The survival and evolution of bacteria in planetary impacts

Current geological evidence places the emergence of life on Earth between 4.1-3.4 Gya, a period notable for its high frequency of hypervelocity impacts by minor bodies left behind from planet formation. These impacts generate extreme shock temperatures and pressures on the scale of thousands of Kelvin and tens of gigapascals that endure for microseconds or less. This raises the question of whether impacts and the associated shock conditions may have influenced the origin and evolution of life by acting as a sufficient selection pressure to generate heritable phenotypic changes among surviving organisms.

To investigate this, planetary impacts were simulated using a two-stage light gas gun capable of accelerating projectiles to velocities in excess of 7 km s-1. Previous work using various methodologies and bacterial species have consistently observed a very small surviving population, with attempts to quantify the survival rate varying in accuracy. This thesis builds on previous work by designing an experimental approach that subjects bacterial samples to impact, quantifies post-impact survival rates, and investigates whether shock conditions influence evolutionary trajectories. The lengthy and complex design and development process identified several essential considerations for work of this nature, including the minimisation of contamination, the ability to retrieve the entire impacted sample in a recoverable state, and the use of appropriate materials to fully transfer the shock conditions to the entire sample.

The finalised method involved a nylon sabot containing an agar suspension of bacterial cells impacting a liquid target. Experiments using the Gram-negative Escherichia coli and the Gram-positive Priestia megaterium and Rhodococcus erythropolis revealed typical survival rates of 0.0001-0.1% following exposure to pressures of 0.6-31 GPa and temperatures of 308-1227 K. Survival rates varied significantly, particularly between the Gram-positive species. R. erythropolis exhibited greater resistance, potentially due to mycolic acids on the cell wall bearing the brunt of the shock and decreasing the frequency of cell lysis. P. megaterium showed less resistance, potentially due to the long thin rod-shaped cell morphology and low surface-area-to-volume ratio increasing the exposure of cells to shock conditions. These results were supported by sonication experiments, where the same bacterial species were subjected to pressures and temperatures of lower magnitudes for longer durations than in impacts.

Screening for auxotrophs post-impact yielded no evidence of heritable phenotypic change, with the short duration of the shock conditions potentially the limiting factor. It is instead hypothesised that planetary impacts may have influenced evolution on a populational level, with advantageous phenotypes increasing in frequency in post-impact environments and passed on to future generations and species, while disadvantageous phenotypes decrease in frequency and are eventually lost.