The innate immune system is the first player involved in the recognition/interaction with nanomaterials. Still, it is not the only system involved. The co-evolution of the microbiota with the innate immune system built an interdependence regulating immune homeostasis that is poorly studied. Herein, the simultaneous interaction of iron-oxide nanoparticles (Fe-oxide NPs), immune cells, and the microbiota associated with the blood of the sea urchin Paracentrotus lividus was explored by using a microbiota/immune cell model in vitro-ex vivo and a battery of complementary tools, including Raman spectroscopy, 16S Next-Generation Sequencing, high-content imaging, NanoString nCounter. Our findings highlight the P. lividus immune cells and microbiota dynamics in response to Fe-oxide NPs, including i) morphological rearrangement and immune cell health status maintenance (intracellular trafficking increasing, no …