The signatures of intact polar lipids in pyrolysis as indicators of viable extraterrestrial life

Distinguishing between viable and deceased life is a crucial objective for both in situ life-detection missions and planetary protection protocols. On Earth, the presence of intact polar lipids (IPLs) has been used as an indicator for the presence of viable life. This is because many IPLs are rapidly degraded following death of the host organism (Harvey et al. , 1986). Typically, IPLs are detected and characterised using liquid-based extraction and chromatography techniques (e.g. Zink et al., 2003) that are not well-suited to extraterrestrial settings due to contamination concerns and difficulties with transporting solvents (Sephton et al., 2023).

Instead, in situ organic analyses are generally performed by raising the sample temperature to thermally release organic material for subsequent characterisation. This pyrolysis process causes thermochemical decomposition, potentially obscuring diagnostic organic compounds. To overcome this issue, laboratory experiments pyrolysing standard compounds of interest can be used to determine the pyrolytic signatures of diagnostic species, so that they can be recognised in the results of future extraterrestrial experiments (Royle et al., 2021).

In this project we aimed to establish the signature products of IPLs in pyrolysis-gas chromatography-mass spectrometry (pyrolysis-GC-MS) experiments, and compare these signatures to those of their post-mortem degradation products. Hence, products identified as being unique to the IPL could serve as markers of viable organisms when using pyrolysis techniques suitable for extraterrestrial environments. Natural samples, including a bacterial culture, were pyrolysed to evaluate the potential for these signatures to be discerned in complex organic assemblages.

We identified monoglycerides as a unique signature of ester-bound IPLs in pyrolysis. Monoglycerides are especially pertinent as they preserve the ester bond in the parent IPL which has been specifically shown to be susceptible to post-mortem enzymatic hydrolysis (Harvey et al., 1986).  Following this recognition, monoglycerides were successfully targeted and detected in the pyrolysis of the bacterial culture.

Ether-bound lipids typical of archaea exhibited less distinctive signatures. The resulting limitations will be outlined along with considerations prompted by the potential for extra-terrestrial biochemistry to vary beyond life as we know it. The potential confounding influence of inorganic minerals will also be explored.

This project has shown that the distinctive signatures of IPLs can be detected using in-situ organic analyses, expanding the capabilities of existing life-detection instrumentation. As a highly sensitive and relatively simple procedure, the use of pyrolysis-GC-MS to detect the signatures of viable life may also have application in planetary protection procedures aiming to minimise the risk of releasing harmful alien organisms into the terrestrial biosphere.

References:

• Harvey, H. R., Fallon, R. D., & Patton, J. S. (1986). The effect of organic matter and oxygen on the degradation of bacterial membrane lipids in marine sediments. Geochimica et Cosmochimica Acta, 50(5), 795-804.
• Royle, S. H., Tan, J. S., Watson, J. S., & Sephton, M. A. (2021). Pyrolysis of carboxylic acids in the presence of iron oxides: implications for life detection on missions to Mars. Astrobiology, 21(6), 673-691.
• Sephton, M. A., Tan, J. S., Watson, J. S., Hickman-Lewis, K., & Madariaga, J. M. (2023). Organic geochemistry of in situ thermal-based analyses on Mars: the importance and influence of minerals. Journal of the Geological Society, 180(5), jgs2022-152.
• Zink, K. G., Wilkes, H., Disko, U., Elvert, M., & Horsfield, B. (2003). Intact phospholipids—microbial “life markers” in marine deep subsurface sediments. Organic geochemistry, 34(6), 755-769.