What is already known on this topic
The gut microbiota can determine how animals respond to their external environment by, for example, influencing the host’s digestion, immune function and metabolism.
What this research adds
To explore whether the gut microbiota could also influence the host’s tolerance to heat and cold, researchers analyzed tadpoles — the larval stage in the life cycle of an amphibian. When exposed to higher temperatures, tadpoles with depleted gut microbiotas were five times more likely to die than tadpoles with heathy microbiotas. That’s likely because animals raised without microbes had an altered metabolism and lower levels of activity of two proteins that help turn food into energy.
Conclusions
The findings highlight the link between the gut microbes of cold-blooded vertebrates and the animals’ tolerance to heat — a factor that should be taken into account when predicting animals’ responses to climate change.
Warming temperatures threaten the survival of amphibians and other cold-blooded animals, whose body temperature varies with that of the environment. Now, researchers have found that gut microbes can help frogs to cope with hot temperatures.
The findings, published in Nature Ecology and Evolution, highlight the link between the gut microbes of cold-blooded vertebrates and the animals’ tolerance to heat and cold. This means it may be important to take into account the gut microbiota when predicting the animals’ responses to climate change, the researchers say.
Unlike mammals, amphibians, reptiles and other cold-blooded animals can’t regulate their own internal temperatures, and a shift of only a few degrees can make a big difference for them. Indeed, global surveys have shown declines in the populations of several cold-blooded vertebrates due to a warming climate.
“Host-associated microbial communities, or microbiomes, have recently emerged as a critical factor that regulates how animals respond to their external environment,” the researchers say. “Interactions between hosts and microorganisms are bidirectional, with microbial communities being shaped by host factors such as the immune system and host genetics, while also impacting host physiology (for example, digestion and immune function) and gene expression.”
Because gut microbes are known to regulate the host’s metabolism, the researchers — led by Samantha Fontaine at the University of Pittsburgh — set out to explore whether the microbiota could also influence the host’s tolerance to heat and cold. To do so, the team analyzed tadpoles — the larval stage in the life cycle of an amphibian.
Warm temperatures
The researchers used laboratory-reared tadpoles of the American green frog (Lithobates clamitans). The tadpoles were raised in water at three temperatures: 14 °C, 22 °C and 28 °C. Half of the tadpoles were raised in pond water, which is naturally rich in microbes, and half in pond water treated with heat and pressure to kill most microbes.
The tadpoles’ gut microbiota was dominated by Firmicutes and Proteobacteria, but animals grown in germ-free water had fewer kinds of bacteria in their guts. In particular, they had lower levels of Dependentiae and Plantomycetes, which are common to water and soil habitats.
Tadpoles raised without microbes were able to tolerate lower temperatures than those raised in pond water. But when exposed to higher temperatures for longer periods of time, they were five times more likely to die than tadpoles raised in pond water. Even in water at a temperature of 14 °C, animals with a healthy microbiota coped better in the long run.
Altered metabolism
Next, the team explored how gut microbes could help tadpoles to handle heat. Animals raised in germ-free water had an altered metabolism and lower levels of activity of two proteins that help turn food into energy. Reductions in the activity of these proteins may be associated with the lower tolerance to heat observed in these animals, the researchers say.
“We definitely need more natural experiments to see how this would actually play out in the wild,” Fontaine says. However, she adds, the study is the first to show a connection between heat tolerance and gut bacteria in amphibians.
The findings could also pave the way to for new conservation approaches for animals threatened by climate change, Fontaine says. “The idea of probiotics is really big in the field: Are there microbes that are really important that we can supplement animals with?” she says. “If so, it’s possible that [we] can help them withstand changes in temperatures more robustly.”
