Australia: The Land Where Time Began

A biography of the Australian continent 

Microbial Biomethylation – Substantial Contribution to Aquifer Arsenic Cycling

A prominent role is played by microbes in the transformation of arsenic to and from mobile forms in aquifers (1). Inorganic forms of arsenic (2) are involved in much of this cycling, but organic forms can also be generated by microbial activity through methylation (3), though in the case of aquifers this activity is often considered to be insignificant (4,5,6,7). In this paper Maguffin et al., identify the presence of dimethylarsinate as well as other species of methylated arsenic in an aquifer that is hosted in volcaniclastic sedimentary rocks. It is shown by their results that dimethylarsinate is widespread in the aquifer and there is a strong correlation with arsenite concentration. Maguffin et al. used laboratory incubation experiments and aquifer injection tests to show that microbes in an aquifer can produce dimethylarsinate at rates of about 0.1 % of dissolved arsenic per day, which is comparable to rates of production of dimethylarsinate in surface environments. They estimate, based on these results, that biomethylation in aquifers globally has the potential to transform 100 tone of inorganic arsenic to species of methylated arsenic per year; compared with the 420-1,250 tons of inorganic arsenic that is biomethylated in soils (8). The conclusion of Maguffin et al. is that a significant contribution to arsenic cycling in aquifers could be made by biomethylation. Because arsine and methylarsines are yielded by biomethylation, both products being more volatile and prone to diffusion than other species of arsenic, they also suggest that biomethylation may act as a link between the surface and subsurface cycling of arsenic.

Sources & Further reading

1. Maguffin, S. C., M. F. Kirk, A. R. Daigle, S. R. Hinkle and Q. Jin (2015). "Substantial contribution of biomethylation to aquifer arsenic cycling." Nature Geosci 8(4): 290-293.