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.
Highlights
The Fe/Mn-oxidizing bacteria such as Leptothrix and Gallionella species are ubiquitous habitants in aqueous environments, especially at groundwater outwelling sites which are characterized by a nearlyBiology 2016, 5, 26; doi:10.3390/biology5020026 www.mdpi.com/journal/biologyBiology 2016, 5, 26 neutral pH, an oxygen gradient, and a source of reduced Fe and Mn minerals [1,2]
Results on the association of Fe complexes preformed in media with the sheath surfaces raised another query: Does the direct deposition of Fe precipitates onto sheaths require the presence of bacterial cells? To answer this question, cell-free sheaths were prepared by lysozyme-EDTA-SDS
Several earlier Energy Dispersive X-Ray Spectroscopy (EDX)- or EELS-mapping studies showed a nearly uniform distribution of inorganics in the L. ochracea sheaths that were harvested from natural environments [6] or from a relatively long-term (7–14 days) culture [12,20], giving the impression that active groups of organics in the bacterial exopolymer fibrils, which construct the basic frame of the sheaths, contribute to attracting and binding aqueous-phase inorganics [27]
Summary
The Fe/Mn-oxidizing bacteria such as Leptothrix and Gallionella species are ubiquitous habitants in aqueous environments, especially at groundwater outwelling sites which are characterized by a nearlyBiology 2016, 5, 26; doi:10.3390/biology5020026 www.mdpi.com/journal/biologyBiology 2016, 5, 26 neutral pH, an oxygen gradient, and a source of reduced Fe and Mn minerals [1,2]. Metal-oxidizing enzymes have been suggested to play a role in the formation and metal encrustation of the Leptothrix sheath [10,11,12,13]. Encrustation of inorganics in sheaths is arguably a result of biotic metal oxidation, and the associated reactions may even drive the chemolithoautotrophic energy metabolism of L. ochracea [14]. In spite of this background knowledge, the precise mechanism of the interactions between bacterial organics and aqueous-phase inorganics for sheath formation has continued to be a matter of debate
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