Abiotic Deposition of Fe Complexes onto Leptothrix Sheaths

Abstract

Bacteria classified in species of the genus Leptothrix produce extracellular, microtubular, Fe-encrusted sheaths. The encrustation has been previously linked to bacterial Fe oxidases, which oxidize Fe(II) to Fe(III) and/or active groups of bacterial exopolymers within sheaths to attract and bind aqueous-phase inorganics. When L. cholodnii SP-6 cells were cultured in media amended with high Fe(II) concentrations, Fe(III) precipitates visibly formed immediately after addition of Fe(II) to the medium, suggesting prompt abiotic oxidation of Fe(II) to Fe(III). Intriguingly, these precipitates were deposited onto the sheath surface of bacterial cells as the population was actively growing. When Fe(III) was added to the medium, similar precipitates formed in the medium first and were abiotically deposited onto the sheath surfaces. The precipitates in the Fe(II) medium were composed of assemblies of globular, amorphous particles (ca. 50 nm diameter), while those in the Fe(III) medium were composed of large, aggregated particles (≥3 µm diameter) with a similar amorphous structure. These precipitates also adhered to cell-free sheaths. We thus concluded that direct abiotic deposition of Fe complexes onto the sheath surface occurs independently of cellular activity in liquid media containing Fe salts, although it remains unclear how this deposition is associated with the previously proposed mechanisms (oxidation enzyme- and/or active group of organic components-involved) of Fe encrustation of the Leptothrix sheaths.

Direct Adherence of Fe(III) Particles onto Sheaths of Leptothrix sp. Strain OUMS1 in Culture

“…Our previous work [23] confirmed that Fe(III) is directly deposited on sheaths of strain SP-6 cultured in MSVP amended with various Fe(II) or Fe(III) salts. Considering the present results, we can reasonably conclude that such a direct deposition of Fe(III) particles onto the sheath surface occurs irrespective of the strain and the medium that contains Fe sources.…”

“…Because abiotic Fe oxidation Fe(II) to Fe(III) in fully oxygenated water at circumneutral pH is very rapid (half-life < 1 min) [22,34], reduction of Fe(II) concentration might result in discontinued Fe(III) generation. We reported that MSVP medium [35] with high Fe(II) concentrations became visually turbid due to Fe(III) precipitation within 20 min after addition of Fe(II) salts [23]. On the basis of the onset of turbidity, Fe(III) precipitation was much more delayed in SGP + Fe than in MSVP.…”

“…Irrelevant to any biotic metaloxidization theory, we recently provided microscopic and spectroscopic evidence that the deposition of Fe(III) precipitates that are generated by abiotic oxidation directly onto sheaths of SP-6 cells in culture media amended with Fe(II) salts is actually independent of vital activity of bacterial cells [23]. Although it is still unclear how this direct adherence opposes the hypothesized modes of sheath encrustation, we must consider the direct nature of this deposition when investigating the interactions between bacterial exopolymers and metal ions, particularly in shake cultures and whether this direct deposition is unique to the SP-6 strain of Leptothrix.…”

Iron minerals within specific microfossil morphospecies of the 1.88 Ga Gunflint Formation

“…Cell-preserving Type 2 Gunflintia is Fe mineralized, whereas cell-free sheaths (Type 1 Gunflintia and Animikiea ) remained iron free. This is, however, at odds with the observed affinity of cell-free sheaths for iron60. In addition, the 3D preservation of adjacent Huroniospora types (Fig.…”

Perspectives on the Biogenesis of Iron Oxide Complexes Produced by Leptothrix, an Iron-oxidizing Bacterium and Promising Industrial Applications for their Functions

“…Hedrich et al [12] noted that ferrous iron was stable in anoxic environments, but was susceptible to spontaneous chemical oxidation by molecular oxygen. Focusing on this finding, Kunoh et al [58] examined interactions of Fe(II) and Fe(III) in shake culture and found that Fe(II) was promptly converted to Fe(III) precipitation and that the precipitated Fe(III) directly adhered to the existing sheath surfaces. Although they failed to determine how this direct adherence interfered with the proposed modes of sheath encrustation, we must consider this phenomenon when investigating the interactions between bacterial exopolymers and metal ions, particularly in shake cultures.…”