Exobiology and Evolutionary Biology

Survival, Growth, and Evolution of Terrestrial Bacteria in Martian Environments (2)

PI: Nicholson Wayne

Background: A central issue in Astrobiology concerns the origin, evolution and distribution of life in our solar system, and Mars has been a focus of searches for evidence of potentially habitable environments, past and present. Planetary surveys indicate that life, if it exists on Mars, is almost certainly microbial. A number of lines of evidence indicate that surface rocks have been exchanged between Earth and Mars by impact processes, stimulating interest in the possibility that microbial passengers could have hitchhiked between the planets during planetary evolution, particularly at an early stage when the impact rate was much higher and the environments of Earth and Mars were more similar. Interchange of microbes between Earth and Mars can still be envisioned to occur naturally as a consequence of impacts, but also more recently as a result of human planetary exploration activities. In either case, the considerations for understanding transfer of microbial life are the same. The probability of a microbial population surviving interplanetary transfer is a function of: (i) the initial population size and composition; survival of (ii) launch, (iii) transit through space, (iv) entry and
deposition; and (v) ability to survive, grow, and adapt to the new environment of the target planet. Recent evidence indicating that liquid water may be present on Mars in the near subsurface, and has occasionally emerged onto the surface within the past decade, has rekindled interest in the existence of habitable environments located near the martian surface.
Our past NASA-funded work has concentrated on survival of terrestrial bacteria and their ability to function after interplanetary transfer and deposition into the Mars environment. Our recent Preliminary Data indicate that the low-pressure (1-10 mbar) CO2-rich atmosphere at the Mars surface poses a significant barrier to survival and growth of terrestrial bacteria. However, the potential for terrestrial bacteria to adapt and evolve the ability to proliferate at low pressure on Mars has not to date been well constrained.
The Central Hypothesis to be tested is that terrestrial bacteria can evolve the ability to survive and proliferate under low pressure atmosphere characteristic of the surface of Mars. We will test this hypothesis using Mars atmospheric simulations and microbial community samples obtained from extreme “Mars analog” Earth environments. Specific target species to be compared in these studies will be the spacecraft contaminant Bacillus pumilus SAFR-032 and the closely-related and well-characterized model bacterium for Astrobiology studies, Bacillus subtilis.
Objectives:
1. Using simulation chambers with atmospheric composition and pressures approaching the Mars surface and both culture and culture-independent techniques, define the selective low-pressure limits for growth of model bacteria and communities from extreme Earth (“Mars analog”) environments.
2. Cultivate selected model and environmental bacterial isolates from Objective 1 for multiple generations under simulations approaching the defined Mars low-pressure limits, and select for fitter variants.
3. Characterize the changes leading to increased fitness under martian conditions.
Perceived Impact of Proposed Work: The proposed study relates directly to the NASA ROSES 2007 goals of Astrobiology: Exobiology and Evolutionary Biology (C.18) Early Evolution of Life and the Biosphere (vi). The proposed study will investigate the evolutionary adaptation of spacecraft-relevant microbial species to the Mars environment. The study will directly address questions relevant to the NASA goals of: the search for life; the extreme limits of life; planetary protection; and interplanetary transport of life. The study capitalizes on the combined strengths of planetary sciences, genomics, and microbiology, and represents an ideal opportunity for training young scientists in Astrobiology.