Introduction: Using terrestrial extremophiles to inform Martian biosignature detection
Microbes from extreme Earth environments offer valuable testbeds for understanding biosignature production under conditions reminiscent of early Mars. Iron reduction is an ancient microbial metabolism that can influence mineralogy and volatile byproducts in ways that may be recognizable by current and future space instruments. In this study, a Shewanella sp. (strain BF02_Schw) isolated from Blood Falls in Antarctica was incubated with ferrihydrite (Fh) as the electron acceptor to simulate iron-reducing conditions relevant to lacustrine and sedimentary environments on Mars.
Methods: An analog system for Mars-relevant biosignatures
The Antarctic Shewanella was exposed to ferrihydrite under controlled laboratory conditions. Mineralogical changes were tracked over two time points (2 weeks and 8 weeks) using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). The goal was to identify biosignatures that could be detectable by instruments planned for Mars missions, including X-ray diffraction, infrared spectroscopy, Mössbauer spectroscopy, and Raman spectroscopy.
Live versus killed-control comparisons enabled attribution of observed features to biotic activity. Transcriptional changes between Fh-present and Fh-absent treatments were analyzed to distinguish true biotic responses from non-biological artifacts. In parallel, soluble and volatile metabolites were profiled to identify indicators of active metabolism that might accompany mineral transformations.
Results: Mineral transformations and potential biosignatures
Mineralogy of the Live FLT treatments revealed clear temporal evolution of mineral phases. After 2 weeks, peaks consistent with magnetite and ferrihydrite dominated, with minor goethite signals observed in both TEM images and SAED patterns. By 8 weeks, goethite features intensified, magnetite persisted, and peaks compatible with halite formation appeared, corroborated by TEM and SAED analyses. This progression demonstrates how iron-reducing bacteria sculpt mineral assemblages in ways that can, in principle, be detected by flight-ready instruments on Mars missions.
Electronic and spectroscopic biosignatures
In addition to mineralogical signals, the team identified biosignatures through multiple instruments and analyses. The suite included X-ray diffraction and infrared spectroscopy, with complementary Mössbauer and Raman measurements. The biotic origin of these signals was supported by transcriptional upregulation in live treatments compared to killed controls and to Fh-free conditions, confirming that observed mineralogical changes were tied to microbial activity.
Metabolic byproducts as contextual indicators
The experiment detected several soluble and volatile metabolites, notably riboflavin and dimethyl sulfide (DMS). These compounds are consistent with active, iron-reducing microbial processes and could serve as contextual astrobiological indicators when interpreted alongside mineralogical data. While no single biosignature provides definitive proof of life, a convergent suite of features—mineral phase transitions, characteristic spectroscopic signals, and metabolic byproducts—enhances the likelihood of correctly inferring biogenic activity in ancient Martian contexts.
Discussion: Implications for Mars exploration and biosignature science
Terrestrial extremophiles like the Antarctic Shewanella BF02_Schw offer a practical framework for validating biosignature production under iron-reducing conditions that are plausible on early Mars. The observed magnetite, goethite, and halite features, together with riboflavin and DMS production, illustrate how microbial processes could imprint detectable signals in mineral and volatile inventories. These findings support the design and interpretation of Mars mission instruments, informing sampling strategies and data analysis pipelines for detecting ancient or extant life on other worlds.
Conclusion: A robust analogue for targeting Martian biosignatures
Culture-based experiments with terrestrial extremophiles advance our understanding of biosignature production under iron-reducing conditions and help refine what future missions should measure. The multi-technique characterization of iron reduction by an Antarctic Shewanella demonstrates a viable analogue system for putative Martian biosignature identification, guiding instrument development and data interpretation as humanity searches for signs of life beyond Earth.
Implications for future research
Further studies can expand the range of analog conditions, explore additional microbial taxa, and integrate in-situ instrumentation simulations to optimize detection thresholds for Martian environments. By correlating mineralogical, spectroscopic, and metabolite data, researchers can establish more robust criteria for distinguishing biogenic signals from abiotic processes on Mars and other rocky planets.
