Anode Biofilm Transcriptomics Reveals Outer Surface Components Essential for High Density Current Production in Geobacter sulfurreducens Fuel Cells

Abstract

The mechanisms by which Geobacter sulfurreducens transfers electrons through relatively thick (>50 µm) biofilms to electrodes acting as a sole electron acceptor were investigated. Biofilms of Geobacter sulfurreducens were grown either in flow-through systems with graphite anodes as the electron acceptor or on the same graphite surface, but with fumarate as the sole electron acceptor. Fumarate-grown biofilms were not immediately capable of significant current production, suggesting substantial physiological differences from current-producing biofilms. Microarray analysis revealed 13 genes in current-harvesting biofilms that had significantly higher transcript levels. The greatest increases were for pilA, the gene immediately downstream of pilA, and the genes for two outer c-type membrane cytochromes, OmcB and OmcZ. Down-regulated genes included the genes for the outer-membrane c-type cytochromes, OmcS and OmcT. Results of quantitative RT-PCR of gene transcript levels during biofilm growth were consistent with microarray results. OmcZ and the outer-surface c-type cytochrome, OmcE, were more abundant and OmcS was less abundant in current-harvesting cells. Strains in which pilA, the gene immediately downstream from pilA, omcB, omcS, omcE, or omcZ was deleted demonstrated that only deletion of pilA or omcZ severely inhibited current production and biofilm formation in current-harvesting mode. In contrast, these gene deletions had no impact on biofilm formation on graphite surfaces when fumarate served as the electron acceptor. These results suggest that biofilms grown harvesting current are specifically poised for electron transfer to electrodes and that, in addition to pili, OmcZ is a key component in electron transfer through differentiated G. sulfurreducens biofilms to electrodes.

Extracellular Quinones Affecting Methane Production and Methanogenic Community in Paddy Soil

“…This was similar to the previous studies that have reported the enrichment of Geobacter species in aquifer sediments by the addition of humics and other extracellular quinones [43]. The genus Geobacter has been surprisingly discovered to be capable of forming highly conductive network of pilis that facilitate long-range electron transfer [34,38,39], which is becoming an important feature of Geobacter species in anaerobic environments [17,26,27]. There is increasing evidence that Geobacter species are important syntrophic microorganisms capable of establishing syntrophic association with methanogens [19,29,45].…”

Microtoming coupled to microarray analysis to evaluate the spatial metabolic status of Geobacter sulfurreducens biofilms

“…As previously reported (Franks et al, 2009), the biofilm covered the entire available graphite anode surface and formed pillar-structured biofilms greater than 50 mm thick (Figures 2a and 3a). Although a low shear and turbulence forces due to a low flow rate may have favored a more complex biofilm structure, this biofilm structure was similar to those observed in a diversity of other microbial fuel cell designs (Reguera et al, 2006;Nevin et al, 2009), suggesting that the flow of medium through the fuel cell did not alter typical biofilm morphology.…”

“…For example, a previous comparison between gene expression in G. sulfurreducens biofilms producing current versus biofilms growing on the same surface material, but using fumarate as the electron acceptor , revealed increased expression of the gene for PilA, the structural protein for the pili that seem to be electrically conductive pili (Reguera et al, 2005) and may be involved in electron transfer through anode biofilms (Reguera et al, 2006;Nevin et al, 2009). Cells in the inner and outer portions of the biofilm did not have a significant difference in pilA expression, but the adjacent gene, GSU1497, was less expressed (2.6-fold) in the outer biofilm .…”

Proteins involved in electron transfer to Fe(III) and Mn(IV) oxides by Geobacter sulfurreducens and Geobacter uraniireducens

“…Gene deletion studies have suggested that the outer membrane surface c-type cytochrome OmcZ (GSU2076) is important for growth on current-harvesting anodes (Inoue et al, 2010Nevin et al, 2009;Richter et al, 2009). omcZ was upregulated during growth on Mn(IV) oxide but not Fe(III) oxide (Table 4).…”