Uptake of arsenic species by turnip (Brassica rapa L.) and lettuce (Lactuca sativa L.) treated with roxarsone and its metabolites in chicken manure

Relationships between arsenic biotransformation genes, antibiotic resistance genes, and microbial function under different arsenic stresses during composting

Although the arsenic contamination and antibiotic resistance genes (ARGs) during composting have been studied separately, there is limited information on their interactions, particularly, the relationship between arsenic biotransformation genes (ABGs) and ARGs. Therefore, the present study used different forms of arsenic stress (organic and inorganic arsenic at 10 and 50 mg/kg) in pig manure and straw co-composting, to evaluate the effects of arsenic stress on microbial community structures, metabolic function, ABGs, and ARGs. The results showed that arsenic stress had different effects on different parameters and promoted the microbial formation of humic acid and the biodegradation of fulvic acid. Inorganic arsenic showed more rapid effects on microbial community structure, visible within about 20 days, while the effects of organic arsenic were later (about 45 days) due to the necessity of transformation. Moreover, the addition of organic roxarsone and inorganic arsenic resulted in higher expression of ABGs and ARGs, respectively. Arsenic addition also caused increased expression of genes associated with replication and repair. A significant relationship was observed between ABG and ARG expression, for instance, genes involved in arsenic reduction and oxidation were influenced by genes involved in aminoglycoside and chloramphenicol resistance genes (p < 0.05). These complex interactions among microorganisms, functional genes, and external parameters contribute to the understanding of the mechanisms underlying cross-contamination.

Phenylarsonics in concentrated animal feeding operations: Fate, associated risk, and treatment approaches

Phenylarsonics are present as additives in animal feed in some countries. As only a small fraction of these additives is metabolized in animals, they mostly end up in the environment. A comprehensive investigation of the fate of these additives is crucial for evaluating their risks. This review aims to provide a clear understanding of the transformation mechanism of phenylarsonics in vivo and in vitro and to evaluate their fate and associated risks. Degradation of phenylarsonics releases toxic As species (mainly as inorganic arsenic (iAs)). Trivalent phenylarsonics are the metabolites or biotic degradation intermediates of phenylarsonics. The cleavage of As groups from trivalent phenylarsonics catalyzed by C-As lyase or other unknown pathways generates arsenite (As(III)). As(III) can be further oxidized to arsenate (As(V)) and methylated to methyl-arsenic species. The half-lives associated with abiotic degradation of phenylarsonics ranged from a few minutes to tens of hours, while those associated with biotic degradation ranged from several days to hundreds of days. Abiotic degradation resulted in a higher yield of iAs than biotic degradation. The use of phenylarsonics led to elevated total As and iAs levels in animal products and environmental matrices, resulting in As exposure risk to humans. The oxidation of phenylarsonics to As(V) facilitated the sorptive removal of As, which provides a general approach for treating these compounds. This review provides solid evidence that the use of phenylarsonics has adverse effects on both human health and environmental safety, and therefore, supports their withdrawal from the global market.

Increase in arsenic methylation and volatilization during manure composting with biochar amendment in an aeration bioreactor

Biochar is widely used as an amendment to optimize the composting process. In this study, we firstly investigated the effects of biochar amendment on methylation and volatilization of arsenic (As), and the microbial communities during manure composting. Biochar amendment was found to increase the concentrations of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) during mesophilic (days 0-10) and early thermophilic (days 11-15) phases, and promote As volatilization during the maturing phase (days 60-80) of composting. In addition, the abundances of As(V) reductase (arsC) and As(III) S-adenosyl-L-methionine methyltransferase (arsM) genes were higher in the biochar treatment than that in the control. Moreover, biochar amendment influenced the microbial communities by promoting As methylation and volatilization via Ensifer and Sphingobium carrying arsC genes, and Rhodopseudomonas and Pseudomonas carrying arsM genes. This study emphasized the considerable role of biochar on methylation and volatilization of As during manure composting and provided an overall characterization of the community compositions of arsC and arsM genes during manure composting. It will broaden our insights in As biogeochemical cycle during manure composting with biochar amendment, which will facilitate the regulation of As during manure composting and its application in agricultural soil.

Natural degradation of roxarsone in contrasting soils: Degradation kinetics and transformation products

Roxarsone (ROX) is transformed to more toxic arsenicals after land application of ROX-containing poultry litter to agricultural soils. To date, no reports have compared ROX degradation in soils with contrasting properties. In this study, the impact of different incubation conditions on ROX degradation was investigated in red (C-Soil) and yellow-brown (H-Soil) soils. The degradation half-lives of extractable ROX in C-Soil and H-Soil were found to be 130-394d and 4-94d, respectively, indicating that the extractable ROX degraded faster in H-Soil. This result stems from the higher organic matter content, more abundant soil microbes, and lower ROX sorption capacity of H-Soil compared to C-Soil. Degradation of extractable ROX in both C-Soil and H-Soil was significantly promoted by soil moisture and exogenous glucose. Exogenous P(V) facilitated degradation of extractable ROX in C-Soil, but limited effects were observed for H-Soil. HPLC-ICP-MS analysis confirmed that ROX and dimethylarsinic acid were the predominant As species in soil extracts from 119-day incubated C-Soil and H-Soil, respectively. Ultimately, minimal transformation of extractable ROX was observed in C-Soil, but the majority of extractable ROX in H-Soil was biologically transformed. The differences in degradation of extractable ROX in C-Soil and H-Soil highlight the key roles of soil properties on the environmental fate of ROX and associated arsenicals. Results from this study inform the need for comprehensive evaluation of the ecological risks in organoarsenical-contaminated soils.

Arsenic Methylation and its Relationship to Abundance and Diversity of arsM Genes in Composting Manure

Although methylation is regarded as one of the main detoxification pathways for arsenic (As), current knowledge about this process during manure composting remains limited. In this study, two pilot-scale compost piles were established to treat manure contaminated with As. An overall accumulation of methylated As occurred during 60 day-composting time. The concentration of monomethylarsonic acid (MMA) increased from 6 to 190 μg kg^-1 within 15 days and decreased to 35 μg kg^-1 at the end of the maturing phase; while the concentration of dimethylarsinic acid (DMA) continuously increased from 33 to 595 μg kg^-1 over the composting time. The arsM gene copies increased gradually from 0.08 × 10⁹ to 6.82 × 10⁹ copies g^-1 dry mass over time and correlated positively to the concentrations of methylated As. 16S rRNA gene sequencing and arsM clone library analysis confirmed the high abundance and diversity of arsM genes. Many of these genes were related to those from known As-methylating microbes, including Streptomyces sp., Amycolatopsis mediterranei and Sphaerobacter thermophiles. These results demonstrated that As methylation during manure composting is significant and, for the first time, established a linkage between As biomethylation and the abundance and diversity of the arsM functional genes in composting manure.

Fenton process-affected transformation of roxarsone in paddy rice soils: Effects on plant growth and arsenic accumulation in rice grain

Batch and greenhouse experiments were conducted to examine the effects of Fenton process on transformation of roxarsone in soils and its resulting impacts on the growth of and As uptake by a rice plant cultivar. The results show that addition of Fenton reagent markedly accelerated the degradation of roxarsone and produced arsenite, which was otherwise absent in the soil without added Fenton reagent. Methylation of arsenate was also enhanced by Fenton process in the earlier part of the experiment due to abundant supply of arsenate from Roxarsone degradation. Overall, addition of Fenton reagent resulted in the predominant presence of arsenate in the soils. Fenton process significantly improved the growth of rice in the maturity stage of the first crop, The concentration of methylated As species in the rice plant tissues among the different growth stages was highly variable. Addition of Fenton reagent into the soils led to reduced uptake of soil-borne As by the rice plants and this had a significant effect on reducing the accumulation of As in rice grains. The findings have implications for understanding As biogeochemistry in paddy rice field receiving rainwater-borne H2O2 and for development of mitigation strategies to reduce accumulation of As in rice grains.

Shewanella oneidensis MR-1-Induced Fe(III) Reduction Facilitates Roxarsone Transformation

Although microbial activity and associated iron (oxy)hydroxides are known in general to affect the environmental dynamics of 4-hydroxy-3-nitrobenzenearsonic acid (roxarsone), the mechanistic understanding of the underlying biophysico-chemical processes remains unclear due to limited experimental information. We studied how Shewanella oneidensis MR-1 –a widely distributed metal-reducing bacterium, in the presence of dissolved Fe(III), affects roxarsone transformations and biogeochemical cycling in a model aqueous system. The results showed that the MR-1 strain was able to anaerobically use roxarsone as a terminal electron acceptor and to convert it to a single product, 3-amino-4-hydroxybenzene arsonic acid (AHBAA). The presence of Fe(III) stimulated roxarsone transformation via MR-1-induced Fe(III) reduction, whereby the resulting Fe(II) acted as an efficient reductant for roxarsone transformation. In addition, the subsequent secondary Fe(III)/Fe(II) mineralization created conditions for adsorption of organoarsenic compounds to the yielded precipitates and thereby led to arsenic immobilization. The study provided direct evidence of Shewanella oneidensis MR-1-induced direct and Fe(II)-associated roxarsone transformation. Quantitative estimations revealed a candidate mechanism for the early-stage environmental dynamics of roxarsone in nature, which is essential for understanding the environmental dynamics of roxarsone and successful risk assessment.

UV irradiation and UV-H₂O₂ advanced oxidation of the roxarsone and nitarsone organoarsenicals

Roxarsone (ROX) and nitarsone (NIT) are used as additives in animal feeding operations and have been detected in animal manure, agricultural retention ponds, and adjacent surface waters. This work investigates treatment of organoarsenicals using UV-based treatment processes, namely UV irradiation at 253.7 nm and the UV-H2O2 advanced oxidation process. The apparent molar absorptivity was mapped for ROX and NIT across pH and wavelength. For UV irradiation at 253.7 nm, the fluence-based pseudo-first order rate constant (kp(')) and effective quantum yield (Φ) for ROX were 8.10-29.7 × 10(-5) cm(2)/mJ and 2.34-8.37 × 10(-3) mol/E, respectively; the corresponding constants were slightly lower for NIT. The observed rate constants are higher during advanced oxidation (e.g., kp,ROX(')=3.92(±0.19)-217(±48) × 10(-4) cm(2)/mJ). Second order rate constants for organoarsenical transformation by hydroxyl radicals were determined to be 3.40(±0.45) × 10(9) and 8.28(±0.49) × 10(8) M(-1)s(-1) for ROX and NIT, respectively. Solution pH and nitrate concentration did not significantly impact ROX transformation during advanced oxidation; however, bicarbonate and dissolved organic matter from chicken litter reduced ROX transformation through hydroxyl radical scavenging. Inorganic arsenic was the predominant transformation product of ROX during UV-H2O2 treatment.

Organoarsenicals in poultry litter: detection, fate, and toxicity

Arsenic contamination in groundwater has endangered the health and safety of millions of people around the world. One less studied mechanism for arsenic introduction into the environment is the use of organoarsenicals in animal feed. Four organoarsenicals are commonly employed as feed additives: arsanilic acid, carbarsone, nitarsone, and roxarsone. Organoarsenicals are composed of a phenylarsonic acid molecule with substituted functional groups. This review documents the use of organoarsenicals in the poultry industry, reports analytical methods available for quantifying organic arsenic, discusses the fate and transport of organoarsenicals in environmental systems, and identifies toxicological concerns associated with these chemicals. In reviewing the literature on organoarsenicals, several research needs were highlighted: advanced analytical instrumentation that allows for identification and quantification of organoarsenical degradation products; a greater research emphasis on arsanilic acid, carbarsone, and nitarsone; identification of degradation pathways, products, and kinetics; and testing/development of agricultural wastewater and solid treatment technologies for organoarsenical-laden waste.

Degradation of roxarsone in a silt loam soil and its toxicity assessment

The land application of poultry or swine litter, containing large amounts of roxarsone, causes serious arsenic pollution in soil. Understanding biotransformation process of roxarsone and its potential risks favors proper disposal of roxarsone-contaminated animal litter, yet remains not achieved. We report an experimental study of biotransformation process of roxarsone in a silt loam soil under various soil moisture and temperature conditions, and the toxicity of roxarsone and its products from degradation. Results showed that soil moisture and higher temperature promoted roxarsone degradation, associating with emergent pentavalent arsenic. Analysis of fluorescein diacetate (FDA) hydrolysis activity revealed that roxarsone does not exert acute toxic on soil microbes. With the release of inorganic arsenic, FDA hydrolysis activity was inhibited gradually, as evidenced by ecotoxicological assessment using Photobacterium leiognathi. The results shade new lights on the dynamic roxarsone biotransformation processes in soil, which is important for guiding appropriate disposal of poultry or swine litter in the environment.

Roxarsone and its metabolites in chicken manure significantly enhance the uptake of As species by vegetables

Roxarsone is an organoarsenic feed additive which can be finally degraded to other higher toxic metabolites after excreted by animal. In this work, the uptake of As species by vegetables treated with chicken manure bearing roxarsone and its metabolites was investigated. It was showed that more than 96% of roxarsone added in chicken feed was degraded and converted to arsenite, monomethylarsonic acid, dimethylarsinic acid, arsenate, 4-hydroxyphenylarsonic acid and other unknown As species. Arsenite and arsenate could be found in roots of vegetables but only arsenite transported up to shoots. Chicken manure bearing roxarsone and its metabolites increased 33-175% of arsenite and 28% ∼ seven times of arsenate in vegetable roots, 68-175% of arsenite in edible vegetable shoots. Arsenite, the most toxic As form, was the major extractable As species in vegetables accounted for 79-98%. The results reflected that toxic element As could be absorbed by vegetables via the way: roxarsone in feed → animal → animal manure → soil → crop and the uptake of As species would be enhanced by using chicken manure bearing roxarsone and its metabolites as organic fertilizer.

Mapping the protein profile involved in the biotransformation of organoarsenicals using an arsenic metabolizing bacterium

Insight into the organoarsenic metabolism of Alkaliphilus oremlandii OhILAs by comprehensive proteomic analysis.