Prevalence of Methylated Arsenic and Microbial Arsenic Methylation Genes in Paddy Soils of the Mekong Delta

Abstract

Microbially mediated inorganic-methylated arsenic (As) transformation in paddy soil is crucial to rice safety; however, the linkages between the microbial As methylation process and methylated As species remain elusive. Here, 62 paddy soils were collected from the Mekong River delta of Cambodia to profile As-related functional gene composition involved in the As cycle. The soil As concentration ranged from <1 to 16.6 mg kg^-1, with average As contents of approximately 81% as methylated As and 54% as monomethylarsenate (MMAs(V)) in the phosphate- and oxalate-extractable fractions based on As sequential extraction analysis. Quantitative PCR revealed high arsenite-methylating gene (<i>arsM</i>) copy numbers, and metagenomics identified consistently high <i>arsM</i> gene abundance. The abundance of As-related genes was the highest in bacteria, followed by archaea and fungi. <i>Pseudomonas</i>, <i>Bradyrhizobium</i>, <i>Burkholderia</i>, and <i>Anaeromyxobacter</i> were identified as bacteria harboring the most genes related to As biotransformation. Moreover, <i>arsM</i> and <i>arsI</i> (As demethylation) gene-containing operons were identified in the metagenome-assembled genomes (MAGs), implying that <i>arsM</i> and <i>arsI</i> could be transcribed together. The prevalence of methylated As and <i>arsM</i> genes may have been overlooked in tropical paddy fields. The As methylation-demethylation cycle should be considered when manipulating the methylated As pool in paddy fields for rice safety.