Princeton University Library Catalog
- Shoenfelt, Elizabeth [Browse]
- Senior thesis
- Myneni, Satish [Browse]
- Princeton University. Department of Geosciences [Browse]
- Class year:
- 61 pages
- Restrictions note:
- Walk-in Access. This thesis can only be viewed on computer terminals at the Mudd Manuscript Library.
- Summary note:
- Thiol chemistry in natural systems is important to Hg biogeochemistry. The sorption of Hg
to high affinity thiols of cell envelopes seems to influence Hg bioavailability and its subsequent
methylation and reduction, since these transformations are primarily intracellular processes. Using
a qBBr fluorescence titration method in water, I have observed that there are 24±2, 49±12, and
240±80 μmoles thiols per gram wet weight on the surface of Bacillus subtilis, Shewanella oneidensis
MR-1, and Geobacter sulfurreducens, respectively, for the growth conditions presented. I also
determined the thiol concentrations on a dry weight basis, as dry weight does not depend on the
centrifugation speed used to harvest the cells: I observed 120±10, 300±70, and 1000±300 μmoles
thiols per gram dry weight for B. subtilis, S. oneidensis MR-1, and G. sulfurreducens, respectively.
These measurements support the hypothesis that the thiol density distribution directs Hg-(thiol)3
complexation on cell surfaces. It seems that clusters of three thiols in close proximity leads the
nonmethylators B. subtilis and S. oneidensis MR-1 to form inert Hg-(thiol)3 complexes at low Hg
concentrations (Mishra et al., unpublished), since they have lower average cell densities than G.
sulfurreducens. The Hg-methylating G. sulfurreducens theoretically does not have such clusters
on its cell envelope, as it does not form tridentate complexes (Mishra et al., unpublished) despite
a relatively high average surface thiol density. To further our understanding of the thiol chemistry
of Hg-contaminated natural systems, there is evidence that total thiols on the surface of S. oneidensis
MR-1 will increase when the cells are grown in the presence of 250nM and 750nM Hg
in Luria-Bertani (LB) broth. I have also observed that the fraction of Hg-thiol complexes that are
Hg-(thiol)3 is largest when S. oneidensis MR-1 is grown with 100nM Hg2+ in LB broth (inoculated
to about 5×106 cells/L) compared to the same concentration of cells grown with 0 nM, 50 nM,
250 nM, 500 nM, and 750nM Hg2+ in LB broth, which preliminarily suggests that at least some of
the additional thiols are in clusters that are conducive to insoluble Hg-(thiol)3 complex formation.