Bibliography - M. Davidson

1. Smith, D., A. C. Schuerger, M. Davidson, Stephen W. Pacala, C. Bakermans, and T. C. Onstott, 2009: Survivability of Psychrobacter Cryohalolentis K5 Under Simulated Martian Surface Conditions. Astrobiology, 9(2), doi:10.1089/ast.2007.0231 221-228
   [ Abstract ]

   Spacecraft launched to Mars can retain viable terrestrial microorganisms on board that may survive the interplanetary transit. Such biota might compromise the search for life beyond Earth if capable of propagating on Mars. The current study explored the survivability of Psychrobacter cryohalolentis K5, a psychrotolerant microorganism obtained from a Siberian permafrost cryopeg, under simulated martian surface conditions of high ultraviolet irradiation, high desiccation, low temperature, and low atmospheric pressure. First, a desiccation experiment compared the survival of P. cryohalolentis cells embedded, or not embedded, within a medium/salt matrix (MSM) maintained at 25°C for 24 h within a laminar flow hood. Results indicate that the presence of the MSM enhanced survival of the bacterial cells by 1 to 3 orders of magnitude. Second, tests were conducted in a Mars Simulation Chamber to determine the UV tolerance of the microorganism. No viable vegetative cells of P. cryohalolentis were detected after 8 h of exposure to Mars-normal conditions of 4.55 W/m2 UVC irradiation (200–280 nm), −12.5°C, 7.1 mbar, and a Mars gas mix composed of CO2 (95.3%), N2 (2.7%), Ar (1.6%), O2 (0.2%), and H2O (0.03%). Third, an experiment was conducted within the Mars chamber in which total atmospheric opacities were simulated at ô = 0.1 (dust-free CO2 atmosphere at 7.1 mbar), 0.5 (normal clear sky with 0.4 = dust opacity and 0.1 = CO2-only opacity), and 3.5 (global dust storm) to determine the survivability of P. cryohalolentis to partially shielded UVC radiation. The survivability of the bacterium increased with the level of UVC attenuation, though population levels still declined several orders of magnitude compared to UVC-absent controls over an 8 h exposure period.

2. Onstott, T. C., Li-Hung Lin, M. Davidson, B. Mislowack, M. Borcsik, J. Hall, G. Slater, J. Ward, B. S. Lollar, J. Lippmann-Pipke, E. Boice, and L. Pratt, et al., 2006: The origin and age of biogeochemical trends in deep fracture water of the Witwatersrand Basin, South Africa. Geomicrobiology Journal, 12(6), doi:10.1080/01490450600875688 369-414
   [ Abstract ]

   Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH>sub>4 and hydrocarbon, occasionally N₂, originally formed at ∼250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H₂, ⁴He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼10 Ka to >1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO₂ and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past∼100 Myr, a process which inoculates previously sterile environments at depths >2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10⁷ times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bio available chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The ⁴He concentrations and theoretical calculations indicate that the H₂ that is sustaining the subsurface microbial communities (e.g. H₂-utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1nMyr⁻¹. Microbial CH₄ mixes with abiogenicCH₄ to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from∼100 at < 1 kmbls, to <0.01nMyr⁻¹ at >3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ¹³C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.

3. Kieft, T. L., S. M. McCuddy, T. C. Onstott, M. Davidson, Li-Hung Lin, B. Mislowack, L. Pratt, E. Boice, B. S. Lollar, J. Lippmann-Pipke, S. M. Pfiffner, and T. J. Phelps, et al., 2005: Geochemically Generated, Energy-Rich Substrates and Indigenous Microorganisms in Deep, Ancient Groundwater. Geomicrobiology Journal, 22(6), doi:10.1080/01490450500184876 325-355
   [ Abstract ]

   Recent studies have shown that the biosphere extends to depths that exceed 3 km, raising questions regarding the age of the microbes in these deep ecosystems and their sources of energy for metabolism. Abiogenic energy sources that are derived from in situ, purely geochemical sources and thus independent from photosynthesis have been suggested.We sampled saline fracture water emanating from a 3.1-km deep borehole in a Au mine in the Witwatersrand Basin of South Africa and characterized the chemical constituents (including stable isotopes), groundwater age, and indigenous microorganisms. Salinity data and ratios of dissolved noble gases indicate that extremely ancient (2.0 Ga) saline fracture water has mixed with meteoric water to yield an average subsurface residence time of 20–160 Ma, the oldest age of any waters collected to date in the Witwatersrand Basin. H₂ isotope data suggest the water originated from a depth of 4 to 5 km. Sulfur isotope fractionation indicates biological sulfate reduction. Calculations of free energies and steady state energy fluxes based on water chemistry data also support sulfate reduction as the dominant terminal electron accepting process. Lipid and flow cytometry data indicate a sparse microbial community (103 cells ml⁻¹), despite the presence of relatively high concentrations of energy-rich compounds (H₂, CH₄, CO, ethane, propane, butane, and acetate). The H₂ can be explained by radiolysis of water. Stable isotopic signatures of the CH₄ and short chain hydrocarbons indicate abiogenic synthesis. The persistence of energy-rich compounds suggests that other factors are limiting to microbial metabolism and growth, e.g., availability of an inorganic nutrient, such as Fe or phosphate.

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