Territories with extreme conditions like Antarctica still conceal many secrets, despite the numerous research carried out in the field. Scientists have notably studied several freshwater lakes covered with ice. These aquatic habitats, cut off from the outside world, are home to very diverse populations of microbes. One team was particularly interested in Lake Enigma, whose geochemistry and microbial diversity were surprising.
The northern foothills of Victoria Land, Antarctica, contain numerous hydrological formations (streams, surface ponds, subglacial lakes). Many studies have been conducted in the McMurdo Dry Valleys region. These are valleys located in the southern part of Victoria Land, west of McMurdo Sound. Their name comes from their very low humidity and lack of snow or ice cover. It is one of the driest places on the planet! Precipitation is minimal and the surrounding mountains block the flow of ice from nearby glaciers.
The region contains more than 20 perennial ice-covered lakes and numerous streams and meltwater ponds. Scientists have studied them intensively since 1957. Their main properties depend on several factors, including the permanent presence of a layer of ice which prevents atmospheric exchange, geochemical and biogeochemical activities in the ice, variations in the supply of water, etc. The strong physical and chemical gradients observed in these lakes create a complex and diverse ecosystem.
Extreme environments that support life
“ These lakes provide an exceptional environment for microbial life, with temperatures around 0°C », write the researchers in Communications Earth & Environment. It is important to fully understand the differences and similarities between the microbial communities that live there to get a more accurate picture of Antarctic biology.
To complete the research, this team focused on Lake Enigma. It lies between the Boulder Clay and Amorphous glaciers in the northern foothills of Antarctica. Some Antarctic lakes have relatively warm bottom temperatures. Others are frozen from top to bottom and it was long believed that this was also the case for Lake Enigma. And for good reason, the average annual air temperature in this region is -14°C (with a minimum of -40.7°C!).
In reality, like the lakes of the McMurdo Dry Valleys, Enigma has a body of unfrozen water beneath its permanent ice cover at near freezing temperatures. This is what researchers discovered using georadar.
Investigation area, thickness of the ice of Lake Enigma and positioning of the drilling points carried out. Credits: Smedile et al., Communications Earth & Environment (2024)
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They therefore set out to explore the geochemistry and microbiology of Lake Enigma to compare them with those of other lakes studied so far. From November 2019 to January 2020, they drilled into the ice cover at several points. This could reach 11 meters thick in places.
Beneath this layer of ice, they discovered a column of water at least 12 meters deep, clearly stratified. They took several samples of surface ice, different layers of the water column and “microbial mats” located at depth.
A unique ecosystem among Antarctica's subglacial lakes
Geochemical analyzes revealed that the lake basin is closely isolated from the atmosphere and is meromictic. This means that its surface and deep waters very rarely mix. In addition, the interface between these water masses (called chemocline) and the thermal transition zone (thermocline) were particularly marked.
Vertical profiling of temperature, dissolved oxygen, total salinity and pH of the water column corresponding to one of the deepest drilling points. Credits: Smedile et al., Communications Earth & Environment (2024)
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Microbiological analyzes revealed that Lake Enigma supports an extremely diverse and abundant microbial ecosystem, unique among ice-covered Antarctic lakes. In total, 18 bacterial and three eukaryotic phyla (all unicellular phototrophs) were identified.
THE Bacteroidota, Actinobacteriota And Pseudomonadota were the most abundant phyla in all lake habitats, accounting for almost 60% of all gene sequences analyzed. The team, however, noted strong differences in the microbial communities of the ice, the water column and the microbial mat. The ice contained a very undiversified microbiota.
The water column had a relatively high abundance of Cryptista-like organisms – a phylum of single-celled eukaryotes. Oxygen-producing cyanobacteria were practically absent from the ice and the water column. On the other hand, they were more abundant in the benthic microbial mats.
Proportional composition of the microbiota at the phylum level reconstructed from each habitat type in Lake Enigma. Credits: Smedile et al., Communications Earth & Environment (2024)
The researchers also made a surprising discovery. “ A remarkable feature of the microbial ecosystem of Lake Enigma is the presence, and sometimes even dominance, of ultra-small bacteria belonging to the superphylum Patescibacteria », They report. These nanometric-sized bacteria are simple organisms with a very small genome. They were present in the water column and the microbial carpet. They had never been found in the McMurdo Dry Valleys lakes, although they have been widely studied.
A time capsule that could hold the secrets of ancient life
The species of Patescibacteria are particularly abundant in microaerobic and/or anaerobic zones of lakes, sediments and groundwater. Their discovery in the hyperoxic water column of Lake Enigma was therefore unexpected.
Researchers believe that these bacteria are important components of the Enigma Lake microbiome and that they expressed unique metabolic characteristics that allow them to thrive in hyperoxic conditions. However, these tiny organisms exhibit minimal cellular activities, including limited metabolic potential. As a result, they necessarily adopted a symbiotic or predatory lifestyle, the researchers explain.
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“ This discovery highlights the complexity and diversity of food webs in Antarctic lakes covered in permanent ice, with symbiotic and predatory lifestyles a possibility not previously recognized », they conclude.
It is now clear that Lake Enigma contains distinct microbial communities, occupying different roles. But above all, it is likely that it contains traces of ancient life. “ The planktonic and benthic microbiota of Enigma Lake, sealed by ice, likely represent persistent biota from the lake's ancient microbial ecosystem before ice-up », Write the researchers.
In addition to low precipitation and intense solar evaporation, the ice of Lake Enigma undergoes strong sublimation due to the powerful winds that blow more than 100 days a year. Yet the lake still exists, indicating a periodic influx of liquid water. Researchers believe melting of the nearby Amorphous Glacier could be the source. The lake could thus host a mixture of ancient organisms, originating from the lake, and contemporary organisms, originating from the glacier. However, further studies are needed to verify this hypothesis.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
