Researchers from Spain, the UK, and Norway used metagenomic sequencing to profile DNA viruses in water samples from half a dozen Norwegian lakes over three years. Within these samples, they saw DNA communities that differed from those previously identified in freshwater samples from the Antarctic.
Nevertheless, the team noted that some features of the viromes at each pole did overlap; apparently arguing against the notion that viral diversity follows a continuous, north-to-south gradient.
"Viral communities in the Arctic and Antarctic freshwater ecosystems share taxonomic composition of viruses, dominated by unknown and small [single-stranded DNA] viruses, but show very low fine-grain genetic overlap," senior author Antonio Alcamí, a researcher with Madrid's Centro de Biología Molecular, and his co-authors wrote.
"Our comparative analysis sheds light into the global biogeography and connectivity of viral communities," they concluded, "and highlights not only the uniqueness of the polar environment but also the differences between Arctic and Antarctic microbial ecosystems, despite their exposure to similar environmental conditions."
Despite their ecological importance, the makeup of viral communities found in many different types of environments is largely uncharacterized, the researchers noted. For the current study, they focused on viromes from freshwater sites in the far north that are microbe-rich and relatively untouched by human development.
"High-altitude freshwater habitats constitute a unique ecological model to understand the influence of viruses on natural microbial communities and the overall ecosystem," the study's authors explained. "However, no large-scale analysis has yet assessed the diversity and composition of the Arctic freshwater virome."
The team did shotgun metagenomic sequencing on purified DNA virus particles in samples from six bodies of water in Norway in 2010, 2011, and/or 2012.
Samples from all but one of the water bodies were sequenced using Illumina HiSeq instruments. The researchers sequenced the remaining samples with Roche 454 FLX technology. They also did Illumina re-sequencing on samples from a lake in the Antarctic that were considered for a study published in Science in 2009.
When the team delved into sequences from the Arctic lakes, it saw an abundance of single-stranded DNA viruses, which made up some 86 percent of the DNA viruses detected in the samples. More than one-third of the single-stranded DNA viruses belonged to the Circoviridae family.
The remaining viral community components included both double-stranded DNA viruses, along with sequences that most closely resembled single-stranded RNA viruses, the researchers reported.
Some of the Arctic lakes shared more virome features than others, they noted. But despite the diversity detected at these sites, the viral sequence contigs in the Arctic samples typically clustered together — and apart from viral communities in samples from the Antarctic, Arctic Ocean, and elsewhere.
Similarities with viromes from freshwater samples collected in the Antarctic during the spring did turn up when the group compared more general taxonomic features such as representation by single-stranded DNA viruses, unknown viruses, and viruses from specific lineages.