A huge array of virus-like particles described as “astonishing” and “unexpected” has been found in the soil of Harvard Forest, Massachusetts. These particles are not only abnormally large, but also stand out as very strange entities with “previously unimaginable” structures that make everything we know about giant viruses and viral diversity questionable.

“The abundance of viral morphotypes found in the Harvard Forest questions our current understanding of the virosphere and its structural heterogeneity,” write the authors of a pre-peer-reviewed pre-print. “This fascinating window into the complex world of soil viruses, “It leaves little doubt that the high genetic diversity of giant viruses matches the diverse and previously unimaginable particle structures whose origins and functions continue to be studied.”

The strange virus particles have equally bizarre extensions and internal structures, some of which have never been seen before and may suggest new ways for viruses to interact with their hosts. They have tubular protrusions, fibres, inner channels, double capsids (the protein shell of a virus), and tails, which gave them strange names like “tortoise”, “haircut” and “Christmas star” viruses.

Virus-like particles are derived from viruses and are very similar to viruses, although they lack genetic material and do not have the capacity to infect a host cell. However, the authors of the study in the preprint argue that the large icosahedral (20-sided) particles with the modified capsids they found can be safely called “virus particles.”

Using transmission electron microscopy, the team found that the soil samples were filled with “an unexpected variety of soil [virus-like particles] in the 0.2 µm to 1.2 µm size fraction.” For comparison, the largest virus ever discovered, resurrected from the permafrost in Siberia in 2014, was 1.5 µm wide and was comparable in size to a small bacterium.

“Surprisingly, we found that several hundred grams of forest soil contained more capsid morphotypes than all giant viruses ever isolated combined,” the team writes. It’s even more surprising when you think about it.”

The findings highlight that there is still much we don’t know about the strange and wonderful world of giant viruses, and researchers hope they can spark new investigations. In particular, they are interested in investigating whether this remarkable diversity is only a feature of soil ecosystems, or whether aquatic environments are also home to abundant large virus particles.

Preprint research is featured on bioRxiv.