Iron microbialites from an acidic Martian analog (Río Tinto, Spain)—Implications for finding life on Mars

The search for life on Mars requires identifying suitable biosignatures detectable by rover missions or in returned samples. To support the progressing astrobiological exploration of Mars, we investigated modern iron microbialites from Río Tinto, Spain, an acidic terrestrial analog to the Meridiani Planum. We used an integrative approach combining a suite of tools (Raman and Mössbauer spectroscopy, micro X-ray diffraction [µXRD], micro X-ray fluorescence [µXRF]) analogous to the instruments onboard various Mars rovers. Our analyses revealed that the ∼10-cm-thick microbialites exhibit alternating porous and dense layers of jarosite and Fe(III) (oxyhydr)oxides, locally common minerals on Mars. The estimated growth rate of the layered structures (1 mm per year) is very high considering the slow precipitation kinetics of the constituent minerals, inconsistent with solely abiotic precipitation. δ56Fe data for the different layers (−0.91‰ to −0.83‰) and calculated isotopic differences (Δ56Fe(III)solid-aq = −0.62‰ to −0.54‰) were consistent with both biotic and abiotic mineral precipitation. Scanning electron microscope−energy-dispersive X-ray spectroscopy (SEM-EDS) and DNA analyses confirmed the presence of microorganisms known for promoting iron mineral formation, including archaea (e.g., Thermoplasmata) and bacteria (e.g., Acidithiobacillus). Our study highlights three lines of biogenicity: (1) non-isopachous layered structures, (2) high growth rate, and (3) the presence of microorganisms that likely promote iron mineral precipitation. Problematically, growth rates and the presence of relevant microorganisms cannot be reliably inferred or detected, either in the geological record or by the rovers, while layered structures can also form abiotically. While highlighting such challenges in interpreting iron microbialites in modern systems, our study provides a robust framework for exploring these structures on ancient Earth. It also underscores the critical role of integrating high-confidence morphological and geochemical biosignatures—ideally from sample return missions—in the search for life on Mars.