“Giant viruses” sound like some sort of terrifying science fiction creation. But while some of the world’s largest viruses can certainly cause problems if they make their way into humans, others content themselves with infecting algae and other microbes. In doing so, they can prove surprisingly beneficial for our environment.
Last week, a paper published in Microbiome announced that the characteristic genetic signatures of several viruses from a superclass known as nucleocytoplasmic large DNA viruses (NCLDVs) have been found in the Arctic, where the authors theorize that they are infecting algae that live on the ice and snow. If so, the viruses may prove an unexpected boon for efforts to reduce the rate at which our polar ice caps are melting.
The term “giant virus” refers to viruses of the phylum Nucleocytoviricota. The distinguishing characteristic of this phylum is, as one might guess, that many of its members are huge. While most viruses are somewhere between 20 and 200 nanometers in size, giant viruses can be up to a thousand times larger. This makes them comparable in size to—and in some cases, larger than—most bacteria. They also have immense genomes—up to 2.5 million base pairs of DNA, compared to between 7,000 and 20,000 for a typical virus—and a host of unusual abilities.
Nucleocytoviricota is a large family, and contains the viruses responsible for African swine fever and various types of pox infections. However, Laura Perini, co-lead author on the new paper, explains to Popular Science that each “[each of] the viruses to which our environmental signatures were assigned belongs to a family—Allomimiviridae, Pithoviridae, Algavirales and Asfarviridae—[that infects] other microbes (either microalgae or protists).” (She also reassures readers that “None of the viruses that we identified on the ice/snow samples have been related to humans!”)
So far, evidence for these viruses’ presence in the Arctic comes from DNA samples taken from several Arctic environments in which snow algae are flourishing, rather than direct observation. However, Perini is confident that the viruses are alive and active today, citing the presence of viral mRNA in the samples taken: mRNA degrades far more quickly than DNA, and finding it suggests that the viral DNA is from a contemporary source, not some long-dead microbe frozen in the ice. She explains that her team “[was] also able to bin for GVMAGs (giant virus metagenome-assembled genomes) that confirmed once again the presence of these viruses and their taxonomic identification.”
If these viruses are infecting Arctic algae, they might provide a curb on one of the lesser-known contributors to the shrinking of our polar ice caps: several species of algae known collectively as “snow algae.” These algae thrive during summer as snow and ice start to melt—and unfortunately, they also contribute to the rate at which that melting proceeds. Snow and ice reflect most of the sunlight that falls on their surface, but the algae darkens that surface, increasing the amount of light that it absorbs. This heats the snow and ice, increasing the rate at which they melt. The result is a sort of feedback loop, and as a paper published in Plant Science in 2021 notes, Antarctic algal blooms during summer are now so large that they’re visible from space.
However, while their contribution to the rate at which our ice caps are melting is certainly a problem, blooms of snow algae are not in and of themselves inherently negative phenomena. As the 2021 Plant Science points out, “Antarctica has relatively little exposed land to support terrestrial vegetation, with 98.7% of its surface area permanently covered in snow or ice … [and] blooms of red, green, and orange snow algae in Antarctica have been revealed as diverse ecosystems that play an active role in biogeochemical cycling of nutrients and carbon.”
Perini and her team’s paper describes the Arctic algae in similar terms: “There’s a whole ecosystem surrounding the algae. Besides bacteria, filamentous fungi and yeasts, there are protists eating the algae, different species of fungi parasitizing them and the giant viruses that we found infecting them.” Full understanding of the intricacies of ecosystems is key to implementing any such strategies and there are risks in introducing one lifeform to combat another.
If nothing else, however, the apparent presence of NCLDV DNA and mRNA in the samples taken by Perini and her team suggests that these giant viruses can survive in the bitter cold of the Arctic winter—and points to one potential way by which the rate at which our ice caps are melting might be reduced.
