Fossil magnetite crystals of magnetotactic bacteria have been described as "magnetofossils" (18), mostly identified by the morphology of single crystals. In a few cases, fragments of fossil chains (among them chains with strong but smooth bends) could be isolated from soft deep-sea sediments and studied by transmission electron microscopy (14). In this study we were able to visualize the in situ spatial orientation of magnetite crystals and chains inside a solid rock substrate.
The syngeneticity of these particles is evident because they are embedded in the carbonate globules, which quite certainly were formed on Mars (1, 19, 20). Furthermore, Fig. 3A shows lobes of a carbonate globule containing magnetite chains (Fig. 3E) embedded in plagioclase glass and orthopyroxene.
The magnetite crystals in ALH84001 form chains similar to those in modern magnetotactic bacteria. This information should satisfy the last of the six criteria postulated (2) as evidence of biological origin. We also describe five additional criteria characteristic of biologically produced magnetite chains, which could not be present in abiotically formed chains of magnetite crystals (no such chains have ever been observed in nature). We suggest that these criteria have now been satisfied: Uniform crystal size and shape within chains is seen in Fig. 1 G and H, Fig. 1 J and K, and Fig. 4A (in the latter the crystals in the chain marked by an arrow are elongated and larger in diameter than those in the numerous chains of smaller and approximately isodiametric crystals), as well as in Fig. 3 B-F, which show chains of different sizes. Fig. 3 E and F (note different scales) show the difference between the largest, approximately isodiametric, and the smallest, elongated, crystals. No abiological process is known that would result in such sorting of crystals from a mixed pool of sizes and shapes. Gaps between crystals are best seen in Fig. 3B, although they are evident in all sufficiently resolved chains as dark lines between crystals. Orientation of elongated crystals along the chain axis is evident in Figs. 3F and 4A (arrow). Halo around chains, the possible remnant of a membrane, is visible in Fig. 1G-I. Flexibility of chains, a character ascribable to the elastic property of the organic substance between but independent from the shape of the crystals, is evident in many of the small chains in Fig. 1 G-I, Fig. 1 J-L, and Fig. 3 B-F. We conclude that the chains of electron-opaque particles in ALH84001 are magnetofossils, as no other consistent explanation would account for these findings.
The ecological implications of the assumption that the magnetite chains in ALH84001 are of biological origin must be examined: Is it likely, on the basis of our knowledge of terrestrial magnetotactic bacteria, that such microorganisms were present in cracks of ALH84001? Numerous endolithic microorganisms exist in both terrestrial (21) and aquatic (22) habitats, yet it is very unlikely that magnetotactic bacteria were ever alive in ALH84001. All known endolithic microorganisms are sessile, attached to the surface; none is motile, understandable because swimming would be impossible in the microscopic spaces in rocks. Magnetotactic bacteria, with magnetosomes to serve as guides in the direction of swimming, are all motile. Furthermore, the non-Gaussian frequency distribution of the number of crystals in chains (Table 1) is incompatible with the variability of chain length in a living population. We suggest the following scenario: First, decomposed remains of dead magnetobacteria suspended in a carbonate-rich fluid penetrated fissures of ALH84001, already crushed by previous asteroid impact, perhaps after the second impact event (I2) postulated (23). Second, evaporation of the liquid led to the deposition of pancake-shaped carbonate globules, and magnetite crystals and chain fragments were deposited in the periphery of the carbonate discs, perhaps through the mechanism for ring deposition of particles dispersed in liquid drops (24).