Biomineralization by a Newly-Isolated Stalk-Forming Fe-oxidizing Bacterium: Towards Interpretation of Putative Fe Microfossils

Diverse aerobic, lithotrophic Fe-oxidizing bacteria (FeOB) produce distinctive extracellular Fe-rich filaments, which resemble putative Fe microfossils dating from recent to 1.7 Ga (Slack et al., 2007, EPSL: 243). The filament morphology, texture, and composition are promising biosignatures for these FeOB; however, somewhat similar morphologies have been shown to result from chemical precipitates. In order to accurately identify and interpret such biosignatures, morphology must described in detail and be linked to physiological function and growth conditions in extant organisms. Towards this goal, we aimed to isolate a novel, stalk-forming microaerophilic FeOB, since there exist few isolates. We successfully obtained a pure strain (named R-1) from a circumneutral, freshwater Fe seep in Christiana Creek, Newark, DE. This strain produces a twisted stalk, similar to Gallionella and Mariprofundus in morphology and in mineralogy. Our work shows that R-1 is a neutrophilic obligate FeOB, unable to oxidize other organic or inorganic substrates. It is a Beta-Proteobacterium in the Gallionellaceae family but is phylogenetically distinct from previously isolated Gallionella sp. and Sideroxydans sp. The closest cultured relative is S. lithotrophicus (97% similar) and the closest environmental clone is 98% similar. We have begun growing R-1 and the marine stalk-forming FeOB Mariprofundus ferrooxydans in microslide cultures, which allow direct microscope observation without disturbing growth. We are monitoring oxygen concentration gradients and FeOB response to oxygen levels. In order to link morphology to biological function and growth conditions, we will observe stalk formation under various conditions and document various morphological and textural parameter (e.g. branching and orientation) to establish criteria for biogenicity. No organisms are known to make stalks under anaerobic conditions, so if these structures are detected in the rock record, they could be used as signatures for oxygen. Robust morphological biosignatures for FeOB would provide a line of evidence for the presence and distribution of Fe oxidation metabolism, and its environmental settings, throughout Earth history.