Within the intestines of all creatures reside remarkable unsung communities of extraordinary microorganisms. Thriving on the nutrition passing through, these digestive heroes also contribute to the general wellbeing of their host, influencing their immunity and metabolism, and even affecting personality. But Jason Dallas and Robin Warne from Southern Illinois University, USA, wondered whether these bacterial inhabitants – the gut microbiome – may also contribute to how cold blooded (ectothermic) creatures cope with warm conditions. ‘Green frogs breed in warmer temperatures than wood frogs and their larvae are active over the summer months’, says Dallas, explaining that green frogs are generally better prepared for warm conditions than wood frogs. But could the frog's intestinal lodgers contribute to their ability to deal with heat? ‘We sought to examine how alterations to the gut microbiome of wood frog tadpoles would influence their ability to persist under high temperatures’, says Dallas, who decided to replace the microbiome of wood frog tadpoles with the microbiome from hardier green frogs, to find out whether the green frog's intestinal community could improve the wood frog's ability to handle heat.
Collecting freshly laid wood frog frogspawn from local ponds, Dallas and Warne gently washed the eggs in antibiotics to remove any trace of the bacteria that their parents may have left, and then bathed some of the eggs in the bacterial soup collected from the intestines of green frogs. After allowing the freshly hatched wood frog tadpoles to graze on their egg jelly, to cement their green frog-donated gut bacteria, the researchers then left the youngsters to grow before transferring some to warm water (23°C) while others were kept cooler at 15°C, to adjust to summer- and spring-like conditions. Then, 3 days later, the team tested the tadpoles’ ability to withstand heat by warming them gradually from 20°C to 40°C, checking whether they were capable of fleeing after a gentle tap on the tail, to find out whether the transplanted gut microbiome affected their ability to withstand the heat.
Impressively, the gut microbiome-swapped wood frog tadpoles that had grown up in 15°C water were much better prepared for the heatwave than wood frog tadpoles equipped with their own natural microbiome. On average, the tadpoles carrying a green frog gut microbiome were able to cope with water temperatures ∼38.3°C while normal wood frog tadpoles could only stand water up to ∼38.1°C. ‘Our study is the first to identify the cross-species gut microbiota transplantations influence the heat tolerance of the recipients’, says Dallas.
Anna Kazarina and Sonny Lee (Kansas State University, USA) then compared the bacterial population from the guts of the transplanted wood frog tadpoles with the gut bacteria of regular wood frog tadpoles. The transplanted tadpoles’ gut microbiome included more Mucinivorans hirudinis bacteria, which produce acetate, a molecule that can protect cells from damaging toxins produced by metabolism as well as boosting energy-producing mitochondria. ‘This could represent a pathway by which the green frog-treated larvae benefited from the bacteria's increased relative abundance’, says Dallas. And when the team compared the gut microbiomes from regular wood frog tadpoles grown in warm and cooler circumstances, it was clear that the temperature significantly changed which microbes were living in the tadpoles’ intestines, potentially affecting how well prepared they are to cope with heatwaves.
So, the community of microbes inhabiting the guts of tadpoles contributes to their ability to withstand rising temperatures, and Dallas suggests that microbiome transplants from more robust species could offer hope to species at risk as global temperatures rise.
