Survival strategies of an anoxic microbial ecosystem in Lake Untersee, a potential analog for Enceladus

Abstract

Lake Untersee located in Eastern Antarctica, is a perennially ice-covered lake. At the bottom of its southern basin lies 20 m of anoxic, methane rich, stratified water, making it a good analog for Enceladus, a moon of Saturn. Here we present the first metagenomic study of this basin and detail the community composition and functional potential of the microbial communities at 92 m, 99 m depths and within the anoxic sediment. A diverse and well-populated microbial community was found, presenting the potential for Enceladus to have a diverse and abundant community. We also explored methanogenesis, sulfur metabolism, and nitrogen metabolism, given the potential presence of these compounds on Enceladus. We found an abundance of these pathways offering a variety of metabolic strategies. Additionally, the extreme conditions of the anoxic basin make it optimal for testing spaceflight technology and life detection methods for future Enceladus exploration.

Figure 1

An Overview of Lake Untersee’s Anoxic Basin. (A) Satellite view of Lake Untersee and a cross-section of the bathymetry of the lake with the location of the anoxic trough in the South basin. (B) Dissolved methane concentrations within the South basin. (C) Dissolved oxygen concentrations and temperature within the South basin. (D) pH and conductivity in the South basin. Image credit: satellite imagery (©Maxar) provided by NextView License, depth profiles modified from Wand et al., 2006 and Dale Andersen, unpublished data.

Figure 2

(A) View of a cross section of Enceladus and the ocean of Enceladus. (A) Enceladus ocean and icy crust. (B) Cross section of Enceladus’s icy crust, ocean and core. Image credit: illustration by N. Y. Wagner, drawing upon schematic renderings from NASA/JPL-Caltech and the Southwest Research Institute.

Figure 3

The taxonomic composition of the communities at the phyla level. Classified organisms in the figure only include phyla that make up more than 1% of the community.

Figure 4

An overview of the methanogenesis pathways. (A) Shows the methanogenesis pathway abundances in the phyla. (B) Indicates the methanogenesis pathway abundances in each sample. Since only 44% of the reads mapped back to the UF99 sample, not much information can be inferred from the data belonging to this sample. Here, we view methane metabolism from carbon dioxide to be the main pathway present in Euryarchaea (the only known anoxic methanogens). Methanogenesis from acetate is another pathway seen abundantly in non-methanogens. This could indicate acetate as a source of energy in this anoxic environment.

Figure 5

An overview of the nitrogen metabolism pathways. (A) Shows the nitrogen metabolism pathway abundances in the phyla. (B) Shows the nitrogen metabolism pathways abundance in each sample. Since only 44% of the reads mapped back to the UF99 sample, not much information can be inferred from the data belonging to this sample. Nitrate reduction, the most abundant nitrogen metabolism reaction, can couple with methane oxidation and facilitate the usage of methane as an energy source.

Figure 6

An overview of the sulfur metabolism pathways. (A) Shows the sulfur metabolism pathway abundances in the phyla. (B) Shows the sulfur metabolism pathways abundance in each sample. Since only 44% of the reads mapped back to the UF99 sample, not much information can be inferred from the data belonging to this sample. Sulfate reduction, the most abundant sulfur metabolism reaction, can couple with methane oxidation and facilitate the usage of methane as an energy source.