Arsenic (As), one of the most toxic elements in the periodic table, is naturally present in the environment where it undergoes extensive microbial cycling. Because of the toxicity of this metalloid, resistance mechanisms developed by Archaean ancestors to thrive under the presence of reduced As during the earliest anoxic ecosystems have been preserved and are present today in many bacteria and archaea genes. An arsenite efflux pump system or the mechanisms for arsenate reduction or oxidation, to name a few.
Arsenic methylation has been proposed as another resistance mechanism but also as an activation pathway or a precursor reaction for the synthesis of more complex arseno-organic molecules. It is a microbially-mediated process resulting in the addition of one or several methyl groups to inorganic As. The reaction is catalyzed by the enzyme arsenite methyltransferase (ArsM), generating volatile and non-volatile arsenicals.
Rice, main staple food for millions of people worldwide, particularly efficient in accumulating arsenic represents one of the main sources of arsenic-exposure for humans. Methylated arsenic, in certain regions represents the predominant species present in rice. However, rice plants are not able to methylate As and soil microorganisms are the responsible mediators of this process.
Several studies have been done to identify the main microbial drivers of As methylation by arsM sequencing and yet very few microbes have been confirmed as active As methylators by pure strains like the Bacteroidete Arsenicibacter rosenii SM-1 and the Firmicute Clostridium sp. BXM. Nonetheless, members of numerous other phyla present in soils harbor this gene and it is unclear whether all represent functional proteins and active methylating organisms. Thus, the goal of this study was to systematically probe the functionality and the in vivo activity of ArsM across different phyla in microorganisms isolated from or whose arsM gene has been found in paddy soils.
By performing time-course experiments, we assessed the As resistance and the capacity for arsenite methylation and volatilization across seven microbial strains encoding the arsM gene: two archaea (Methanosarcina mazei Gö1, Methanosarcina acetivorans C2A); two Firmicutes (Anaeromusa acidaminophila DSM 3853, Clostridium pasteurianum DSM 525); a Streptomycete (Streptomyces vietnamensis DSM 41927); a Deltaproteobacterium (Geobacter metallireducens GS-15); and a Bacteroidete (Arsenicibacter rosenii SM-1). In addition, all arsM genes were cloned into the arsenic sensitive Escherichia coli AW3110(DE3) and As speciation performed.
The results show that most of the strains were not able to methylate As despite harboring arsM genes that encode functional ArsM proteins. We hypothesize that more efficient As detoxification pathways might be prevalent, precluding methylation. We conclude that the presence of arsM does not equate As methylation activity and more work is undergoing to deconvolute arsM regulation.