Zero-Valent Iron on Mars: An Alternative Energy Source for Methanogens Courtesy: NASA Brendon K. Chastain University of Arkansas Dept. of Biological Sciences Methanogens – Good Model for Possible Subsurface Life on Mars • Can grow in temperatures below 0 C (Reid et al., 2006) • Can tolerate exposure to extremely low pressures (Altheide and Kral, 2008) • Phylogenetically primitive *Mars in a laboratory! Æ Fe0 on Mars • Several large iron meteorites composed primarily of Fe0 found on Mars’ surface. (Schroder et al., 2008) • Stony-irons containing substantial Fe0 also found in what might be a strewn field. (Schroder et al., 2008) Courtesy: NASA/CORNELL Micrometeorite Delivery • Just like Earth, Mars is bombarded with meteoritic debris, much of it Fe0. – Substantially thinner atmosphere increases the amount reaching the surface • Flynn and McKay (1990) estimate as much as 29% of Martian soil from meteorites. – Up to 59,000 tons per year delivered to the surface Shock-reduction from Impacts • Impacts can lead to 1 shock-reduction of iron- bearing minerals resulting in nanophase Fe0. • Numerous studies confirm Fe0 in SNC meteorites. (e.g. Kurihara et al., 2009) • This has been mimicked in the laboratory using JSC Mars-1 regolith 1) TEM image of NWA 2737 olivine. Small, dark (electron-dense) spherules simulant. (Moroz et al., are consistent with α-iron kamacite [after 2009) Treiman et al., 2007] (Pieters et al., 2008) Fe0 Source Summary • Large meteorites deliver exogenous Fe0 • Micrometeorites deliver exogenous Fe0 • Large impactors generate and spread nanophase Fe0 due to shock-reduction of iron-bearing minerals • May lead to substantial buildup of Fe0 over time Fe0 Relationship to Methanogens • Protons in solution can react with Fe0 to produce H2, but the reaction is not thermodynamically favorable. • However, a sink for H2 can drive the reaction. • Daniels et al. (1987) show that methanogens in nutritive medium can utilize the H2. Daniels et al. (1987) This Study: •Fe0/any H+ in solution (energy) • Bicarbonate buffer/CO2 (carbon/water) • Montmorillonite clay (micronutrients) • Does methanogenic metabolism take place in these Mars-relevant conditions? *Why montmorillonite? 14 30 M. formicicum M. barkeri Transfer #1 12 Montmorillonite Montmorillonite 25 Transfer #1 10 Transfer #2 JSC Mars-1 JSC Mars-1 20 8 JSC Mars-2 JSC Mars-2 15 Transfer #2 6 Jarosite Jarosite Methane (% vol.) Methane Methane (%vol.) 10 4 Nontronite Nontronite 2 Buffer-only 5 Buffer-only 0 0 1 3 5 7 9 1113151719212325272931 1 3 5 7 9 1113151719212325272931 Time (weeks) Time (weeks) 90 M. wolfeii 80 Montmorillonite Montmorillonite 70 Transfer #1 JSC Mars-1 60 clay can supply 50 Transfer #2 JSC Mars-2 the micronutrient 40 Jarosite Methane (% vol.) (% Methane 30 Nontronite requirements of 20 Buffer-only 10 methanogens. 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 Time (weeks) 0 Results: Fe vs. no H2 40 35 30 25 Feº-included 20 Control 15 Methane Concentration (% vol.) (% Concentration Methane 10 5 0 0 102030405060708090100 Time (days) Obvious oxidation to Fe3+ *Is the darker color of the montmorillonite an indication of Fe(OH)2? Comparison to Previous Work 40 30 Initial study before transfer 20 Initial study after transfer Feº-included Methane Concentration (% vol.) Concentration (% Methane 10 0 020406080100 Time (days) Concluding Remarks •Fe0 is available on Mars •CO2 is available on Mars • Montmorillonite-like clays are available on Mars • Given the right environmental conditions, these three materials can support methanogenic metabolism • The temperature was not exactly Mars- relevant, BUT….