Microbial life at −13 °C in the brine of an ice-sealed Antarctic lake

Abstract

The permanent ice cover of Lake Vida (Antarctica) encapsulates an extreme cryogenic brine ecosystem (−13°C; salinity, 200). This aphotic ecosystem is anoxic and consists of a slightly acidic (pH 6.2) sodium chloride-dominated brine. Expeditions in 2005 and 2010 were conducted to investigate the biogeochemistry of Lake Vida's brine system. A phylogenetically diverse and metabolically active Bacteria dominated microbial assemblage was observed in the brine. These bacteria live under very high levels of reduced metals, ammonia, molecular hydrogen (H 2 ), and dissolved organic carbon, as well as high concentrations of oxidized species of nitrogen (i.e., supersaturated nitrous oxide and ∼1 mmol·L −1 nitrate) and sulfur (as sulfate). The existence of this system, with active biota, and a suite of reduced as well as oxidized compounds, is unusual given the millennial scale of its isolation from external sources of energy. The geochemistry of the brine suggests that abiotic brine-rock reactions may occur in this system and that the rich sources of dissolved electron acceptors prevent sulfate reduction and methanogenesis from being energetically favorable. The discovery of this ecosystem and the in situ biotic and abiotic processes occurring at low temperature provides a tractable system to study habitability of isolated terrestrial cryoenvironments (e.g., permafrost cryopegs and subglacial ecosystems), and is a potential analog for habitats on other icy worlds where water-rock reactions may cooccur with saline deposits and subsurface oceans.astrobiology | geomicrobiology | microbial ecology | extreme environment T he observation of microbes surviving and growing in a variety of icy systems on Earth has expanded our understanding of how life pervades, functions, and persists under challenging conditions (e.g., refs. 1-3). Studies of the physical characteristics, the geochemical properties, and microbes in ice (triple point junctions, brine channels, gas bubbles) have also changed our perceptions of the environments that may contain traces of, or even sustain, life beyond Earth [e.g., Mars (4), Europa (5), and Enceladus (6)].Solute depression of ice crystal formation or solar radiation melting of water ice are key processes that provide liquid waterthe key solvent that makes life possible-within icy systems. Microbial communities in these conditions are often sustained by a supply of energy that ultimately derives from photosynthesis (present or past). The understanding of ecosystems based on energy sources other than the Sun comes mainly from realms where hydrothermal processes have provided reduced compounds necessary to fuel chemosynthetically driven ecosystems. Methane derived from thermogenic or biogenic sources can also support microbial communities in deep sea (7) and high arctic cold saline seeps (8). More recently, discoveries of life and associated processes in deep terrestrial subsurface ecosystems (9) provide compelling evidence of subsurface life that in some cases is fueled by nonphotosynthetic processes. Ou...

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