Tolerance strategy of Chlorella sorokiniana to combined antibiotic stress revealed by transcriptomic analysis

The pervasive co-occurrence of distinct antibiotic classes in aquatic environments necessitates an understanding of microalgal adaptive strategies under combined stress, a knowledge gap that persists beyond single-antibiotic studies. This study focused on the adaptive responses of a tolerant Chlorella sorokiniana strain, which was isolated and then exposed to a gradient of combined aminoglycoside kanamycin (KAN) and tetracycline (TET) stress. Physiological analysis revealed a tolerance threshold of 60 mg L^-1 for each antibiotic, characterized by initial growth and photosynthetic inhibition followed by partial recovery, suggesting an active acclimation process in this strain. Transcriptomic analysis revealed a coordinated "defense-over-growth" molecular reprogramming. The metabolic response in this strain was characterized by the strategic suppression of anabolic pathways, the fortification of the Calvin cycle, and the dramatic upregulation of glycolysis, the TCA cycle, and oxidative phosphorylation, collectively fueling a multi-layered defense system. To counteract antibiotic stress, C. sorokiniana deploys a multi-pronged strategy within its defense system: active efflux (ABC transporters), an enhanced antioxidant enzyme network, and the upregulation of putative catabolic enzymes like aldehyde dehydrogenase. We propose a novel integrative model where tolerance in C. sorokiniana emerges from the synergistic regulation of a critical system that coordinates photosynthesis, energy metabolism, and detoxification to counteract combined antibiotic stress. These findings delineate the tolerance and underlying adaptive mechanisms of microalgae under combined antibiotic stress. This work provides a genetic blueprint for engineering enhanced phytoremediation strains, and highlights microalgae as active bioremediators capable of deploying sophisticated biochemical machinery to mitigate complex antibiotic pollution.