Microbial growth in actual martian regolith in the form of Mars meteorite EETA79001

The study addresses whether actual martian regolith contains bioavailable nutrients and lacks bactericidal agents such that it can support microbial growth. Prior missions have shown Mars’ surface to be harsh due to radiation and oxidants (for example, oxychlorine species), making surface life unlikely, though terrestrial extremophiles can thrive in analogous extremes. Early Mars likely had thicker CO2 atmosphere and liquid water, and modern subsurface or microenvironments could satisfy habitability criteria (bio-nutrients, water, energy). Most prior microbiology-on-Mars studies relied on terrestrial simulants, which cannot fully capture martian chemistry. This work overcomes that limitation by using actual martian material: sawdust generated when cutting the Mars meteorite EETA79001, testing whether it supports growth of two cyanobacteria (Eucapsis sp., a Chroococcidiopsis isolate “Chr20-20201027-1”) and two bacteria (E. coli, the extremophile Planococcus halocryophilus) under terrestrial conditions across a range of regolith:water ratios. The EETA79001 composition and its similarity to in situ Phoenix Wet Chemistry Laboratory (WCL) results support its relevance as a proxy for martian regolith chemistry, including available ions such as K+, Na+, Mg2+, Ca2+, SO4 2−, Cl−, and lower perchlorate compared to martian soils at Phoenix.

• Multiple Mars missions reveal a surface with high radiation and oxidants (oxychlorine compounds), challenging surface habitability, yet Earth extremophiles tolerate such extremes.
• Laboratory studies using Mars regolith simulants (e.g., MGS, JSC, MMS) have tested microbial survival and growth, with mixed outcomes depending on species and conditions. Prior work showed Chroococcidiopsis-like isolates maintain growth better than some cyanobacteria in simulants, while Eucapsis often declines.
• Phoenix WCL analyses characterized martian regolith aqueous chemistry (notably SO4 2−, NO3 −, Mg2+, Ca2+, and perchlorate at the Phoenix site), providing a benchmark for assessing meteorite leachates.
• Perchlorate’s prevalence on Mars impacts microbial habitability; some halo- and perchlorate-tolerant microbes can grow in relevant brines.
• Actual martian meteorites are scarce and rarely available in sufficient uncontaminated quantities for growth assays. EETA79001 is relatively young and well-curated, and its sawdust offers an opportunity for direct testing of microbial growth on genuine martian material.
• Comparisons indicate EETA79001 leachate ion profiles resemble WCL regolith (dominant SO4 2−, NO3 −, Mg2+, Ca2+), though at generally lower concentrations; perchlorate appears lower than Phoenix-site averages and above typical terrestrial levels.

Material: EETA79001 sawdust generated during NASA-JSC cutting of the meteorite in a nitrogen cabinet with a diamond saw (no oil/water). Prior studies indicate minimal, understood contamination. The meteorite is basaltic/gabbroic with olivine megacrysts and shock glass veins; detailed leachate composition previously published (see Supplementary Table S1).

Microorganisms: Cyanobacteria: Eucapsis sp. and a Chroococcidiopsis isolate (Chr20-20201027-1) from the Atacama Desert. Bacteria: Escherichia coli (commercial strain) and Planococcus halocryophilus (gift culture). Selection rationale in Supplementary Note 1.

Regolith:water ratios and controls:

• Cyanobacteria: 4:1 (dry regolith control; inoculum only), 2:1, 1:2, 1:5, 1:10; plus 0:1 (water-only control).
• Bacteria: 4:1 (dry regolith control), 2:1, 1:1, 1:2, 1:5; plus 0:1 (water-only control).
• For all, 20 mg EETA79001 sawdust per vial; triplicates per condition and timepoint.

Cyanobacterial culturing and enumeration:

• Axenic stock cultures maintained in Alga-Gro media at ~23 °C under 16 h light/8 h dark (455–650 nm). Cells were washed (centrifuged and resuspended) into sterile DI water. OD750 adjusted to achieve ~10^5–10^6 cells mL−1 in inoculum.
• Inoculation: 5 µL inoculum added; regolith:water ratios established by adding appropriate DI water volumes (except 4:1 dry control).
• Enumeration by Most Probable Number (MPN): At scheduled days (e.g., 2, 6, 10, 18, 22), regolith-water mixtures were vortex-resuspended; serial 10-fold dilutions were made into 48/96-well plates containing Alga-Gro. Positive (green) vs negative (colorless) wells recorded across dilutions; MPN calculated per FDA BAM Appendix 2. Technical replicates in quadruplicate per dilution; positive/negative control wells included. MPN is suitable for low counts and particulate-containing samples.

Bacterial culturing and enumeration:

• E. coli and P. halocryophilus revived on TSA; 20 h cultures prepared at 25 °C. Cells suspended in DI water; OD600 adjusted to ~10^7–10^8 cells mL−1. Viable counts at day 0 established.
• Inoculation into 20 mg regolith with specified DI water volumes to reach 2:1, 1:1, 1:2, 1:5; 4:1 dry regolith control (inoculum only); 0:1 water-only control (5 µL inoculum into 1 mL DI water). Triplicates per ratio/timepoint.
• At each timepoint (up to 23 days), 1 mL DI water added to the vial, vortexed; serial 10-fold dilutions prepared. From each dilution, 50 µL plated on TSA. Plates with countable colonies used to compute CFU mL−1 by accounting for dilution factor and plated volume. Replicate plates averaged; standard deviations used for error bars.

General: All assays used sterile deionized water and standard microbiological equipment (autoclave, incubator, spectrophotometer). Figures illustrate appearance of regolith at different ratios and growth curves over time.

Data availability: All data in main text/Supplementary; raw data to be archived on Harvard Dataverse.

• Actual martian regolith material (EETA79001 sawdust) supported growth/survival of four microorganisms under terrestrial conditions for weeks across a range of regolith:water ratios.

Cyanobacteria (Eucapsis sp. and Chr20-20201027-1):

• Eucapsis: MPN decreased by about one order of magnitude from ~10^5 to ~10^4 cells mL−1 across ratios after initial exposure; over days 2–22, counts fluctuated between ~10^3–10^6 cells mL−1. More stable counts were observed at 1:5 and 1:2 ratios on several days (6, 10, 18), but overall a notable decline occurred by day 22 (average ~10^4 cells mL−1).
• Chr20 (Chroococcidiopsis isolate): After small decreases at days 2 and 6, it recovered and increased by up to ~1 order of magnitude, plateauing around ~10^5 cells mL−1. It survived and proliferated beyond initial counts across ratios, with some fluctuations at higher regolith ratios (2:1, 1:2). Multiple ratios yielded MPN values near the upper counting range, indicating regolith was beneficial.
• Cyanobacterial controls: Eucapsis showed minimal growth in both regolith-only (4:1) and water-only (0:1), dropping up to three orders of magnitude over the control period. Chr20 decreased in the regolith-only control until day 10, showed some growth by day 18, but exhibited no growth in water-only control.

Bacteria (E. coli and Planococcus halocryophilus):

• E. coli: CFU mL−1 decreased relative to day 0 viable counts. Initial growth was better at 1:1, 1:2, 1:5 than at 2:1. By day 4, 2:1 fell below the limit of quantification (LOQ); by day 10 only the 1:5 ratio still showed growth, remaining 1–2 orders of magnitude below initial levels thereafter. Controls: In water-only (0:1), E. coli reached a plateau by day 10 and maintained stable growth; in regolith-only (4:1) control, no growth was detected, suggesting water availability is more critical than regolith for E. coli survivability.

• P. halocryophilus: Displayed strong growth and survival across all regolith:water ratios. Even at 2:1, significant growth occurred between days 2–14 before dropping below LOQ; the 1:5 samples showed robust, continued growth through days 17–23. Regolith-only controls exhibited no continued growth.

• Geochemical relevance: EETA79001 leachate contains bioavailable nutrients (e.g., PO4 3−, K+, SO4 2−, Ca2+, Mg2+) and ion profiles broadly similar to Phoenix WCL results, lending confidence that the meteorite-derived regolith can represent martian regolith chemistry for growth assays.

• Overall inference: The regolith does not contain bactericidal agents at levels that prevent growth under the tested terrestrial conditions, and added water likely increases nutrient bioavailability.

The findings demonstrate that genuine martian regolith material can support microbial growth and survival for weeks under Earth-like conditions. Cyanobacteria, particularly the Chroococcidiopsis isolate, performed well across regolith:water ratios, while Eucapsis generally declined but persisted. Among bacteria, the extremophile Planococcus halocryophilus exhibited sustained growth, in contrast to E. coli, which required higher water availability and generally declined in regolith. These outcomes align with expectations from prior simulant studies, where Chroococcidiopsis-like strains outperform other cyanobacteria and extremophiles tolerate martian-like chemistries better than non-extremophiles.

The similarity of EETA79001 leachate ion profiles to those measured by Phoenix WCL suggests that the nutrients present are relevant to martian conditions, and their bioavailability is sufficient to support growth, especially when water is present. The absence of observed bactericidal effects indicates that regolith chemistry alone may not preclude microbial habitability in favorable niches (e.g., subsurface microenvironments) where liquid water and shielding from radiation exist. Moreover, the observation that E. coli can maintain growth in water-only controls but not in dry regolith underscores the primacy of water availability over mineral nutrient supply for some organisms.

Implications extend to planetary protection (Earth microbes could survive in martian materials under favorable conditions) and to astrobiology, supporting the hypothesis that past or present subsurface martian habitats could host microbial life. For human exploration, cyanobacteria’s ability to grow in martian regolith suggests potential for in-situ resource utilization (ISRU): oxygen production and provision of nutrients and biomass to support heterotrophic microbes and plants in bioregenerative life-support systems.

This study is the first to directly demonstrate microbial growth using actual martian regolith material (EETA79001 sawdust) as substrate. Across regolith:water ratios from 4:1 to 1:10 (cyanobacteria) and to 1:5 (bacteria), both a Chroococcidiopsis isolate and Planococcus halocryophilus showed sustained growth for up to 22–23 days, while Eucapsis declined and E. coli showed limited survival that depended strongly on water availability. The EETA79001-derived regolith provides bioavailable nutrients and lacks detectable bactericidal effects under terrestrial conditions. If broadly representative of martian regolith, these results imply that martian microbial life could have existed in the past and may persist today in suitable subsurface aqueous environments. Practically, cyanobacterial growth on regolith supports concepts for bio-sustainable human habitats on Mars via ISRU (oxygen generation, nutrient provisioning).

Future research directions include: testing a broader diversity of microorganisms; extending experiments to longer durations; evaluating growth under simulated martian conditions (low pressure, low temperature, relevant radiation/UV flux, CO2 atmosphere); assessing additional meteorites and regolith analogs to capture spatial variability; and quantifying nutrient bioavailability dynamics and potential inhibitory species such as perchlorate across different martian-like chemistries.

• Experiments were conducted under terrestrial conditions (temperature, pressure, atmosphere, and light), without martian radiation, UV flux, or low-pressure/low-temperature stresses.
• Short to moderate duration (up to 22–23 days) limits inference on long-term survival and adaptation.
• The meteorite sawdust, while well-curated, may not fully represent the heterogeneity of martian surface regolith; composition (including perchlorate) differs from some landing sites.
• Only four microorganisms were tested; results may not generalize across taxa.
• Sawdust particulates and past handling introduce potential, albeit minimal, contamination concerns.
• Growth assessments relied on MPN (cyanobacteria) and CFU (bacteria), which have inherent variability and detection limits; some timepoints fell below LOQ.
• Control conditions indicate water availability is a key factor; disentangling mineral nutrient effects from hydration effects requires additional controlled studies.