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Distinct morphological characteristics of magnetite formed intracellularly by magnetic bacteria (magnetosome) are invoked as compelling evidence for biological activity on Earth and possibly on Mars. Crystals of magnetite produced extracellularly by a variety of bacteria including Geobacter metallireducens GS-15, thermophilic bacteria, and psychrotolerant bacteria are, however, traditionally not thought to have nearly as distinct morphologies. The size and shape of extracellular magnetite depend on the culture conditions and type of bacteria. Under typical CO2-rich culture conditions, GS-15 is known to produce superparamagnetic magnetite (crystal diameters of approximately <30 nm). In the current study, we were able to produce a unique form of tabular, single-domain magnetite under nontraditional (low-CO 2) culture conditions. This magnetite has a distinct crystal habit and magnetic properties. This magnetite could be used as a biosignature to recognize ancient biological activities in terrestrial and extraterrestrial environments and also may be a major carrier of the magnetization in natural sediments.

Transmission electron microscopy studies have been used to argue that magnetite crystals in carbonate from Martian meteorite ALH84001 have a composition and morphology indistinguishable from that of magnetotactic bacteria. It has even been claimed from scanning electron microscopy imaging that some ALH84001 magnetite crystals are aligned in chains. Alignment of magnetosomes in chains is perhaps the most distinctive of the six crystallographic properties thought to be collectively unique to magnetofossils. Here we use three rock magnetic techniques, low-temperature cycling, the Moskowitz test, and ferromagnetic resonance, to sense the bulk composition and crystallography of millions of ALH84001 magnetite crystals. The magnetic data demonstrate that although the magnetite is unusually pure and fine-grained in a manner similar to terrestrial magnetofossils, most or all of the crystals are not arranged in chains.A debate has been raging for the last 7 years over whether magnetite crystals in carbonate in Martian meteorite ALH84001 are four-billion-year-old fossils of magnetotactic bacteria (1-5) or are instead inorganic assemblages (6-9). have identified six properties that they claim are collectively unique to magnetosomes (intracellular magnetite crystals) produced by the modern terrestrial magnetotactic bacterium strain MV-1: unusual truncated hexa-octahedral morphology, few crystallographic defects, elongated habit, narrow size distribution restricted mainly to the single domain field, high purity, and alignment in chains. From their transmission electron microscopy (TEM) analyses of individual crystals acid-extracted from ALH84001 carbonates, they have argued that some of the magnetite crystals share the first five of these properties in common with MV-1. They conclude that Ϸ25% of the magnetite crystals in ALH84001 zoned carbonate are most likely magnetofossils intimately mixed with a population of Ϸ75% inorganic magnetite. Thomas-Keprta et al. could not comment on the sixth property (alignment in chains) because they did not analyze the magnetite crystals in situ.Friedmann et al. (5) have used stereo backscattered scanning electron microscopy (SEM) imaging of the surfaces of ALH84001 carbonate to argue that some of the magnetite crystals are in fact arranged in chains. If so, this would provide dramatic support for the hypothesis that the magnetite is biogenic. This is because a chain of equant magnetite crystals has higher magnetostatic energy than a ring of the crystals and is therefore not commonly observed for abiogenic magnetite in nature. Magnetosome chains are thought to be stabilized in the bacteria by a rigid biomechanical structure, because when removed from the cell they often collapse into the lower-energy ring or clumped configuration (10-12). However, the proposed chains in the images of Friedmann et al. (5) do not appear to be isolated from surrounding magnetite crystals, which calls into question the appropriateness of their being labeled ''chains'' at all rather than members of a three-dimen...

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Manganese oxide (Mn oxide) minerals from bacterial sources produce electron paramagnetic resonance (EPR) spectral signatures that are mostly distinct from those of synthetic simulants and abiogenic mineral Mn oxides. Biogenic Mn oxides exhibit only narrow EPR spectral linewidths (∼500 G), whereas abiogenic Mn oxides produce spectral linewidths that are 2-6 times broader and range from 1200 to 3000 G. This distinction is consistent with X-ray structural observations that biogenic Mn oxides have abundant layer site vacancies and edge terminations and are mostly of single ionic species [i.e., Mn(IV)], all of which favor narrow EPR linewidths. In contrast, abiogenic Mn oxides have fewer lattice vacancies, larger particle sizes, and mixed ionic species [Mn(III) and Mn(IV)], which lead to the broader linewidths. These properties could be utilized in the search for extraterrestrial physicochemical biosignatures, for example, on Mars missions that include a miniature version of an EPR spectrometer.

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A scaled‐up dielectric barrier discharge (DBD) reactor has been developed and demonstrated for the production of hydrogen from steam methane reforming (SMR) by catalytic nonthermal plasma (CNTP) technology. Compared to SMR, CNTP offers conversion at ambient pressure (101.325 kPa), low temperature with better efficiency, making it suitable for distributed hydrogen production with small footprint. There have been several lab‐scale DBD reactors reported in the literature. Dimension of the scaled‐up DBD reactor is about six times the lab‐scale version and can produce 0.9 kg H2/day. The scale‐up is, however, nonlinear; several technical innovations were required including spray nozzle for homogeneous introduction of steam, perforated tube central electrodes for generation of homogeneous plasma. Conversion efficiency of the scaled‐up DBD reactor is 70–80% at 550°C and 500 W. A continuous run of 8 hr was demonstrated with typical product gas composition of 69% H2, 6% CO2, 15% CO, 10% CH4.

Kinetics of the reactions of the principal radical species, the tertiary alkyl and peroxy radicals, generated on photooxidation of poly(n-butyl acrylate) (PnBA) were studied at room temperature under different oxygen pressures. A simplified mechanism of photooxidation, similar to that proposed earlier (Liang, R. H.; Tsay, F.-D.; Gupta, A. Macromolecules 1982,15,974), was used to interpret the data. The kinetic rate parameters as well as the radical concentrations developed under steady-state illumination conditions were estimated by a least-squares fit to the observed data by using kinetic equations based on such a mechanism. It was found that at least two different types of tertiary alkyl radicals (i.e., radicals of different reactivities) were being formed during photooxidation of PnBA.