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Unified theory relates microbial metabolism to lab and field

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.

University of Illinois at Urbana-Champaign. November 2000.

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