Towards Determining Biosignature Retention in Icy World Plumes

Abstract

With the discovery of the persistent jets of water being ejected to space from Enceladus, an understanding of the effect of the space environment on potential organisms and biosignatures in them is necessary for planning life detection missions. We experimentally determine the survivability of microbial cells in liquid medium when ejected into vacuum. Epifluorescence microscopy, using a lipid stain, and SEM imaging were used to interrogate the cellular integrity of E. coli after ejected through a pressurized nozzle into a vacuum chamber. The experimental samples showed a 94% decrease in visible intact E. coli cells but showed a fluorescence residue in the shape of the sublimated droplets that indicated the presence of lipids. The differences in the experimental conditions versus those expected on Enceladus should not change the analog value because the process a sample would undergo when ejected into space was representative. E. coli was selected for testing although other cell types could vary physiologically which would affect their response to a vacuum environment. More testing is needed to determine the dynamic range in concentration of cells expected to survive the plume environment. However, these results suggest that lipids may be directly detectable evidence of life in icy world plumes.

Putative Methanogenic Biosphere in Enceladus's Deep Ocean: Biomass, Productivity, and Implications for Detection

“…Although our predictions of cell density in the ocean plume are more often than not above detectable levels, a number of processes could affect the density of cells in the space plume material relative to cell density in the ocean plume. First, a significant amount of cells ejected in the plume could be destroyed owing to depressurization (Bywaters et al 2020 report a potential cell destruction rate of 94% in depressurization experiments). Due to the linear relation between the concentration factor and the required sample volume shown in Figure 4, such an approximately 10-fold decrease in the intact cell abundance in the plume translates into an approximately 10-fold increase in the required minimal sample volume to about 1 mL compared to the case where abundance of cells in the plume directly reflects cell abundance in the ocean plume above the hydrothermal vent.…”

Science Objectives for Flagship-Class Mission Concepts for the Search for Evidence of Life at Enceladus

“…Bywaters et al ( 2020 ), for example, show that a few percent of Escherichia coli survive transport through a pressurized nozzle into vacuum, and Cosciotti et al ( 2019 ) documented the survivability of cyanobacteria in low temperatures and salinities expected for Ocean Worlds. For the purposes of this study, we chose to avoid the requirement of a viable, motile cell by targeting only morphology and autofluorescence (Bhartia et al , 2010 ; Hand et al , 2017 ), though future work should investigate which dyes might be included to conduct a more encompassing fluorescence investigation.…”

The ETNA mission concept: Assessing the habitability of an active ocean world

“…Thus, in combination with chemical measurements of plume materials (Objective 2A), ETNA would also search for visual biomarkers (Simoneit, 2002(Simoneit, , 2004. All living cells possess a membrane made of lipids that aid in the exchange of material, communication, energy conservation, and protection from thermal and mechanical stresses, and these structures can be visually identified (Stoeckenius, 1962;Bretscher, 1985;Watteau and Villemin, 2018) and experimental work suggests that the morphological integrity of cells could be preserved during plume ejection (Bywaters et al, 2020). While the detection of cell-like structures would not offer conclusive evidence of life (Schopf, 1993;McKay et al, 1996), such a detection in combination with various chemical biosignatures would present a more robust characterization.…”