Antibiotic Stress Reshapes Physiological Homeostasis, Lipid Metabolism, and Culture-Associated Bacterial Communities in Chlorella pyrenoidosa.

Antibiotics are widely used to decontaminate microalgal cultures, but their effects on algal physiology and associated bacterial communities remain unclear. We examined the concentration- and compound-dependent responses of Chlorella pyrenoidosa to ampicillin sodium, sulfanilamide, chloramphenicol, and streptomycin sulfate. Growth, culturable bacterial abundance, pigment content, photosynthetic performance, and fatty-acid composition were assessed across a mixed-antibiotic concentration gradient and under individual treatments. Higher antibiotic concentrations suppressed bacteria but inhibited algal growth, reduced chlorophyll accumulation, and altered fatty-acid composition. Physiological disruption increased in the order ampicillin sodium < sulfanilamide < chloramphenicol < streptomycin sulfate. Streptomycin sulfate produced the strongest response, reducing maximum quantum yield of PSII photochemistry (Fv/Fm) and light-use efficiency by 51.4% and 44.4%, respectively, while increasing total polyunsaturated fatty acids from 59.68% to 79.57%. 16S rRNA gene sequencing and untargeted metabolomics showed that streptomycin sulfate reduced bacterial diversity and increased the relative abundance of Proteobacteria from 44.54% to 90.66%. Lipid-related pathways were altered, particularly α-linolenic acid and glycerophospholipid metabolism. Bacterial-community and metabolomic changes covaried under STRS exposure, and responsive taxa correlated with phospholipid- and oxylipin-related metabolites. These findings identify antibiotic acts as a selective stressor that alters algal physiology, lipid metabolism, and associated bacterial communities, highlighting the need for species-specific decontamination strategies.