Variability in Arsenic Methylation Efficiency across Aerobic and Anaerobic Microorganisms

Microbially-mediated
methylation of arsenic (As) plays an important
role in the As biogeochemical cycle, particularly in rice paddy soils
where methylated As, generated microbially, is translocated into rice
grains. The presence of the arsenite (As­(III)) methyltransferase gene
(arsM) in soil microbes has been used as an indication
of their capacity for As methylation. Here, we evaluate the ability
of seven microorganisms encoding active ArsM enzymes to methylate
As. Amongst those, only the aerobic species were efficient methylators.
The anaerobic microorganisms presented high resistance to As exposure,
presumably through their efficient As­(III) efflux, but methylated
As poorly. The only exception were methanogens, for which efficient
As methylation was seemingly an artifact of membrane disruption. Deletion
of an efflux pump gene (acr3) in one of the anaerobes, Clostridium pasteurianum, rendered the strain sensitive
to As and capable of more efficiently methylating As. Our results
led to the following conclusions: (i) encoding a functional ArsM enzyme
does not guarantee that a microorganism will actively drive As methylation
in the presence of the metalloid and (ii) there is an inverse relationship
between efficient microbial As efflux and its methylation, because
the former prevents the intracellular accumulation of As.