Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures

Abstract

McKay et al. [(1996)Science 273, 924 -930] suggested that carbonate globules in the meteorite ALH84001 contained the fossil remains of Martian microbes. We have characterized a subpopulation of magnetite (Fe3O4) crystals present in abundance within the Fe-rich rims of these carbonate globules. We find these Martian magnetites to be both chemically and physically identical to terrestrial, biogenically precipitated, intracellular magnetites produced by magnetotactic bacteria strain MV-1. Specifically, both magnetite populations are single-domain and chemically pure, and exhibit a unique crystal habit we describe as truncated hexaoctahedral. There are no known reports of inorganic processes to explain the observation of truncated hexa-octahedral magnetites in a terrestrial sample. In bacteria strain MV-1 their presence is therefore likely a product of Natural Selection. Unless there is an unknown and unexplained inorganic process on Mars that is conspicuously absent on the Earth and forms truncated hexaoctahedral magnetites, we suggest that these magnetite crystals in the Martian meteorite ALH84001 were likely produced by a biogenic process. As such, these crystals are interpreted as Martian magnetofossils and constitute evidence of the oldest life yet found. W e report here the presence of single-domain, chemically pure, truncated hexa-octahedral magnetite crystals in terrestrial samples and Martian meteorite ALH84001. We suggest that the truncated hexa-octahedral magnetite crystals in the Martian meteorite ALH84001 were likely formed by a biogenic process. These magnetite crystals are embedded in Ϸ3.91 (Ϯ0.05)-Ga-old carbonate globules (1) that fill cracks and pore space in the 4.5-Ga-old Martian meteorite ALH84001. These truncated hexa-octahedral magnetite crystals are identical to those produced intracellularly by the marine magnetotactic bacterium strain MV-1 (2-7); natural selection has optimized the magnetic moment of MV-1 magnetite particles (2,(8)(9)(10)(11)(12). There is no known natural terrestrial inorganic mechanism that can explain the observation of truncated hexa-octahedral magnetite crystals associated with MV-1. Therefore, unless there is an unknown inorganic process on Mars, which seems to be absent on the Earth, we suggest that ALH84001 truncated hexaoctahedral magnetites formed by a similar mechanism to their terrestrial biogenic counterparts. As such, these crystals are interpreted as Martian magnetofossils and they constitute evidence of the oldest life yet found. In support of this, we note that early Mars likely had free-standing bodies of liquid water (13,14), and both organic (15) and inorganic carbon and energy (e.g., atmospheric CO 2 13 ) sources. Furthermore, early Mars also likely possessed a substantial planetary magnetic field (16), which would have been sufficient to support the evolution and growth of magnetotactic bacteria. Characteristics of Biogenic MagnetiteMagnetotactic bacteria produce well ordered membranebounded intracellular crystals of magnetite (Fe 3 O 4 ) and͞or greigi...

Magnetovibrio blakemorei gen. nov., sp. nov., a magnetotactic bacterium ( Alphaproteobacteria : Rhodospirillaceae ) isolated from a salt marsh

“…Greigite was never observed in magnetosomes. The specific crystal habit of magnetite is truncated hexaoctahedral, with eight 111 octahedral faces, six 110 hexagonal faces and six 100 cubic faces (Thomas-Keprta et al, 2001). The dimensions of each crystal are approximately 356 35653 nm, with the axis of elongation corresponding to the alignment of crystals within the chain (Fig.…”

“…Strain MV -1 T has been the subject of several previous studies, centred mostly on the production and morphology of magnetite by this strain in culture (Bazylinski et al, 1988(Bazylinski et al, , 1995Thomas-Keprta et al, 2001;Dubbels et al, 2004), including the generation of spontaneous nonmagnetotactic mutants (Dubbels et al, 2004), and autotrophic growth of strain MV-1 T via the CalvinBenson-Bassham (CBB) cycle . Nevertheless, this important magnetotactic strain has not yet been characterized in the literature.…”

Magnetosome formation in prokaryotes

“…Tooth-shaped Fe 3 S 4 crystals have also been observed 20 . Elongated crystals are so unusual that their presence in recent and ancient sediments and in the Martian meteorite ALH84001 has led to their designation as magnetofossils 115 , and is cited as evidence for the past presence of magnetotactic bacteria in aquatic habitats and sediments [115][116][117] and life on ancient Mars [118][119][120][121] . However, elongated crystals of and ELECTRON HOLOGRAPHY 44 (FIG.…”

Origins of magnetite nanocrystals in Martian meteorite ALH84001

“…We have then used these new observations to evaluate the merits of both theoretical and experimental attempts to reproduce ALH84001-like disks with embedded magnetites through partial thermal decomposition of Fe-bearing carbonate. In the larger context, resolving or at least constraining the origin of the magnetites within ALH84001 carbonate disks has importance in that it has been previously suggested that the precipitation of the carbonate disks may have been facilitated through biogenic activity (McKay et al, 1996) and that a fraction of the embedded magnetites exhibit chemical and physical features identical to those produced by contemporary magnetotactic bacteria (McKay et al, 1996;Thomas-Keprta et al, 2000a;Thomas-Keprta et al, 2001;Thomas-Keprta et al, 2002). Table 1 lists the ALH84001 rock splits used in the research presented herein.…”