Links Between Environmental Quality, Gut Microbiota, and Host Performance in Nutrient-Impacted Springs

Microbial symbionts may enhance adaptation to environmental stress, yet their role in nutrient-enriched freshwater habitats remains unclear. We conducted a full-factorial laboratory experiment with least killifish, Heterandria formosa, manipulating water chemistry from four Florida springs and gut microbiota inoculations from conspecifics at those sites. Sequencing of 16S rRNA revealed that gut microbial richness was higher in fish reared in less nutrient-loaded waters, but inoculation treatments, if successful, did not produce significant shifts in composition. Gut communities consistently differed from ambient water, with enrichment of Pseudomonadota and Planctomycetota. Females grew more slowly in the most polluted water. In a parallel result, males displayed higher mass-specific metabolic rates in the most nutrient-loaded water. Reproductive output per female nearly doubled when experimental microbiota and water originated from the same site, despite stable microbial diversity, though parentage and inoculation success were not verified; therefore, these results should be interpreted cautiously. These results indicate that environmental quality shapes gut microbiota and host performance, and that matching microbial communities to habitat conditions can boost reproduction. Such interactions may contribute to freshwater fish resilience and merit integration into conservation strategies.