Only a small fraction of the microscopic life that inhabits soil has been identified, which makes quantifying what’s out there nearly impossible. But a significant leap forward was made last year when scientists studying artificially warmed soils at Harvard Forest accidentally discovered 16 giant viruses in soil microbes.
According to Jeffrey Blanchard, a biology professor at the University of Massachusetts and one of the scientists involved in the study, giant viruses got their name because they have unusually large genomes: some have as many as 2,000 genes, compared to the 5 to 10 genes found in most viruses. Their physical size is also much larger than that of other viruses – as much as 100 times larger than the influenza virus, for example.
“Giant viruses had never been discovered in a forest ecosystem before,” said Blanchard, who collaborated with scientists at the Department of Energy’s Joint Genome Institute. “They’ve mostly been found in marine environments inside of amoebas, and they’ve recently been discovered in fish. We can only hypothesize their role in the ecosystem, but they’re a natural part of the food web.
“People tend to think of viruses as bad, as infectious agents that might kill crops or other things,” he added. “But they’re not all bad guys. Just like most bacteria, they’re just a part of life.”
Only about 100 giant viruses had been identified around the world prior to the discovery at Harvard Forest, and the 16 new giant viruses were all found within one teaspoon of soil.
“That suggests that there is an incredible diversity of giant viruses in this one place, and if we went to other places in Harvard Forest and other places around the world, we’re sure to find more giant virus biodiversity,” said Blanchard. “It’s amazing that we had no idea that a major group of organisms like this was even there. We probably could have taken a teaspoon of soil from anywhere in the world and discovered something.”
The giant viruses were discovered as part of an effort to understand the differences in microbial communities that might occur as the climate warms. For nearly 30 years, a test plot at Harvard Forest has had heating cables buried a few inches underground to keep the soil temperature five degrees warmer than in nearby control areas. Blanchard used a new method for sorting through individual cells in samples and sequencing the genomes of those cells. The process, which broke open some microbes to liberate the viruses inside them, is expensive and hadn’t been used previously on forest soils, which is why the giant viruses had not been detected until now.
As a result of the discovery of the giant viruses, Blanchard plans to develop new approaches to look for them in a more targeted manner at Harvard Forest and elsewhere. “We weren’t trying to discover giant viruses,” Blanchard concluded, “but it has been a welcome distraction.”