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Unified theory relates
microbial metabolism to lab and field
Kloeppel, Physical Sciences Editor
(217) 244-1073; firstname.lastname@example.org
CHAMPAIGN, Ill. -- The ability
to describe the rates at which microbial populations metabolize in the
natural environment has been limited by the lack of a general theory
of microbial kinetics. Now, researchers at the University of Illinois
have found an approach that holds significant promise for extending
the results of laboratory experiments to better understand microbial
metabolism in nature.
"The growth of microbial
populations can have profound affects on the chemistry of groundwater,
from acid-mine drainage in the West to arsenic poisoning in wells in
Bangladesh," said Craig Bethke, a UI professor of geology. "The bulk
of the world's microbial biomass operates by eating rocks -- taking
inorganic chemicals and using them to produce energy. By constructing
quantitative models of that reaction process, we might find more effective
solutions and control measures."
While various kinetic-rate
laws currently exist, their empirical nature means they must be selected
to match a given set of experimental results.
"There is no guarantee that
a rate law chosen to describe behavior observed in a laboratory culture
will apply in a given geochemical environment," Bethke said. "Also,
the available rate laws do not account for the amount of energy that
might be derived by a given metabolism, further limiting their usefulness
in modeling natural environments."
Graduate student Qusheng
Jin and Bethke have devised a new description of microbial kinetics
based upon the internal mechanisms of microbial respiration in terms
of chemiosmotic theory.
"In our approach, a cell's
metabolism is represented by a multi-step, enzymatically catalyzed reaction
that is directly coupled to energy production by the development of
a proton-motive force and the consequent synthesis of adenosine triphosphate
from adenosine diphosphate," Bethke said. "We derive a rate law that
accounts for the reaction's thermodynamics and the energy required to
produce ATP, as well as the abundance of microbes and the concentrations
of substrate species and reaction products in solution."
The overall respiration reaction
can be simplified into three steps: an electron-donor oxidation step,
a rate-determining step and an electron-acceptor reduction step. The
reactions between electron donors and acceptors are mediated by central
metabolic pathways and electron transfer.
Because the researchers based
their equation for electron transport on first principles, it provides
a fundamental description of microbial metabolism and can be applied
over a broad range of parameters. Under specific conditions, the generalized
solution simplifies to the rate laws now in common use, Bethke said.
"Our unified theory predicts
the results of experiments conducted under a variety of conditions,
and offers a simple explanation for threshold substrate concentrations
-- a phenomenon that, in many cases, can be shown to result directly
from kinetic and thermodynamic principles."
The researchers presented
their unified theory of microbial kinetics at the annual meeting of
the Geological Society of America, held Nov. 9-18 in Reno, Nev.