Missing for more than three decades, a fungus that infects microscopic creatures has made an unexpected comeback. This discovery highlights how the collections at the U-M Herbarium continue to drive new scientific discoveries.

Scientists usually expect discoveries to happen in the field. For a University of Michigan researcher, Alden Dirks, one of their most exciting discoveries arrived in…an email.

Alden spent years searching for a mysterious fungus known to infect tardigrades, microscopic animals famous for surviving some of the harshest conditions on Earth. These tiny creatures can endure extreme cold, intense radiation, dehydration, and even the vacuum of space by entering a suspended state known as cryptobiosis. 

Yet tardigrades are not invincible. These organisms have a weakness, which Alden discovered when a tardigrade biologist working in Finland reached out. "He didn't know we had been looking for it," said Alden. "He reached out because he thought he had found something interesting and knew U-M researcher Tim James is an expert in this group of fungi."

That message led to the rediscovery of Sorochytrium milnesiophthora, a rare fungus that infects and kills tardigrades. The species had not been documented since it was first described from moss samples in the Appalachian Mountains of North Carolina in 1985. Sorochytrium milnesiophthora is one of the few known fungi capable of infecting and killing tardigrades.

Alden first became interested in the organism because it belongs to Blastocladiomycota, a poorly understood group of fungi studied by their at-the-time Ph.D. advisor, Tim, professor, curator of fungi at the U-M. For several years, Alden searched for the fungus in lichen and moss samples collected from Michigan and even traveled to North Carolina to visit the original collection site where infected tardigrades were first discovered. Despite repeated efforts, they came up empty-handed.

Imagine their surprise when the breakthrough Alden was looking for came from thousands of miles away – a lichen sample collected in Finland. When the sample arrived in Ann Arbor, Alden carefully extracted tardigrades and monitored them over time. A technique shared by former lab member Kensuke Seto proved critical. Instead of searching for visibly infected tardigrades, Alden isolated healthy-looking individuals and observed them for several weeks.

"The initial infection is almost impossible to see," they said. "The fungus becomes obvious only after it develops." Eventually, infected tardigrades began showing clear signs of fungal growth. "It was really exciting," said Alden. The rediscovery ultimately led to the first genome sequence for the species and revealed several unexpected findings. Among the biggest surprises was the fungus's internal transcribed spacer (ITS) region, a DNA sequence commonly used by scientists to identify fungal species.

At roughly 5,500 base pairs long, the ITS region of S. milnesiophthora is the largest ever documented in fungi. Researchers suspect the unusual genetic feature may help the fungus survive long periods of desiccation, similar to how some lichens survive being dried out.

"It raises fascinating questions," Alden said. "Can the fungus survive drying out the same way tardigrades do? Could it remain dormant and resume infecting its host once conditions improve?"

Those questions remain unanswered, but they point to intriguing directions for future research. The rediscovery also transformed scientists' understanding of where these fungi live. Originally known only from North Carolina, Sorochytrium-related DNA sequences were later identified in environmental samples from Brazil and across Europe. The findings suggest that the group is far more widespread and diverse than previously believed.

For Alden, one of the study's biggest lessons extends beyond a single fungus. The experience highlighted the possibility that many other organisms remain hidden within biodiversity datasets, simply because they are too rare or too difficult to detect. "Even within all the unknown diversity we're discovering through environmental DNA, there may be another layer of biodiversity that we're still overlooking," they said.

The discovery also highlights an unusual aspect of herbarium collections. Unlike many preserved specimens, lichens and tardigrades can survive for years, possibly even decades, in a dried state. In some cases, researchers have successfully rehydrated old lichen specimens and brought them back to life. "We could potentially go back to historical specimens and study organisms that have been sitting dormant for decades," Alden said. That means collections may serve as more than archives; they can act as biological time capsules.