Mercury methylation coupled to iron reduction by dissimilatory iron-reducing bacteria

• Iron reduction occurred with DIRB. Hg(II) restrained dissimilatory Fe(III) reduction.
• Mercury methylation could be mediated by iron-reducing bacteria (DIRB).
• Mercury methylation rate was positively correlated with iron reduction rate.

Iron reduction and mercury methylation by dissimilatory iron-reducing bacteria (DIRB), Geobacter sulfurreducens and Shewanella oneidensis, were studied, and the relationship of mercury methylation coupled to iron reduction was determined. The ability of both bacteria for reducing iron was tested, and Fe(III) reduction occurred with the highest rate when ferric oxyhydroxide was used as a terminal electron acceptor. G. sulfurreducens had proven to mediate the production of methylmercury (MeHg), and a notable increase of MeHg following the addition of inorganic Hg was observed. When the initial concentration of HgCl2 was 500 nM, about 177.03 nM of MeHg was determined at 8 d after G. sulfurreducens inoculation. S. oneidensis was tested negligible for Hg methylation and only 12.06 nM of MeHg was determined. Iron reduction could potentially influence Hg methylation rates. The increase in MeHg was consistent with high rate of iron reduction, indicating that Fe(III) reduction stimulated the formation of MeHg. Furthermore, the net MeHg concentration increased at low Fe(III) additions from 1.78 to 3.57 mM, and then decreased when the added Fe(III) was high from 7.14 to 17.85 mM. The mercury methylation rate was suppressed with high Fe(III) additions, which might have been attributable to mercury complexation and low availability.