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Abstract Title: Controls on MeHg Uptake and Degradation by Several Anaerobic Bacteria
Presenter Name: Andrew Graham
Company/Institution: Grinnell College
Session: Special Session - Meta-omic and geochemical approaches to linking microbial activity to biogeochemical mercury cycling
Co-Authors: Andrew Graham,Sarah Weber,Ania Szlembarska,Linda Peng

Abstract Information :

MeHg- and inorganic Hg(II) (Hg(II)i)-cell association and MeHg degradation were evaluated in three organisms: the SRB Desulfovibrio desulfuricans and Pseudodesulfovibrio mercurii ?hgcAB and the FeRB Geobacter bemidjiensis. The two SRB lack the merA, merB, and hgcAB genes present in G. bemidjiensis. We explored the impact of ligands (cysteine, thioglycolate, methionine, and EDTA) and competitive divalent metals (Cd(II), Cu(II), and Zn(II)) on Hg species uptake and transformations. Across all conditions, little MeHg was converted to Hg(II)i. For the SRB, Hg(II)i produced was 0 to 15% over 48 h. Cell-free and heat-killed controls showed similar MeHg loss as active treatments, implicating abiotic processes in MeHg loss. MeHg degradation by the FeRB G. bemidjiensis was also slow, with less than 10% of added MeHg transformed to Hg0 over 24 h incubations. In the presence of thiol ligands, less than 15% of MeHg associated with cells compared to 80-100% for Hg(II)i. In the presence of methionine and EDTA, MeHg cell-association increased to 30 to 90%, but MeHg degradation did not increase. With G. bemidjiensis, this pattern held for both externally added MeHg and MeHg produced intracellularly via methylation of Hg(II)i. We hypothesize that slow MeHg degradation is due to slow MeHg uptake and/or rapid export of intracellular MeHg. Addition of 50 æM Cu(II) resulted in inhibition of MeHg conversion to Hg0 and Hg(II)i conversion to MeHg and Hg0 by G. bemidjiensis. Addition of 50 æM Cd(II) caused less inhibition and addition of 50 æM Zn(II) no inhibition. Interestingly, Cu(II) addition also resulted in a marked increase in MeHg cell-association. The best explanation for these results is that Cu(II) inhibits both Hg(II)i import and MeHg export. Overall, our results suggest that microbial MeHg degradation by SRB and FeRB is likely slow under conditions where MeHg is present at low concentrations and complexed by thiolate ligands.

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