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Researchers examining the soil in a Central Massachusetts forest for changes in bacteria say they’ve accidentally discovered 16 new giant viruses, organisms that have only been known to science since 2003.

It is the first time giant viruses have been found in a forest ecosystem, said University of Massachusetts Amherst biology professor Jeff Blanchard, who oversaw the research. Most have been found in aquatic environments. With the discovery, a total of 242 giant viruses have now been found.

Researchers were looking into the possible effects of climate change on bacteria in the soil when they made the discovery.

“We were not looking for giant viruses,” Blanchard said in a statement from the university. “Our goal was to isolate bacteria directly from the environment to understand how microbial communities are changing in response to soil warming.”

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Researchers from UMass, the Department of Energy’s Joint Genome Institute, and Stanford University made the discovery in Harvard Forest in Petersham. They published their findings Monday in the journal Nature Communications.

The viruses were discovered using a kind of DNA analysis called mini-metagenomics.

An analysis of a tiny amount of forest soil found 16 genomes that were previously unknown, but could be identified as belonging to new giant viruses, Blanchard said.

“It just amazed me. . . . All this was literally in a teaspoon of soil in Harvard Forest,” he said in a telephone interview. “The diversity of these viruses out there in the world is almost unfathomable to me right now.”

Viruses were long thought to be much smaller than bacteria and too small to be seen under a light microscope. But in 2003, “giant viruses” that were large enough to be visible by microscopes were discovered, The discovery of the viruses, which contain larger genomes than smaller viruses, has prompted scientists to reexamine their thinking about viruses.

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The resolution of a light microscope is 0.25 micrometers. The typical size of a bacteria is 0.5 to 5.0 micrometers. (A micrometer is one-millionth of a meter.) The largest giant virus found so far has stretched 1.5 micrometers, Blanchard said. That would still mean 1,000 could fit across a 1.5-millimeter pinhead.

So far, Blanchard said, “No giant virus has ever been known to be a pathogen of humans. That doesn’t mean they can’t. But as far as we know, they’re not a danger to humans. They’re more of a danger to small, single-celled microbes.”

Blanchard said he’s interested in how they affect the carbon cycle — which includes the process by which plants in a forest take in carbon dioxide and the carbon is later released. The carbon cycle is being affected by climate change. Blanchard said giant viruses are already known to play a role in the carbon cycle in the oceans.

The discovery was made during soil-warming experiments at the forest, which is about 28 miles northeast of the UMass Amherst campus.

At the site, heating cables similar to those used to keep football and soccer fields from freezing are buried about 4 inches under the ground, keeping the surface warmer than the ambient temperature, simulating the warming expected from climate change, the university said.

Blanchard said the viruses were found in both the heated soil and in unheated control soil. A new study would be needed to see whether there were more or different viruses in the warmer plots, he said.

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The study’s lead coauthors were UMass grad student Lauren Alteio and JGI bioinformaticist Frederik Schulz. The work was funded by the US Department of Energy, the US Department of Education, and the National Science Foundation. The Harvard Forest is a 4,000-acre forest owned by Harvard University and used for research.