Silicon-Rich, Iron Oxide Microtubular Sheath Produced by an Iron-Oxidizing Bacterium, Leptothrix sp. Strain OUMS1, in Culture

Eco-Benign Orange-Hued Pigment Derived from Aluminum-Enriched Biogenous Iron Oxide Sheaths

Inorganic pigments have been widely used due to their low cost of production, strong hiding power, and chemical resistance; nevertheless, they have limited hue width and chromaticity. To eliminate these disadvantages, we herein propose the use of an ingenious biotemplate technique to produce Al-enriched biogenic iron oxide (BIOX) materials. Spectrophotometric color analysis showed that high levels of Al inclusion on heat-treated BIOX samples produced heightened yellowish hues and lightness. The Al-enriched BIOX sheaths exhibited a stable tubular structure and excellent thermal stability of color tones after heating at high temperatures and repetitive heat treatments. Ultrastructural analysis and mechanical destruction experiments revealed that the highly chromatic orange-hue of these pigments are ascribed probably to an ingenious cylindrical nanocomposite architecture composed of putative Fe-included low crystalline Al oxide regions and hematite particles embedded therein. The present work therefore demonstrates that the bioengineered material can serve as an epochal orange-hued inorganic pigment with low toxicity and marked thermostability that should meet large industrial demand.

Reclassification of Sphaerotilus natans subsp. sulfidivorans Gridneva et al. 2011 as Sphaerotilus sulfidivorans sp. nov. and comparative genome analysis of the genus Sphaerotilus

Filamentous iron oxides accumulating bacteria Sphaerotilus natans subsp. natans and S. natans subsp. sulfidivorans were described as subspecies based on 99.7% identity of their 16S rRNA sequences, in spite of important physiological difference. The ANI between their genomes was 94.7%, which indicate their assignment to different species. S. natans subsp. sulfidivorans and S. montanus possess genes for a complete SOX system, while S. natans subsp. natans encode only SoxYZ. There are genes for the Calvin cycle in the genomes of S. hippei DSM 566^T, S. natans subsp. sulfidivorans D-501^T, and S. montanus HS^T. Lithoautotrophy on reduced sulfur compounds is probably possible for S. natans subsp. sulfidivorans and S. montanus, but not for S. natans subsp. natans. Considering significant differences in the genome characteristics and metabolic potential of S. natans subsp. sulfidivorans and S. natans subsp. natans, we propose their classification as different species, S. natans and S. sulfidivorans sp. nov.

Preparation and Characterization of Additional Metallic Element-Containing Tubular Iron Oxides of Bacterial Origin

Biogenic microtubular iron oxides (BIOXs) derived from Leptothrix spp. are known as promising multifunctional materials for industrial applications such as ceramic pigments and catalyst carriers. Here, we report unprecedented BIOX products with additive depositions of various metallic elements prepared by a newly devised "two-step" method using an artificial culture system of Leptothrix cholodnii strain OUMS1; the method comprises a biotic formation of immature organic sheaths and subsequent abiotic deposition of Fe and intended elements on the sheaths. Chemical composition ratios of the additional elements Al, Zr, and Ti in the respective BIOXs were arbitrarily controllable depending on initial concentrations of metallic salts added to reaction solutions. Raman spectroscopy exemplified an existence of Fe-O-Al linkage in the Al-containing BIOX matrices. Time-course analyses revealed the underlying physiological mechanism for the BIOX formation. These results indicate that our advanced method can contribute greatly to creations of innovative bioderived materials with improved functionalities.

High-Quality Inorganic Red Pigment Prepared by Aluminum Deposition on Biogenous Iron Oxide Sheaths

Naturally occurring tubular iron oxides produced by aquatic bacteria in Leptothrix spp. are promising raw materials for hematite-based red pigments because of the higher heat resistance as compared with chemically synthesized hematite compounds. Here, we report iron oxide red pigments prepared through an additive deposition of aluminum on culture-based biogenous iron oxide (cBIOX) sheaths using an artificial culture system of L. cholodnii strain OUMS1. The heat-treated Al-containing cBIOXs exhibited elevated chroma and lightness along with increasing Al contents and enhanced thermal stability of color tones to repetitive heat treatments. XRD analysis showed a monophasic pattern of hematite in the Al-rich cBIOX after heating at a wide range of high temperatures. Micromorphology analyses revealed that putative Al oxide regions present among hematite particles plausibly prevented the grain growth of hematite during heat treatments. The results therefore demonstrate that the bioderived Al-rich iron oxide sheaths can serve as innovative inorganic red pigments feasible for industrial applications.

Rapid and reversible lithiation of doped biogenous iron oxide nanoparticles

Certain bacteria produce iron oxide material assembled with nanoparticles (NPs) that are doped with silicon (Fe:Si ~ 3:1) in ambient environment. Such biogenous iron oxides (BIOX) proved to be an excellent electrode material for lithium-ion batteries, but underlying atomistic mechanisms remain elusive. Here, quantum molecular dynamics simulations, combined with biomimetic synthesis and characterization, show rapid charging and discharging of NP within 100 fs, with associated surface lithiation and delithiation, respectively. The rapid electric response of NP is due to the large fraction of surface atoms. Furthermore, this study reveals an essential role of Si-doping, which reduces the strength of Li-O bonds, thereby achieving more gentle and reversible lithiation culminating in enhanced cyclability of batteries. Combined with recent developments in bio-doping technologies, such fundamental understanding may lead to energy-efficient and environment-friendly synthesis of a wide variety of doped BIOX materials with customized properties.

Biosorption of metal elements by exopolymer nanofibrils excreted from Leptothrix cells

Leptothrix species, aquatic Fe-oxidizing bacteria, excrete nano-scaled exopolymer fibrils. Once excreted, the fibrils weave together and coalesce to form extracellular, microtubular, immature sheaths encasing catenulate cells of Leptothrix. The immature sheaths, composed of aggregated nanofibrils with a homogeneous-looking matrix, attract and bind aqueous-phase inorganics, especially Fe, P, and Si, to form seemingly solid, mature sheaths of a hybrid organic-inorganic nature. To verify our assumption that the organic skeleton of the sheaths might sorb a broad range of other metallic and nonmetallic elements, we examined the sorption potential of chemically and enzymatically prepared protein-free organic sheath remnants for 47 available elements. The sheath remnants were found by XRF to sorb each of the 47 elements, although their sorption degree varied among the elements: >35% atomic percentages for Ti, Y, Zr, Ru, Rh, Ag, and Au. Electron microscopy, energy dispersive x-ray spectroscopy, electron and x-ray diffractions, and Fourier transform infrared spectroscopy analyses of sheath remnants that had sorbed Ag, Cu, and Pt revealed that (i) the sheath remnants comprised a 5-10 nm thick aggregation of fibrils, (ii) the test elements were distributed almost homogeneously throughout the fibrillar aggregate, (iii) the nanofibril matrix sorbing the elements was nearly amorphous, and (iv) these elements plausibly were bound to the matrix by ionic binding, especially via OH. The present results show that the constitutive protein-free exopolymer nanofibrils of the sheaths can contribute to creating novel filtering materials for recovering and recycling useful and/or hazardous elements from the environment.