Microbial Arsenic Methylation in Soil and Rice Rhizosphere

• Published in: Environmental science & technology Vol. 47; no. 7; pp. 3141 - 3148
• Main Authors: Jia, Yan, Huang, Hai, Zhong, Min, Wang, Feng-Hua, Zhang, Li-Mei, Zhu, Yong-Guan
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 02.04.2013
• Subjects: Agronomy. Soil science and plant productions; Arsenic; Arsenic - metabolism; Arsenic content; arsenites; Bacteria - genetics; Bacteria - metabolism; Base Sequence; Biodiversity; Biological and medical sciences; DNA Primers - metabolism; essential amino acids; Food toxicology; Fundamental and applied biological sciences. Psychology; Gene Dosage - genetics; Genes; Genes, Bacterial - genetics; horizontal gene transfer; Medical sciences; Methylation; methyltransferases; microorganisms; Nonnative species; Oryza - metabolism; Oryza - microbiology; paddy soils; Phylogeny; Plant Roots - microbiology; Polymorphism; restriction fragment length polymorphism; Rhizosphere; Rice; roots; S-adenosylmethionine; Soil - chemistry; Soil and water pollution; Soil Microbiology; Soil science; soil solution; Soils; Solutions; Toxicology; Microbial activity; Monocotyledones; Arsenic; Pollutant behavior; Cereal crop; Trace element; Carcinogen; Oryza sativa; Rhizosphere; Soils; Gramineae; Biotransformation; Angiospermae; Poison; Spermatophyta; Methylation; Food; Chemical contamination; Rice
• DOI: 10.1021/es303649v
• ISSN: 0013-936X; 1520-5851

Methylated arsenic (As) species are a common constituent of rice grains accounting for 10–90% of the total As. Recent studies have shown that higher plants are unlikely to methylate As in vivo suggesting that As methylation is a microbial mediated process that occurs in soils prior to plant uptake. In this study, we designed primers according to the conserved essential amino acids and structural motifs of arsenite S-adenosylmethionine methyltransferase (ArsM). We report for the first time the successful amplification of the prokaryotic arsM gene in 14 tested soils with wide ranging As concentrations. The abundance and diversity of the arsM gene in the rice rhizosphere soil and roots were analyzed using the designed primers. Results showed that microbes containing arsM genes were phylogenetically diverse, as revealed by the clone library and terminal restriction fragment length polymorphism (T-RFLP) analysis, and were branched into various phyla. Concentration of methylated As species in the soil solution was elevated in the rhizosphere soil and also by the addition of rice straw into the paddy soil, corresponding to the elevated abundance of the arsM gene in the soil. These results, together with evidence of horizontal gene transfer (HGT) of the arsM gene, suggest the genes encoding ArsM in soils are widespread. These findings demonstrate why most rice, when compared with other cereals, contains unusually high concentrations of methylated As species.