Tiny bacteria that usher dissolved zinc into a solid form may help make the removal of mining waste more efficient in groundwater and wetlands, according to a study in the 1 December issue of Science.
The bacteria can clean up contaminated water to meet drinking standards after they strip water of impurities and re-package them into zinc sulfide crystals called sphalerite.
Characterized by their ability to naturally form sulfides, these bacteria may eventually explain how many low temperature zinc ore deposits formed throughout geologic time in complex, natural systems, according to the interdisciplinary U.S. and Australian research team.
Scuba divers collected the bacteria from a flooded tunnel in Tennyson, Wisconsin. The scientists found that members of this particular family of bacteria, Desulfobacteriaceae, grow and help mineralize microscopic beads of sphalerite within a protective "biofilm" that holds together a microbial community.
These tiny spheres, known as "aggregates," cluster from the bacteria and measure up to 10 nanometers in diameter each. With a zinc concentration a million times more than the surrounding water, they can each hold up to a billion zinc sulfide particles and even contain trace amounts of selenium and arsenic.
Many sulfate-reducing bacteria thrive in environments that totally lack oxygen, but some species actually thrive at low levels of aeration, in which sulfide precipitation should offer an effective means to biologically remove elements in contaminated groundwater and wetlands, suggest Matthias Labrenz of University of Wisconsin-Madison and co-authors.
The activity of these bacteria may have played a role in prehistoric geochemical cycles when the Earth had not yet accumulated significantly great amounts of atmospheric oxygen. Presence of the bacteria may be indicated by the deposits of zinc sulfide ores corresponding to this geologic period.
The team of scientists used x-ray experiments to analyze and determine that in the bacteria of study, aggregates found within the biofilm comprise of essentially pure zinc sulfide. Geochemical modeling showed that concentrations of zinc in the groundwater solution as low as one part per million in the vicinity of the bacteria induced sphalerite precipitation at low temperatures.
Genetic tests and unique bacterial lipid signatures, which compare the structures of fatty acid molecules in the biofilm, showed that the sphalerite-forming bacteria came from the same taxonomic group. Fluorescent probing further confirmed their identification, and showed that the bacteria most responsible for zinc sulfide precipitation grew in parts of the microbe community that experienced lower oxygen levels.
Joining Matthias Labrenz in this research were Gregory K. Druschel, Tamara Thomsen-Ebert, Genjamin Gilbert, Susan A. Welch, Gelsomina De Stasio, Philip L. Bond and Jillian F. Banfield at the University of Wisconsin-Madison in Madison, WI; Kenneth M. Kemner, Barry Lai and Shelly D. Kelly at Argonne National Laboratory, Argonne, IL; and Graham A. Logan and Roger E. Summons at Australian Geological Survey Organisation, Canberra, Australia.
The research was funded by the U.S. Department of Energy Office of Basic Energy Sciences, National Science Foundation Division of Earth Sciences Programs, and the NASA Jet Propulsion Laboratory Astrobiology Institute.
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