Superfund sites are in famous for their hazardous,
stubborn chemical wastes, but one cleanup solutions
may be to put the right mix of plants and microbes
together in the soil, according a new study ventured
by the University of Florida . The study examined
the interaction between two of the likeliest candidates
for cleanup duty the loblolly pine and soil dwelling,
methane-eating bacteria.
The practice of planting chemically resilient trees
and plants in contaminated sites to absorb harmful
chemicals from the soil, known as phytoremediation,
is highly appealing both to environmental cleanup
agencies and to the communities near the hazardous
sites. Phytoremediation is still in its infancy but
has the potential to be relatively safe, sustainable
and efficient and the trees are aesthetically pleasing
as well.
Microbes in the soil also play a key role. They help
plants not only to absorb nutrients through their
roots but also to soak up contaminants. However, the
interaction between plants microbes is not necessarily
symbiotic, according to the study, which appeared
in the January issue of the Bulletin of Environmental
Contamination and Toxicology. "we are trying to see
what the role of the microbes is in the rhizosphere,
the soil region around the plant roots," said Adriana
Pacheco, a graduate student in UF's environmental
engineering department and the lead author of the
paper. "It seems to be one of the most important processes
occurring."
Pacheco's research focused on methanotrophs, bacteria
that consume methane in soils and in the process can
also consume and break down a range of harmful organic
compounds that may be present, such as the carcinogenic
polychlorinated biphenyls, commonly known as PCB's
and trichloroethylenes, or TCE's. However, not all
trees work equally well with all kinds of microbes,
and knowing how the different plant species affect
the bacteria may be the key to effective and efficient
cleanup, Pacheco said. In her study, she focused on
one tree species, the loblolly pine, a prime candidate
for phytoremediation at a number of Superfund sites,
particularly in the southeastern United States .
Loblolly pines are well known for supporting thriving
populations of bacteria near their roots, possibly
because of the piles of needles littering the soil
and releasing a pungent group of chemicals called
monoterpenes. "These terpenes have been shown to inhibit
bacteria," Pacheco said. "But metanotrophs can also
be in really high concentrations in the rhizosphere
of the pines, and they are degrading TCE's. So one
of the questions is, are monoterpenes helping them
in some way?"
To answer this question, Pacheco isolated several
different species of methanotrophic bacteria, fed
them methane and added monoterpenes as well as TCE's
and then measured the bacteria's response to the monoterpenes
by observing how oxygen levels in the samples changed
over time. She found that while some species of the
bacteria thrived when the pine chemical was added,
the chemical appeared to be toxic to others. That,
she said, suggests environmental engineers will need
to choose both plants and microbes carefully when
planning phytoremediation based cleanup.
Scientists also want to address what happens to the
contaminants after they've passed through the microbe
plant system. "It is a very aesthetic treatment,"
said Angela Lindner, a UF professor of environmental
engineering and co-author of the paper. "Trees and
plants are very resilient. They can accumulate the
chemicals, and many times they will also transform
the chemicals within the plant, and then the products
as well as the chemical can volatilize through the
leaves. But we need to know where the stuff is going."
For methanotrophic bacteria, at lease, the fate of
the chemicals is known. The bacteria produce an enzyme
that breaks down harmful chlorinated compounds into
harmless carbon dioxide, oxygen and water. The study
provides environmental engineers with more of the
information they'll need to design effectie cleanup
programs, said Robin Brigmon, principal engineer in
the Environmental Biotechnology section of the Savannah
River National Laboratory. "Information like this
on the terpene activity will really help perhaps give
us another monitoring device to look at environmental
restoration," he said.
Furthermore, understanding the interactions between
methanotrophic bacteria, the root systems of plants
and chemicals in the soil has larger implications,
Brigmon added.
" These kinds of practices are extremely important
and significant from a global perspective," he said.
"Different methanotrophs each have their own ecological
niche, and this will work will help to further characterize
the organisms responsible for the activity. That can
have implications for (the fate of) other chemical,
scuh as greenhouse gases, that percolate through the
rhizosphere."