Metabolic reprogramming in Oocystis borgei drives the biosynthesis of gold nanoparticles

Microalgae-mediated biosynthesis of gold nanoparticles (AuNPs) offers a sustainable alternative for nanomaterial synthesis; however the metabolic mechanisms underlying the reduction of HAuCl₄ and the formation of AuNPs remain unclear. In this study, the green microalga Oocystis borgei was employed to synthesize AuNPs, and associated cellular metabolic alterations were systematically examined using transmission electron microscopy (TEM) and untargeted LC-MS-based metabolomics. TEM revealed AuNP accumulation in the chloroplasts, accompanied by substructural disorganization. Metabolomic profiling identified 1,871 upregulated and 1,681 downregulated ions in positive ion mode, and 1,092 upregulated and 1,032 downregulated ions in negative ion mode. Among the significantly upregulated metabolites, fatty acyls, glycerophospholipids, and oxidanesulfonic acid were associated with lipid peroxidation and membrane remodeling. In contrast, downregulated metabolites, including carbohydrates, nucleic acids, and chlorophyll derivatives, indicated suppressed growth and metabolic activity. The differentially abundant metabolites(DAMs) were enriched in purine metabolism, porphyrin and chlorophyll biosynthesis, and plant hormone signaling pathway. Notably, porphyrin and chlorophyll degradation and amino acid/protein metabolism suppression correlated with growth inhibition, whereas oxidised lipids (19-hydroxy-nonadecanoic acid) and stress-responsive metabolites (brassinosteroids and carotenoids) facilitated AuNP synthesis. These findings elucidate the metabolic trade-offs in O. borgei during HAuCl₄ detoxification and AuNP biogenesis, enhancing our mechanistic understanding of algal-based nanomaterial synthesis.