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

Abstract

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.