Microbial Role in Environmental Magnetism

Abstract:

Laboratory and field investigations have documented transient magnetic susceptibility related to microbial iron cycling. It is desirable to advance our knowledge about the complex interactions between microorganisms and magnetite to reach our goal of using geophysical methods such as magnetic susceptibility measurements to foster our understanding of the biogeochemical cycling of iron. This contribution to the panel discussion will focus on the ability of different microorganisms to bio-synthesize and to both reduce and oxidize the mixed valent mineral magnetite (Fe²⁺Fe³⁺₂O₄) and how these abilities can influence geophysical measurements. One prominent example is the magnetotactic bacteria, which synthesize magnetite inside their cells, orienting the bacterial cells in Earth’s magnetic field and helping the bacteria to find the ideal redox conditions in the oxic-anoxic transition zone. Another example for magnetite synthesis is the iron-reducing bacteria. When iron-reducing bacteria such as Geobacter use solid iron (ferrihydrite) as terminal electron-acceptor, magnetite precipitates extracellularly. On the other hand, strains of the iron-reducing Shewanella putrefaciens bacteria have been found to be able to reduce Fe(III) in magnetite resulting in magnetite’s dissolution. Also iron-oxidizing bacteria can interact with magnetite, oxidizing Fe(II) from the magnetite. Therefore, magnetite can serve as a “biogeochemical battery”, being alternatively used as electron donor and acceptor. Moreover, magnetite enhances microbial electron exchange during “electric syntrophy”, which occurs between bacteria that ferment organic compounds and relay electrons to methanogenic Archaea. The conductive properties of magnetite are probably also the reason for the enhanced microbial degradation of recalcitrant compounds and increased methane production when magnetite nanoparticles are added to anaerobic sludge digesters.