Iron reduction as a viable metabolic pathway in Enceladus' ocean

Roche, Matthew J.; Fox-Powell, Mark G.; Hamp, Rachael E. and Byrne, James M. (2023). Iron reduction as a viable metabolic pathway in Enceladus' ocean. International Journal of Astrobiology, 22(5) pp. 539–558.

DOI: https://doi.org/10.1017/S1473550423000125

Abstract

Recent studies postulated the viability of a suite of metabolic pathways in Enceladus’ ocean motivated by the detection of H₂ and CO₂ in the plumes – evidence for available free energy for methanogenesis driven by hydrothermal activity at the moon's seafloor. However, these have not yet been explored in detail. Here, a range of experiments were performed to investigate whether microbial iron reduction could be a viable metabolic pathway in the ocean by iron-reducing bacteria such as Geobacter sulfurreducens. This study has three main outcomes: (i) the successful reduction of a number of crystalline Fe(III)-bearing minerals predicted to be present at Enceladus was shown to take place to differing extents using acetate as an electron donor; (ii) substantial bacterial growth in a simulated Enceladus ocean medium was demonstrated using acetate and H_2(g) separately as electron donors; (iii) microbial iron reduction of ferrihydrite was shown to partially occur at pH 9, the currently accepted value for Enceladus’ ocean, whilst being severely hindered at the ambient ocean temperature of 0°. This study proposes the possibilities for biogeochemical iron cycling in Enceladus’ ocean, suggesting that a strain of iron-reducing bacteria can effectively function under Enceladus-like conditions.