Electronic dialogue between proteins is expected to be a key component of charge transport at the microbe-mineral interface (MMI) and requires complex structures. Microbial nanofilaments are one such structure produced in energetically engineered environments. These nanostructures consist of natural protein electronic conduits which can target the microbe-mineral interface and facilitate charge transport over a distance. Nanofilaments are phylogenetically diverse inducible extracellular appendages, and have the potential to serve as organic electronic conductors. However, recent investigations on such microbial nanofilaments have been confined to a few bacterial genera such as Geobacter, Shewanella and Synechocystis. Here, we report the evidence for longitudinal electron transport through inducible nanofilaments produced by another genus, the metabolically versatile photosynthetic, iron(III) respiring bacterium Rhodopseudomonas palustris strain RP2, in photic, iron(III) oxide-rich environments.In contrast, chemosynthetic dark-grown anoxic cells are weak in their ability to reduce ferric-oxide and no longer produce extracellular structures. Independent evaluation techniques illustrate the induction of extracellular filaments and their electrical properties. Scanning probe and nanofabricated electrode measurements provide conclusive evidence for the occurrence of direct charge transfer along the length and radius of nanofilaments from strain RP2. These findings not only expand our knowledge of the range of bacteria known to produce nanofilaments but also provide further research opportunities in the field of bionanotechnology, sustainable remediation (bioelectrochemical remediation systems) in contaminated sites (petroleum hydrocarbons) and MMI process at photic environments.

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An electrophototrophic, hydrocarbonoclastic bacterium Rhodopseudomonas palustris stain RP2 was isolated from the anodic biofilms of hydrocarbon fed microbial electrochemical remediation systems (MERS). Salient properties of the strain RP2 were direct electrode respiration, dissimilatory metal oxide reduction, spore formation, anaerobic nitrate reduction, free living diazotrophy and the ability to degrade n-alkane components of petroleum hydrocarbons (PH) in anoxic, photic environments. In acetate fed microbial electrochemical cells, a maximum current density of 305 ± 10 mA/m2 (1000Ω) was generated (power density 131.65 ± 10 mW/m2) by strain RP2 with a coulombic efficiency of 46.7 ± 1.3%. Cyclic voltammetry studies showed that anaerobically grown cells of strain RP2 is electrochemically active and likely to transfer electrons extracellularly to solid electron acceptors through membrane bound compounds, however, aerobically grown cells lacked the electrochemical activity. The ability of strain RP2 to produce current (maximum current density 21 ± 3 mA/m2; power density 720 ± 7 μW/m2, 1000 Ω) using PH as a sole energy source was also examined using an initial concentration of 800 mg l-1 of diesel range hydrocarbons (C9-C36) with a concomitant removal of 47.4 ± 2.7% hydrocarbons in MERS. Here, we also report the first study that shows an initial evidence for the existence of a hydrocarbonoclastic behavior in the strain RP2 when grown in different electron accepting and illuminated conditions (anaerobic and MERS degradation). Such observations reveal the importance of photoorganotrophic growth in the utilization of hydrocarbons from contaminated environments. Identification of such novel petrochemical hydrocarbon degrading electricigens, not only expands the knowledge on the range of bacteria known for the hydrocarbon bioremediation but also shows a biotechnological potential that goes well beyond its applications to MERS.

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