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Frog-Killing Fungus Meets Its Match in Hidden World of Tiny Predators

The views expressed are those of the author and are not necessarily those of Scientific American.


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A donkey hauls Pyrenees lake water laden with fungus-seeking microbial predators. Image courtesy Dirk Schmeller; Used with permission.

As I reported in a feature story in Scientific American last December , some fungi have been behaving badly of late, attacking bats, plants, amphibians, reptiles, and people with gusto, driving many species to extinction and others to the brink. It’s all quite depressing. But today in Scientific American online I report some good news: we may have allies in our efforts to fight at least one of these rampaging fungi. Tiny, armed, hungry allies.

Dirk Schmeller, Adeline Loyau, Frank Pasmans, Mark Blooi, and their colleagues have discovered that, at least in the alpine lakes of the Pyrenees between France and Spain, tiny predatory microbes can put the disease-causing fungus Batrachochytrium dendrobatidis, or Bd, soundly in its place. They do this because the fungus has relatives in the lake that look and act similarly, and these fungi make up a natural part of these microbes’ diets. These native microscopic fungi dine on detritus, not frogs. But to micropredatores, the spores of Bd look just like their regular breakfasts, and are evidently just as tasty.

You can read more about these findings and their implications for reining in the epidemic over at my Sci Am news story.

The microbial predators themselves (and the fungus they eat) are fascinating, but I didn’t have room to describe them in as much detail as I’d like in the article. So over here I thought I’d share a little more about them.

Bd is a chytrid (ki’-trid), a group of fungi that are often small and relatively simple. They are common in the environment and feed on detritus, animals, plants, algae, protists, or other fungi; there are species that specialize in parasitizing – and cramming almost their entire bodies into – single grains of pine pollen.

Although they grow and feed via tubular filaments like the rest of the fungi, they tend to be small and aqueous. That’s because their asexual spores do something unique among fungi: they have tails and they swim. If you didn’t know any better, you might mistake them for sperm. Because most chytrids are aquatic, it makes sense for their spores to be propeller-powered.

In frogs, the chytrid fungi motor around a pond until they find a likely victim, then settle down and sprout filaments into the skin of their hosts. When mature, they grow a spherical sporangium embedded in their host’s skin that emits its own swimming spores when ripe.

I couldn’t find a good video of B. dendrobatidis doing this, but here is a chytrid called Rhizophydium gramanis — a parasite of wheat roots — releasing its swimming spores from its round sporangia. At about :33, you can actually see them spilling out en masse.

So that’s our disease-causing organism. On the hunt for it, it turns out, are a slew of creatures. Though feeding strategies of the microbes that Schmeller et al. tested involved both active predation or passive filtration, the most effective Bd killers were predators. Among the most important were a tiny animal called a rotifer and several species of the single-celled protist Paramecium, although nearly every microbe they tested demonstrated fairly substantial spore-eating powers.

Fig. 3 from Schmeller et al., 2014. Click image for source.

As you can see, poor Stentor (a filter-feeding, horn-shaped protist) was so bad at catching them that is actually led to an increase in Bd spores. The two species the team actually isolated from the mountain lakes with Bd they sampled are marked “Pyrenees”.

Though frequently less than a millimeter long, rotifers come complete with stomachs, gastric glands, bladders, snapping jaws, and eyespots. They possess a crown of cilia that beat both to propel the animal and to draw in prey. Some rotifers are trappers that catch prey in funnels or other snares; one species nabs other protists with its jaws, drills through their shell, and sucks out the goodies like a spider. Here’s a short video of a rotifer that appears to be related to the one in the study. Notice its little clamping tail at the left, which some rotifers can use to latch on to things.

Paramecium species often look like shaggy slippers (their bodies are covered with propulsive tails called cilia which give the group they belong to – the ciliates – their name) that catch and swallow their prey whole. Many other ciliates are predatory; some sport filaments called trichocysts that allow them to grasp and paralyze prey prior to devouring it. Here are a few Paramecia cavorting in phase contrast microscopy, which makes their little internal organelles show up better. Watch for the beating of the cilia on their surface, and the distinctive oral groove. Warning: this video contains sound.

To test the abilities of individual species of rotifer and Paramecium to dispatch swimming Bd spores, the team housed them with tadpoles and Bd spores. The presence of rotifers resulted in not a single tadpole being infected with Bd, while tadpoles with a Paramecium roommate were much less likely to get infected with Bd than a control.

But to be absolutely sure that the predators were consuming Bd spores, one thing was left to do. When the scientists put two of the Paramecium species in containers with artificially glowing Bd spores, this was the happy result.

Two Paramecia that seem to have visited the All-You-Can-Eat Bd Buffet. At left, two individuals under regular illumination, and at right are the same organisms under fluorescence. C+D Paramecium caudatum. E+F Paramecium aurelia. Fig. S4 from Schmeller et al. 2014. Click image for source.

So if micro predators are so good at killing Bd, why has it gotten out of hand in some lakes and ponds around the world? To learn more about that question and about the possible use of these micro predators in fighting the global chytridiomycosis sourge, head over to my article.

Reference

Schmeller D., Blooi M., Martel A., Garner T.J., Fisher M., Azemar F., Clare F., Leclerc C., Jäger L. & Guevara-Nieto M. & (2014). Microscopic Aquatic Predators Strongly Affect Infection Dynamics of a Globally Emerged Pathogen, Current Biology, 24 (2) 176-180. DOI:

Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ. Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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