Giant viruses infect cyanobacteria in Arctic epishelf lake | Polarjournal
The Milne Fjord Epishelf lake lies hidden under a layer of ice and floats on the denser and heavier waters of the Arctic Ocean. Photo: Denis Sarrazin, Center for Northern Studies

In the far north of Canada, on the north coast of Ellesmere Island, in the Milne Fjord sits a unique freshwater lake that rests on the Arctic Ocean and is covered by glacial ice. Only the ice shelf of the Milne Glacier acts as a barrier at the fjord exit, holding the lake in place. A very special community of microorganisms, dominated by cyanobacteria, lives in the epishelf lake. These are often infected by unusual “giant viruses”. In a study published in the journal Applied and Environmental Microbiology, researchers have now quantified the viruses in the lake for the first time.

The epishelf lake in Milne Fjord is an extremely rare ecosystem – there are only two such lakes in the Arctic. In contrast, they are more common in Antarctica. The special feature is that meltwater from the glacier collects between the ice and the Arctic Ocean. The fresh water of the lake floats on the seawater because it has a lower density. Mixing of the two bodies of water is also prevented by the upward closure of the epishelf lake, where the overlying glacier tongue protects the freshwater from wind and waves.

To understand polar aquatic ecosystems, it is essential to study the viruses that live in them, because these ecosystems are dominated by microorganisms that are often infected with viruses. In the current study, the research team determined the diversity and distribution of viruses and, more importantly, aims to understand how giant viruses affect the ecology of the lake through their interactions with the cyanobacteria they infect.

The freshwater lake is held in place only by the Milne Ice Shelf. If it collapses due to warming, the epishelf lake will also disappear. Graphic: Labbé et al. 2022

“Just as the lake’s freshwater ecosystem is different from the Arctic Ocean ecosystem, it has its own viral community,” Mary Thaler, a microbiologist at the University of Laval, Quebec, and co-author of the study, told Live Science.

However, microbiologists do not have too much time left to get a picture of biodiversity and biogeochemical cycles as the Arctic is warming rapidly. “The ice shelf holding the lake in place is getting smaller every year, and when it breaks up, the lake will drain into the Arctic Ocean and be lost,” said Alexander Culley, a microbiologist at the University of Laval, Quebec, and corresponding author of the study.

“Our results highlight the uniqueness of the viral community in the freshwater lake compared to the marine fjord water, particularly in the halocline,” Culley said. The halocline, or salinity jump layer, is the transition zone between water layers with different salinity. According to Culley, it is precisely this environment that provides niches for viruses and potential hosts that are not found in freshwater or in seawater layers with uniform salinity.

Milne Fjord is located on the north coast of Ellesmere Island in the Canadian Arctic. Map: GoogleEarth

The research team took water samples from the lake and sequenced all the DNA in the lake water to identify the microorganisms and viruses it contained. Among them were “giant viruses” of the genus Megaviricetes, which are many times larger than typical viruses.

“One of the characteristics of viruses in general is that they are tiny, much smaller than the smallest bacterium, and have few genes to help them replicate,” Thaler said. “In the last 20 years, however, scientists have discovered giant viruses the size of a bacterium with genomes that may contain many interesting genes.”

“The high bacterial density combined with a possible prevalence of the lytic lifestyle at this depth suggests that viruses play an important role in biomass turnover,” Thaler continues. “Lytic lifestyle” means that the microbial host cell is destroyed releasing virus daughter particles.

According to the authors, the study lays a foundation for a better understanding of viral ecology in different regions around the world and particularly in the high Arctic. But the details of this ecosystem remain hidden. Researchers do not know about most viruses, how they affect the microbes they infect, or which viruses affect which microbes.

Julia Hager, PolarJournal

Link to the study: Labbé, M., Thaler, M., Pitot, T. M. et al. (2022). Climate-Endangered Arctic Epishelf Lake Harbors Viral Assemblages with Distinct Genetic Repertoires. Applied and Environmental Biology.

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