Analogue experiments for the detection of bacterial biosignatures in ice grains relevant to ocean worlds

The identification of biosignatures on extraterrestrial ocean worlds is key to the search for life on these bodies. Saturn’s icy moon Enceladus, and possibly Jupiter’s moon Europa, eject plumes containing gas and ice grains formed from subsurface salty water into space [1,2,3,4]. The emitted ice grains can be analyzed by impact ionization mass spectrometers, such as the Cosmic Dust Analyzer (CDA) onboard the earlier Cassini-Huygens mission [5] or the Surface Dust Analyzer (SUDA) onboard the upcoming Europa Clipper mission [6] rendering the ocean’s composition accessible for analysis during spacecraft flybys [7]. CDA data collected in the Saturnian system revealed that Enceladus’ ocean is salty [8] and contains a variety of organic material, including complex macromolecules [9] and low mass volatile compounds [10]. These low mass compounds can potentially act as amino acid precursors and could be capable of interacting within or near Enceladus’ hydrothermal vent systems [11], or Enceladus’ porous rocky core [12]. These findings enhance Enceladus’ relevance as a habitable environment potentially able to support microbial life in its subsurface ocean. However, biosignatures have not yet been identified in extraterrestrial environments.

The Laser Induced Liquid Beam Ion Desorption (LILBID) experiment is proven to accurately reproduce impact ionization mass spectra of ice grains recorded in space [13]. Previous analogue experiments using this setup have shown that amino acids, fatty acids and peptides could successfully be detected in simulated ocean world scenarios [14,15]. The next step is to investigate whether the characteristic mass spectral fingerprints of bacterial life forms, if enclosed in ice grains, could also be detected and identified by SUDA-type instruments.

We therefore conducted high sensitivity LILBID experiments on extracts of two bacteria, Sphingopyxis alaskensis (S.alaskensis) and Escherichia coli (E.coli), to simulate their characteristic spectral features in cationic and anionic impact ionization mass spectra. E. coli is a well-studied model bacterium, and S. alaskensis is a model oligotroph, abundant in marine environments. The predictable, small size of S. alaskensis (<0.1 µm³) and its ability to utilize low concentrations of nutrients [16] makes it a suitable analogue bacterium for potential lifeforms on extraterrestrial oceans. Laboratory spectra of extracted bacterial DNA, lipids and the corresponding aqueous phases produced during their extraction - potentially containing polar molecules - were performed. To simulate the salty Enceladean or Europan ocean water, the extracts have been investigated in background solutions with increasingly high NaCl concentrations.

In the mass spectra, we observe mass lines identified as microbial fragments, corresponding to fatty acids derived from the bacteria’s membrane lipids. In the S. alaskensis and E. coli DNA mass spectra we also identify nucleobases and compounds produced from the phosphate deoxyribose backbone. The recorded mass spectra are stored in a growing database of laboratory LILBID spectra (Klenner et al., in prep.), designed to aid in the interpretation of results from earlier missions, such as Cassini, as well as help planning future missions to icy ocean worlds in the Solar System, such as Europa Clipper [17].

References

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