The divide that unites us: Cell membranes as biomarkers of living organisms
Several space exploration missions currently underway for NASA, the European Space Agency (ESA) and other space agencies are seeking signs of life elsewhere in our cosmos, and Mars is a prime target. In addition to technical and analytical know-how, scientists must know what they are looking for – what is a sign of life even in a sample that is no longer living. Cell membranes or their remnants are excellent candidates. With the support of the Marie Skłodowska-Curie programme, MSCA, the NanoMembR project applied novel spectroscopic methods to characterise complex model membranes under Mars-like degradation conditions. Knowledge gained and techniques developed will soon be exploited on two space exploratory platforms currently under development by ESA.
What separates living organisms from inorganic molecules
Compartmentalisation is required for life. All life on Earth is based on cells that separate the interior from the surrounding environment with a semipermeable membrane composed of lipid molecules. That makes cell membranes great universal biomarkers of life as we know it. In addition, due to their bipolar nature with one end hydrophobic and the other hydrophilic, the lipids self-assemble – great for ‘creating’ life – into a bilayer in an aqueous environment (water being another ‘marker’ of life). Further, although only a few nanometres thick, membranes and membrane patches are very robust and can persist over long periods of time without degrading significantly, in contrast to DNA and other molecules.
Life and death in space
According to Andreas Elsaesser of the Free University of Berlin and MSCA Fellow: “The key objective of NanoMembR was to study how environmental influences mimicking, for example, Martian conditions determine degradation pathways in increasingly complex model membranes of various compositions.” NanoFTIR, nanoscale Fourier transform infrared spectroscopy, combines very high spatial resolution with the analytical power for nanoscale chemical identification. Scattering-type scanning near-field microscopy (s-SNOM) enables spectroscopic imaging with spatial resolution way below the diffraction limit. Using these techniques, Elsaesser was able not only to monitor the stability of membranes spectroscopically but also to study structural changes on the nanoscale. According to Elsaesser: “The stability of membranes is significantly affected by the molecular composition of the membranes and environmental factors. Oxygen in combination with ultraviolet radiation is a key driver for membrane degradation.” These results are currently in preparation for publication. Equally important, NanoMembR established nanoFTIR as a novel tool for membrane investigation and proved its application to natural as well as artificial membranes. The latter serendipitously opened the door to its use to analyse actual space samples.
Off to nature’s lab in the sky
The outcomes are being incorporated into two space exposure platforms currently under development by ESA. The ‘labs’, external to the International Space Station or as free-flying nanosatellite, combine the advantages of long-term exposure with near-real-time in situ monitoring. The Exocube platform will be part of ESA’s novel exobiology facility outside the International Space Station in low earth orbit. SpectroCube is a free-flying CubeSat-based miniaturised in situ space exposure platform in a highly elliptical orbit around Earth for astrochemistry and astrobiology research. Results from NanoMembR could help us identify search targets for life detection missions in our solar system and beyond.
Keywords
NanoMembR, life, space, membrane, ESA, degradation, nanoscale, in situ, cell,spectroscopic, orbit, nanoscale Fourier transform infrared spectroscopy (nanoFTIR), Mars, scattering-type scanning near-field microscopy (S-SNOM), ultraviolet