Q-295. Geobacter Gets a Sweet Tooth: Awakening Latent Metabolic Pathways Via Adaptive Evolution

Z. M. Summers, S. A. Haveman, M. Izallalen, D. R. Lovley;
Univ. of Massachusetts, Amherst, MA.

The limited range of electron donors that Geobacter species metabolize restricts the range of substrates that can be used to promote in situ uranium bioremediation and requires that mixed communities be employed for producing current from complex organic wastes. For example, the culture-collection strain of G. sulfurreducens is unable to utilize energy/electron-dense compounds, such as sugars and glycerol, and grows poorly on lactate. However, the genome-based in silico metabolic model for Geobacter sulfurreducens predicts that it should grow well on all of these substrates. Therefore, it was hypothesized that by exerting the appropriate selective pressure on G. sulfurreducens it might possible to increase the range of substrates utilized. Repeated transfer of five parallel cultures of G. sulfurreducens in medium with lactate as the electron donor and fumarate as the electron acceptor for over 500 generations reduced the doubling time from 24 h to 5 h, much closer to the 2.6 h that the model predicted. The doubling time on acetate increased twofold, suggesting decreased fitness for acetate metabolism. A lactate-adapted strain was recovered that could also utilize pyruvate, as well as a number of precursors to pyruvate, such as a variety of sugars and glycerol. Preliminary resequencing of the genome of these strains uncovered three mutations in the succinyl-CoA synthetase operon and it’s regulator. This enzyme, which is necessary for growth on substrates other than acetate, is active only in the adapted strains. The evolution of a strain of G. sulfurreducens that can utilize electron-dense fuels, such as sugars and glycerol, that are important components of biomass or wastes (glycerol is a major waste product of biodiesel production) has important implications for current production and bioremediation. Furthermore, it raises an important question of why Geobacter species do not utilize these compounds in soils and sediments, but rather rely on fermentative microorganisms to convert these substrates to acetate and other fermentation products that Geobacter species then utilize.