Preservation of Extracellular Sheaths Produced by Iron-Oxidizing Bacteria: An Analog for Potential Morphological Biosignatures on Mars

In the search for life on Mars, evaluating the biogenicity of morphological structures may be important, as they can provide a primary independent line of evidence for past life and can be used to target areas to focus further analyses. However, our experience with terrestrial materials indicates that the deleterious effects of diagenetic processes regularly make the assessment, and even detection, of microfossils and other microscopic biosignatures challenging. To improve our understanding of these effects on Mars, we collected samples that contained sheath-shaped extracellular structures produced by iron-oxidizing bacteria (FeOB) from a Mars analog circumneutral iron deposit and subjected them to artificial maturation by hydrous pyrolysis. Simulated diagenesis induced a phase change in the mineralogy of the structures, from ferrihydrite to crystalline iron oxides. We found that conditions associated with the onset of this phase change were correlated with the start of significant degradation of the extracellular structures. Our results reveal the sensitivity of remains of FeOB to diagenesis, which provides insights for improved targeting of astrobiological missions to areas on Mars that are most conducive to morphological biosignature preservation. Additionally, these results compel increased scrutiny of FeOB-like purported biosignatures if their mineralogy is dominated by crystalline iron oxides.