Abstract:
Biogenic Iron (Oxyhydr)oxides (BIOS) are organo-minerals formed as a result of Fe(II) oxidation by Fe(II)-Oxidizing Bacteria (FeOB). The mineral part is typically poorly crystalline Fe(III) (oxy-hydr)oxides (e.g., ferrihydrite) and goethite, while the organic part is mainly polysaccharides moi-eties and cells. Their composition and precipitation vary depending on the metabolism of the FeOB. Nitrate-dependent FeOB show high degree of cell encrustation, whereas phototrophic FeOB have developed mechanisms to avoid encrustation because encrustation prevents the pho-ton uptake for photosynthesis. BIOS play an important role in the environment as the Fe(III) can be used as electron acceptor while the associated carbon can act as an electron donor and/or carbon source. Moreover, BIOS are potential biosignatures (e.g., microfossils or molecular). Here we present a general overview of FeOB and FeRB metabolisms, BIOS and their reactivity, with a focus on their use as biosignatures of past and present like on Earth, and in other Fe-rich bodies, such as Mars and icy moons (Chapter 2).
However, we acknowledge that little is known on the bioavailability of the Fe and C for Fe(III)-reducing bacteria, the effect of the BIOS source and the transformation products. Therefore, in this PhD thesis we chose three FeOB representatives of (a) phototrophic metabolism (Rhodop-seudomonas palustris TIE-1), (b) chemoautotrophy (enrichment culture KS) and (c) mixotrophy (Acidovorax sp. BoFeN1). We described the mineralogy and organic bonds of their BIOS and test their bioavailability for Fe(III) reduction and C oxidation using two Fe(III)-reducing bacteria (FeRB) and fermentative microbes. We used Geobacter sulfurreducens and Shewanella oneidensis MR-1, which are representatives of two different Fe(III) reduction mechanisms, being direct cell-min-eral contact and via electron shuttles, respectively. To test the C degradation we enriched fer-mentative microbes from sediments of Lake Constance using different carbon sources (yeast extract, autoclaved cells of S. oneidensis and formaldehyde fixed cells of S. oneidensis).
We found lepidocrocite and poorly crystalline Fe oxides to be the dominant minerals and carbo-hydrates and Fe-C bonds to be present in similar proportion in the three BIOS, although the total organic carbon was 2.21 wt%, 0.2 wt % and 1.29 wt % for TIE-1, KS and BoFeN1 respectively. Reduction experiments showed that the presence of available organic carbon was the main factor affecting the extent of reduction, while the reduction rate highly depended on the bacteria involved (both FeOB and FeRB). In the presence of an additional carbon source (acetate/lactate for G. sulfurreducens and S. oneidensis MR-1, respectively) the BIOS-Fe(III) were reduced to a 100%, although 3-10 times faster by G. sulfurreducens. Without additional C, the maximum extent of reduction was 50% and G. sulfurreducens reduced the Fe(III) 2-7 faster (Chapter 3).
The BIOS-Fe(III) reduction resulted in mineral transformation to vivianite in all cases. Interestingly, the reduction of the BIOS from BoFeN1, the FeOB with the highest degree of encrustation, re-sulted in cells being embedded in
