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
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 dont 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
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.
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
|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
- it appears that antibiotic
resistance may be a good indicator of level of contamination and
thus need for cleanup.
|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
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
|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.
to Research Snapshots)