Chemolithoautotrophic bacteria flourish at dark water-ice interfaces of an emerged Arctic cold seep.

Below their ice shells, icy moons may offer a source of chemical energy that could support microbial life in the absence of light. In the Arctic, past and present glacial retreat leads to isostatic uplift of sediments through which cold and methane-saturated groundwater travels. This fluid reaches the surface and freezes as hill-shaped icings during winter, producing dark ice-water interfaces above water ponds containing chemical energy sources. In one such system characterized by elevated methane concentrations - the Lagoon Pingo in Adventdalen, Svalbard (~10 mg/L CH4, <0.3 mg/L O2, -0.25°C, pH 7.9), we studied amplicons of the bacterial and archaeal (microbial) 16S rRNA gene and transcripts in the water pond and overlaying ice. We show that active chemolithoautotrophic sulfur-oxidizing microorganisms (Sulfurimonas, Thiomicrorhabdus) dominated a niche at the bottom of the ice in contact with the anoxic water reservoir. There, the growing ice offers surfaces interfacing with water, and hosts favorable physico-chemical conditions for sulfide oxidation. Detection of anaerobic methanotrophs further suggests that the ice led to a steady-state dark and cold methane sink under the ice throughout winter, in two steps: first methane is oxidized to carbon dioxide and sulfates concomitantly reduced to sulfides by the activity of ANME-1a and SEEP-SRB1 consortia, in a second time energy from sulfides is used by sulfur- oxidizing microorganisms to fix carbon dioxide into organic carbon. Our results underline ice- covered and dark ecosystems as a hitherto overlooked oasis of microbial life and emphasize the need to study microbial communities in icy habitats.