Centimeter-long electron transport in marine sediments via conductive minerals

© 2015 International Society for Microbial Ecology All rights reserved. Centimeter-long electron conduction through marine sediments, in which electrons derived from sulfide in anoxic sediments are transported to oxygen in surficial sediments, may have an important influence on sediment geochemistry. Filamentous bacteria have been proposed to mediate the electron transport, but the filament conductivity could not be verified and other mechanisms are possible. Surprisingly, previous investigations have never actually measured the sediment conductivity or its basic physical properties. Here we report direct measurements that demonstrate centimeter-long electron flow through marine sediments, with conductivities sufficient to account for previously estimated electron fluxes. Conductivity was lost for oxidized sediments, which contrasts with the previously described increase in the conductivity of microbial biofilms upon oxidation. Adding pyrite to the sediments significantly enhanced the conductivity. These results suggest that the role of conductive minerals, which are more commonly found in sediments than centimeter-long microbial filaments, need to be considered when modeling marine sediment biogeochemistry

Expressing the Geobacter metallireducens PilA in Geobacter sulfurreducens Yields Pili with Exceptional Conductivity

ABSTRACT The electrically conductive pili (e-pili) of Geobacter sulfurreducens serve as a model for a novel strategy for long-range extracellular electron transfer. e-pili are also a new class of bioelectronic materials. However, the only other Geobacter pili previously studied, which were from G. uraniireducens , were poorly conductive. In order to obtain more information on the range of pili conductivities in Geobacter species, the pili of G. metallireducens were investigated. Heterologously expressing the PilA gene of G. metallireducens in G. sulfurreducens yielded a G. sulfurreducens strain, designated strain MP, that produced abundant pili. Strain MP exhibited phenotypes consistent with the presence of e-pili, such as high rates of Fe(III) oxide reduction and high current densities on graphite anodes. Individual pili prepared at physiologically relevant pH 7 had conductivities of 277 ± 18.9 S/cm (mean ± standard deviation), which is 5,000-fold higher than the conductivity of G. sulfurreducens pili at pH 7 and nearly 1 million-fold higher than the conductivity of G. uraniireducens pili at the same pH. A potential explanation for the higher conductivity of the G. metallireducens pili is their greater density of aromatic amino acids, which are known to be important components in electron transport along the length of the pilus. The G. metallireducens pili represent the most highly conductive pili found to date and suggest strategies for designing synthetic pili with even higher conductivities. IMPORTANCE e-pili are a remarkable electrically conductive material that can be sustainably produced without harsh chemical processes from renewable feedstocks and that contain no toxic components in the final product. Thus, e-pili offer an unprecedented potential for developing novel materials, electronic devices, and sensors for diverse applications with a new “green” technology. Increasing e-pili conductivity will even further expand their potential applications. A proven strategy is to design synthetic e-pili that contain tryptophan, an aromatic amino acid not found in previously studied e-pili. The studies reported here demonstrate that a productive alternative approach is to search more broadly in the microbial world. Surprisingly, even though G. metallireducens and G. sulfurreducens are closely related, the conductivities of their e-pili differ by more than 3 orders of magnitude. The ability to produce e-pili with high conductivity without generating a genetically modified product enhances the attractiveness of this novel electronic material

Roadmap on emerging concepts in the physical biology of bacterial biofilms: from surface sensing to community formation

Bacterial biofilms are communities of bacteria that exist as aggregates that can adhere to surfaces or be free-standing. This complex, social mode of cellular organization is fundamental to the physiology of microbes and often exhibits surprising behavior. Bacterial biofilms are more than the sum of their parts: single-cell behavior has a complex relation to collective community behavior, in a manner perhaps cognate to the complex relation between atomic physics and condensed matter physics. Biofilm microbiology is a relatively young field by biology standards, but it has already attracted intense attention from physicists. Sometimes, this attention takes the form of seeing biofilms as inspiration for new physics. In this roadmap, we highlight the work of those who have taken the opposite strategy: we highlight the work of physicists and physical scientists who use physics to engage fundamental concepts in bacterial biofilm microbiology, including adhesion, sensing, motility, signaling, memory, energy flow, community formation and cooperativity. These contributions are juxtaposed with microbiologists who have made recent important discoveries on bacterial biofilms using state-of-the-art physical methods. The contributions to this roadmap exemplify how well physics and biology can be combined to achieve a new synthesis, rather than just a division of labor

Investigations of electron transport and storage mechanisms in microbial biofilms

Electron transport is a fundamental mechanism in a variety of biological systems such as photosynthesis and aerobic respiration. However, the transport has long been considered to occur only over short distances (\u3c 1 μm), primary by metalloproteins. Recently, the conduction of electrons over large distances (\u3e 10 μm) along networks of microbial pilin filaments known as microbial nanowires has been invoked to explain a wide range of important redox phenomena that influence carbon and mineral cycling in soils and sediments, bioremediation, corrosion, interspecies electron transfer and anaerobic conversion of organic wastes to methane or electricity. However, there has never been any direct experimental demonstration of this long-distance electron transport. In fact, previous measurements of microbial biofilms have noted just the opposite: that biofilms act as insulators, not conductors. In this thesis, we reconcile these confounding observations with the demonstration that biofilms of several species of commonly studied microorganisms do function as insulators, whereas biofilms of Geobacter sulfurreducens , common in soils and sediments can form a conductive matrix, with a conductivity comparable to synthetic conductive polymers. We show that biofilms are capable of conducting electrons over 1.25 cm, many thousands of times the size of a cell. Biofilm conductivity was found to be proportional to the abundance of pilin filaments and the conductivity of sheared pilins was comparable to biofilms. We also found that biofilm conductivity regulates fuel cell current density. We demonstrate that electron transport in the biofilms does not occur via localized charge carriers known as cytochromes, as almost universally predicted, but rather through delocalized electronic states. Moreover, we report a quantum mechanical interference phenomenon of weak localization in pilin nanowires. Additionally, we demonstrate that cytochromes can be used to store electrons with capacitance comparable to commercial supercapacitors. Furthermore, the degree of conductivity and capacitance within the films can be tuned via changes in gene expression or gate bias. This study demonstrates that pilin-associated long-distance electron transport through a microbial matrix is feasible, establishes approaches that could be used for evaluating the possibility of electron flow through natural microbial communities, and demonstrates the potential for developing novel bioelectronic materials

Comment on “On electrical conductivity of microbial nanowires and biofilms” by S. M. Strycharz-Glaven, R. M. Snider, A. Guiseppi-Elie and L. M. Tender, Energy Environ. Sci., 2011, 4, 4366

A paper published in Energy and Environmental Science by Strycharz-Glaven et al. suggests that electron transport along the pili and through the conductive biofilms of Geobacter sulfurreducens proceeds via electron superexchange rather than metallic-like conductivity. Multiple lines of evidence disprove the superexchange hypothesis, but are consistent with metallic-like conductivity

Biofilm conductivity is a decisive variable for high-current-density Geobacter sulfurreducens microbial fuel cells

Current outputs of microbial fuel cells (MFCs) are too low for most perceived practical applications. Most efforts for further optimization have focused on modifications of fuel cell architecture or electrode materials, with little investigation into the properties of microorganisms that are most essential for maximal current production. Geobacter sulfurreducens produces the highest current densities of any known pure culture; is closely related to the Geobacter species that often predominate in anode biofilms harvesting electricity from organic wastes; and produces highly conductive anode biofilms. Comparison of biofilm conductivities and current production in different strains of G. sulfurreducens revealed a direct correlation between biofilm conductivity and current density. Electrochemical impedance spectroscopy measurements demonstrated that higher biofilm conductivity not only reduced resistance to electron flow through the biofilm, but also lowered the activation energy barrier for electron transfer between the biofilm and the anode. These results demonstrate the crucial role of biofilm conductivity in achieving high current density in MFCs and suggest that increasing biofilm conductivity can boost MFC performance

Lack of cytochrome involvement in long-range electron transport through conductive biofilms and nanowires of Geobacter sulfurreducens

Two competing models for long-range electron transport through the conductive biofilms and nanowires of Geobacter sulfurreducens exist. In one model electrons are transported via pili that possess delocalized electronic states to function as protein wires with metallic-like conductivity. In the other model electrons are transported by more traditional electron transfer via electron hopping/tunneling between the c-type cytochromes in G. sulfurreducens biofilms and pili. The cytochrome hypothesis was further examined. Quantifying c-type cytochromes in G. sulfurreducens biofilms and pili indicated that there are insufficient cytochromes to account for electron transport through the bulk of the biofilm or pili and demonstrated that there is a negative correlation between cytochrome abundance and biofilm conductivity. Direct imaging using atomic force microscopy revealed that cytochromes were not packed close enough on pili to permit electron hopping/tunneling along the pili. Inactivating cytochromes had no impact on biofilm conductivity. The results of electrochemical gating studies were inconsistent with electron transport via cytochromes. Theoretical considerations suggest that a cytochrome model cannot explain the previously reported response of biofilm conductivity to temperature changes. These multiple lines of evidence, which rely on approaches with different sets of assumptions, demonstrate that the hypothesis that long-range electron transport through G. sulfurreducens biofilms and nanowires can be attributed to electron hopping/tunneling between c-type cytochromes is incorrect. In contrast, these multiple lines of evidence are consistent with long-range electron transport through the biofilms via networks of pili that possess metallic-like conductivity