Final report - Microbial pathways for the reduction of mercury in saturated subsurface sediments Page: 2 of 6
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Barkay, Young & Zylstra (DE-FG02-05ER63969)
Final report - p. 2
HgCl2, nitrate reduction was not detected even on the 220'h day. These findings are being
prepared for publication (Wiatrowski et al., in preparation).
Increasing Hg concentrations also decrease the number of dominating species
selected in the enrichments as revealed by tRFLP patterns. The enrichment that was
supplemented with 312 nM of HgCl2, had a single tRFLP peak was most closely related
to that of the alphaproteobacterium Bradyrhizobium sp., a known denitrifier, and a PCR
fragment amplified from this enrichment by merA-specific primers, had 89% inferred
sequence identity to MerA in Acinetobacter sp. suggesting the selection of Hg resistant
bradyrhizobia in denitrifying enrichments. The role for this genus in the response to Hg
in the FRC subsurface sediments was supported when Hg resistant denitrifying isolates
from the enrichments were related to bradyrhizobia, one of which had a merA with 67%
inferred sequence identity to MerA of the actinobacterium Nocardioides sp. Thus,
denitrifying bradyrhizobia might be major players in Hg detoxification in the subsurface.
These findings are being prepared for publication (Wang et al., in preparation). Together,
these results suggest that (i) denitrification in subsurface sediments is highly
sensitive to inhibition by Hg; (ii) denitrification in the enrichments was restored
upon selection and growth of Hg resistant bacteria; (iii) horizontally transferred
merA genes might have contributed to this enrichment. While initial experiments with
iron reducing enrichments did not provide reproducible results, later incubations were
successful and the analyses and further examination of these incubation is currently
progressing.
The reduction of Hg(II) by metal reducing anaerobic bacteria: One of the major findings
of our project is that anaerobic metal reducing bacteria such as Shewanella oneidensis
MR-1 and Geobacter spp. reduce Hg(II) to Hg(0) by a novel pathway that is not related
to the mer-mediated resistance (Wiatrowski et al., 2006). These strains are of great
interest for bioremediation in the subsurface due to their ability to reduce toxic metals
and radionuclides. Additionally, these strains do not display Hg resistance typical of an
organism with a mer operon. In the presence of 200 nM Hg(II), live cells of strain MR-1
grown aerobically reduced 83 6.3% of the Hg(II), whereas autoclaved cells reduced
21 0.9% in 24 hours. Hg(II) reduction by MR-1 also occurred with iron oxyhydroxide
and fumarate as terminal electron acceptors. Reduction of Hg(II) showed a strong
dependence on the presence of an electron donor and an electron acceptor, as incubation
of cells in media which lacked either resulted in activity that is not significantly different
from that of autoclaved cells (p > 0.01). Unlike mer-mediated Hg(II) reduction, this
activity is not inducible, as exposed cells and unexposed cells had a specific activity for
reduction of Hg(II) of 3.14 0.25 and 3.07 0.35 nmol min1 mg protein respectively.
Live and autoclaved cells of G. sulfurreducens PCA reduced Hg(II) with specific
activities of 2.8 1.3 nmol Hg(II) min1 mg protein and 0.34 0.5 nmol Hg(II) min1 mg
protein 1, respectively. Live and autoclaved cells of strain G. metallireducens GS-15 had
specific activities of 6.05 1.4 nmol Hg(II) min1 mg protein and 1.65 0.9 nmol Hg(II)
min mg protein 1, respectively. However, Hg(II) reduction is not universal among
DMRB or anaerobes, as it was absent in Anaeromyxobacter dehalogenans strain 2CP-C,
which can reduce iron, and the nitrate reducer Pseudomonas stutzeri OXi.
One of the most intriguing finding of these studies was based on the observation
when grown using iron oxyhydroxide as a terminal electron acceptors, Hg(II) reduction
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barkay, Tamar; Young, Lily & Zylstra, Gerben. Final report - Microbial pathways for the reduction of mercury in saturated subsurface sediments, report, August 25, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc928394/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.