Abstract EANA2025-59

Enceladus and Europa are key targets for present and future space missions looking for habitable conditions (e.g., JUICE, Europa Clipper, and the future ESA L4 mission[1]). The surface of these moons experiences ionizing radiation[2][3], and therefore ionizing radiation-resistant extremophiles, capable of surviving similar conditions to those of these icy moons, should be studied. In this work, the bacteria Stenotrophomonas chelatiphaga, Flavobacterium frigidarium, Brevundimonas staleyi, and Janibacter limosus were selected due to their survivability at freezing temperatures, and the presence of pigments associated with radioresistance[4]. All these bacteria were sampled from sea ice prokaryotic communities in the subarctic[5]. Bacterial strains stored in glycerol stocks at -80ºC were cultivated, and suspensions of each isolate in physiologic serum were prepared at 106 CFU/mL to study its inactivation kinetics by gamma radiation. The samples were exposed to gamma radiation in a Co-60 experimental equipment at the doses of 0.1kGy and 0.17kGy. The surviving fraction were estimated by direct plating technique onto Tryptic Soya Agar (TSA) after 7 days of incubation at 25ºC. Several technical and biological replicates were performed, and a non-irradiated sample followed all the assays to control the inactivation response. The D10 values for each isolate, which represent the radiation dose required to inactivate 90% of the population, were derived. Our results showed that Flavobacterium was less resistant to gamma radiation (D10 0.032 kGy), followed by Stenotrophomonas (D10 0.059 kGy), Janibacter (D10 0.064 kGy), and Brevundimonas (D10 0.067 kGy). Our findings showed that the bacteria retrieved from ice (Brevundimonas and Janibacter), possess higher resistance than those retrieved from water (Stenotrophomonas and Flavobacterium). Janibacter showed a pigment darkening after one week of growth, suggesting a stress response, which could explain its second delayed growth phase that was not seen on the other bacteria. Ionizing radiation levels can vary widely depending on the scenario; based on reference values, underneath 10 cm of ice on Europa, the ionizing radiation dose would be 60.3 Gy/year[2]. On Enceladus, at the trailing hemisphere with a shielding of 10 cm of ice would receive 0.19 Gy/year[3]. At these rates, reaching the D10 dose of these bacteria would take between 194 to 405 days on the Europa with 10 cm of shielding, and 168 to 351 years on the trailing hemisphere of Enceladus with a 10 cm of ice shielding. Results from this study may help to more critically evaluate the potential of future mission to detect putative microbial life on Europa and Enceladus.

 

Acknowledgments

The authors are grateful to João Canário for providing the subarctic samples, and the Centre for Northern Studies for the support during fieldwork. CQE, iBB and CTN acknowledge the financial support of FCT.

 

References

[1] Martins Z. et al. (2024) https://cosmos.esa.int/documents/1866264/1866292/ESA_L4_Expert_Committee_report_Voyage_2050_Moons_of_the_Giant_Planets.pdf

[2] Nordheim T. A. et al. (2018) Nat. Astron. 2, 673–679.    

[3] Nordheim T. A. et al. (2017) Icarus 286, 56–68.

[4] D'Ischia M. et al. (2021) Phys. Life Rev. 37, 65–93.

[5] Coelho L. F. et al. (2022) Sci. Total Environ. 827, 154286.