Hojatollah Vali

Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and iron sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

Elongated prismatic magnetite crystals in ALH84001 carbonate globules: Potential Martian magnetofossils

Using transmission electron microscopy (TEM), we have analyzed magnetite (Fe3O4) crystals acid-extracted from carbonate globules in Martian meteorite ALH84001. We studied 594 magnetites from ALH84001 and grouped them into three populations on the basis of morphology: 389 were irregularly shaped, 164 were elongated prisms, and 41 were whisker-like. As a possible terrestrial analog for the ALH84001 elongated prisms, we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain MV-1. By TEM again, we examined 206 magnetites recovered from strain MV-1 cells. Natural (Darwinian) selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment. In this study, we describe six properties of magnetite produced by biologically controlled mechanisms (e.g., magnetotactic bacteria), properties that, collectively, are not observed in any known population of inorganic magnetites. These criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories. Of the ALH84001 magnetites that we have examined, the elongated prismatic magnetite particles (similar to 27% of the total) are indistinguishable from the MV-1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals.

Comprehensive genetic dissection of the magnetosome gene island reveals the step-wise assembly of a prokaryotic organelle.

Although membrane-bounded compartments are commonly considered a unique eukaryotic characteristic, many species of bacteria have organelles. Compartmentalization is well studied in eukaryotes; however, the molecular factors and processes leading to organelle formation in bacteria are poorly understood. We use the magnetosome compartments of magnetotactic bacteria as a model system to investigate organelle biogenesis in a prokaryotic system. The magnetosome is an invagination of the cell membrane that contains a specific set of proteins able to direct the synthesis of a nanometer-sized magnetite crystal. A well-conserved region called the magnetosome island (MAI) is known to be essential for magnetosome formation and contains most of the genes previously implicated in magnetosome formation. Here, we present a comprehensive functional analysis of the MAI genes in a magnetotactic bacterium, Magnetospirillum magneticum AMB-1. By characterizing MAI deletion mutants, we show that parts of its conserved core are not essential for magnetosome biogenesis and that nonconserved genes are important for crystal formation. Most importantly, we show that the mamAB gene cluster encodes for factors important for magnetosome membrane biogenesis, for targeting of proteins to this compartment and for several steps during magnetite production. Altogether, this genetic analysis defines the function of more than a dozen factors participating in magnetosome formation and shows that magnetosomes are assembled in a step-wise manner in which membrane biogenesis, magnetosome protein localization, and biomineralization are placed under discrete genetic control.

Records of an ancient Martian magnetic field in ALH84001

Although Mars does not presently appear to have a global dynamo magnetic field, strong crustal fields have recently been detected by the Mars Global Surveyor above surfaces formed ∼3 or more Ga. We present magnetic and textural studies of Martian meteorite ALH84001 demonstrating that 4 Ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization. Because ^(40)Ar/^(39)Ar thermochronology demonstrates that most ALH84001 carbonates have probably been well below the Curie point of magnetite since near the time of their formation [Weiss et al., Earth Planet. Sci. Lett. (2002) this issue], their magnetization originated at 3.9–4.1 Ga on Mars. This magnetization is at least 500 million years (Myr) older than that known in any other planetary rock, and its strong intensity suggests that Mars had generated a geodynamo and global magnetic field within 450–650 Myr of its formation. The intensity of this field was roughly within an order of magnitude of that at the surface of the present-day Earth, sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in ALH84001 and possibly for the production of the strong crustal anomalies. Chromite in ALH84001 may retain even older records of Martian magnetic fields, possibly extending back to near the time of planetary formation.

Magnetotactic bacteria and their magnetofossils in sediments

Abstract Living magnetotactic bacteria from freshwater environments show, under natural and laboratory conditions, a great variety of morphological appearances. Their magnetosomes vary in number, shape, and size. One species of bacteria yields up to 1000 magnetosomes per cell. Individual particles reach a size of up to 200 nm. As a rule, they form elongated chains. In bacteria which are held under laboratory conditions, loop-shaped arrangements and nearly unordered clusters are also found. During the laboratory experiments, different species of bacteria periodically dominated. Freshwater samples always exhibited a great variety of different magnetosomes. On the other hand, samples from marine environment clearly showed the preponderance of one type of particle. These observations may reflect specific properties of the physico-chemical milieu in which the bacteria grew. At the end of our experiments north- and south-seeking bacteria co-existed in roughly equal quantities. Quaternary unconsolidated sediments from the Ammersee (Bavaria) and the Antarctic, Quaternary to Tertiary deep-sea sediments from the Atlantic and Pacific, and Jurassic limestones from the Sonnwendgebirge (Tyrol, Austria) were analyzed for fossil magnetosomes. We found that fossil magnetosomes were morphologically similar to those from living bacteria, but seemed to be corroded in some cases. The observed differences in quantity, size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites, but are not determined by geographical latitude.

Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures

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 hexa-octahedral. 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 hexa-octahedral 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.

Magnetofossils from Ancient Mars: a Robust Biosignature in the Martian Meteorite ALH84001

Evidence of biogenic activity on Mars has profound scientific implications for our understanding of the origin of life on Earth and the presence and diversity of life within the Cosmos. Analysis of the Martian meteorite Allan Hills 84001 (ALH84001) revealed several lines of evidence that has led some investigators to suggest that microbial life existed on Mars approximately 4 billion years ago (45). One of the strongest lines of evidence is the presence of tens-of-nanometer-size magnetite (Fe3O4) crystals found within carbonate globules and their associated rims in the meteorite (57, 58). Approximately one-quarter of these magnetites have remarkable morphological and chemical similarities to magnetite particles produced by magnetotactic bacteria, which occur in aquatic habitats on Earth. Moreover, these types of magnetite particles are not known or expected to be produced by abiotic means either through geological processes or synthetically in the laboratory. We have therefore argued that these Martian magnetite crystals are in fact magnetofossils (57, 58). If this is true, such magnetofossils would constitute evidence of the oldest life forms known. In this respect, we note there is now considerable uncertainty concerning when the earliest terrestrial life forms existed. Until recently, results from the ~3.5-billion-year-old Apex cherts of the Warrawoona group in western Australia held this record (52), although this work is now in question (12).

A highly water-dispersible/magnetically separable palladium catalyst based on a Fe3O4@SiO2 anchored TEG-imidazolium ionic liquid for the Suzuki–Miyaura coupling reaction in water

A novel ionic liquid functionalized magnetic nanoparticle was prepared by anchoring an imidazolium ionic liquid bearing triethylene glycol moieties on the surface of silica-coated iron oxide nanoparticles. The material proved to be an effective host for the immobilization of a Pd catalyst through a subsequent simple ion-exchange process giving a highly water dispersible, active and yet magnetically recoverable Pd catalyst (Mag-IL-Pd) in the Suzuki–Miyaura coupling reaction in water. The as-prepared catalyst displayed remarkable activity toward challenging substrates such as heteroaryl halides and ortho-substituted aryl halides as well as aryl chlorides using very low Pd loading in excellent yields and demonstrating high TONs. Since the catalyst exhibited extremely low solubility in organic solvent, the recovered aqueous phase containing the catalyst can be simply and efficiently used in ten consecutive runs without significant decrease in activity and at the end of the process can be easily separated from the aqueous phase by applying an external magnetic field. This novel double-separation strategy with negligible leaching makes Mag-IL-Pd an eco-friendly and economical catalyst to perform this transformation.

Biogeochemical and environmental factors in Fe biomineralization: Magnetite and siderite formation

The formation of siderite and magnetite by Fe(III)-reducing bacteria may play an important role in C and Fe geochemistry in subsurface and ocean sediments. The objective of this study was to identify environmental factors that control the formation of siderite (FeCO3) and magnetite (Fe3O4) by Fe(III)-reducing bacteria. Psychrotolerant (<20°C), mesophilic (20–35°C) and thermophilic (>45°C) Fe(III)-reducing bacteria were used to examine the reduction of a poorly crystalline iron oxide, akaganeite (β-FeOOH), without a soluble electron shuttle, anthraquinone disulfuonate (AQDS), in the presence of N2, N2-CO2(80:20, V:V), H2 and H2-CO2 (80:20, V:V) headspace gases as well as in

Metal Reduction and Iron Biomineralization by a Psychrotolerant Fe(III)-Reducing Bacterium, Shewanella sp. Strain PV-4

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