A sealed ecosystem under the ice
Lake Untersee is a freshwater lake in the Gruber Mountains of Antarctica, permanently sealed from the atmosphere by an ice cover 2 to 4 meters thick. This immense ice lid creates a stable, isolated environment, shielding the waters from Antarctica's fierce winds. The lake, covering 11.4 square kilometers and reaching a maximum depth of nearly 170 meters, is isolated. Its water chemistry is extreme. The upper water column is highly alkaline, with a pH that can range from 9.8 to 12.1, comparable to household cleaning products. This high pH is partly due to carbon dioxide being stripped from the water by the photosynthetic microbes below.
The water column itself is ultra-oligotrophic, meaning it is very low in nutrients. However, it is supersaturated with oxygen, reaching 150% of normal levels, a byproduct of the life thriving on the lakebed. At the southern end of the lake, a separate, deeper basin is completely anoxic, or oxygen-free. This anoxic basin contains some of the highest concentrations of methane found in any lake on Earth, produced by microbes in the sediment.
Living fossils on the lake floor
The notable features of Lake Untersee are the microbial structures carpeting its floor. Discovered in 2008 by a team led by geobiologist Dale Andersen, these structures are modern versions of stromatolites, some of the earliest fossil evidence of life on Earth. They appear in several forms, including flat mats and small, centimeter-scale pinnacles. Most impressively, the lake hosts large, conical stromatolites that grow up to 70 centimeters tall, a size and shape not seen in any other modern environment.
These structures are built layer by layer by communities of cyanobacteria, primarily species of Phormidium and Leptolyngbya. The microbes photosynthesize, trapping fine clay and sediment to form thin laminations, each between 0.2 and 0.8 millimeters thick. Carbon dating suggests these layers accumulate over near-decadal timescales. The only animals in this ecosystem are microscopic invertebrates like rotifers, nematodes, and tardigrades, which do not disturb the slow-growing microbial architecture.
A portal to other worlds
The unique conditions of Lake Untersee make it a valuable natural laboratory for astrobiologists. The combination of a perennially ice-covered body of water, extreme chemistry, and a simple, microbial-dominated ecosystem provides a compelling analogue for environments that may have existed on early Earth, before oxygen became plentiful. The stromatolites show what microbial landscapes might have looked like more than 3 billion years ago.
Scientists also use the lake to model conditions on other worlds. The anoxic, methane-rich basin is is an analogue for the subsurface ocean of Enceladus, Saturn's icy moon. The challenges of studying life sealed under thick ice in Antarctica help researchers develop strategies and technologies for future missions to search for life on Mars or the ice-covered moons of Jupiter and Saturn. Research at Lake Untersee tests robotics, telepresence, and remote operations that could one day be deployed millions of miles from Earth.