Abstract_EANA2026-82

Abstract EANA2026-82

How to detect bacteria growing at Enceladus’ ocean surface

Marie Dannenmann (1), David Burr (2,3), Mirandah Ackley (1), Karen Olsson-Francis (4) and Frank Postberg (1)

(1) Planetary Sciences and Remote Sensing, Institute of Geological Sciences, Freie Universität Berlin, Berlin, Germany. (2) Experimental Biophysics and Space Sciences, Department of Physics, Freie Universität Berlin, Berlin, Germany. (3) Institute for Biology - Microbiology, Freie Universität Berlin, Berlin, Germany (4) AstrobiologyOU, Faculty of Science, Technology, Engineering & Mathematics, The Open University, Milton Keynes, United Kingdom.

Detecting biosignatures of potential extraterrestrial life is a major goal of current space missions to icy moons. Enceladus likely harbors habitable conditions in its liquid subsurface ocean and has therefore been selected as the target of ESA’s next large-class mission (L4). Biosignatures of a putative biosphere in this ocean could be detected by impact ionization mass spectrometers (such as HIFI [1] with heritage from SUDA onboard Europa Clipper and CDA onboard Cassini). These instruments rely on fast impact speeds during flybys to analyze the composition of single ice grains that are ejected from the moon. At Enceladus, these particles form at the ocean surface and are delivered to space in form of icy jets driven by water vapour through fractures in the ice shield [2].

Enceladus’ ocean is thought to be cold (0-2°C), alkaline (pH 9-11) [3], and moderately saline (0.05-2 M) [2]. While hydrothermal environments could provide warmer temperatures and organics from chemosynthetic reactions at the ocean floor, the ocean surface environment also presents key advantages: (1) oxygen forms by radiolytic reactions on top of the icy crust and could be transported down through the ice to the ocean allowing potential aerobic metabolisms [4]. (2) Organic compounds likely accumulate in a hydrophobic layer on top of the ocean comparable to the surface microlayer on top Earth’s oceans (neuston) providing elevated concentrations of organic substrates for growth [5]. (3) This niche is located close to the site of ice grain formation and ejection. If putative lifeforms inhabit the ocean surface, those cells may predominantly serve as nucleation cores for the ice grains and dominate detectable biosignatures therein.

We chose a bacterial model organism, Rhodonellum psychrophilum, to (i) demonstrate growth under the assumed conditions at the ocean surface and (ii) investigate the detectability of its biosignatures in ice grains by impact ionization mass spectrometers. R. psychrophilum grew to 10^8 cells/mL at 3°C and pH 10 in an aerobic ocean simulant containing yeast extract at concentrations representing hydrocarbon concentrations in the neuston on Earth. We simulated impact ionization as employed by HiFi or SUDA with laser desorption coupled to time-of-flight mass spectrometry [6]. In cation and anion mass spectra (10^9 cells/mL, i.e., ≤1 cell per ice grain), we identified amino acids and fatty acids, respectively, in biotic abundance ratios. We could also assign more complex compounds, such as dipeptides or lipid fragments. Finally, in experiments employing different growth temperatures, we observed a higher relative abundance of unsaturated fatty acids with lower temperature, representing a cold-adaptation that may be typical for the ocean surface environment.

Our results help define biosignatures that can be expected in an icy moon environment and guide the interpretation of data obtained from impact ionization mass spectrometers on current (SUDA, Europa Clipper) and upcoming space missions such as L4.

[1] Kempf S. et al. (2025). EGU25-20361.

[2] Postberg, F. et al. (2009). Nature, 459(7250), 1098-1101.

[3] Glein C. R. and Truong N. (2025). Icarus, 441(15), 0019-1035.

[4] Ray C. et al. (2021). Icarus, 364, 114248.

[5] Postberg F. et al. (2018). Nature, 558(7711), 564-568.

[6] Klenner, F. et al. (2019). Rapid Commun. Mass Spectrom., 33(22), 1751-1760.