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Bacteria are found everywhere that researchers have been clever enough to sample. They are found in the deepest ocean sediments, the highest atmospheric altitudes, at temperatures that cook most other organisms, imbedded and active in Antarctic ice, associated with the most heavily polluted sites and the most pristine--in short everywhere. Their biological diversity probably exceeds that of all other organisms. For example, one gram (approximately 1 teaspoon full) of forest soil has been estimated to contain over 4,000 species of bacteria, most of which are unknown to science. Bacteria are particularly adept at breaking down complex organic compounds, both biological and man made. On the Savannah River Site (SRS), as at many other industrial locations, this ability to break down complex molecules or otherwise remediate contamination has been the focus of research for possible application to solving environmental problems.

bacteria.jpg (13211 bytes)While bacteria have been effectively stimulated under field conditions to remediate certain sites, the technology often is not transferable to other sites. Because little is known about the basic ecology of bacteria in nature, it is not surprising that practices that work at one location don’t always work at other locations or at the same location at different times. At the Savannah River Ecology Laboratory (SREL) scientists are seeking to understand the basic ecology of bacteria under natural and stressed (contaminated) conditions.

SREL studies can be placed under three general headings: ecology of trichloroethylene (TCE)-degrading bacteria, ecology of bacteria associated with coal pile run-off, and use of bacteria as indicators of industrial pollution and cleanup.

Ecology of TCE-degrading bacteria

The solvent TCE is one of the most frequently encountered organic pollutants and is a carcinogen. Groundwaters on the SRS have become contaminated with this material. Groundwater is the primary source of water found in streams and rivers. The contamination plumes found in SRS groundwater will eventually come to the surface along streams and thus contaminate water that flows into the Savannah River. The goal of this research program is to explore the community dynamics of trichloroethylene-degrading microbial populations. Interestingly, bacteria that "eat" methane (methanotrophs) can also break down TCE, even though they get no energy or carbon from the process. Previous work on the SRS suggested that injecting methane and other gaseous nutrients via horizontal wells stimulated the native bacterial community and greatly increased the TCE degradation rate. To further investigate the feasibility of using microbes to degrade TCE, SREL research is focusing on gaining an understanding of the basic ecology of methane-degrading bacteria. These studies include determining whether methane/TCE-degrading bacteria are a natural component of groundwater and surface water ecosystems. Other studies are investigating which gaseous nutrients and which concentrations cause the greatest sustained stimulation of TCE-degrading bacteria. In addition, a protocol has been developed to assess the abundance of genes used in the degradation of TCE. Such an assessment will allow predictions on the effectiveness of gaseous injection and may help explain why this technology is not easily trans-ported to other sites, even within the SRS.

Use of bacteria as indicators of industrial pollution and cleanup

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Areas of TCE contamination on the Savannah River Site.

Because of their ability to break down complex molecules under stressful conditions, bacteria are the primary focus of many bioremediation studies. While some bacteria respond to perturbations with increased growth rates, it is not clear what effect various forms of pollution have on bacterial species or genetic diversity in the native communities. For example, one result of metal pollution is an increase in the numbers and kinds of metal-resistant bacteria. The genes that code for metal resistance are often carried on plasmids, or small mobile pieces of DNA. Coincidentally, these same plasmids often carry genes that confer antibiotic resistance. In a survey of bacterial assemblages collected from Four Mile Creek on the SRS, we found that:

  • highest levels of antibiotic resistance were found in bacteria in a tributary stream that drains Central Shops and C-Reactor,
  • there may be significant industrial contamination in this tributary but not in the main stream channel, and
  • it appears that antibiotic resistance may be a good indicator of level of contamination and thus need for cleanup.
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Sampling sites on tributaries of Four Mile Creek that drain C-Area and Central Shops (N-Area) on the SRS.  At each location, levels of bacterial resistance to two antibiotics are indicated by the colored proportion of the circle; at site B1 all bacteria sampled were resistant to streptomycin.

Given the large areas of the earth that have industrial pollution, these results may be even more significant. Because antibiotic resistance is carried on mobile DNA elements, it can be distributed easily to unrelated bacteria, including pathogens. Our current research is investigating whether antibiotic resistant bacteria can escape from contaminated streams into the atmosphere. If so, we are trying to determine how far they are transported into the atmosphere (i.e., meters, kilometers, worldwide). This is the first study that is attempting to link the indirect effect of industrial pollution to the resurgence of antibiotic resistance in human pathogens.

Ecology of bacteria associated with coal pile run-off

The storage of sulfur-rich coal and the combustion products of coal represents a pollution source that has severely impacted numerous ecosystems. Specifically, the exposure of coal deposits to oxygen and water results in the conversion of pyrite (FeS2) to sulfuric acid. The resulting acidic leachate is enriched with salts and heavy metals, forming a type of pollution referred to as acid mine drainage (AMD). Despite the fundamental role of bacteria as both causative agents and as potential bioremediators of AMD, their ecology in these systems has not been studied in a comprehensive manner, due primarily to an inability to culture the majority of environmental bacteria.

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Run-off associated with coal piles in D-Area on the SRS.

D-Area on the SRS has a 20-year-old exposed reject coal pile. Acidic, metal-rich leachate from this pile has contaminated the sediments of an adjacent forested wetland, causing vegetation die-off. Historically, because of the need to culture or grow bacteria, it has been extremely difficult to monitor changes in the total bacterial community since less than 1% of the bacteria in a sample can be cultured. However, recent advances in molecular biology allow scientists to track, under field conditions, various bacterial genes or gene sequences without the requirement of culturing the bacteria. Our research project is using some of these molecular biological tools to determine the bacterial community composition within a contamination gradient. This information will be used to determine if and how bacterial community composition and diversity has changed in response to the acidic, metal-rich coal leachate. Additionally, this experiment could provide data to test the hypothesis that as yet uncultured bacteria, in addition to the well-studied bacteria Thiobacillus ferrooxidans and Leptospirillum ferrooxidans, could be causative agents of AMD.

Cleanup of contaminated sites within the DOE complex is a major budgetary concern. Bacteria have been shown to be effective indicators and processors of contamination. SREL studies will provide information on the basic ecology of bacteria that will allow managers to make informed decisions on the feasibility of technology transfer to other sites. In addition, these studies may demonstrate a potential new dimension to risk assessment, i.e., the risk of the dissemination of antibiotic resistance and its subsequent effects on human health.

 

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