(Left to right) Scientists have discovered previously unknown giant viruses, nicknamed “turtle,” “plumber,” and “Christmas star” based on their shapes. (Image credit: Fisher et al. DOI: 10.1101/2023.06.30.546935 (license CC-BY-NC-ND 4.0 International))
Giant viruses found in New England forest soil have features not seen in other viruses of similar size, from star-shaped outer shells to exotic tubular appendages, according to preliminary research published in bioRxiv.
“We have discovered a whole new diversity of forms that we have never seen before,” study co-author Matthias Fischer, a virologist at the Max Planck Institute for Medical Research in Germany, told Live Science. “I am confident that many, if not most, of these are completely new and the first viruses that we have ever seen.”
Giant viruses typically range in size from 0.2 to 1.5 micrometers and have complex genomes that can contain up to 2.5 million DNA base pairs, the study found. That’s significantly larger than most viruses, such as influenza viruses, which range in diameter from 0.08 to 0.12 micrometers. So far, giant viruses have been found to primarily infect single-celled organisms such as amoebae, rather than animals or humans. These viruses have been found in a variety of ecosystems around the world, including oceans, Arctic lakes, and even melting permafrost.
In 2018, a separate team of researchers discovered giant viruses in the soil of the Harvard Forest in central Massachusetts using metagenomic analysis, which compares genetic material found in the soil with sequences already in global genetic databases. For the new study, the scientists instead analyzed soil samples from the same forest using a transmission electron microscope, which uses beams of electrons to study the embedded viruses in incredible detail.
“The genome can only tell you so much about an organism, so you can’t see what it looks like,” Fisher said, citing a 2018 study. “Giant viruses are not only different in their genomes, but also in the particles and structures they form, and that was a completely new discovery.”
However, because Fisher and his colleagues did not perform genomic analyses, they cannot accurately call the newly discovered particles “viruses,” he explained, and instead called them “virus-like” in their report. However, they are likely viruses, he said.
The researchers observed a double-shelled virus-like particle that had a distinctive star-shaped appearance, which they called the “Christmas star morphotype.” The team also identified viruses with appendages of varying length, thickness, and density protruding from the surface of the particles, and they called these unusual structures the “haircut” morphotype.
While Fisher and his team don’t yet know what these strange features do, he speculated that some of the viruses’ characteristics may help them attach more efficiently to host cells. The researchers also speculate that similar giant viruses could be found in the soils of other deciduous forests around the world.
“This fascinating window into the complex world of soil viruses confirms that the high genetic diversity of giant viruses is associated with diverse and previously unimagined particle structures,” the researchers wrote in a June 30 preprint that has not yet been published in a peer-reviewed journal.
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