The right bacterial mix could help frogs take the heat

Some of a green frog’s heat tolerance might come from its microbiome.

Put a wood frog tadpole and a green frog tadpole in water and turn up the heat. The two species live in similar environments, but green frog tadpoles just keep swimming in water that’s up to 1.5 degrees Celsius warmer.

But when scientists transferred the gut microbiome of a green frog (Lithobates clamitans) to the eggs of a wood frog (L. sylvaticus), the wood frog larvae could handle short stints at higher water temperatures, Jason Dallas and colleagues report September 7 on bioRxiv.org. The study reveals one reason some species might be more, or less, sensitive to climate change.

Green frogs and wood frogs are closely related but lead different lives. Wood frogs go from egg to adult in only three months in the spring. Green frog larvae can remain as tadpoles through the whole summer, swimming in higher water temperatures as a result. Dallas, a herpetologist at Middle Tennessee State University in Murfreesboro, was particularly interested in the frogs’ critical thermal maxima — the hottest temperature at which the animal can continue to move.

“Amphibians [and] reptiles are globally imperiled, but still remain understudied,” Dallas says, and it’s not clear just how their microbiomes contribute to how they live their lives. To find out what role the microbiome might play in frog temperature tolerance, Dallas headed out to a local wetland in early spring in search of wood frog eggs. Timing is key. “They’ll breed, and it’ll all be over in basically one or two nights,” he says. “The next day you go, [and] there’s hundreds of egg masses, if not thousands.”

Dallas scooped a few egg masses into a large plastic tub and drove back to campus. Slowly. “You go a little bit slow, which the other drivers may not appreciate but the wood frogs like.” Back at the lab, he and his colleagues washed the eggs in an antibiotic solution to rid them of their own microbiome and inoculated some of them with gut bacteria from green frogs.

When the eggs hatched, the tadpoles ate a little of their egg jelly — gaining a dose of pond water or green frog bacteria in the process. Dallas found that those who had received a dose of green frog gut bacteria could take short periods of water heated to 38.3° C, while those inoculated with their normal pond water or no bacteria at all topped out at 37.7° C. When the researchers analyzed the microbiomes of the larvae, they found that larvae inoculated with green frog bacteria had more diverse microbiomes. 

The temperature difference is small, but could be meaningful, says Timothy Colston, a herpetologist at the University of Puerto Rico at Mayagüez who was not involved in the study. “Even 1 degree of difference over a short period of time, if that gives you an advantage to being able to survive and reproduce, then that’s going to increase your fitness,” he says.

How exactly a frog’s microbiome helps the animals take more heat remains a bit of a mystery. The right bacterial population could be good for the frog’s overall health. If they’re protected, say from viruses or fungal infections, they might be better able to resist a heatwave or two, Colston says. Gut microbes could also affect how a frog’s metabolism functions, or whether and how they express proteins that resist heat.

“There’s probably a lot of cellular mechanisms and other mechanisms in play,” Dallas says. Certainly, this is not an excuse to go around dosing frog populations with bacteria during heatwaves; he and his colleagues were only able to identify broad groups of microbes involved. But it does show one way that some frogs might get hot — or not — in the face of climate change.