Amazonian Fish and Their Bacterial Symbionts: An Intricate Co-Dependence Relationship in a Changing Environment

Fish are hosts to symbiotic bacteria that form diverse communities, which play key functions for their host immunity, nutrition, development, and physiology. Numerous studies documented the existence of these bacterial communities in the guts, skin mucus, and gills of Amazonian fish. Dominated by Betaproteobacteria, Amazonian fish microbiotas play critical roles for their host, notably in the modulation of ionoregulatory processes, which enable fish to thrive in physiologically challenging acidic and ion-poor blackwater found throughout the Amazonian basin. In this report, we detail the composition and roles of Amazonian fish microbial symbionts, and we discuss their sensitivity to various host- and environment-specific factors such as host genotype, parasitism level, diet, and environmental physicochemistry and bacterioplankton composition. All these factors will be affected by future anthropogenic perturbations such as deforestation, dam construction, riverine agriculture, mining, oil exploitation, and climate change. Downstream effects on fish microbiotas are suspected to include a dysbiosis, an increase in the abundance of pathogenic bacteria, a shift in community diversity, and a perturbation of the equilibrium in fish-microbiota-parasite interactions. We conclude that Amazonian fish can instantly acquire adaptive functions via the recruitment of new microbial symbionts, which could benefit their ability to cope with present and future threats.