Variability in Arsenic Methylation Efficiency across Aerobic and Anaerobic Microorganisms

Area of Science:

• Environmental microbiology
• Biogeochemistry
• Arsenic research

Background:

• Microbial methylation of arsenic (As) is key to the As biogeochemical cycle, especially in rice paddy soils.
• Methylated As is translocated into rice grains, impacting food safety.
• The arsenite (As(III)) methyltransferase gene (arsM) indicates microbial methylation potential.

Purpose of the Study:

• To evaluate the As methylation capabilities of seven microorganisms encoding active ArsM enzymes.
• To understand the relationship between microbial arsenic resistance mechanisms and methylation efficiency.
• To investigate the role of efflux pumps in limiting microbial arsenic methylation.

Main Methods:

• Assessing As methylation by seven microbial species with active ArsM.
• Comparing methylation efficiency between aerobic and anaerobic microorganisms.
• Investigating the effect of deleting an efflux pump gene (acr3) in *Clostridium pasteurianum*.

Main Results:

• Only aerobic microorganisms efficiently methylated arsenic.
• Anaerobic microorganisms showed high arsenic resistance due to efficient As(III) efflux but poor methylation.
• Methanogens' methylation activity appeared to be an artifact of membrane disruption.
• Deleting the *acr3* efflux pump gene in *Clostridium pasteurianum* increased arsenic sensitivity and methylation.

Conclusions:

• Possession of a functional ArsM enzyme does not guarantee active arsenic methylation.
• An inverse relationship exists between microbial arsenic efflux and methylation efficiency.
• Efficient efflux prevents intracellular arsenic accumulation, thereby limiting methylation.