The microbiome as a link between diet quality, health, and escape behavior in amphibian larvae under heat stress

What animals eat is fundamental to their growth and development, and one of the main factors modulating their microbiomes. Microbiomes adjust to environmental conditions and influence host physiology and behavior. Thus, they can improve the ability of the host to deal with environmental stressors, such as those resulting from climate change, which include increased temperatures and altered food quantity and quality. 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, respectively), two rearing temperatures (18 C or 24.5 C), and exposure or not to a heat wave (28 C for 48h) shaped the bacterial community in the guts of amphibian larvae (Rana temporaria). We then examined how the treatments, associated shifts in larval gut bacterial communities, and predicted metabolic pathways were related to 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 and with increasing diet quality (i.e., reduced herbivory) at this temperature. In contrast, previous studies have shown that ectotherms prefer herbivorous diets under increased temperatures. High-quality food may not be available ad libitum in the wild, making an herbivorous diet the best available option. The intermediate-quality diet resulted in a decreased ability to react to an aversive stimulus at 24.5 C, but not in larvae exposed to the heat wave. The heat wave may have triggered an increase in the abundance of Klebsiella, with concomitant increase in the myo-inositol degradation pathway, which plays a role in controlling cell membrane fluidity and signaling and may increase attention levels. Similar outcomes in host performance under most experimental conditions highlight the possible existence of plasticity of the bacterial community and alternative enterotypes with functional redundancy that are compatible with host health.