Universal Magnetic Assay For Biogenicity

As mentioned above, Thomas-Keprta et al. (2002) proposed a MAB, including six distinctive properties displayed by bacterial magnetosomes, that allow to distinguish them from inorganic magnetic particles, including defect-free crystals, consistent shapes and aspect ratios, elongated crystal morphologies and chemical purity. Yet some magnetite-producing bacteria synthesize magnetosomes with crystallographic defects similar to that of synthetic magnetite crystals (Devouard et al., 1998; Taylor et al., 2001), and shapes and aspect ratios of magnetosomes from different strains of magnetotactic bacteria differ from one another, and do not always show elongated morphologies (Buseck et al., 2001). Among the greig-ite-producing magnetotactic bacteria, magnetosomes often lack the chemical purity and present crystallographic defects (Posfai et al., 1998a, b). These exceptions question the universality of the MAB proposed by Thomas-Keprta et al. (2002) and suggest that different criteria are needed to distinguish between bio-genic and non-biogenic nanophase magnetic iron minerals.

It is important to note that organisms lacking several traits listed in the previous MAB, are still able to efficiently move along magnetic field lines. This indicates that compositional purity, defect-free crystals and elongated crystal morphologies are not strictly necessary for the purpose of magnetotaxis. These traits are highly desirable from the point of view of magnetic optimization, since they maximize the saturation magnetization of the particles and thus the degree of efficiency when aligning with the geomagnetic field lines, yet they are not necessary for magnetotaxis itself. If these traits are typical, but not universal, among magnetite-producing magnetotactic organisms on Earth, and do not generally apply to greigite producing ones, then the tenet that bacterial magnetic precipitates can be unambiguously identified by the MAB needs to be reconsidered.

There are however two essential properties common to all magnetotactic bacteria that may constitute a universal MAB: (1) the use exclusively of magnetic SD crystals and (2) the arrangement of these crystals in chains or chain-like structures. These traits are not fortuitous but obey to universal physical laws, independent of environmental or biological factors. A chain of magnetic SD crystals has a net magnetic moment that results from the addition of each crystal's magnetic moment within the chain (Blakemore, 1982; Bazylinski et al., 1995; Dunin-Borkowski et al., 1998). Smaller magnetic particles are unstable because of thermally induced fluctuations of their magnetic moment, whereas larger particles have a low net magnetic moment due to magnetic domain formation. In either case, the resultant chain would have a null or very low magnetic moment. This suggests that microorganisms analogous to terrestrial magnetotactic prokaryotes, if ever present on Mars, likely developed a similar orientation mechanism to magnetotactic prokaryotes on Earth. Support for this idea comes from phylogenetic studies, which have shown that different species of MB belong to different phylogenetic lineages, suggesting that magnetotaxis in prokaryotes evolved separately more than once throughout life history (DeLong et al., 1993), yet always based on chains of magnetic SD particles. Additionally, in any given population of fossil chains of magnetosomes, likely some traits concerning the shape, crystallography and composition of the magnetosomes listed in the previous MAB, may be present, and could then be considered as additional evidence to support the biogenic origin of the magnetic crystals. Fossil chains of magnetic single-domain particles are therefore an ideal biomarker and ought to be searched for in returned samples from the Martian surface such as ancient sedimentary deposits formed in standing waters (McKay et al., 2003).

5. Summary

The paper presented by McKay et al. (1996) did not only inspire a large number of studies aiming to prove or disprove the evidence of life hypothesis, but it also stirred new debates about life on Earth and opened new avenues for the search of life outside our planet. Together with other technological and scientific innovations in the 1990s, it precipitated the foundation of the NASA Astrobiology Institute (NAI), and inspired a renewed interest in the exploration of Mars, which was practically forgotten since the Viking mission in the 1970s. Of the different lines of evidence for life in ALH84001 suggested by McKay et al. (1996), the presence of fossil nanobacteria and of chains of magnetic single-domain particles remain plausible albeit controversial. Irrespectively of their origin in ALH84001, these structures ought to be considered as important biomarkers and therefore be searched for in samples returned from Mars in the future.

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Biodata of David C. Fernández-Remolar, author (with other coauthors) of "Preservation Windows for Paleobiological Traces in the Mars Geological Record"

Dr. David C. Fernández-Remolar is currently researcher in the Centro de Astrobiología (INTA-CSIC). He obtained his Ph.D. from the Complutense University at Madrid in 1999 studying Lower Cambrian phosphatized skeletons of Sierra de Córdoba. At the Centro de Astrobiología he is currently researching geobiology and biogeochemistry of extreme environments and geohistorical Mars analogs such as Río Tinto focused in the surface and subsurface astrobio-logical exploration of Mars. Other areas of interest are the astrobiological exploration of Europa and the geobiology of the Proterozoic and Lower Cambrian deposits of Spain.

E-mail: [email protected]

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