Adaptation strategies of cyanobacteria under trace tetracycline exposure: A comparative study of toxigenic and non-toxigenic strains.

Trace antibiotics exert unclear ecological impacts on cyanobacteria, especially the differential responses between toxigenic and non-toxigenic strains. To address this, we established a paired-strain comparative framework exposing toxigenic Microcystis aeruginosa and non-toxigenic Synechocystis sp. to environmentally relevant tetracycline (0.2, 2, 20 μg/L) for 14 days. Results showed that TC induced hormetic growth in both strains, with final cell density increases of 2.47-9.79% (M. aeruginosa) and 4.98-8.59% (Synechocystis sp.) relative to controls, but exhibited divergent adaptation strategies. M. aeruginosa optimized photosystem efficiency, shifted to peroxidase (POD) and non‑enzymatic antioxidant defense, and down-regulated phosphate uptake. Synechocystis sp. expanded light‑harvesting complexes, co‑activated superoxide dismutase (SOD) and POD, and maintained elemental homeostasis. Transcriptomics revealed shared up-regulation of core metabolism and translation, providing a conserved molecular basis for the hormetic effect. Weighted gene co‑expression network analysis showed 11 modules in M. aeruginosa versus 5 modules in Synechocystis sp., indicating intrinsic molecular differences underlying the divergent adaptation strategies. Notably, TC promoted the synthesis and accumulation of microcystins (MCs) in M. aeruginosa but suppressed extracellular release, and the MC‑LR proportion decreased extracellularly but increased intracellularly. Thus, monitoring dissolved MCs alone may underestimate toxin risks and miss the risk of secondary contamination upon cell lysis. Overall, these findings reveal divergent adaptation strategies and shared regulatory mechanisms between toxigenic and non-toxigenic cyanobacteria, along with MCs metabolic remodeling under trace TC, offering new insights into bloom management under antibiotic pollution.