Change of magnetic mineralogy related to groundwater fluctuation and hydrocarbon contamination
Final Report Abstract
Due to the abundance of iron in soils and sediments, iron cycling between Fe(III)-reducing and Fe(II)- oxidizing bacteria is a widespread and important process, affecting not only carbon, nitrogen and phosphate biogeochemical cycles but also influencing the formation and dissolution of iron minerals. At the hydrocarbon-contaminated field site in Hradcany, which shows an increased magnetic susceptibility due to former fluctuations of the ground water table, we were able to isolate a new strain of Geothrix fermentans. Strain HradG1 is now the second published strain from the genus Geothrix and is capable not only to live under microoxic conditions but also to reduce ferrihydrite to magnetite, possibly contributing directly to the measured magnetic properties. Our data collected from the field site shows that Geothrix and other Fe(III)-reducing bacteria are probably the main drivers for magnetite formation at this site. Magnetite is a strong magnetic mineral, containing both Fe(II) and Fe(III). We could show that the iron in magnetite is accessible for both Fe(II)-oxidizing and Fe(III)- reducing bacteria, changing the Fe(II)/Fe(III) ratio over time and simultaneously changing the magnetic properties of the magnetite particles. We observed iron cycling in magnetite between an phototrophic Fe(II)-oxidizer and an Fe(III)-reducer in a co-culture, indicating the potential of magnetite to store and accept electrons as a “biogeobattery”, which underlines the importance of this mineral in the bacterial iron cycle in the environment. Additionally, our experiments with different nitrate-dependent Fe(II)-oxidizers and also denitrifying bacteria indicate that abiotic side effects caused by the biologically produced nitrite might contribute significantly to the overall observed Fe(II) oxidation in cultures and thus also to magnetite formation. To avoid artifacts during sampling, we developed a revised ferrozine assay using sulfamic acid instead of HCl to avoid chemical Fe(II) oxidation by nitrite, caused by rapid decomposition of the nitrite at acidic pH and the formation of strong oxidants such as NO and NO2. Furthermore, we showed that cell encrustation can be commonly observed in different nitrate-reducing bacteria growing at high Fe(II) concentrations. This shows that abiotic side effects caused by nitrite are an important factor in Fe(II)- oxidizing cultures and questions the enzymatic ability of nitrate-dependent Fe(II)-oxidizers. Overall our data suggest that iron-metabolizing bacteria strongly influence the geochemical and also magnetic properties in the environment by forming and changing magnetic minerals and also by producing reactive nitrogen species.
Publications
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(2011). Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations and Fe(II):Fe(total) ratio. Geochimica Cosmichimia Acta, 75, 6844–6858
Piepenbrock, A., Dippon, U., Porsch, K., Appel, E., Kappler, A.
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(2012) Potential function of added minerals as nucleation sites and effect of humic substances on mineral formation by the nitratereducing Fe(II)-oxidizing strain Acidovorax sp. BoFeN1. Environmental Science and Technology, 46, 6556−6565
Dippon, U., Pantke, C., Porsch.K., Larese-Casanova, P., Kappler, A.
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(2013). Abiotic oxidation of Fe(II) by reactive nitrogen species in cultures of the nitrate-reducing Fe(II) oxidizer Acidovorax sp. BoFeN1 – questioning the existence of enzymatic Fe(II) oxidation. Geobiology 11(2): 180-190
Klueglein, N., Kappler, A.
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(2013). Magnetite formation by the novel Fe(III)-reducing Geothrix fermentans strain HradG1 isolated from a hydrocarbon-contaminated sediment with increased magnetic susceptibility. Geomicrobiology Journal 30(10): 863-873
Klueglein, N., Loesekann-Behrens, T., Obst, M., Behrens, S., Appel, E., Kappler, A.
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(2014). Potential role of nitrite for abiotic Fe(II) oxidation and cell encrustation during nitrate reduction by denitrifying bacteria. Applied and Environmental Microbiology, 80(3): 1051-1061
Klueglein, N., Zeitvogel, F., Stierhof, Y. D., Floetenmeyer, M., Konhauser, K. O., Kappler, A., Obst, M.
