Tardigrades (also known as "water bears") have built up a reputation for being pretty much invulnerable (even though Bdelloid rotifers does a lot of tardigrades' tricks and more), but while they can survive being launched into space, they are at the mercy of some fungi that can turn their little bodies into living incubators. One such fungi is a chytrid fungus named Sorochytrium milnesiophthora. And while most people who have heard of chytrid only know about the two species which infects frogs and other amphibians, most chytrids are actually parasites of invertebrates and algae, with some playing important ecosystem roles such as controlling harmful algal blooms and converting otherwise inedible phytoplankton into food for zooplankton, like water fleas
 |
| Left: A tardigrade infected with Sorochytrium milnesiophthora, Top Right: S. milnesiophthora zoosporangia (indicated by arrow) in the body of an infected tardigrade, Bottom Right: S. milnesiophthora grown on nutrient agar. Photos from Figure 1 of the paper |
In this study, researchers found S. milnesiophthora growing in tardigrades that were living amongst salted shield lichens collected from Kuljunmaa Island, Finland. Samples of the lichen were dried out for a year, then rehydrated and shaken to isolate the infected tardigrades and extract their fungal cells. When samples of the fungus were grown on a petri dish filled with nutrient agar, it grew into a type of lumpy yellow mould. But S. milnesiophthora takes on a very different form upon encountering a tardigrade.
The story of a water bear's fungal downfall begins with a single zoospore. Unlike most fungi, Sorochytrium and other chytrid-type fungi produce spores which have a flagellum-like tail that allows them to swim through water. Upon contact with the unfortunate tardigrade, the spore fires a tube which punctures the victim's body wall and invades its interior. At this stage, the fungal cells look like round balls which are indistinguishable from regular cells that float around in the tardigrade's body cavity. But over the course of a week, these fungal cells start proliferating throughout the water bear's body, which develops a reddish-brown colour. Once the host dies from being completely taken over by growing fungal cells, S. milnesiophthora squirts newly produced zoospores out of the carcass to begin the infection cycle anew
Aside from its ability to take down the seemingly invulnerable tardigrades, S. milnesiophthora also has something else which makes it stands out - an internal transcribed spacer gene that is 55 thousand megabases long, which is among the longest known for any organisms. This section of DNA, also known as ITS, is commonly used for identifying and classifying different species of organisms, and can serve as a DNA "barcode" to identify different types of fungi. Because it is a relatively short sequence, traces of it can be easily amplified and detected in a sample. Hence they are also often used in metabarcoding, a molecular technique for detecting DNA from a whole bunch of different organisms in a single sample, often used for detecting microorganisms which would be difficult and laborious to find using conventional microscopy.
But because the ITS gene of S. milnesiophthora is so long, this messes with the primers which are used for metabarcoding, as they were designed for amplifying short sections of DNA. Which means S. milnesiophthora would have been missed by conventional metabarcoding since the primers can't amplify its DNA for detection. So Sorochytrium and other similar fungi might actually be more common than previously thought. Case in point, while examining tardigrades for Sorochytrium, the researchers also found two other species of fungi along with two species of water molds associated with the water bears.
Water bears are charming-looking critters, and it may seem sad to see them die such a brutal death, but all this tardigrade slaying does serve an important ecosystem function. In addition to regulating the tardigrade population to make room for other microscopic animals, S. milnesiophthora may inadvertently be providing food for the ecosystem that exists on lichens. By converting water bears into zoospores, Sorochytrium could be providing other microscopic animals, such as rotifers, with a free meal. While deadly to their hosts, for everything else, chytrid zoospores can be tasty snacks for filter-feeding animals which relish those flagellated spores, as they are more nutritious than those animals' usual fare of algae, bacteria, and organic detritus,
There are vibrant ecosystems everywhere for those with the eyes (and microscope) to see. So next time you see a patch of moss or lichen, take a moment to appreciate the drama of life, death, and rebirth being played out in those microscopic worlds.
Reference:
Dirks, A. C., Vecchi, M., Orozco-Quime, M., Schwarz, E., Calhim, S., & James, T. Y. (2026). Rediscovery of the tardigrade pathogen Sorochytrium milnesiophthora, a blastoclad boasting the longest-known fungal internal transcribed spacer. Mycologia 118: 623-637.
