Microbial Isolates from Enceladus Medium 042a
Evidence from the Cassini mission confirmed that Saturn's moon Enceladus hosts a global subsurface ocean. Modeling of plume derived particles and gases suggests that this ocean is alkaline to hyperalkaline, influenced by serpentinization, and capable of sustaining redox disequilibria that could support microbial metabolisms. With currently available geochemical data from Enceladus' plume, we used thermodynamic modeling to develop a solute composition consistent with one possible composition of Enceladus' ocean. We then used reasonable assumptions about energy sources that may be available to microorganisms on Enceladus to convert this solute composition to a microbial growth medium. To explore the potential adaptations microorganisms may have to putative Enceladus ocean conditions, the medium was inoculated with samples from diverse ocean world analog environments on Earth, including subseafloor crustal fluids and cave biofilms. The microorganisms capable of persisting in this geochemically bounded medium were heterotrophs with low carbon requirements and unexpectedly broad substrate utilization profiles. Although acetate was supplied to approximate plausible abiotic carbon sources on Enceladus, genomic evidence indicated isolates can metabolize a wide spectrum of organic compounds, indicating that most of the organisms enriched under these conditions were generalists rather than specialists. Genomic and physiological analyses further showed the presence of stress-response pathways, sodium-based bioenergetic systems, osmoregulation strategies, and other adaptations consistent with survival in alkaline, low-nutrient settings. These results suggest that metabolically versatile, stress-tolerant microorganisms may serve as useful model organisms for life in Enceladus' subsurface ocean and demonstrate the value of geochemically modeled media as a framework for constraining habitability, identifying relevant biosignatures, and probing potential microbial survival strategies beyond Earth.