Super-Shewanella
Introducing Shewanella oneidensis strain MR-1, a versatile bacterium that scientists at Pacific Northwest National Laboratory are studying in the Microbial Cell Project as a potential biological solution to Department of Energy sites contaminated during the manufacture of nuclear weapons.
"The Microbial Cell Project has two main objectives," said Yuri Gorby, a PNNL microbiologist involved in the project. "First we would like to find out how Shewanella works so it can be used to help remediate contaminated DOE sites. We also believe that understanding Shewanella may help explain how early life on earth developed and functioned."
Like human cells, microbes need fuel to survive. Both human and bacteria cells create adenosine triphosphate (ATP), the substance that fuels all cellular activity, in a process called respiration. Similar to a human breathing in oxygen and exhaling carbon dioxide, Shewanella has the ability to "inhale" certain metals as electron acceptors and "exhale" them in an altered state. For example, it can change oxidized uranium, which is water soluble, into reduced uranium, which is insoluble.
"Shewanella can enzymatically reduce metals like uranium, technetium and chromium--the same metals that are contaminating groundwater at DOE sites," Gorby said. "Because Shewanella can transform these metals from soluble form into precipitates, we're hoping to use it in a controlled system to help prevent contaminated groundwater from spreading into the Columbia River."
But first scientists working on the project must understand the intricacies of cell respiration in Shewanella. "We want to understand how this organism controls its ability to use different electron acceptors for respiration, how it senses these electron acceptors in the environment and what cellular components are involved in the electron transfer process that is the basis of respiration," Gorby said. The approach taken by PNNL scientists and their collaborators is to study the genes of Shewanella to determine all possible processes that can occur in the cell and then to study the proteins that actually catalyze electron transport and metal reduction.
Beyond investigating Shewanella's potential for remediating subsurface sediments and groundwaters contaminated with heavy metals and radionuclides, research on this scientifically evocative organism may yield insights into early life on earth and possibly life on other planets.
Iron, the principal electron acceptor for Shewanella and other metal-reducing bacteria thriving in environments lacking oxygen, is the fourth most abundant element on earth. Before bacteria and plants produced oxygen via photosynthesis, iron was likely the most abundant electron acceptor on prehistoric earth. Hence, metal-reducing organisms, like Shewanella, were likely to have developed before other respiratory organisms.
Recent hypotheses also suggest that life on other planets, such as the iron-rich planet Mars, probably includes metal-reducing bacteria. In any case, the Microbial Cell Project is contributing to the understanding of the fundamental processes of a scientifically fascinating and potentially useful bacterium, Shewanella oneidensis strain MR-1.
The Microbial Cell Project is part of the Genomes to Life program supported by DOE's Office of Biological and Environmental Research. Other participating institutions include Michigan State University, the University of Southern California, the University of Hawaii, Baylor College of Medicine, Oak Ridge National Laboratory and the Institute for Systems Biology.