Abstract
The formation of methylmercury (MeHg), which is biomagnified in
aquatic food chains and poses a risk to human health, is effected by
some iron- and sulfate-reducing bacteria (FeRB and SRB) in anaerobic
environments. However, very little is known regarding the
mechanism of uptake of inorganic Hg by these organisms, in part
because of the inherent difficulty in measuring the intracellular Hg
concentration. By using the FeRB Geobacter sulfurreducens and the
SRB Desulfovibrio desulfuricans ND132 as model organisms, we
demonstrate that Hg(II) uptake occurs by active transport. We also
establish that Hg(II) uptake by G. sulfurreducens is highly dependent
on the characteristics of the thiols that bind Hg(II) in the external
medium, with some thiols promoting uptake and methylation and
others inhibiting both. The Hg(II) uptake system of D. desulfuricans
has a higher affinity than that of G. sulfurreducens and promotes Hg
methylation in the presence of stronger complexing thiols. We observed
a tight coupling between Hg methylation and MeHg export
from the cell, suggesting that these two processes may serve to
avoid the build up and toxicity of cellular Hg. Our results bring up
the question of whether cellular Hg uptake is specific for Hg(II) or
accidental, occurring via some essential metal importer. Our data
also point at Hg(II) complexation by thiols as an important factor
controlling Hg methylation in anaerobic environments.
