<p>Dinoflagellates are major contributors to marine phytoplankton and many species emit blue bioluminescence from specialized organelles called scintillons in response to mechanical stimulation and transient intracellular acidification. Because pollutants can disrupt this pathway, changes in light output provide a sensitive proxy for environmental stress. We utilized the bioluminescence of dinoflagellate <i>Pyrocystis lunula</i> to develop a rapid, high-throughput toxicity assay. Cultures aged 30-40 days were exposed to a range of metal concentrations for 24 hours, and stimulus-evoked emission was quantified in 96-well plates using a microplate luminometer. All metals tested produced dose-dependent inhibition of bioluminescence, with sensitivity varying by contaminant. Non-essential metals showed the strongest inhibitory effects: Cd²⁺ (half-maximal inhibitory concentration, IC₅₀, 24 h = 0.014 mg L⁻¹) and Pb²⁺ (IC₅₀, 24 h = 0.016 mg L⁻¹). Essential metals, Cu²⁺ and Zn²⁺, also reduced emission but at higher concentrations; differences were significant by one-way analysis of variance with Tukey’s post hoc test. This assay is rapid, low-cost, and scalable, offering a practical tool for monitoring trace-metal contamination and supporting ecotoxicological assessments.</p>