Mercury uptake and export in Shewanella oneidensis

Abstract

Given methyl mercury's negative health effects, recent research regarding mercury has focused on mercury methylating bacteria. However, mercury uptake is a precursor to mercury methylation. We found that the facultative anaerobe, Shewanella oneidensis MR-1, which is known to not methylate mercury, has an uptake system similar to that of the mercury methylator Geobacter sulfurreducens, pointing to a possibility that there is a universal mercury uptake mechanism present across Proteobacteria. Namely, we found that mercury uptake in S. oneidensis in the presence of the thiol cysteine is an active process, is substrate specific, and zinc transporters are implicated in the uptake. Through S. oneidensis, we also see that on a per cell basis, mercury uptake is greater in the presence of oxygen. However, when normalized on a per protein basis, the presence of oxygen has minimal effects on cellular mercury uptake. Our data also points to a potential export mechanism in S. oneidensis. Unlike G. sulfurreducens, where mercury can be converted into methyl mercury and exported out of the cell, S. oneidensis has an export mechanism that does not rely on methylation. We see that cellular mercury in S. oneidensis decreases when mercury is not present in the external medium, pointing to possible Hg(II) export as a complex with a thiol such as glutathione.