Bugbuster - survivability of living bacteria upon shock compression

Shock recovery experiments were conducted on suspensions of 10(6)/ml E. coli bacteria contained in a water-based medium that is emplaced within stainless steel containers. The water is shocked and recovered. These experiments simulate the environment of bacteria residing either in surface bodies of water or in subsurface water-filled cracks in rocks. Early Earth life is likely to have existed in such environments. However, the E. coli are not believed to be representative of early life and are merely used here for initial experiments. Some 10(-2) to 10(-4) of the bacteria population survived initial (800 ns duration) shock pressures in water of 220 and 260 MPa. TEM images of shock recovered bacteria indicate cell wall rupture and delamination. This appears to be the mortality mechanism. The TEM images indicate cell wall indentations may be occurring as would be consistent with Rayleigh-Taylor or Richtmyer-Meshkov fluid instabilities. In the present case, we consider the experiments as representing three layers of fluids: (1) The waterbased medium, a stronger and possibly denser cell wall medium, and the interior of the cell cytoplasm. Variations of only 10-15% are expected in density. (2) A second mechanism that may cause cell wall failure is the multiple shock (nearly isentropic) compression freezing of liquid water medium into ice VI or ice VII high pressure phase that are 20% to 25% denser than the liquid. The decrease in volume associated with the transformation is expected to induce overpressures in the still liquid cell cytoplasm. Cell dynamic tensile wall strength thus appears to be a critical parameter from either of the above failure modes. Because the strain rate dependence of cell wall tensile strength is unstudied, we utilize the Grady and Lipkin [D.E. Grady, L. Lipkin, Criteria for impulsive rock fracture, Geophys. Res. Lett. 7 (1980) 255-258] model of tensile failure versus time scale (strain rate). Our single datum is fit to this law and we assume that at low strain rates, overpressures exceeding the cell Turgor pressure require on the order of similar to 103 s. This model which has been applied to brittle media and metals for describing failure may permit application of short duration laboratory experiments as in the present ones to infer responses of organisms to much lower shock pressures, but for longer time scales (100 to 103 s) of planetary impacts. Using the present data for E. coli and applying the Grady and Lipkin model, we find that a 1.5 km diameter impactor will cause mortality of bacteria within a radius of 10(2) km but upon stress related attenuation the subsurface bacteria outside of this radius should survive. (c) 2006 Published by Elsevier B.V.