Redox cycling of Fe(II) and Fe(III) in magnetite accelerates aceticlastic methanogenesis by Methanosarcina mazei

It has been recently shown that magnetite nanoparticles (nanoFe₃ O₄ ) can facilitate methanogenic syntrophy but the effect of magnetite on methanogenesis alone remains elusive. Here we show that aceticlastic methanogenesis by Methanosarcina mazei is accelerated by magnetite and is correlated with the redox cycling of structural Fe(II) and Fe(III) in the mineral. An enrichment and its closest pure culture relative, Ms. mazei zm-15, both obtained from a natural wetland of the Tibetan plateau were tested in this experiment. The Fe(II) to Fe(III) ratios in magnetite, as measured by multiple approaches, show an initial increase in both the methanogenic cultures and the blank preparations containing no microbes. The Fe(II)/Fe(III) ratio then displays a distinct decline followed by an increase towards the end of incubation only in the enrichment and pure culture cultivations. This redox cycling of magnetite is in accordance with the stimulation of aceticlastic methanogenesis. Microscopic observation reveals the precipitation of nanoFe₃ O₄ on methanogen cell surface. The genomic analysis predicts that in addition to electron transfer components essential for aceticlastic methanogenesis, Ms. mazei zm-15 contains an outer-surface multiheme c-type cytochrome (MHC) and a few function-unknown surface proteins that harbour monoheme motif. We hypothesize that the redox cycling of nanoFe₃ O₄ delivers a positive influence via the MHC to the membrane electron transfer chain and hence promote the aceticlastic methanogenesis.

Electrostatic potential and valence modulation in La0.7Sr0.3MnO3 thin films

The Mn valence in thin film La_0.7Sr_0.3MnO₃ was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. These results were supported by first principles calculations. The surface valence increases to up to Mn^3.7+, whereas the interface valence reduces down to Mn^2.5+. The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality.

Spatially Resolved Distribution of Fe Species around Microbes at the Submicron Scale in Natural Bacteriogenic Iron Oxides

Natural bacteriogenic iron oxides (BIOS) were investigated using local-analyzable synchrotron-based scanning transmission X-ray microscopy (STXM) with a submicron-scale resolution. Cell, cell sheath interface (EPS), and sheath in the BIOS were clearly depicted using C-, N-, and O- near edge X-ray absorption fine structure (NEXAFS) obtained through STXM measurements. Fe-NEXAFS obtained from different regions of BIOS indicated that the most dominant iron mineral species was ferrihydrite. Fe(II)- and/or Fe(III)-acidic polysaccharides accompanied ferrihydrite near the cell and EPS regions. Our STXM/NEXAFS analysis showed that Fe species change continuously between the cell, EPS, and sheath under several 10-nm scales.

Field-based Raman spectroscopic analyses of an Ordovician stromatolite

Raman spectrometers are being miniaturized for future life-detection missions on Mars. Field-portable Raman spectrometers, which have similar spectral parameters to the instruments being developed for Mars rovers, have been used to examine extant biosignatures, but they have not yet been used to examine ancient biosignatures. Here, a portable Raman spectrometer was used to analyze an Ordovician stromatolite at the outcrop, revealing both its mineralogy and the presence of sp² carbonaceous material. As stromatolites are often used as proof of the presence of life in Archean rocks and are searched for on Mars, the ability to analyze them in the field with no sample preparation has important ramifications for future Mars missions. However, these results also reveal that a 785 nm excitation source, rather than the 532 nm excitation source planned for future missions, might be a better choice in the search for fossil biosignatures.

Controlled cobalt doping in biogenic magnetite nanoparticles

Cobalt-doped magnetite (CoxFe3 -xO4) nanoparticles have been produced through the microbial reduction of cobalt-iron oxyhydroxide by the bacterium Geobacter sulfurreducens. The materials produced, as measured by superconducting quantum interference device magnetometry, X-ray magnetic circular dichroism, Mössbauer spectroscopy, etc., show dramatic increases in coercivity with increasing cobalt content without a major decrease in overall saturation magnetization. Structural and magnetization analyses reveal a reduction in particle size to less than 4 nm at the highest Co content, combined with an increase in the effective anisotropy of the magnetic nanoparticles. The potential use of these biogenic nanoparticles in aqueous suspensions for magnetic hyperthermia applications is demonstrated. Further analysis of the distribution of cations within the ferrite spinel indicates that the cobalt is predominantly incorporated in octahedral coordination, achieved by the substitution of Fe(2+) site with Co(2+), with up to 17 per cent Co substituted into tetrahedral sites.