Fossil bacterial magnetite in deep-sea sediments from the South Atlantic Ocean
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Keywords:
Detritus
Natural remanent magnetization
Greigite
Magnetotactic Bacteria
Magnetosome
Sapropel
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Although nearly 60 mineral-like compounds are known to be synthesized biochemically by living organisms, only two (magnetite Fe{sub 3}O{sub 4}, and greigite Fe{sub 3}S{sub 4}) have ferromagnetic properties capable of providing a stable remanent magnetization to sediments. In particular, the magnetotactic bacteria are now known to provide a significant supply of fine-grained, single-domain magnetite to surface sediments in environments ranging from deep-sea sediments to terrestrial soils. In many such environments, including the deep-sea sediments shallow-water platform carbonates, and perhaps soils, the fossilized bacterial magnetosomes are clearly the source of the stable remanent magnetization (NRM) upon which paleomagnetic studies and magnetic polarity stratigraphies are based. The crystal structures of these bacterial magnetofossils, revealed through the use of high-resolution TEM techniques, confirms both their biological origin and documents some of the postdepositional dissolution and diagenetic effects to which they are subjected. Recent discoveries have revealed an amazing diversity of magnetite-precipitating micro-organisms, many of which produce up to several hundred times more fine-grained magnetite per cell than known previously. These organisms include some of the largest bacterial cells yet found, which are up to 15 micrometers in length and contain several thousand magnetite crystals in their magnetosome chains. The contribution of these organismsmore » to the magnetization of sediments is as yet unknown.« less
Magnetotactic Bacteria
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Greigite
Natural remanent magnetization
Rock magnetism
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Magnetosome
Magnetotactic Bacteria
Greigite
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Magnetotactic bacteria contain magnetosomes--intracellular, membrane-bounded, magnetic nanocrystals of magnetite (Fe(3)O(4)) or greigite (Fe(3)S(4))--that cause the bacteria to swim along geomagnetic field lines. We isolated a greigite-producing magnetotactic bacterium from a brackish spring in Death Valley National Park, California, USA, strain BW-1, that is able to biomineralize greigite and magnetite depending on culture conditions. A phylogenetic comparison of BW-1 and similar uncultured greigite- and/or magnetite-producing magnetotactic bacteria from freshwater to hypersaline habitats shows that these organisms represent a previously unknown group of sulfate-reducing bacteria in the Deltaproteobacteria. Genomic analysis of BW-1 reveals the presence of two different magnetosome gene clusters, suggesting that one may be responsible for greigite biomineralization and the other for magnetite.
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Sulfate-Reducing Bacteria
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Magnetotactic Bacteria
Greigite
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Magnetotactic Bacteria
Magnetosome
Environmental Magnetism
Rock magnetism
Natural remanent magnetization
Magnetism
Greigite
Single domain
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Summary For magnetic orientation, magnetotactic bacteria biosynthesize magnetosomes, which consist of membrane‐enveloped magnetic nanocrystals of either magnetite ( Fe 3 O 4 ) or greigite ( Fe 3 S 4 ). While magnetite formation is increasingly well understood, much less is known about the genetic control of greigite biomineralization. Recently, two related yet distinct sets of magnetosome genes were discovered in a cultivated magnetotactic deltaproteobacterium capable of synthesizing either magnetite or greigite, or both minerals. This led to the conclusion that greigite and magnetite magnetosomes are synthesized by separate biomineralization pathways. Although magnetosomes of both mineral types co‐occurred in uncultured multicellular magnetotactic prokaryotes ( MMP s), so far only one type of magnetosome genes could be identified in the available genome data. The MMP C andidatus M agnetomorum strain HK ‐1 from coastal tidal sand flats of the N orth S ea ( G ermany) was analysed by a targeted single‐cell approach. The draft genome assembly resulted in a size of 14.3 M b and an estimated completeness of 95%. In addition to genomic features consistent with a sulfate‐reducing lifestyle, we identified numerous genes putatively involved in magnetosome biosynthesis. Remarkably, most mam orthologues were present in two paralogous copies with highest similarity to either magnetite or greigite type magnetosome genes, supporting the ability to synthesize magnetite and greigite magnetosomes.
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Greigite
Magnetotactic Bacteria
Multicellular organism
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Magnetotactic bacteria contain chains of magnetically interacting crystals (magnetosomes), which aid navigation (magnetotaxis). To improve the efficiency of magnetotaxis, magnetosome crystals (which can consist of magnetite or greigite) should be magnetically stable single domain (SD) particles. Larger particles subdivide into nonuniform multidomain (MD) magnetic structures that produce weaker magnetic signals, while small SD particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic (SP) behavior. In this study, we determined the stable SD range as a function of grain elongation and interparticle separation for chains of identical greigite grains using fundamental parameters recently determined for greigite. Interactions significantly increase the stable SD range. For example, for cube‐shaped greigite grains the upper stable SD threshold size is increased from 107 nm for isolated grains to 204 nm for touching grains arranged in chains. The larger critical SD grain size for greigite means that, compared to magnetite magnetosomes, greigite magnetosomes can produce larger magnetic signals without the need for intergrain interactions.
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Greigite
Magnetotactic Bacteria
Superparamagnetism
Single domain
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