Study of bacterial communities may provide climate-change clues
 s part of the world carbon cycle, bacterial communities
in freshwater lakes break down carbon in decaying organic matter, converting
it into carbon dioxide that is released into the atmosphere.
However, in humic lakes—darkly stained, bog-rimmed bodies of water
that contain high levels of decaying organic matter—this process
creates even higher carbon-dioxide emission levels. “There’s
a lot of concern that, as the climate changes, more carbon will be turned
into carbon dioxide in these kinds of lakes,” says Assistant Professor
Katherine
McMahon.
With $1.5 million from the National Science Foundation, McMahon is among
a multidisciplinary group of UW-Madison and University of Illinois Urbana-Champaign
researchers that is studying the composition of bacterial communities
in humic lakes and how these microorganisms respond to changes in their
environment.
This fundamental “systems” knowledge
may help researchers develop more accurate ecosystem-level models, which
enable them to predict carbon or nutrient flow through the system. It
also may give high carbon-dioxide-emitting humic lakes greater weight
in climate-change models.
Because they are giant receptacles for fallen leaves, tree limbs and
other organic debris, lakes are prime carbon-storage areas, says McMahon.
“They store a lot of carbon in their sediments,” she says.
“But we don’t understand very well how that carbon is then
mobilized to become CO2 by the bacteria.”
For years, ecosystem scientists have studied the process of ecological
succession; for example, after a fire destroys a section of forest,
the established order in which ecosystem members—grass, bushes,
softwoods, hardwoods, and so on—repopulate the area.
Recently, McMahon and her graduate students Stuart Jones, Ashley Shade
and Ryan Newton discovered that, while bacterial populations differ
in composition and function based on lake type, these microorganisms
engage in a similar succession process—but on an annual time scale.
So, barring disturbances such as increased storms or land use changes,
bacteria in a water sample taken on a given date in one year will resemble
closely those in a sample taken on the same date in the following years.
“We want to know what’s going to happen to those kinds of
patterns, or those kinds of trajectories, when the climate changes,
when land use changes, when we have increased storm events—because
the bacteria are very sensitive to the physical environment,”
says McMahon.
That sensitivity is what makes bacteria difficult to study; in fact,
they are so adaptive to their environment that culturing them for laboratory
research is of little value. “They lose whole parts of their genetic
material when you bring them into the lab,” says McMahon.
Taking weekly water samples from a northern Wisconsin humic lake, the
researchers will use new DNA-based tools to study how these quick-growing
microorganisms change in response to their environment. They also hope
to learn how bacteria speciate. In addition, they will study how bacteria
interact with algae in humic lakes and whether the organisms affect
which bacterial species are present.
At this point, says McMahon, the goal is understanding bacterial processes
and ecology. “Solutions will come once we understand the systems
behind the problems,” she says.
McMahon is leading the project in collaboration with eight other principal
investigators. At UW-Madison, her colleagues include Associate Professor
Chin
Wu; Timothy Kratz, Trout Lake research station director and a Center
for Limnology senior scientist; Center for Limnology Senior Scientist
Barbara Benson, Trout Lake Senior Scientist James Rusak, Soil Science
and Civil and Environmental Engineering Associate Professor Joel
Pedersen, and Botany Professor Linda Graham. The University of Illinois
researchers include Natural Resources and Environmental Sciences Assistant
Professor Angela Kent and Microbiology Assistant Professor Rachel Whitaker.
As part of the grant, the researchers will conduct a variety of related
educational and outreach initiatives in collaboration with UW-Madison
Center for Biology Education Outreach Program Director Robert Bohanan;
staff in the UW-Madison Center for the Integration of Research, Teaching
and Learning; and staff in the Center for Limnology. These initiatives
will target educational materials about microbial science, water resources
and aquatic environments to K-12 students and provide professional-development
resources to their teachers. Building on established initiatives, the
researchers’ educational activities also will include underrepresented
high school students who participate in the UW-Madison Pre-college Enrichment
Opportunity Program for Learning Excellence (PEOPLE) program, and undergraduate
and graduate students.
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