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Project Progress

Ferrous iron (Fe(II)) can serve as an electron donor for chemolithotrophic iron-oxidizing microorganisms under both oxic and anoxic conditions. It may also have played a role in past (and possibly, present) life on Mars, whose crust has a relatively rich content of Fe(II)-bearing silicate minerals (e.g. ultramafic basalt rocks) (McSween et al., 2009). Our initial goals were to determine whether an established chemolithoautotrophic Fe(II)-oxidizing, nitrate-reducing culture (Blothe & Roden, 2009 ; Straub et al., 1996 ; Weber et al., 2001) can grow by oxidation of Fe(II) in basalt glass. Previous and ongoing NAI-supported studies have demonstrated that this culture is capable of growing with insoluble Fe(II)-bearing phyllosilicate phases such as biotite, smectite, and illite. Our interest in the potential for microbial oxidation of Fe(II) in basalt glass stems from recent suggestions that near-surface hydrothermal venting may have occurred during past periods of active volcanic/tectonic activity on Mars (Neukum et al., 2010). Such activities could have produced basalt glass phases that might have served as energy sources for chemolithotrophic microbial activity.

Several experiments were conducted to determine whether the Fe(II)-oxidizing, nitrate-reducing culture can oxidize Fe(II) in basaltic glass. Previous and ongoing NAI-supported studies have demonstrated that this culture is capable of growing with insoluble Fe(II)-bearing phyllosilicate phases such as biotite, smectite, and illite. Our interest in the potential for microbial oxidation of Fe(II) in basalt glass stems from recent suggestions that near-surface hydrothermal venting may have occurred during past periods of active volcanic/tectonic activity on Mars. Such activities could have produced basalt glass phases that might have served as energy sources for chemolithotrophic microbial activity. Thus we sought to test whether the culture could oxidize the Fe(II) in basalt glass in a manner analogous to its ability to oxidize Fe(II) in insoluble Fe(II)-bearing phyllosilicate phases. The glass was obtained from an active lava flow from the Kilauea volcano in Hawaii.

As a shield volcano, Kilauea has long stood as a Mars analog. The culture was initially grown on clay-sized particles of crushed glass suspended in anaerobic liquid media with nitrate as the electron acceptor. The results indicate chemolithotrophic Fe(II) oxidation took place: the dilute HCl-extractable Fe(II) content of the mineral suspensions declined

over time in inoculated suspensions, whereas no significant Fe(II) loss took place in uninoculated controls. Near-term future work will involve X-ray diffraction and electron microscopic analysis of the Fe phases produced during microbial Fe(II) oxidation. Parallel analyses will be conducted on glass samples oxidized abiotically by H₂O₂ to determine whether enzymatic activity produces compositionally or morphologically unique end-products.

References Cited

Blothe M, Roden EE (2009) Composition and activity of an autotrophic Fe(II)-oxidizing, nitrate-reducing enrichment culture. Applied and Environmental Microbiology, 75, 6937–6940.
McSween HY, Taylor GJ, Wyatt MB (2009) Elemental composition of the Martian crust. Science, 324, 736-739.
Neukum G, Basilevsky AT, Kneissl T, Chapmam MG, van Gasselt S, Michael G, Jaumann R, Hoffmann H, Lanz JK (2010) The geologic evolution of Mars: Episodicity of resurfacing events and ages from cratering analysis of image data and correlation with radiometric ages of Martian meteorites. Earth Planet. Sci. Lett., 294, 204-222.
Straub KL, Benz M, Schink B, Widdel F (1996) Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. Applied and Environmental Microbiology, 62, 1458-1460.
Weber KA, Picardal FW, Roden EE (2001) Microbially-catalyzed nitrate-dependent oxidation of biogenic solid-phase Fe(II) compounds. Environmental Science & Technology, 35, 1644-1650.