Giant virus DNA may help polar algae survive harsh environments

 In the game of survival, you can't always go it alone. Tiny algae living in the harsh conditions of the world's polar oceans appear to be better able to withstand the cold, high salinity, and extreme UV radiation, thanks in part to giant virus DNA in their genome, according to a study published in Current Biology.

Compared to tiny viruses like the flu or the common cold, giant viruses known as nucleocytoviricota (NCV) are massive. Their genomes can be larger than those of some bacteria and can carry hundreds of genes. Instead of relying entirely on a host's cells to replicate, like most viruses, they often carry genetic instructions involved in processes such as metabolism and DNA repair.

Giant virus DNA has already been detected in common algae, but researchers from the Rosenstiel School of Marine, Atmospheric and Earth Science at the University of Miami wanted to know whether it was present in polar algae, helping them survive. And they didn't have to go anywhere to get their answers.

The team analyzed the already-sequenced genomes of nine different polar algae. They used a computer program called ViralRecall to scan algae DNA for sequences that didn't match typical algae sequences but did resemble those of giant viruses. In particular, the software searched for specific viral protein families almost exclusively found in giant viruses called giant virus orthologous groups (GVOGs). And when the researchers found them in the algae, they labeled them giant endogenous viral elements (GEVEs).

Genetic record breaker

The study found giant virus DNA in all of the polar algae, especially a unicellular green algae called ICE-L, a member of the Chlamydomonas genus. It contains more than 400 giant virus-derived regions that encode over 25,000 viral genes. This is the most extensive viral endogenization (where viral DNA becomes a permanent part of a host's genetic code) in any eukaryote (organisms with cells that have a true nucleus).

To see whether and how these viral genes help the algae survive, the scientists studied publicly available RNA sequencing data from a previous experiment in which ICE-L was subjected to stressful conditions.

Repurposing viral DNA

Approximately 40% of the viral genes were expressed (active) during these tests. Hundreds of these genes were switched on in direct response to the harsh conditions. For example, under high UV, the algae increased expression of a viral protein normally used for DNA replication. The researchers believe the algae has repurposed the viral DNA for its own needs.

"The stress-responsive expression of diverse GEVE-encoded genes suggests that some viral elements may have been co-opted to enhance host fitness under challenging conditions," wrote the team in their paper.

This suggests that viruses are not always the enemy but can actually help an organism survive.

"Our findings position giant viral endogenization as a key driver of genome content, regulatory complexity, and environmental adaptation in polar algae."

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