Insights into antibiotic resistance gene dynamics during Tanfloc-induced flocculation-storage process for...

Tannin-based flocculants (TA) are increasingly promoted as green polymeric alternatives for cyanobacteria removal in algae-laden drinking water sources, yet their potential to influence antibiotic resistance gene (ARG) dissemination during subsequent flocculation-storage remains unclear. This study compared TA with polyaluminum chloride (PACl) to assess ARG fate in both supernatant and cyanobacteria-laden drinking water sludge throughout flocculation and 8-day storage. Results showed that TA achieved over 97% removal efficiency for both cyanobacteria and ARGs at a low dosage of 20 mg/L, outperforming PACl. Moreover, TA treatment led to markedly reduced release of microcystin-LR and dissolved organic matter (DOM) after storage. Nevertheless, elevated biomass within TA-induced flocs promoted ARG proliferation, primarily due to enhanced production of triplet-state DOM and suppression of carotenoid synthesis. Metagenomic evidence indicated elevated abundances of CAZyme genes (e.g., GH43, CE4, CE1, GH9, CE11), highlighting an increased functional potential for TA-associated polymer breakdown and consequent weakening of the floc coating after 8 days, which in turn promoted ARG escape from flocs. Meanwhile, increased motility of phycosphere-associated antibiotic-resistant bacteria (e.g., Pseudomonas) promoted ARG transfer into the supernatant, accompanied by enrichment of mobile genetic elements and virulence factor genes, which collectively amplified ecological risks. These findings underscore that ARG release and dissemination should be explicitly integrated into safety assessments of TA-based cyanobacteria control, and they provide mechanistic guidance for mitigating ARG hazards in algal-affected drinking water supplies.