Horizontally acquired mitochondrial alternative oxidase contributes to springtail bioenergetics and life belowground

Springtails are tiny hexapods that occupy habitats ranging from above the soil surface to hypoxia-prone belowground environments, yet the physiological mechanisms enabling this ecological expansion have remained unresolved. Here, I take an evolutionary bioenergetics approach to show that a mitochondrial alternative oxidase (AOX) acquired by horizontal gene transfer in the springtail ancestor became functionally integrated into animal physiology and was preferentially retained in lineages occupying low-oxygen habitats. Surveying 202 springtail genome assemblies, I identified 65 high-confidence AOX loci in 48 species, each embedded within otherwise typical springtail genomic neighborhoods. Phylogenetic and motif-based analyses support an oomycete donor and indicate ancestral acquisition followed by repeated loss, especially in aboveground taxa. High-resolution respirometry and hypoxia-exposure experiments further show that AOX is active only in AOX-positive species and is associated with hypoxia tolerance. These results identify horizontal gene transfer as a source of ecophysiological innovation in animals and suggest that acquired mitochondrial functions can help shape ecological sorting across environmental gradients, with implications for soil ecosystem processes.