Interplay of diet, heat stress, and the microbiome shapes health and escape behavior in amphibian larvae

What animals eat modulates their microbiome and is fundamental to their health. Microbiomes can improve hosts’ ability to cope with environmental stressors, including increased temperatures and altered food quantity and quality associated with climate change. Using a multifactorial experimental design, we tested whether three diets with increasing amounts of protein, fat, and components of animal origin (designated low-, intermediate-, and high-quality), two rearing temperatures (18 °C or 24.5 °C), and exposure or not to a heat wave (28 °C for 48 h) shaped the gut bacterial community of amphibian larvae (Rana temporaria). We then examined how the treatments, associated shifts in gut bacterial communities, and predicted metabolic pathways related to larvae nutrient assimilation (isotopic signatures), health (body condition and developmental rate), and escape behavior. Larvae maintained their body condition and developed faster at 24.5 °C, with higher diet quality (i.e., reduced herbivory) further accelerating development at this temperature. The intermediate-quality diet reduced the ability of larvae to react to an aversive stimulus at 24.5 °C, but this effect did not occur in larvae exposed to the heat wave. The heat wave may have triggered an increase in the abundance of Klebsiella, together with an increase in the myo-inositol degradation pathway, which influences cell membrane fluidity and signaling and may increase attention levels. Similar outcomes in host performance under most experimental conditions highlight the potential plasticity of the bacterial community and the presence of alternative enterotypes with functionally redundant metabolic capacities compatible with host health.