Exposure to Environmental Microbes Alters Responsiveness of Tadpole Gut Microbiome to Dietary Tannins

Amphibian larvae consume variable diets in the wild, which can include tannin-rich plant material. Tannins are secondary metabolites that, when consumed, could have complex effects on herbivorous amphibian larvae, including altering their microbiome. Previous studies on the effects of dietary tannic acid on tadpoles were performed using laboratory water, largely devoid of natural microbes. Given that tadpoles acquire much of their gut microbiota from the aquatic environment, we sought to understand the effect of dietary tannins on tadpole size and the gut microbiome when animals were raised in natural pond water, supplying a diverse repertoire of microbes. We raised Green Frog (Lithobates clamitans) tadpoles in autoclaved (microbially depleted) or natural (microbially rich) pond water treatments for 4 weeks. Tadpoles were fed a control diet or a diet containing 2% tannic acid. Tadpoles raised in natural pond water had a greater body mass and length than those raised in autoclaved pond water, but dietary tannins had no effect on body size. Gut bacterial diversity was profiled using 16S rRNA sequencing. The gut microbiome from tadpoles raised in natural pond water was more diverse than that of tadpoles raised in autoclaved pond water. In general, dietary tannins caused a decrease in bacterial diversity and a net reduction in the relative abundance of potentially pathogenic bacterial genera in tadpoles raised in natural pond water but not autoclaved pond water. This study highlights the importance of replicating natural microbial contexts in captive experiments to better investigate biological interactions, such as plant-herbivore and host-microbe interactions.