by Sergio E. Morales, Christina Hulbe, Clara Martínez-Pérez, Federico Baltar, The Conversation
Antarctica represents one of the last frontiers of discovery on Earth. We focus on what lies beneath the Ross Ice Shelf, Antarctica’s huge wedge of floating ice that harbors the southernmost extent of the Southern Ocean.
This ice-covered cavity contains an ocean almost equal in volume to the North Sea. But here, the ice forms a permanent and impenetrable canopy over a completely dark and cold environment (about -1.9℃).
As part of a multidisciplinary research project to explore this world under the ice, we have discovered a thriving microbial community that is distinct and well adapted to surviving without light and the organic matter that rains down in the open ocean.
Instead, this food web is built on inorganic nitrogen and sulfur compounds as sources of chemical energy. Microbes use these alternative energy sources to fix dissolved carbon dioxide into complex organic molecules and biomass which, in turn, fuel this underwater world.
Our discovery echoes the earliest hints of microscopic life beneath the ice, first recognized during the 19th century voyages of James Clark Ross, who gave this southernmost ocean and ice shelf its name. .
During the summers between 1840 and 1842, the crews aboard HMS Erebus and HMS Terror dodged icebergs, managed fickle winds and chipped frozen spray from their rigging and decks as they pushed south through the Ross Sea. Their goal was as simple as that of the Polynesian travelers who had preceded them centuries earlier: discovery.
Captain Ross’ voyage of discovery and research destined to find and explore the southernmost limit of the ocean. Everywhere — icebergs, seabed mud, and even the guts of larger organisms — they found evidence of microbial life. “The remains of microscopic animalcules […] countless myriads of an entirely new and minute organic life form.”
For Captain Ross, the vast, floating extension of the continental ice sheet, now called the Ross Ice Shelf, was a barrier that his ships could not overcome. Today, we can pursue the ocean further south, traverse the surface of the ice in tracked vehicles, and use specially designed drilling systems to break through the ice cover over a largely uncharted ocean.
Mystery of life under the ice
Elsewhere, marine ecosystems are primarily fueled by photosynthetic organisms that use sunlight to turn nutrients in the water into biomass. At depths where sunlight does not reach, sinking organic particles transfer carbon and energy in a process known as the biological carbon pump.
But under the ice cover there is no rain of organic particles from above. And once the water flows into the ocean cavity under the ice, it can take up to five years for it to see sunlight again. Yet when scientists first observed this environment in 1977, they found microbes, amphipods and fish.
Analytical methods at the time were limited, leaving open the question of whether what they found was a working food web. Our team’s recent expedition solved the mystery.
In December 2017, as part of a large interdisciplinary project, drillers from Victoria University of Wellington Te Herenga Waka used a hot water drill to melt a 30cm wide borehole in the central region of 360 m thick Ross Ice Shelf (at about 80.7S, 174.5W), about 300 km offshore. We used this unique access point to sample microbial life in the ocean cavity.
Reveal what is hidden
We didn’t know how abundant the microbial community would be, but we expected the ocean conditions at different depths to be important. To make sure we collected enough biomass, we lowered a battery-powered filtration pump through the borehole into the freezing environment below.
Collecting samples was a waiting game. We worked all night slowly pumping hundreds of gallons of water through a filter paper in the center of the pump. Each filtration was repeated three times, at depths of 30 m, 180 m and 330 m from the base of the floating ice, covering the entire water column between the pack ice and the ocean floor.
The chamber containing the filter paper had to be carefully opened each time, as a thin film of water froze when the pump hit the cold air. Residual liquid water was siphoned from the pump as a “soft” water sample.
While the filter paper samples could be frozen, the liquid water had to be transported as quickly as possible to the nearest laboratory, 400 km from the Scott base. Poor visibility meant there was no flying for almost three weeks, and finally we decided to drive – in a 24-hour marathon, 20 km/h through the pack ice in a Hägglund tracked vehicle .
Once safely back in our labs, we shared samples with colleagues in Austria, New Zealand, Spain, Australia and the United States. They used an array of cutting-edge genomic techniques and biogeochemical measurements to identify the microscopic organisms living in the ocean cavity, where their energy comes from, and what they do with it.
Our efforts have shed light on microbes with great metabolic flexibility that allows them to harvest energy from multiple sources and, in doing so, maintain a complex food web living in complete darkness.
Antarctica is surrounded by 1.6 million square kilometers of ice shelves, each with its own microbial community. Together, they represent a significant source of unaccounted-for energy and carbon.
Everywhere we look we find microbial communities using any available source of energy, creating the basis for all of Earth’s ecosystems. Just as Captain Ross predicted 180 years ago, understanding life in this remote system helps us make sense of life everywhere else on Earth.
Probing the mysteries of Antarctica’s deep, dense seawater
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