Biomineralization (Pt 2): Fe Hydroxide, Magnetite, Mn Oxides, Clays, Amorphous Silica | GEO GIRL

Microorganisms can contribute to mineral formation in 2 ways: biologically induced biomineralization or biologically controlled biomineralization. This video goes through examples of biominerals formed by induced biomineralization. Microbes passively facilitate mineral formation by providing reactive cell surfaces as nucleation sites for mineral precipitation and produce metabolites or compounds that react with other constituents in the water to precipitate minerals. Iron hydroxides can form biologically by Fe oxidizing bacteria, which use oxygen to oxidize ferrous Fe(III) to ferric Fe(II). Sometimes even dead bacteria can induce Fe hydroxide formation by providing charged cell surfaces on which Fe hydroxide can adsorb and precipitate. Such Fe hydroxide phases often convert into more stable phases, such as goethite (FeOOH) & hematite (Fe2O3), which typically occur abiotically but can result in mineral encrusted cells. Magnetite (Fe3O4) can also form by biologically induced mineralization. Magnetite contains both ferrous and ferric iron. Studying magnetite producing organisms is important for determining whether the biological magnetism signature in rocks is different from abiotic magnetic sediment as this may serve as the basis for biosignature development & help with the search for life on other worlds as well as on ancient Earth. Interestingly, biologically induced magnetite is nano-sized and nano-magnetite is not magnetic like larger magnetite crystals. Manganese oxides form in similar environments to Fe oxides and hydroxides. In fact, Fe and Mn often coprecipitate in ferromanganese deposits. Mn nodules grow by accretion in deep sea environments. But ferromanganese deposition can also occur on land in desert varnish (flat & shiny black coating on rocks in desert environments). Most of these Mn oxide deposits are formed by biological induced biomineralization. The formation of clay minerals can also be biolgically induced, either by increasing silicate weathering rates, raising or decreasing pH, providing charge cell surfaces for clay precipiation, or creating microenvironments in which conditions are favorable for clay precipitation. Biofilms likely contribute to forming such microenvironments that induce mineral formation. Such bio-induced clays are typically amorphous in contrast to their structured abiotic forms. Amorphous silica can form in a similar way, typically in hot springs and geyser vents which has led to the name geyserite for this mineral. The silica adsorbs to reactive cell surfaces and eventually encrusts the cell, this can lead to entire colonies cemented together in a large siliceous matrix. Some microbes can use biofilms and sheaths as a protective barrier against silica precipitation so that they don't get enfulfed by such minerals and die. Hope you enjoy!

References: Geomicrobiology: https://amzn.to/2WAloTR
Biology of Microorganisms: https://amzn.to/3kQqIuD

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0:00 Induced biomineralization recap
1:36 Induced biominerals
1:56 Fe hydroxide
9:12 Fe oxides: magnetite
13:11 Mn oxides
14:59 Ferromanganese deposits
16:18 Desert varnish
17:25 Clays
22:07 Amorphous silica
25:15 Upcoming content!
26:25 bloopers!

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