Arsenic methylation and microbial communities in paddy soils under alternating anoxic and oxic conditions.

Arsenic (As) methylation in soils affects the environmental behavior of As, excessive accumulation of dimethylarsenate (DMA) in rice plants leads to straighthead disease and a serious drop in crop yield. Understanding the mobility and transformation of methylated arsenic in redox-changing paddy fields is crucial for food security. Here, soils including un-arsenic contaminated (N-As), low-arsenic (L-As), medium-arsenic (M-As), and high-arsenic (H-As) soils were incubated under continuous anoxic, continuous oxic, and consecutive anoxic/oxic treatments respectively, to profile arsenic methylating process and microbial species involved in the As cycle. Under anoxic-oxic (A-O) treatment, methylated arsenic was significantly increased once oxygen was introduced into the incubation system. The methylated arsenic concentrations were up to 2-24 times higher than those in anoxic (A), oxic (O), and oxic-anoxic (O-A) treatments, under which arsenic was methylated slightly and then decreased in all four As concentration soils. In fact, the most plentiful arsenite S-adenosylmethionine methyltransferase genes (arsM) contributed to the increase in As methylation. Proteobacteria (40.8%-62.4%), Firmicutes (3.5%-15.7%), and Desulfobacterota (5.3%-13.3%) were the major microorganisms related to this process. These microbial increased markedly and played more important roles after oxygen was introduced, indicating that they were potential keystone microbial groups for As methylation in the alternating anoxic (flooding) and oxic (drainage) environment. The novel findings provided new insights into the reoxidation-driven arsenic methylation processes and the model could be used for further risk estimation in periodically flooded paddy fields.

Soil Microbiomes and their Arsenic Functional Genes in Chronically High-Arsenic Contaminated Soils.

Microbial arsenic transformations play essential roles in controlling pollution and ameliorating risk. This study combined high-throughput sequencing and PCR-based approaches targeting both the 16 S rRNA and arsenic functional genes to investigate the temporal and spatial dynamics of the soil microbiomes impacted by high arsenic contamination (9.13 to 911.88 mg/kg) and to investigate the diversity and abundance of arsenic functional genes in soils influenced by an arsenic gradient. The results showed that the soil microbiomes were relatively consistent and mainly composed of Actinobacteria (uncultured Gaiellales and an unknown_67 - 14 bacterium), Proteobacteria, Firmicutes (particularly, Bacillus), Chloroflexi, and Acidobacteria (unknown_Subgroup_6). Although a range of arsenic functional genes (e.g., arsM, arsC, arrA, and aioA) were identified by shotgun metagenomics, only the arsM gene was detected by the PCR-based method. The relative abundance of the arsM gene accounted for 0.20%-1.57% of the total microbial abundance. Combining all analyses, arsenic methylation mediated by the arsM gene was proposed to be a key process involved in the arsenic biogeochemical cycle and mitigation of arsenic toxicity. This study advances our knowledge about arsenic mechanisms over the long-term in highly contaminated soils.

Active microbial arsenic methylation in saline-alkaline paddy soil.

Seawater rice has been cultivated to ensure food security. The salt-tolerant rice strains are resistant to saline and alkali but may be vulnerable to elevated arsenic (As) near coastal regions. Herein, the saline-alkaline paddy soil was incubated with natural irrigation river for three months to explore the mobility and transformation of As. The incubation results showed that 65 ± 1.2 % solid-bound As(V) was reduced to As(III) within two weeks with the release of As(III) to porewater. The dissolved As(III) was methylated after two weeks, resulting in dimethyl arsenate (DMA) as the dominant As species (87 %-100 %). The elevated As methylation was attributed to the most abundant arsenite methyltransferase gene (arsM) (4.1-10.4 × 107/g dry soil), over three orders of magnitude higher than As redox-related genes. The analysis of arsM operational taxonomic units (OTUs) suggested the highest sequence similarity to Proteobacteria (25.7-39.5 %), Actinobacteria (24.9-30.5 %), Gemmatimonadetes (7.5-11.9 %), Basidiomycota (5.1-12.5 %), and Chloroflexi (4.1-8.7 %). Specifically, Chloroflexi and Actinobacteria are salt-tolerant bacteria, probably responsible for As methylation. The As in grain was within a safe regulatory level, and the dominance of methylated As in porewater did not enhance its accumulation in rice grains.

Impacts of silicon addition on arsenic fractionation in soils and arsenic speciation in Panax notoginseng planted in soils contaminated with high levels of arsenic.

Arsenic (As) is a well-known carcinogenic substance whose biological toxicity in soils and plants depends on its concentration and chemical forms. Silicon (Si) generally can alleviate biotic and abiotic stresses, including As stress. However, its effects vary depending on As chemical form, plant species and other factors. A pot experiment was performed to investigate the effects of Si addition on the content and forms of As in red soil and its uptake, transport and speciation in Panax notoginseng. The results showed that additions of 25 and 75 mg kg-1 of Si both significantly decreased the concentrations of water-soluble As and exchangeable As in soil and therefore decreased the bioavailability of soil As. However, the As uptake by Panax notoginseng (PN) was increased, which resulted in increases in As concentration by 18.5% and 2.3% in roots and by 56.7% and 58.3% in shoots, respectively, when compared with the control. Arsenate (As(V)) was the dominant As species in all the treatment soils (99.8-100%), whereas arsenite (As(III)) was prevalent in plant roots (75.2-92.4%), shoots (74.1-87.9%) and leaves (73.9-84.3%). Si addition (25 and 75 mg kg-1) significantly increased As(III) concentration in roots by 167.5% and 83.3%, respectively. Monomethylarsonic acid (MMA) was the only detected methylated As but at low concentrations (0.01-0.29 mg kg-1) and only in PN leaves. Si addition (25 and 75 mg kg-1) significantly increased the copy number of the arsenite methyltransferase (arsM) gene by 31.0% and 47.2% but did not increase the methylated As species content in PN leaves. The detected copy number of the arsM gene did not represent the capacity of soil to methylate As, and the sources of MMA in leaves need to be explored in further research.

Diversity and abundance of arsenic methylating microorganisms in high arsenic groundwater from Hetao Plain of Inner Mongolia, China.

Arsenic methylation is regarded as an effective way of arsenic detoxification. Current knowledge about arsenic biomethylation in high arsenic groundwater remains limited. In the present study, 16 high arsenic groundwater samples from deep wells of the Hetao Plain were investigated using clone library and quantitative polymerase chain reaction (qPCR) analyses of arsM genes as well as geochemical analysis. The concentrations of methylated arsenic (including monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) varied from 2.40 to 16.85 µg/L. Both bacterial and archaeal arsenic methylating populations were detected in the high arsenic aquifer. They were dominated by Proteobacteria, Firmicutes, Gemmatimonadetes, Nitrospirae, Methanomicrobia and a large unidentified group. The abundances of predominant populations were correlated positively to either total organic carbon or total arsenic and arsenite concentrations. The arsM gene abundances in high arsenic groundwater ranged from below detection to 5.71 × 106 copies/L and accounted for 0-3.32‰ of total bacterial and archaeal 16S rRNA genes. The arsM gene copies in high arsenic groundwater showed closely positive correlations with methylated arsenic concentrations. The overall results implied that arsenic methylating microorganisms were abundant and diverse in high arsenic groundwater. This was the first study of arsenic methylating microbial communities in high arsenic groundwater aquifers and might provide useful information for arsenic bioremediation in groundwater systems.