Giant viruses have been discovered infecting microscopic algae in a rare lake in the Arctic Ocean, a new study finds.

The Milne Fiord epishelf lake is a body of freshwater that sits on top of seawater less than 500 miles (800 kilometers) from the North Pole.

Researchers studying the lake found that the freshwater had a richer and more diverse range of viruses than the salt water beneath it.

They also found "giant" viruses – several times larger than typical viruses – affecting microscopic algae just below the boundary between fresh water and salt water.

"Just as the freshwater ecosystem of the lake is distinct from the ecosystem of the Arctic Ocean, it also has its own distinct community of viruses," study co-author Mary Thaler, a microbiologist at Laval University in Quebec, told Live Science in an email.

An epishelf lake is held in place by ice but has no physical bottom. The lake's freshwater floats above the seawater because fresh water is less dense than salt water. The top of the lake is covered in ice, protecting the freshwater from waves or wind that would otherwise force the two water types to mix.

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The researchers drilled through the ice and collected water samples from the lake. Then, they sequenced the DNA found in these samples to identify a variety of viruses, including some belonging to a group of giant viruses called Megaviricetes.

"One of the characteristics of viruses in general is how tiny they are, much smaller than the smallest bacterium, and carrying only a few genes to help them replicate," Thaler said.

"However, in the past 20 years, scientists discovered giant viruses that are as big as a bacterium, with genomes that could potentially carry many interesting genes."

The researchers don't know how most of the viruses affect the microscopic algae, or even which viruses infect which organisms, according to a statement released by The American Society for Microbiology.

The study authors hope to learn more detailed information about the ecosystem in the future, but they're in a race against time; rising temperatures threaten to destroy the ice dam holding the fresh water in place.

"Epishelf lakes used to be more common in the Arctic, but now they are extremely rare," Thaler said. "If the ice dam breaks apart – which has happened in other fiords – then Milne Fiord Epishelf Lake will be lost."

The study was published online Aug. 25 in the journal Applied and Environmental Microbiology.

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