Microbial pre-conditioning of perchlorate-bearing Martian regolith simulant: Geochemical evolution toward a plant-compatible substrate

Sustained human presence on Mars depends on reliable in situ food production. However, Martian regolith contains perchlorate (ClO₄⁻) salts that inhibit plant and microbial growth. The Plant Trek project tests whether perchlorate-reducing microbial consortia can pre-condition perchlorate-bearing regolith, initiate early soil-forming processes, and improve substrate suitability. A perchlorate-reducing microbial community sourced from hypersaline sediments in Utah was introduced into the Plant Trek Mars Mix (PTMM). PTMM is a basaltic, phyllosilicate- and sulfate-bearing regolith simulant amended to ~2.25 wt.% Ca- and Mg-perchlorate. The material was incubated under aerobic and anaerobic conditions, reducing water-extractable perchlorate to approximately 7–9% of the initial amended Day 0 proxy. This paper emphasizes Stage I geochemical outcomes, with Stage II included only for substrate-development indicators such as pH and organic matter. Plant-available major cations (Ca, Mg, K) remained high, and toxic metals were largely absent from the exchangeable fraction. Aerobic conditioning produced a pronounced increase in plant-available phosphorus (222.2 ± 24.16 mg kg⁻¹). In contrast, anaerobic samples remained near zero, indicating that oxygen-dependent microbial processes strongly enhanced P availability in PTMM. Measurable total carbon and nitrogen were already established after Stage I, averaging ~0.557 wt.% C and ~0.00770 wt.% N across aerobic and anaerobic treatments. The pH declined modestly (~0.2–0.4 units) after 45 days of plant growth in Stage II. Together, these observations indicate the persistence of an early substrate-forming reservoir within an initially organic-free mineral matrix. Collectively, these results show that microbial pre-conditioning can couple perchlorate remediation with nutrient mobilization and early chemical and biological substrate development.