Although interactions of metallic nanoparticles (NPs) with various microorganisms have been previously explored, few studies have examined how metal sensitivity impacts NP toxicity. The present study investigated the effects of copper NPs (Cu-NP) exposure on the model alga Chlamydomonas reinhardtii in the presence and absence of the essential micronutrient copper. The toxic ranges for Cu-NPs and the ionic control, CuCl2 , were determined using a high-throughput adenosine triphosphate (ATP)-based fluorescence assay. The Cu-NPs caused similar mortality in copper-replete and copper-deplete cells (median inhibitory concentration [IC50]: 14-16 mg/L) but were less toxic than the ionic control, CuCl2 (IC50: 7 mg/L). Using this concentration range, the Cu-NP impacts on cell morphology, copper accumulation, chlorophyll content, and expression of stress genes under both copper supply states were assessed. Osmotic swelling, membrane damage, and chloroplast and organelle disintegration were observed by transmission electron microscopy at both conditions. Despite these similarities, copper-deplete cells showed greater accumulation of loosely bound and tightly bound copper after exposure to Cu-NPs. Furthermore, copper-replete cells experienced greater loss of chlorophyll content, 19% for Cu-NPs, compared with only an 11% net decrease in copper-deplete cells. The tightly bound copper was bioavailable as assessed by reverse-transcriptase quantitative polymerase chain reaction analysis of CYC6, a biomarker for Cu deficiency. The increased resistance of copper-deplete cells to Cu-NPs suggests that these cells potentially metabolize excess Cu-NPs or better manage sudden influxes of ions. The results suggest that toxicity assessments must account for the nutritional status of impacted organisms and use toxicity models based on estimations of the bioavailable fractions.