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Project Summary

Metal oxides have been studied widely in the biogeochemical literature for understanding the adsorption and other surface interactions of dissolved organic and inorganic molecules with mineral surfaces. The goal of our study is to understand whether the earliest lipid membranes or “protocells” would have been stable in contact with different mineral surfaces on early Earth, and whether the surface properties of the minerals control their relative affinity to cell membranes. In previous years of this study, we used bulk adsorption isotherms and classical DLVO theory modeling approaches to examine the stability lipid bilayers in contact with micron-sized quartz (α-SiO2), rutile (α-TiO2) and corundum (α-Al2O3) particles. By understanding the role of natural geochemical parameters such as mineral surface chemistry, solution chemistry and temperature cycling on protocell membrane stability, we attempted to model potential aqueous environments where life may have originated such as lacustrine, tidal pool, and sub-aerial or submarine hydrothermal vents. In the present project year, we used neutron reflectivity to determine if the geometry of the mineral surface (sub-spherical particles versus planar single crystal surfaces) affects membrane stability. The results of our various approaches were consistent showing that lipid membrane stability depends on (1) lipid head-group charge and (2) surface charge of the mineral, which in turn depend on pH, ionic strength, presence or absence of Ca2+, (3) van der Waals interactions, and (4) relative hydrophobicity of the surface, as well as purely physical parameters such as relative size of the model membrane relative to the mineral surface. Our project addresses NASA Astrobiology Institute’s (NAI) Roadmap goals of understanding the origins of cellularity and the evolution of mechanisms for survival at environmental limits, and NASA’s Strategic Goal of advancing scientific knowledge of the origin and evolution of the Earth’s biosphere and the potential for life elsewhere.

Keywords: lipid, protocell, vesicle, self-assembly, pre-biotic, mineral surface, hydrophilic, hydrophobic, bilayer