A fantastical world of potential giant viruses lurks beneath the soil

Giant viruses may try out all sorts of funky lewks.

New images reveal the varied — and sometimes whimsical — shapes of hundreds of potential soil-dwelling giant viruses. One shape is dubbed “haircut” for its fibers that bristle like freshly buzzed hair. “Gorgon” has tubelike appendages snaking from its shell. And flaps poking out of “turtle” resemble the reptile’s head, limbs and tail, virologist Matthias Fischer and colleagues report June 30 at bioRxiv.org.

These and other peculiar-looking shapes “clearly tell us that we’ve underestimated how structurally diverse these viruses are,” says Fischer, of the Max Planck Institute for Medical Research in Heidelberg, Germany.

Since the discovery of the first giant virus in 2003, scientists collecting genetic material from the environment have uncovered a wide world of giant viruses (SN: 3/21/18). These viruses are roughly 10 to 50 times the diameter of viruses that cause the common cold. The genetic data suggest that giant viruses are diverse, widespread and abundant.

But genetics can’t tell us everything about a virus’s biology, says Steven Wilhelm, a microbiologist at the University of Tennessee in Knoxville. “We don’t know what we’re looking at, who it infects or what it could possibly be doing.”

Some of the viruslike particles that virologist Matthias Fischer and colleagues identified resemble a turtle (one shown), with flaps that appear to protrude from the corners of its purported shell. Fischer says he “has absolutely no idea what these flaps are doing.” It’s possible they are extended openings of some sort. M.G. Fischer, U. Mersdorf and J.L. Blanchard/bioRxiv.org 2023
Many of the giant viruslike particles discovered — such as one dubbed “haircut” (shown) — have fibers attached to their exteriors. The fibers often have sugar molecules, making for a potentially sweet exterior. That kind of candy-coated viral shell might entice amoebic hosts to consume the virus. M.G. Fischer, U. Mersdorf and J.L. Blanchard/bioRxiv.org 2023
One group of potential giant viruses comes in the “gorgon” shape (shown), with long tubular-looking structures around a central shell. If the tubes are hollow, it’s possible a virus could use them for injection or suction. Or perhaps they’re used as something entirely different: a kind of flotation device, says virologist Matthias Fischer. At this point, he says, “it’s all speculation.” M.G. Fischer, U. Mersdorf and J.L. Blanchard/bioRxiv.org 2023

The new work could help change that, Fischer says. Using transmission electron microscopy, his team analyzed about half a kilogram of soil from Harvard Forest in Petersham, Mass., to produce an image gallery of giant virus diversity — potentially.

Fischer is careful not to call the virus look-alikes “viruses” just yet. The researchers have seen the particles only with a microscope; they haven’t confirmed that the potential viruses can infect particular organisms.

Still, looking at the structures Fischer’s team identified, microbiologist Frederik Schulz of the Joint Genome Institute in Berkeley, Calif., says he’s “convinced that many of these are actual virus particles.”

Scientists can only speculate why giant viruses might form tubular, bristly or turtlelike appendages. They may help the virus infect a host or perhaps move through the environment, Fischer says. “It’s going to be a wild ride … to see what each of these structures do.”

Whatever function they may have, Fischer thinks even more peculiar shapes remain to be discovered. “If a handful of forest soil already contains so many different virus particles,” he says, “this is clearly just the very tip of the viral Mount Everest.”