Less than 500 miles from the North Pole, Lake Milne Fiord Epishelf is a unique freshwater lake that floats above the Arctic Ocean, held in place only by a sheet of ice. The lake is dominated by single-celled organisms, including cyanobacteria, which are frequently infected with unusual “giant viruses.” Researchers from Laval University, Quebec, Canada have produced the first assessment of virus abundance in this lake. The research is published in Applied and Environmental Microbiology, a journal of the American Society for Microbiology.
Viruses are essential for understanding polar aquatic ecosystems because these ecosystems are dominated by single-celled microorganisms, which are frequently infected by viruses. These viruses, and their diversity and distribution in Lake Milne Fiord, have rarely been studied. The team is currently working on sequencing the giant viruses, an effort that will likely lead to understanding how the viruses influence the ecology of the lake through their interactions with the cyanobacteria they infect.
Rapidly increasing temperatures limit the time left for microbiologists to develop a clear picture of the biodiversity and biogeochemical cycles of these ice-dependent environments, as well as the consequences of rapid and irreversible temperature changes. “The pack ice that holds the lake in place is deteriorating every year, and when it breaks, the lake will spill into the Arctic Ocean and be lost,” said corresponding author Alexander I. Culley.
“Our results highlight the uniqueness of the viral community in the freshwater lake, compared to marine fjord water, particularly in the halocline community,” Culley said. The halocline is an area where the salinity drops rapidly as one goes up the water column. According to Culley, this environment provides niches for viruses and hosts that are found neither in freshwater layers nor in marine layers of uniform salinity.
The remote High Arctic lake could only be reached by helicopter, weather permitting. The research team collected water samples and sequenced all the DNA from the lake water, allowing them to identify viruses and microorganisms in it. The study establishes a foundation for advancing the understanding of viral ecology in various global environments, particularly in the High Arctic.
“High bacterial abundance coupled with a possible prevalence of the lytic lifestyle at this depth suggests that viruses play an important role in biomass turnover,” said team member Mary Thaler, Ph.D. Culley at Laval University. The “lytic lifestyle” refers to the release of daughter viral particles when the host microbial cell is destroyed.
The most dramatic change observed in Epishelf Lake in Milne Fiord has been a multi-year decline in the abundance of cyanobacteria. The researchers attributed the decline to increasing marine influence in the freshwater lake, “since cyanobacteria are very low in abundance in the Arctic Ocean,” they wrote.
Nevertheless, the details of this ecosystem remain obscure, as so far most of its viruses are only known through fragments of their sequences. So, in most cases, scientists don’t yet know how viruses influence the microbes they infect, or which viruses inhabit which microbes.
Reference: Labbé M, Thaler M, Pitot TM, Rapp JZ, Vincent WF, Culley AI. The climate-threatened Arctic Epishelf Lake harbors viral assemblages with distinct genetic repertoires. Appl Env Microphone. 2022;0(0):e00228-22. doi:10.1128/aem.00228-22
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