Introduction The impact of ocean acidification (OA) and nanoplastics (NPs) on harmful algal blooms (HAB) has emerged as a major global concern. However, the combined effects of OA and NPs on the HAB species are poorly understood. Methods In this study, dinoflagellate Prorocentrum donghaiense , a typical HAB species, was exposed to varying concentrations of NPs (108.15 ± 8.52 nm) (0, 5, 10, and 15 mg L −1 ) and CO 2 (low CO 2 :...