Aquaculture oxidant (ClO2) or antibiotic disinfection induces unique bimodal aggregation and boosts exDNA sedimentation: A disinfection-driven great spatial shift of antibiotic resistance risk

ClO₂ has been ever-increasingly used as an alternative disinfectant to alleviate antibiotic resistance risk in aquaculture. However, the feasibility of ClO₂ disinfection in reducing antibiotic resistance has not been clarified yet. We comparatively explored the aggregation mechanisms and their effect on extracellular DNA (exDNA) partition and settlement in disinfected aquaculture waters and natural waters. In contrast to the unimodal aggregation in natural non-aquaculture waters, a unique bimodal size distribution pattern of micron-sized aggregates was found in aquaculture waters regardless of the disinfectants (macro-aggregates – 200-700 μm in diameter and micro-aggregates – 2-200 μm in diameter). The bimodal aggregates had 2-4 orders of magnitude higher content of Ferron cations and enriched hundred-fold exDNA in aquaculture waters than in natural waters. ExDNA was adsorbed on the surface of aggregates and conglutinated mainly by carbohydrates and coagulative cations. Macro-aggregates had lower fractal dimension but greater sedimentation velocities compared with micro-aggregates. Polylithionite was the key ballast mineral facilitating fast sedimentation of aggregates in aquaculture waters. Loading more antibiotic resistance genes and mobile gene elements, the aquaculture aggregates sank more rapidly from water to sediments than the natural-water aggregates. It indicates that disinfection with ClO₂ or antibiotics facilitated the spatial transfer of antibiotic resistance risk with high horizontal transfer potential from water column to sediment through forming bimodal aggregates. These findings imply that the adoption of antibiotic alternatives such as the oxidant of ClO₂ is far from sufficient to alleviate antibiotic resistance in aquaculture.