Newly discovered giant viruses look like stars, turtles, and perfume bottles

May 18, 2023

Matthias G. Fischer et al., 2023/bioRxiv

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It is estimated that there are over 100 billion trillion (1024) stars in space, but do you know the number of stars in the entire universe is way less than the number of viruses that currently inhabit Earth?

A report suggests that our planet is home to 1031 viruses, and they are probably the most powerful beings in terms of population. They come in all kinds of shapes and forms, however, some are more bizarre than others.

Recently, researchers from the Max Planck Institute and the University of Massachusetts discovered giant virus-like particles (VLPs) in soil samples from Harvard Forest. They are unlike any other viruses humans have ever known.

"What we found is a whole new diversity of shapes that we have never seen before. I would bet that many of those, if not the majority, are completely new and the first sightings of viruses,” Dr. Matthias Fischer, one of the study authors and a scientist at the Max Planck Institute, told Live Science.

As per their shape, the researchers call them:

Christmas star (star-shaped viruses), Turtle (viruses that have a turtle-like form), Plumber (viruses with tubular or pipe-shaped tentacles), Supernova (viruses that look like exploding stars), Gorgon (viruses having multiple tubular appendages), Hair-cut (viruses with spiky hair-like structures), and Flacon (viruses that have a shape like a perfume bottle).

Most viruses found in nature are 20 to 200 nanometres (200 x 10-9 m) in size, but the newly discovered virus-like particles have body measurements ranging between 0.2 to 1.5 micrometers (1.5 x 10-6 m).

The good news is — such giant viruses are known to infect only unicellular organisms and not complex life forms like humans. According to the researchers, the VLPs inhabit soil and may play an essential role in the carbon cycle by limiting the number of other microbes, such as bacteria.

This way, they might regulate the amount of carbon in the soil at any given place. For the carbon cycle to work well, it is important that the amount of carbon freely available in the atmosphere is in balance with the carbon going back to Earth. This balance is crucial for ensuring a stable environment and climate conditions.

However, the researchers are still not sure why the giant VLPs have such unusual shapes. For instance, Turtles have lobed appendages, Christmas star viruses come equipped with an outer star-shaped shell, and Gorgons have tubular outgrowths.

They assume perhaps their bizarre shape helps them stick and stay attached to their host. Further research is required to confirm these speculations and their role in the environment.

Matthias G. Fischer et al., 2023/bioRxiv

Although the viruses discovered by the researchers have unique physical features, they are not the first giant viruses known to humanity. Scientists have previously recovered such viruses from oceans, frozen lakes, and even from other organisms.

For instance, in 2003, scientists discovered a virus that mimicked bacteria. It is known as the Mimivirus and was found hidden inside the cytoplasm of an amoeba. The virus also has a big brother called the Mamavirus.

While a mimivirus can grow up to 600 nm, the biggest VLP discovered by the researchers measures 690 nm in length.

However, these size comparisons stand nowhere against Pithovirus sibericum, a 1,500 nm long virus found buried inside a 30,000-year-old ice structure in Siberia. To this date, it is the biggest virus ever discovered.

Nevertheless, Dr. Fischer and his team believe the newly discovered giant VLPs will enhance our knowledge of the great virus biodiversity around us. It will also help us explore how we could use these microorganisms to benefit our ecosystem.

“The cornucopia of viral morphotypes found in Harvard Forest alone questions our current understanding of the virosphere and its structural heterogeneity. With this visual display of viral diversity, we hope to inspire other researchers to explore different microcosms by electron microscopy and to isolate more virus-host systems for detailed characterizations,” the researchers note.

The study is published in bioRxiv.

Study Abstract:

Large DNA viruses of the phylum Nucleocytoviricota infect diverse eukaryotic hosts from protists to humans, with profound consequences for aquatic and terrestrial ecosystems. While nucleocytoviruses are known to be highly diverse in metagenomes, knowledge of their capsid structures is restricted to a few characterized representatives. Here, we visualize giant virus-like particles (VLPs, diameter >0.2 µm) directly from the environment using transmission electron microscopy. We found that Harvard Forest soils contain a higher diversity of giant VLP morphotypes than all hitherto isolated giant viruses combined. These included VLPs with icosahedral capsid symmetry, ovoid shapes similar to pandoraviruses, and bacilliform shapes that may represent novel viruses. We discovered giant icosahedral capsids with structural modifications that had not been described before including tubular appendages, modified vertices, tails, and capsids consisting of multiple layers or internal channels. Many giant VLPs were covered with fibers of varying lengths, thicknesses, densities, and terminal structures. These findings imply that giant viruses employ a much wider array of capsid structures and mechanisms to interact with their host cells than is currently known. We also found diverse tailed bacteriophages and filamentous VLPs, as well as ultra-small cells. Our study offers a first glimpse of the vast diversity of unexplored viral structures in soil and reinforces the potential of transmission electron microscopy for fundamental discoveries in environmental microbiology.