Biological Iron‐Monosulfide Production for Efficient Electricity Harvesting from a Deep‐Sea Metal‐Reducing Bacterium

Abstract

Micro-organisms inhabit a broad range of natural environments through the exploitation of multiple metabolic strategies and are able to perform most thermodynamically possible redox reactions to obtain energy for their growth. Present concerns about the depletion of natural resources has increased the need to understand inherent microbial ingenuities, and search for methodologies for utilizing (or replicating) their metabolism to harness renewable sources of energy. Herein, we report the biological production of single-phase iron monosulfide (mackinawite) for efficient harvesting of bacterial metabolized electrons in electrochemical cells. Iron sulfides (FeS), such as mackinawite, greigite, marcasite, and pyrite, are ubiquitous and abundant minerals in anoxic marine environments formed as by-products of microbial metabolism or a consequence of geothermal activity. Several biogeochemical processes associated with microbial interaction with FeSs have been reported, with the discovery of highly active biospheres around hydrothermal vents illustrating the significance of FeSs as an energy source for microbial activities in environments isolated from solar irradiation. Moreover, a number of biological functions of FeSs, including structural support, antimicrobial agents, and magnetic-, optical-, and gravity-sensing devices, have been identified; this highlights the diverse microbial mechanisms that exist for harnessing cell-surface-associated nanosized FeSs. Although the biomineralization of nanosized FeSs is a wellknown phenomenon, only a few previous studies have considered its biological function from the view point of electron-conducting properties. FeSs have a wide range of compositions and structures, fulfilling the diverse electrochemical functionalities required for solid-state batteries, electrocatalysis, photovoltaics, and other industrial applications. In an effort to further explore microbial ingenuity and understand the importance of FeSs in microbial metabolism and electron transfer, we examined the biomineralization of FeSs by the Fereducing bacterium Shewanella loihica PV-4, which was originally isolated from iron-rich microbial mats near a deep-sea hydrothermal vent (1325 m below sea level). This study demonstrated that S. loihica PV-4 has the ability to exploit the Fe 3d electrons of biologically synthesized iron monosulfide (FeS) as extracellular long-distant electron-transfer conduits. Self-organizing, electrically conducting cell–FeS assemblies enabled the generation of a microbial current two orders of magnitude higher than in cell cultures lacking the biogenic minerals. S. loihica PV-4 cells were cultured anaerobically at 25 8C in a medium containing both ferric ions (FeCl3) and thiosulfate (Na2S2O3) as terminal electron acceptors, and lactate as a carbon source and electron-donating compound. The cell suspension instantly turned from yellow to brown, and a black precipitate was generated after approximately 5 h of inoculation. The X-ray diffraction (XRD) pattern of the yellow precipitate present before the addition of cells showed a broad undefined region with no significant peaks (Figure 1, bottom). Upon incubation with cells, however, several peaks appeared in the XRD pattern, which were assigned to elemental sulfur (S), goethite (a-FeOOH), and mackinawite (FeS; Figure 1, 1 day). The peak intensities for both elemental sulfur and goethite decreased with increasing incubation times, while a concomitant increase in the peaks assigned to mackinawite was observed. SEM observations (Figure 2) combined with confocal microscopy on GFP-labeled cells (Figure S2) showed populations of rod-shaped cells surrounded by nanosized FeS colloids, with the nanocolloids serving to interconnect the cells (Figure 2, bottom). The black precipitate was not observed in the suspension lacking cells ; this confirms that the formation of mackinawite was a consequence of the microbial reduction of ferric ions and thiosulfate. Mackinawite is one of the major constituents of black sedimentary minerals in anoxic marine systems and has a struc-