ZEOLITIC LIFEBOATS: AN ANALOGUE FOR INVESTIGATING LIFE ON MARS

Hypersaline -alkaline lake environments and their evaporite deposits have long been suggested as terrestrial analogues for studying life on Mars. Recent studies by Gibson et al (2003) and Bish et al (2003) suggest the presence of zeolites in Martian regolith. Trona-producing alkaline-saline lakes on Earth provide an aqueous environment with alkalinity capable of producing authigenic sedimentary zeolite minerals, such as erionite, by alteration of trachyte glass. These environments also support prolific populations of haloalkaliphilic archaea and bacteria.

Periodic precipitation – desiccation cycles result in dilution and evaporative concentration of pore waters in these systems, with pore fluid chemistry fluctuating in ion concentration and pH. Ultimately, evaporation to dryness results in alkaline carbonate mineral precipitation. Within these biologically harsh fluctuating environments, microbial archaea must maintain osmotic balance by adjusting their intracellular ion concentration through the accumulation and discharge of KCl.

Erionite consists of a three dimensional framework of (Si,Al)O₄ tetrahedra linked to form channel structures. Substitution of Al³⁺ for Si⁴⁺ results in a net negative charge that may be balanced by extra-framework cations such as Na⁺ or K⁺ held within the channel structures. Erionite has a strong preference for K⁺ along with H₂O molecules that also occupy the channel structures. Extra-framework cations and H₂O are commonly exchangeable. As evaporation and desiccation progress, microorganisms may associate with erionite to exploit the mobile K⁺ and H₂O, in a sense using the erionite as a lifeboat for survival.

To investigate this relationship, Natrialba magadii, a microbial archaea isolate from Lake Magadi in East Africa, was cultured in the laboratory in the presence and absence of erionite. Samples were evaporated to dryness under aerobic conditions and analyzed using SEM/EDS.

Analyses reveal preservation of microbes in samples containing erionite whereas no evidence of preservation is observed in the absence of erionite. Results indicate an interaction between these microbes and erionite that leads to preservation. This research suggests that zeolite bearing rocks should be targets for the exploration of life on future Mars sample return missions.