Lakes beneath Europe's largest glacier are warmed by geothermal heat. They host psychrophilic and thermophilic bacteria simultaneously, with temperature gradients creating distinct microbial zones.
Boaworm, CC BY-SA 3.0
A world beneath the ice
Underneath Vatnajökull, Europe's largest ice cap by volume, lies a hidden world of liquid water. Several active volcanic systems, including Grímsvötn and Bárðarbunga, sit directly beneath the ice. The constant geothermal heat they produce melts the base of the glacier, forming dozens of subglacial lakes insulated from the surface by hundreds of meters of ice.
The best-known of these are the Grímsvötn lakes, which lie within a volcanic caldera, and the two Skaftárkatlar lakes. The ice above the Skaftárkatlar is 350 to 400 meters thick. These lakes are not static; they grow as meltwater accumulates and then drain periodically in immense glacial outburst floods known as jökulhlaups. A major jökulhlaup from Grímsvötn in 1996 released water at a peak flow of 50,000 cubic meters per second, temporarily making it one of the largest rivers on Earth by volume. These powerful events are a fundamental force in shaping the Icelandic landscape, creating the vast sandur plains south of the glacier.
A paradox of life
The complete darkness, low nutrient levels, and immense pressure under the ice create an extreme environment. Yet, these lakes host unique microbial ecosystems. Scientists have found communities of bacteria that are specially adapted to the cold, dark conditions. These organisms are chemosynthetic, meaning they derive energy from chemical reactions involving sulfur and iron compounds released by the geothermal vents, rather than from sunlight. This process is a basis for life completely independent of the sun.
The most unusual feature is the coexistence of two opposite types of extremophiles. The bulk of the lake water is near freezing, ideal for psychrophiles (cold-loving microbes). Simultaneously, geothermal hotspots on the lakebed create zones of much warmer water, providing a habitat for thermophiles (heat-loving microbes). Studies of accretion ice from Lake Vostok in Antarctica—ice formed from the lake water freezing to the bottom of the glacier—have also revealed signatures of thermophilic bacteria, suggesting geothermal systems may be a common feature of subglacial lakes. The Vatnajökull lakes, with their sharp temperature gradients, show how these two distinct life strategies can operate in the same isolated system. The low-diversity bacterial communities found here are dominated by just a handful of taxa, including Sulfuricurvum and Acetobacterium. These subglacial ecosystems are used as analogs to study how life might exist in the sub-surface oceans of icy moons like Jupiter's Europa and Saturn's Enceladus.
💡Fun Facts
The Icelandic word *jökulhlaup*, meaning "glacier run," has been adopted internationally by scientists to describe any glacial outburst flood.
Some of the bacteria found in these lakes, like *Geobacter* and *Desulfuosporosinus* species, use sulfur and iron for anaerobic respiration.
During the massive 1996 *jökulhlaup*, the floodwaters transported 1,000-tonne blocks of ice and deposited enough sediment to extend the coastline by up to 800 meters in some areas.
Scientists use radar and seismic instruments from the ice surface to map the hidden lakes and monitor their water levels.
The national park is accessible 24/7, but activities on the glacier itself are weather-dependent and require guided tours.
Admission
The national park is free to enter, but guided tours for glacier hiking or visiting ice caves have associated costs.
Accessibility
The subglacial lakes are not accessible to the public. Visitors can experience the glacier through physically demanding guided tours like glacier walks and ice caving. Visitor centers like the one at Skaftafell are generally accessible.
