Enceladus as a Potential Niche for Methanogens and Estimation of Its Biomass

Abstract

Enceladus is a potential target for future astrobiological missions. NASA’s Cassini spacecraft demonstrated that the Saturnian moon harbors a salty ocean beneath its icy crust and the existence and analysis of the plume suggest water–rock reactions, consistent with the possible presence of hydrothermal vents. Particularly, the plume analysis revealed the presence of molecular hydrogen, which may be used as an energy source by microorganisms ( e.g., methanogens). This could support the possibility that populations of methanogens could establish in such environments if they exist on Enceladus. We took a macroscale approximation using ecological niche modeling to evaluate whether conditions suitable for methanogenic archaea on Earth are expected in Enceladus. In addition, we employed a new approach for computing the biomass using the Monod growth model. The response curves for the environmental variables performed well statistically, indicating that simple correlative models may be used to approximate large-scale distributions of these genera on Earth. We found that the potential hydrothermal conditions on Enceladus fit within the macroscale conditions identified as suitable for methanogens on Earth, and estimated a concentration of 1010–1011 cells/cm3.

Observations of Elemental Composition of Enceladus Consistent with Generalized Models of Theoretical Ecosystems

“…Our model predicts ranges of steady-state biomass concentrations (Figure 1) given Enceladus conditions that match previous, independent estimates [52, 64, 69] and would be comparable to eutrophic environments on Earth [75] (see Extended Data Table 1). At the upper end of estimated P concentrations, we find ranges of estimates for higher growth efficiencies that match the estimates offered by [69] and [64] for life growing in hydrothermal fluid. Meanwhile, our low energy estimates match the biomass estimate of 10 5 cells/mL from [52].…”

“…Here we use multiple modeling approaches all centered around the meta-framework of considering the relative ratios of different elements in an environment as a biosignature. Our chemostat model, which leverages a few free parameters (energetic limits on maximum growth rate, washout rate - see Methods) to calculate steady state cell density as a function of steady state phosphorus concentration, predicted ranges of supportable cell densities comparable to those previously offered for the Enceladus ocean [52, 64, 69] at high phosphate concentrations, but at the low end of the range of phosphate concentrations suggested by Postberg et al [55], we predict lower supportable cell densities - potentially a signature of phosphate limitation at these low concentrations. We used genomic data on methanogenic Archaea to predict the macromolecular composition of an ecosystem comprised of these organisms, and found that our estimated ecosystem N:P ratio matches, within an order of magnitude, the ammonium:phosphate ratio implied by the maximum concentration from Postberg et al [55].…”

“…Previous geochemical modeling studies analyzing the potential rates of methanogenesis and metabolism of putative Enceladus life have suggested imperfect total conversion of hydrogen and carbon dioxide into methane, leaving a residual energy signature [26], while other modeling efforts have suggested that total cell counts in hydrothermal vent communities would higher than average population densities on Earth if all available substrates for metabolism were consumed [69]. However, it is possible that even in the low-energy circumstances of dark, anaerobic methanogenesis as the mode of primary production (compared to oxygenic photosynthesis on Earth [12]), resources to synthesize biomass may be limiting to life [40, 60].…”

“…Previous studies have offered selection criteria for Earth analogues of ocean worlds and other life detection candidates have centered around energetic parallels, such as light availability, temperature, carbon sources, and sources of chemical energy [14, 20, 42]. In particular, Earth environments such as deep subsurface sediments, the Lost City hydrothermal vent system, and cold gas hydrates [20, 42, 69]. Studies attempting to mirror Enceladus conditions to test the viability of terrestrial methanogens have also emphasized temperature, pressure, and relative supply of redox components as key variables [64, 68, 69].…”

“…We propose combining biogeochemical modeling with recent flight mass spectral data to include biomass composition in addition to metabolic energy flux in our suite of life detection approaches [1, 69]. Namely, we present: 1) chemostat models incorporating nutrients, 2) macromolecular allometric scaling for ecosystems with size distributions corresponding to putative terrestrial analogs, 3) generalized scaling of elemental ratios as a function of cellular physiology and ecosystem size-structure.…”