Pelagic Shuttles of Antibiotic Resistance Genes: Zooplankton as Overlooked Vectors Across Space and Food Webs.

Antibiotic resistance genes (ARGs) accumulate in aquatic environments, where they create reservoirs and transmission pathways that can undermine antimicrobial treatments and alter the microbial community structure in ways that ultimately affect human and animal health. However, the contribution of zooplankton in these pathways remains critically overlooked. Emerging evidence shows that compared with surrounding water, copepods and cladocerans accumulate ARG loads that are one to two orders of magnitude greater, acting as microbial hotspots that disperse resistant bacteria across seasons and depths. Inside protistan vacuoles, densely packed prey cells undergo conjugation, rapidly accelerating horizontal ARG transfer. Long-term archives reveal persistent ocean-wide dissemination of the class-1 integron integrase (intI1) and sul2 genes since at least the 1970s. Here, I synthesize mechanistic and field evidence, pinpoint knowledge gaps, and recommend priorities: integrate zooplankton into routine ARG surveillance, quantify biofilm-mediated exchanges, and mitigate contamination from coselective pollutants to curb zooplankton-driven ARG propagation. By framing zooplankton-associated ARG dynamics within the broader community ecology of antimicrobial resistance, this mini-review highlights how aquatic food-web processes feed back into the emergence, evolution, and transmission of resistance that concerns for One Health outcomes beyond the clinic.