Jarosite Biomineralization under a CO2-Rich, O2-Limited Atmosphere: Potential Microbial Contributions to Martian Sulfate Deposits

Abstract

Jarosite-group minerals are ubiquitously observed in Martian sulfate-bearing deposits and serve as diagnostic proxies for past acidic aqueous environments. Proposed formation mechanisms include evaporation of acidic saline waters, magmatic vapor alteration, subglacial weathering, and microbially mediated Fe2+ oxidation. Among these, microbial formation has drawn astrobiological interest. To assess the effects of Martian atmospheric composition, we constructed a laboratory atmospheric simulation system and examined the Fe2+ oxidation and mineralization capacity of the acidophilic iron-oxidizing bacterium Acidithiobacillus ferrooxidans under O2-depleted, CO2-rich conditions. Biomineralization experiments were performed across four CO2/O2 gas treatments, with initial dissolved O2 (DO) saturation values of 14.6%, 8.0%, 3.7%, and 1.4%, respectively. Fe2+ oxidation and jarosite precipitation were markedly delayed in the two lower-DO-saturation treatments, whereas biomineralization occurred in all inoculated treatments. XRD, SEM, EDS, and FTIR analyses revealed time-dependent mineralogical and compositional changes in the jarosite-group precipitates, with NH4-related features becoming more apparent during later mineralization. Collectively, these findings demonstrate that microbial jarosite formation is viable under CO2-rich, O2-limited aqueous conditions, thereby lending empirical support to the potential for biological contributions to Martian jarosite deposits. The observed mineralogical and compositional features are not uniquely diagnostic of a biological origin, making the products difficult to distinguish from abiotic counterparts and suggesting that reliable identification of biosignatures in Martian jarosite will require converging evidence from multiple independent lines of investigation.