Investigations on the Interplanetary Transfer of Microorganisms

Both of the potential ways in which organisms could be transferred from one planet to another; natural (on and in rocks) and artificial (on spacecraft) are of great interest to the PPARC mission. In the first instance, understanding the survival of microorganisms in space experiments allows us, for the first time, to empirically test Lord Kelvin's pioneering ideas on the transfer of life on rocks from one planet to another. In the second case, contamination of sample return or outbound spacecraft is of crucial interest to PPARC's involvement in Aurora, Rosetta and other space missions. Our recent work has shown that important fundamental science questions can be addressed during the course of these investigations. The work we propose here will continue our investigations aimed at understanding what dispersal filters (barriers) act on the transfer of microorganisms from one planet to another. We will take advanatge of four ESA space experiments - STONE - to simulate meteoritic infall and the effects of this process on microorganisms, BIOPAN - a 16 day space exposure experiment, and EXPOSE - two 1 year space exposure experiments. The experiments we propose to undertake focus on photosynthetic organisms for several reasons: 1) To be active, photosynthetic organisms must be near the surface of a rock to gather energy from sunlight. Frequently this habit is manifest as endolithic growth (discreet layers of organisms growing just under the rock surface). This makes them subject to ablation during atmospheric entry and thus using them yields insights into the various selection pressures ('dispersal filters') that act against different metabolic groups during transfer, 2) many of them have resting states that would allow some cells to survive buried deeper in the rock in a metabolically inactive state. These experiments help us to understand how these organisms might survive transfer and they can be used as a proxy for all other types of organisms that live buried deep in rocks, and 3) photosynthesis is the most productive form of metabolism, yielding the great diversity of life we see on Earth today. Uderstanding the potential of this mode of metabolism to be transferred between planets is of astrobiological interest because it would yield insights in to the ability of a star to host multiple productive biospheres. One potential criticism is that photosynthetic organisms would not be expected on spacecraft. However, these organisms can be used as model systems to understand the effects of space conditions on any cellular materials and the fate of biomolecules. Our work will be of interest to astronomers, planetary scientists and also biologists.