[M-IS17] Aqua planetology

[MIS17-04] Growth and molecular selection due to freeze-thaw cycles: Artificial evolution experiments simulating icy ocean worlds

Kazumu Kaneko¹, Asako Narita¹, *Yasuhito Sekine¹, Tomoaki Matsuura¹ (1.Earth-Life Science Insitute, Tokyo Institute of Technology)

Keywords:icy satellites, vesicle, synthetic biology

Geologically–active icy satellites, such as Europa and Enceladus, are known to have subsurface oceans under the icy crust. These icy ocean worlds have attracted attention due to the potential habitability. Knowledge on thermodynamic stability of functional organic compounds under the subsurface ocean environments is critical to predict potential life signatures in future missions. Cell membranes can isolate interior materials from external environments and, thereby, are thought to be a necessary function for life on Earth and beyond. The composition and structure of cell membranes of Earth’s life may have evolved from primitive membrane structure, called vesicles, depending on environmental changes. A major environmental change in the subsurface ocean is freeze–thaw cycles at the interface between the ocean and overlying icy crust. Although vesicle growth and division are known to occur thorough freeze–thaw cycling, the conditions for vesicle growth and their effects on size distributions are poorly investigated. In addition, physico–chemical properties of vesicles could change depending on types of amphiphiles, such as length and degree of unsaturation of the carbon chain. No experimental study has been done to investigate the compositional changes of vesicles through freeze–thaw cycles.
Here, we investigate changes in both morphology and composition of vesicles upon freeze–thaw cycles possibly occurred at the ice–ocean interface of icy satellites. We first prepared two types of vesicles: One was made of eggPC composed of multiple types of phospholipids and the other was made of PLPC(34:2–16:0/18:2) and POPC(34:1–16:0/18:1). Freeze–thaw cycling was then conducted with two different methods concerning freezing rate: flash freezing and slow freezing.
We found that the size of vesicles increased from ~100 nm to up to 2000 nm for both flash freezing of solutions with highly–concentrated vesicles and slow freezing of solutions with moderately–concentrated vesicles, suggesting that slow freezing at the ice-ocean interface of icy satellite can concentrate vesicles, if exist, in oceanic water for growth. We also found that, in grown large vesicles, a fraction of POPC(34:1–16:0/18:1) decreased both in flash and slow freezing. During slow freezing, this decrease was accompanied by an increase in PLPC(34:2–16:0/18:2). This suggests that membrane properties with unsaturated hydrophobic groups, such as POPC(34:1–16:0/18:1), in vesicles may be preferred for growth in freeze–thaw cycles due to their high fluidity. Our results suggest that freeze–thawing may be an important environmental pressure to promote the size and compositional evolution of membranes in the subsurface ocean of icy satellites.