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Viruses in the cold waters of the Arctic are strongly seasonal and are also detected in the Antarctic. This surprising discovery comes from a multi-year time-series study led by the GEOMAR Helmholtz Center for Ocean Research Kiel. The results offer new insights into the fragile balance of polar ecosystems—with implications for the role of viruses as indicators of change in the ocean, about which baseline knowledge is still lacking. The study is in Nature Communications.

The polar regions experience the strongest seasonal changes on the planet. The Arctic Ocean is known as an extreme, often ice-covered environment. But on closer inspection, it harbors a wealth of life—much of it microbial. Viruses, in particular, are closely intertwined with their hosts, mostly bacteria. These partnerships shift dramatically with the seasons, depending on light, temperature and nutrient availability.

Same viruses at both poles—an unexpected finding

The international research team, coordinated by GEOMAR, found that the composition of virus communities in the Arctic Ocean is strongly seasonal—and also unexpectedly similar to viruses in the Southern Ocean, surrounding Antarctica. This challenges the view that polar virus populations should differ markedly between the northern and southern hemispheres. Notably, the same viral groups were not found in warmer regions.

"It was completely unexpected to find such similar viral patterns at both poles, despite the huge geographical distance between them," says Alyzza Calayag, marine ecologist at GEOMAR and lead author of the study. "Understanding how this similarity arises is one of the big questions for future research."

A multiannual viral catalog

Samples for the study were collected in the Arctic using automated water samplers at the HAUSGARTEN Observatory, operated by the Alfred Wegener Institute (AWI). Over four years (2016–2020), the devices continuously collected seawater samples in the Fram Strait—the ocean passage between Greenland and Svalbard.

To detect viruses, the researchers searched millions of long DNA sequences with computational tools that identify viral DNA signatures. This enabled detecting viruses both inside of and attached to bacteria. The team also applied network analysis techniques to link specific viruses to their preferred hosts.

To determine whether these viruses also occur beyond the Arctic, the team compared their findings to global metagenomic datasets—that is, environmental DNA collected from various ocean regions. They found that 42% of the Arctic viruses also appear in Antarctic waters.

Summer surge: 30 viruses per bacterium

Another striking result was the dramatic seasonal difference in virus abundance and composition. "In winter, the number of viruses and bacteria was roughly equal" explains Calayag. "But in summer, especially between August and September, virus numbers surged. On average, we found 30 viruses for every single bacterium."

This sharp seasonal peak had gone unnoticed until now, because previous studies lack the context of continuous ecosystem observation utilized in the study, and do not sample during the dark winter periods.

Calayag adds, "We see that both the abundance and the composition of the viral communities shift with the seasons. Different environmental conditions lead to the dominance of different virus types—and these in turn have distinct effects on the microbial food web."

Viruses specifically infect certain bacteria, regulating their growth and spread. In doing so, they shape nutrient cycling and energy flow in the ocean.

Climate change could reshape polar microbial dynamics

This delicate microbial balance could be disrupted by climate change. "As temperature, salinity or sea ice cover change, so do the living conditions for viruses," says Calayag.

"Cold-adapted viruses could be displaced, and new types may emerge. This would affect the entire ecological interplay in polar waters. That's why viruses are important early indicators of change in the polar oceans."