Yesterday an article appeared in the Scientific American on exciting new research that suggests that predatory microorganisms can have a significant impact on the occurrence of the chytrid fungus Batrachochytrium dendrobatidis. This chytrid fungus is the cause of the disease chytridiomycosis, responsible for wiping out amphibian populations worldwide. So the discovery that natural predators exist, able to significantly reduce the number of these fungi present in an ecosystem, could have interesting repercussions for amphibian survival.
Common frog tadpoles
The study started out with the simple observation of one of the scientists that the water from high-altitude lakes in the Pyrenees contained a higher concentration of Bd-zoospores than the low-altitude lakes. To understand how this could be, they first turned to water quality and tested whether water quality differed between low- and high-altitude lakes. When this showed to give no explanation, they looked at the presence of micropredators: this time they found low numbers of these in high-altitude lakes and high numbers of micropredators in low-altitude lakes. This indicated that these micropredators are controlling the numbers of Bd-zoospores. Further testing in the lab showed that Bd zoospores survived longer in water brought back (by donkeys!) from lakes high up in the Pyrenees than in water from low-altitude lakes. Filtering water from low-altitude lakes reduced its ability to kill Bd-zoospores and because probably the microorganism had been filtered out, this supports the idea that this fungus is being eaten by the micropredators.
The effects of the presence of the microorganisms on tadpoles were also investigated: tadpoles housed in water from low-altitude lakes had lower infection rates and less severe infections than tadpoles residing in water from higher up the mountains. More support for the theory that microorganisms are responsible for a lower rate of infections, came from the observation that sterilizing the water from the low-altitude lakes led to higher rates of infection. Presumably because all microorganisms have been killed in this way and cannot predate anymore on the Bd-zoospores, who are causing the infection (and were released into the water after sterilization). Finally, when the researchers put the tadpoles in water together with individual species of a microscopic predatory animal, the same decrease in infection rates was noticed.
A donkey hauls Pyrenees lake water laden with fungus-seeking microbial predators. Credit: Dirk Schmeller
This all amounts to some compelling evidence for the predation of chytrid fungus by microorganisms. In the past, similar research has been done with releasing Bd-zoospores in experimental ponds stocked with microorganisms. However, no conclusive results had been found on the effects of the microorganisms, because not enough Bd-zoospores were present in these ponds to detect an effect. In retrospect, this might be due to the surprising ability of these microorganisms to consume a lot of the chytrid fungus. The biggest surprise from this research definitely came from the large amount of chytrid fungus that these small predators seem to consume. Of course it was known that these micropredators like to feast on prey the size of the Bd-zoospores (even if they do not naturally occur in these ponds), but that this predation could significantly impact the population size of the chytrid fungus, had never been shown before. So the appraisal from the scientific world for this research is mainly related to the convincing results indicating that indeed Bd-prevalence diminishes significantly whenever enough microorganisms are around.
And most importantly, the decline is significant enough to lower the rates of infection of tadpoles by the chytrid fungus, and thereby promoting survival of these tadpoles. Adding ‘a cup of microorganisms’ to every lake might therefore have a very positive impact on amphibian numbers by getting rid of these myotic creatures that are so violently attacking them. Further good news is that cleaning lakes completely of Bd-zoospores doesn’t seem to be necessary, which would be practically impossible. Only above a certain threshold does Bd-prevalence start to result in infections in amphibians.
So what does this teach us. Well, to quote Scientific American: “In principle (although such a practice requires much further testing), these results point to a simple, relatively natural solution to ponds plagued with Bd: boost native micro predator populations”. And to do that, we must ensure to keep these lakes in a relatively pristine state and keep invasive species from getting in. And, wherever fish have been introduced, who are predators of these microorganisms, we should try to transfer these to their natural habitats. Because boosting the microbial biodiversity in the long run will contribute to conserving our wonderful amphibian biodiversity.
I encourage you to read the wonderful original article and the blog post written about it