November 11, 2020

Soil microbes vary in their ability to chemically transform toxic arsenic

Arsenic is naturally present in the soil and groundwater of many countries, posing a health risk to local populations specially in South East Asia. In rice paddy fields, notably, arsenic accumulates in the grain, as its cultivation under flooded conditions favors arsenic release from soil.

Yet, the fate of arsenic in rice fields is highly interconnected with the metabolic activity of the soil microbiome and some microbes can perform arsenic methylation, which facilitates arsenic accumulation in rice grains. To complicate the matter, microbial methylation can render arsenic compounds ultimately more or less toxic – this depending on the microbial species and whether their habitat is aerated or not.

Now, a new study from the group of Rizlan Bernier-Latmani (NCCR Microbiomes WP4 leader) sheds light on how different species of soil bacteria and archaea vary in their capacity to accumulate and transform toxic arsenic.

In this study, the lead author Dr. Karen Viacava and her co-authors examined seven species – 2 aerobes and 5 anaerobes – that potentially methylate arsenic and are likely to be found in rice paddy soils. They found that anaerobic microbes better resisted arsenic toxicity but methylated arsenic less efficiently than their aerobic counterparts. This was associated with an active pumping out of arsenic from the cell, a mechanism that is less efficient or even absent in aerobes.

Their results have also led the authors to postulate on the evolutionary maintenance of arsenic methylation in arsenic-resistant anaerobes. If not used for detoxification, could these microbes produce toxic methylated arsenic compounds to compete with other members of the soil microbiome? This intriguing hypothesis remains to be explored further.

doi: 10.1021/acs.est.0c03908