Arsenic Methylation in Soils and Its Relationship with Microbial arsM Abundance and Diversity, and As Speciation in Rice

Abstract

Methylation of arsenic in soil influences its environmental behavior and accumulation by plants, but little is known about the factors affecting As methylation. As speciation was determined in the pore waters of six soils from diverse geographical locations over 54 days of incubation under flooded conditions. The concentration of methylated As (monomethylarsonic acid, MMA, and dimethylarsinic acid, DMA) varied from 0 to 85 μg L −1 (0 − 69% of the total As in pore water). Two Bangladeshi paddy soils contaminated by irrigation of As-laden groundwater produced large concentrations of inorganic As but relatively little methylated As. Two contaminated paddy soils from China produced a transient peak of DMA during the early phase of incubation. Methylated As represented considerable proportions of the total soluble As in the two uncontaminated soils from the UK and U.S. The copy number of the microbial arsenite methyltransferase gene (arsM) correlated positively with soil pH. However, pore-water methylated As correlated negatively with pH or arsM copy number, and positively with dissolved organic C. GeoChip assay revealed considerable arsM diversity among the six soils, with 27−35 out of 66 sequences in the microarray being detected. As speciation in rice plants grown in the soils generally mirrored that in the pore water. The results suggest that methylated As species in plants originated from the soil and As methylation in soil was influenced strongly by the soil conditions. ■ INTRODUCTIONArsenic (As) is a ubiquitous contaminant in the environment originating from both natural and anthropogenic sources. Because its biogeochemical behavior and toxicity vary greatly among different chemical species, it is important to understand how the speciation of As changes in the environment and what drives such changes. A particularly important case is the paddy rice system because it is now recognized that rice is a major source of As in the human diet. 1 The anaerobic conditions in paddy soils are conducive to the mobilization of arsenite, 2,3 which is taken up inadvertently by rice roots through the strong uptake pathway for silicic acid. 4 Rice grain contains both inorganic As (arsenate and arsenite) and organic As (mostly dimethylarsinic acid, DMA; occasionally also trace amounts of monomethylarsonic acid, MMA, and tetramethylarsonium). 5−9 As speciation in rice varies widely among different riceproducing regions. Market-basket surveys show that Asian rice generally is dominated by inorganic As with DMA typically accounting for about 20% of the total As. [5][6][7]10 In contrast, rice produced in the U.S. and Europe is more variable in As speciation with many samples containing more organic than inorganic As. 5,7,8,10 It is possible that that this geographical pattern reflects the relative bioavailability of inorganic versus organic As in different paddy environments. 10 Compared with inorganic As, methylated As species are more easily accumulated in rice grain. 10 Although pentavalent methylated As species are ...

Arsenic Methylation and Volatilization by Arsenite S -Adenosylmethionine Methyltransferase in Pseudomonas alcaligenes NBRC14159

“…It may therefore play an important role in the biogeochemistry of As cycling. arsM genes have been detected in soils using degenerate primers (29) or Geochip which contains probes for more than 60 arsM genes but not PaarsM (30). However, the degenerate primers used by Jia et al (29) could not amplify PaarsM from the genomic DNA of P. alcaligenes, likely due to the phylogenetic diversity and the limited As methylation sequences available.…”

“…More recently, degenerate primers based on the known diversity of arsM sequences from both prokaryotic and eukaryotic microbes have been designed for the purpose of identifying additional arsM genes (29). However, Pseudomonas arsM sequences were not included in the design of degenerate primers due to the lack of reference sequences from isolates in the genus (29,30).…”

Arsenic Methylation Dynamics in a Rice Paddy Soil Anaerobic Enrichment Culture

“…12 Distinct geographic patterns exist in methylated arsenic abundance in rice grains, with rice grown in the southcentral U.S. tending to have a large fraction of methylated arsenic, while Asian-grown rice accumulates primarily inorganic As (iAs). 2,8,13 The reasons for these patterns are unknown. Microbial arsenic methylation is catalyzed by arsenite Sadenosylmethionine methyltransferase (ArsM) enzymes 14 which mediate the sequential methylation of As(III) to mono-, di-, and trimethylated species.…”

Arsenic Transformation in Swine Wastewater with Low-Arsenic Content during Anaerobic Digestion

“…For example, environmental conditions influence the As methylation activity of microbes, while the abundance of As methylation microbes has little effect on As methylation. A previous study reported that the arsM gene copy number correlated positively with the soil pH [25]. Although there were more As methylated microbes in neutral alkaline soils, the activity of As methylation was higher in acidic soils.…”