Nanoparticle-Induced Hormesis in Microalgae: Insights from Transcriptomics and Soft X-ray Tomography

The combination of near-native, three-dimensional (3D) cryo-soft X-ray tomography (cryo-SXT) with transcriptomics establishes a multiscale platform to link organelle-level structural remodeling to dose-dependent gene expression changes from contaminant exposure. Applying this platform to AgNPs-induced hormesis in Chlamydomonas reinhardtii, we directly correlate organelle remodeling, such as alterations in lipid droplets and starch-associated structures, with transcriptional reprogramming of energy metabolism pathways across exposure gradients. This approach reveals how molecular regulation translates into structural adaptation under hormetic versus toxic conditions. Comparison with silver ions (Ag⁺) controls indicates that the observed biphasic effects are largely attributable to dissolved Ag⁺. At low dose (25 μg/L), AgNPs trigger hormetic adaptation, as evidenced by starch sheath thickening and lipid droplets (LDs) shrinkage, driven by upregulation of starch synthesis genes and suppression of triacylglycerol synthesis. Conversely, high-dose (300 μg/L) exposure induces cytotoxicity, wherein triacylglycerol synthesis is promoted and stress-associated metabolic reallocation occurs, accompanied by LDs expansion, starch granule enlargement, pyrenoid shrinkage, and starch sheath thinning. Furthermore, under high-dose stress, intracellular carbon pools are diverted toward starch biosynthesis, resulting in a compositional shift toward highly branched amylopectin. Intriguingly, LDs and starch sheath display biphasic dose-dependent structural changes, identifying them as sensitive biomarkers of cellular state. By quantitatively linking transcriptional reprogramming with organelle remodeling, this study establishes a correlative imaging-omics platform that enables direct association between molecular regulation and structural phenotypes in organisms exposed to nanoparticles.