Metal Limiting Habitability in Enceladus? Availability of Trace Metals for Methanogenic Life in Hydrothermal Fluids

Enceladus' ocean could support methanogenic life in terms of the availability of chemical energy (H₂ and CO₂) and nutrients (N and P). However, excess energy and nutrients in the ocean raise the question of why they remain abundant if Enceladus is inhabited. Terrestrial methanogens require trace metals, such as Co, Ni, Cu, Zn, and Mo, for their enzyme activation; nevertheless, the availability of these trace metals is largely unknown in Enceladus' ocean. Here, we investigate concentrations of dissolved trace metals in Enceladus based on hydrothermal experiments and thermodynamic equilibrium calculations in order to understand the minerals that control their concentrations in water-rock interactions. Our results show that Ni and Co concentrations in hydrothermal fluids can be controlled by dissolution of a sulfide mineral, pentlandite, in chondritic rocks. In a pH range for Enceladus' ocean, our calculations show that hydrothermal environments would be the source of dissolved Ni and Co. Given a suggested range of water chemistry (pH and dissolved species) of Enceladus' ocean, Ni, Zn, and Mo concentrations in hydrothermal fluids would be comparable to the levels required for terrestrial methanogens. However, both Co and Cu concentrations would be depleted compared with the levels required for terrestrial methanogens. We suggest that if methanogenic life in Enceladus requires trace metals at the same levels as for terrestrial methanogens, the availability of Co and Cu could control the activity of methanogenesis, possibly leaving excess chemical energy and nutrients in the ocean.