuing strain BP-3 from pyruvic acid-induced growth inhibition. The two bacteria further enhance their collective competitiveness against other microbes by using arsenic as a weapon. Strain DT-2 reduces relatively non-toxic methylarsenate [MAs(V)] to highly toxic methylarsenite [MAs(III)], which kills or suppresses competitors, while strain BP-3 detoxifies MAs(III) by methylation to non-toxic dimethylarsenate [DMAs(V)]. These two arsenic transformations are enhanced when strains DT-2 and BP-3 are grown together. The two strains, along with their close relatives, widely co-occur in soils and their abundances increase with the soil arsenic concentration. Our results reveal that these bacterial types employ a two-tiered mutualism to ensure their collective metabolic activity and maintain their ecological competitive against other soil microbes. These findings shed light on the intricateness of bacterial interactions and their roles in ecosystem functioning.

Dietary intake of household cadmium-contaminated rice caused genome-wide DNA methylation changes on gene/hubs related to metabolic disorders and cancers

Cadmium (Cd) contamination in food has raised broad concerns in food safety and human health. The toxicity of Cd to animals/humans have been widely reported, yet little is known about the health risk of dietary Cd intake at the epigenetic level. Here, we investigated the effect of a household Cd-contaminated rice (Cd-rice) on genome-wide DNA methylation (DNAm) changes in the model mouse. Feeding Cd-rice increased kidney Cd and urinary Cd concentrations compared with the Control rice (low-Cd rice), whereas supplementation of ethylenediamine tetraacetic acid iron sodium salt (NaFeEDTA) in the diet significantly increased urinary Cd and consequently decreased kidney Cd concentrations. Genome-wide DNAm sequencing revealed that dietary Cd-rice exposure caused the differentially methylated sites (DMSs), which were mainly located in the promoter (32.5%), downstream (32.5%), and intron (26.1%) regions of genes. Notably, Cd-rice exposure induced hypermethylation at the promoter sites of genes Caspase-8 and interleukin-1β (Il-1β), and consequently, their expressions were down-regulated. The two genes are critical in apoptosis and inflammation, respectively. In contrast, Cd-rice induced hypomethylation of the gene midline 1 (Mid1), which is vital to neurodevelopment. Furthermore, 'pathways in cancer' was significantly enriched as the leading canonical pathway. Supplementation of NaFeEDTA partly alleviated the toxic symptoms and DNAm alternations induced by Cd-rice exposure. These results highlight the broad effects of elevated dietary Cd intake on the level of DNAm, providing epigenetic evidence on the specific endpoints of health risks induced by Cd-rice exposure.

Multi-site field trials demonstrate the effectiveness of silicon fertilizer on suppressing dimethylarsenate accumulation and mitigating straighthead disease in rice

Rice accumulates both inorganic arsenic (iAs) and organic As species such as dimethylarsenate (DMA). Although DMA is less toxic to humans, it has been shown in hydroponic studies to induce rice straighthead disease, a physiological disorder prevalent in some rice growing regions causing large yield losses. We investigated the effects of different amendments on As species dynamics in soil porewater, accumulation of As species in rice husks and grains, and the incidence of straighthead disease in five field experiments conducted over 2 years at three sites where straighthead disease was observed in previous seasons. The amendments included silicon (Si) fertilizer, micronized zero valent iron (μZVI), sulfate, nitrate, Si-rich biochar, and a mixture of trace and major elements. Straighthead disease was observed in all five experiments. Rice panicles showing the straighthead disease symptoms contained much higher DMA concentrations in husks and grains than normal panicles. Silicon fertilizer was highly effective at decreasing the disease incidence rate and increasing seed setting rate, resulting in 14.9-58.1% increase in grain yield. Silicon fertilizer increased soil porewater iAs and DMA concentrations, but decreased iAs and DMA accumulation in husks and grains, suggesting that Si suppressed the uptake of iAs and DMA by rice plants. Other amendments alleviated straighthead disease to smaller extents than Si fertilizer, with the effect of biochar and the mixture of trace and major elements likely also being attributed to the addition of Si. Results from this field-based study demonstrate that excessive accumulation of DMA is the main cause of rice straighthead disease and Si fertilizer is highly effective at mitigating this disease by suppressing DMA accumulation.

Adsorption Characteristics of Dimethylated Arsenicals on Iron Oxide-Modified Rice Husk Biochar

In this study, the adsorption characteristics of dimethylated arsenicals to rice husk biochar (BC) and Fe/biochar composite (FeBC) were assessed through isothermal adsorption experiments and X-ray absorption spectroscopy analysis. The maximal adsorption capacities (q_m) of inorganic arsenate, calculated using the Langmuir isotherm equation, were 1.28 and 6.32 mg/g for BC and FeBC, respectively. Moreover, dimethylated arsenicals did not adsorb to BC at all, and in the case of FeBC, q_m values of dimethylarsinic acid (DMA(V)), dimethylmonothioarsinic acid (DMMTA(V)), and dimethyldithioarsinic acid (DMDTA(V)) were calculated to be 7.08, 0.43, and 0.28 mg/g, respectively. This was due to the formation of iron oxide (i.e., two-line ferrihydrite) on the surface of BC. Linear combination fitting using As K-edge X-ray absorption near edge structure spectra confirmed that all chemical forms of dimethylated arsenicals adsorbed on the two-line ferrihydrite were DMA(V). Thus, FeBC could retain highly mobile and toxic arsenicals such as DMMTA(V) and DMDTA(V)) in the environment, and transform them into DMA(V) with relatively low toxicity.

Arsenic perception and signaling: The yet unexplored world

Arsenic is one of the most potent carcinogens in the biosphere, jeopardizing the health of millions of people due to its entrance into the human food chain through arsenic-contaminated waters and staple crops, particularly rice. Although the mechanisms of arsenic sensing are widely known in yeast and bacteria, scientific evidence concerning arsenic sensors or components of early arsenic signaling in plants is still in its infancy. However, in recent years, we have gained understanding of the mechanisms involved in arsenic uptake and detoxification in different plant species and started to get insights into arsenic perception and signaling, which allows us to glimpse the possibility to design effective strategies to prevent arsenic accumulation in edible crops or to increase plant arsenic extraction for phytoremediation purposes. In this context, it has been recently described a mechanism according to which arsenite, the reduced form of arsenic, regulates the arsenate/phosphate transporter, consistent with the idea that arsenite functions as a selective signal that coordinates arsenate uptake with detoxification mechanisms. Additionally, several transcriptional and post-translational regulators, miRNAs and phytohormones involved in arsenic signaling and tolerance have been identified. On the other hand, studies concerning the developmental programs triggered to adapt root architecture in order to cope with arsenic toxicity are just starting to be disclosed. In this review, we compile and analyze the latest advances toward understanding how plants perceive arsenic and coordinate its acquisition with detoxification mechanisms and root developmental programs.

Double-edged effects of elevating temperature on the aging of exogenous arsenic in flooded paddy soils

Temperature variation can have a significant impact on arsenic (As) bioavailability in paddy soils. However, details regarding the transformation of exogenous As during the aging process in paddy soils at various temperatures remain unclear. This work investigated the effects of temperature on the As extractability and As species transformation of three paddy soils spiked with exogenous arsenate at 60 mg kg^-1 under flooded aging and explored the related chemical and microbial mechanisms. The results showed that 0.05 M NH₄H₂PO₄-extractable As decreased over time during flooded aging for 192 days, and it decreased by approximately one-third at 35 °C compared with 15 °C and 25 °C at the same aging time, indicating that higher temperatures facilitated the decrease in As extractability. As(V) reduction mainly occurred at 35 °C because the abundance and As(V)-reducing capacity of the predominant indigenous bacteria, the Bacillus sp strains, and the abundance of the arrA gene were significantly higher than those at 15 °C and 25 °C. The reduction of As(V) to As(III) and aging occurred simultaneously. The kinetic models were established, and the rate constants of the reduction and aging processes were obtained. Soil properties significantly affected the aging and reduction processes of extractable As(V). Our study indicated that elevating temperature had dual effects on the environmental risk of As in the flooded aging process. The previous definition of "aging" based on cationic metals needs to be updated according to the transformation characteristics of As species in flooded conditions. Our results addressed the necessity of impeding the reduction of As(V) in paddy soils under global warming.

Dimethylated Thioarsenates: A Potentially Dangerous Blind Spot in Current Worldwide Regulatory Limits for Arsenic in Rice

Arsenic (As) occurrence in rice is a serious human health threat. Worldwide, regulations typically limit only carcinogenic inorganic As, but not possibly carcinogenic dimethylated oxyarsenate (DMA). However, there is emerging evidence that "DMA", determined by routine acid-based extraction and analysis, hides a substantial share of dimethylated thioarsenates that have similar or higher cytotoxicities than arsenite. Risk assessments characterizing the in vivo toxicity of rice-derived dimethylated thioarsenates are urgently needed. In the meantime, either more sophisticated methods based on enzymatic extraction and separation of dimethylated oxy- and thioarsenates have to become mandatory or total As should be regulated.

Dimethylmonothioarsenate Is Highly Toxic for Plants and Readily Translocated to Shoots

Arsenic is one of the most relevant environmental pollutants and human health threats. Several arsenic species occur in soil pore waters. Recently, it was discovered that these include inorganic and organic thioarsenates. Among the latter, dimethylmonothioarsenate (DMMTA) is of particular concern because in mammalian cells, its toxicity was found to exceed even that of arsenite. We investigated DMMTA toxicity for plants in experiments with Arabidopsis thaliana and indeed observed stronger growth inhibition than with arsenite. DMMTA caused a specific, localized deformation of root epidermal cells. Toxicity mechanisms apparently differ from those of arsenite since no accumulation of reactive oxygen species was observed in DMMTA-exposed root tips. Also, there was no contribution of the phytochelatin pathway to the DMMTA detoxification as indicated by exposure experiments with respective mutants and thiol profiling. RNA-seq analysis found strong transcriptome changes dominated by stress-responsive genes. DMMTA was taken up more efficiently than the methylated oxyarsenate dimethylarsenate and highly mobile within plants as revealed by speciation analysis. Shoots showed clear indications of DMMTA toxicity such as anthocyanin accumulation and a decrease in chlorophyll and carotenoid levels. The toxicity and efficient translocation of DMMTA within plants raise important food safety issues.

Widespread Occurrence of the Highly Toxic Dimethylated Monothioarsenate (DMMTA) in Rice Globally

Arsenic (As) accumulation in rice is of global concern for human health and international trade. Rice is typically reported to contain inorganic As (iAs) and dimethylated arsenate (DMA), with current food guidelines limiting toxic iAs but not less-toxic DMA. Here, we show that the highly toxic dimethylated monothioarsenate (DMMTA) is also found in rice worldwide and has been unknowingly determined as less-toxic DMA by previous routine analytical methods. Using enzymatic extraction followed by high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) analysis with a C18 column, DMMTA was detected in rice grains (n = 103) from a field survey from China and in polished rice grains (n = 140) from a global market-basket survey. Concentration ranged from <0.20 to 34.8 μg/kg (median 10.3 μg/kg), accounting for 0 to 21% of total As. A strong linear correlation was observed in all rice samples between DMA and DMMTA (being 30 ± 8% of DMA) concentrations. This robust relationship allows an estimation of DMMTA in rice grains from the DMA data reported in previous market-basket surveys, showing a general global geographical pattern with DMMTA concentration increasing from the equator toward high-latitude regions. Based on the global occurrence and potential high toxicity, DMMTA in rice should be considered in health risk assessments and for setting food regulations.

Toxic metals and metalloids: Uptake, transport, detoxification, phytoremediation, and crop improvement for safer food

Agricultural soils are under threat of toxic metal/metalloid contamination from anthropogenic activities, leading to excessive accumulation of arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in food crops that poses significant risks to human health. Understanding how these toxic metals and their methylated species are taken up, translocated, and detoxified is prerequisite to developing strategies to limit their accumulation for safer food. Toxic metals are taken up and transported across different cellular compartments and plant tissues via various transporters for essential or beneficial nutrients, e.g. As by phosphate and silicon transporters, and Cd by manganese (Mn), zinc (Zn), and iron (Fe) transporters. These transport processes are subjected to interactions with nutrients and the regulation at the transcriptional and post-translational levels. Complexation with thiol-rich compounds, such as phytochelatins, and sequestration in the vacuoles are the common mechanisms for detoxification and for limiting their translocation. A number of genes involved in toxic metal uptake, transport, and detoxification have been identified, offering targets for genetic manipulation via gene editing or transgenic technologies. Natural variations in toxic metal accumulation exist within crop germplasm, and some of the quantitative trait loci underlying these variations have been cloned, paving the way for marker-assisted breeding of low metal accumulation crops. Using plants to extract and remove toxic metals from soil is also possible, but this phytoremediation approach requires metal hyperaccumulation for efficiency. Knowledge gaps and future research needs are also discussed.