Competitive advantage of oxygen-tolerant bioanodes of in bioelectrochemical systems.

Oxidative stress greatly limits current harvesting from anode biofilms in bioelectrochemical systems yet insufficient knowledge of the antioxidant responses of electricigens prevents optimization. Using PCA as a model electricigen, we demonstrated enhanced oxygen tolerance and reduced electron losses as the biofilms grew in height on the anode. To investigate the molecular basis of biofilm tolerance, we developed a genetic screening and isolated 11 oxygen-tolerant (oxt) strains from a library of transposon-insertion mutants.

Genetic analysis of electroactive biofilms.

Geobacter biofilms synthesize an electroactive exopolysaccharide matrix with conductive pili and c-cytochromes that spatially organizes cells optimally for growth and electron transport to iron oxide substrates, soluble metal contaminants, and current-harvesting electrodes. Despite its relevance to bioremediation and bioenergy applications, little is known about the developmental stages leading to the formation of mature (>20 μm thick) electroactive biofilms. Thus, we developed a transposon mutagenesis method and a high-throughput screening assay and identified mutants of Geobacter sulfurreducens PCA interrupted in the initial stages of surface colonization (attachment and monolayer formation) and the vertical growth and maturation of multilayered biofilms.

Genetic Identification of a PilT Motor in Reveals a Role for Pilus Retraction in Extracellular Electron Transfer.

The metal-reducing bacterium requires the expression of conductive pili to reduce iron oxides and to wire electroactive biofilms, but the role of pilus retraction in these functions has remained elusive. Here we show that of the four PilT proteins encoded in the genome of , PilT3 powered pilus retraction in planktonic cells of a PilT-deficient strain of and restored the dense mutant biofilms to wild-type levels. Furthermore, PilT3 and PilT4 rescued the twitching motility defect of the PilT-deficient mutant.

Enhanced uranium immobilization and reduction by Geobacter sulfurreducens biofilms.

Biofilms formed by dissimilatory metal reducers are of interest to develop permeable biobarriers for the immobilization of soluble contaminants such as uranium. Here we show that biofilms of the model uranium-reducing bacterium Geobacter sulfurreducens immobilized substantially more U(VI) than planktonic cells and did so for longer periods of time, reductively precipitating it to a mononuclear U(IV) phase involving carbon ligands. The biofilms also tolerated high and otherwise toxic concentrations (up to 5 mM) of uranium, consistent with a respiratory strategy that also protected the cells from uranium toxicity.

Fermentation of glycerol into ethanol in a microbial electrolysis cell driven by a customized consortium.

The in situ generation of ethanol from glycerol-containing wastewater shows promise to improve the economics of the biodiesel industry. Consequently, we developed a microbial electrolysis cell (MEC) driven by the synergistic metabolisms of the exoelectrogen Geobacter sulfurreducens and the bacterium Clostridium cellobioparum, which fermented glycerol into ethanol in high yields (90%) and produced fermentative byproducts that served as electron donors for G. sulfurreducens.

Consolidated bioprocessing of AFEX-pretreated corn stover to ethanol and hydrogen in a microbial electrolysis cell.

The consolidated bioprocessing (CBP) of corn stover pretreated via ammonia fiber expansion (AFEX-CS) into ethanol was investigated in a microbial electrolysis cell (MEC) driven by the exoelectrogen Geobacter sulfurreducens and the CBP bacterium Cellulomonas uda. C. uda was identified in a screening for its ethanologenic potential from AFEX-CS and for producing electron donors for G.

Electron donors supporting growth and electroactivity of Geobacter sulfurreducens anode biofilms.

Geobacter bacteria efficiently oxidize acetate into electricity in bioelectrochemical systems, yet the range of fermentation products that support the growth of anode biofilms and electricity production has not been thoroughly investigated. Here, we show that Geobacter sulfurreducens oxidized formate and lactate with electrodes and Fe(III) as terminal electron acceptors, though with reduced efficiency compared to acetate. The structure of the formate and lactate biofilms increased in roughness, and the substratum coverage decreased, to alleviate the metabolic constraints derived from the assimilation of carbon from the substrates.

Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism.

The in situ stimulation of Fe(III) oxide reduction by Geobacter bacteria leads to the concomitant precipitation of hexavalent uranium [U(VI)] from groundwater. Despite its promise for the bioremediation of uranium contaminants, the biological mechanism behind this reaction remains elusive. Because Fe(III) oxide reduction requires the expression of Geobacter's conductive pili, we evaluated their contribution to uranium reduction in Geobacter sulfurreducens grown under pili-inducing or noninducing conditions.

Anaerobic cell culture.

Although the ability of some microorganisms to grow without O(2) has long been recognized, the application of new methodologies has greatly expanded the known diversity and potential of anaerobic microorganisms and processes. In particular, anaerobic techniques that permit the successful cultivation of microorganisms on solid media have opened new avenues for the study of the physiology and metabolic potential of many new microorganisms using molecular, genomic, and proteomic tools. One technique above all has proven instrumental for anaerobic studies over the years: the use of the anaerobic chamber.