Astaxanthin and cellular metabolites production in Haematococcus lacustris exposed to silver nanoparticles

Introduction Silver nanoparticles (AgNPs) can modulate microalgal metabolism in a dose-dependent and stage-specific manner. Haematococcus lacustris, a key astaxanthin-producing microalga, is highly sensitive to environmental stressors that regulate the transition from the green vegetative stage to the red aplanospore stage. Materials and Methods The effects of 10 nm and 20 nm citrate-stabilized AgNPs (0.01–5 mg L-1) were assessed on biomass, cellular metabolites, photosynthetic pigments, lipids, and astaxanthin. Nanoparticles were introduced on day 3, with analyses performed at the end of the green (day 9) and red stages (day 16). Results Low to moderate AgNPs concentrations (0.01–1 mg L⁻¹) increased biomass, proteins, carbohydrates, and lipids during the green stage, while 5 mg L⁻¹ inhibited growth and pigments and elevated MDA levels. In the red stage, all concentrations reduced final biomass; however, 10 nm AgNPs at 0.01–0.5 mg L⁻¹ boosted astaxanthin (by up to ~30%) and lipids (by up to ~96%). Higher doses, along with all 20 nm AgNP treatments, suppressed astaxanthin accumulation. Conclusions H. lacustris exhibits a hormetic response to AgNPs: mild exposure stimulates key metabolites, while higher concentrations become inhibitory. Nanoparticle size, dose, and timing are crucial for precisely directing metabolic pathways and improving astaxanthin yield.