Infectious agents shape fish populations by inducing lethal and sublethal changes that alter nutrient metabolism and metal bioaccumulation. These shifts can manifest as changes in the ionome—the specific combination of essential and non-essential chemical elements defining the whole-body composition of an individual. Understanding how pathogens shape the fish ionome is critical for developing advanced monitoring tools and clarifying the ecological roles of hosts and their pathogens. This study reports the first documented outbreak of Citrobacter freundii, Pseudomonas aeruginosa, Pseudomonas mosselii, and Shewanella xiamenensis bacterial infections in wild-caught eastern mosquitofish (Gambusia holbrooki) from three populations in Extremadura, southwestern Spain. Under laboratory-controlled conditions, we established associations between these bacterial outbreaks and the whole-fish body ionome of G. holbrooki. We compared 19 chemical elements and seven elemental ratios among diseased fish, healthy fish at the outbreak, and individuals fully recovered 100 days post-infection following antibiotic treatment. The fish ionome clearly discriminated between diseased and healthy states, and the response was consistent across all three populations. Our findings support the utility of whole-fish body elemental composition in G. holbrooki as a biomarker for environmental monitoring. Furthermore, as the bacterial infections were associated with the capture and transport-induced stress of wild individuals, this study provides critical data on the opportunistic pathogens that may be co-introduced into recipient ecosystems through the release of this widely distributed invasive fish species.
Cano-Rocabayera et al. (Thu,) studied this question.