Nanoplastics are pervasive pollutants in the environment and pose a significant threat to the health of ecosystems. Their presence in the marine environment raises concerns about their impact on marine biota, including microalgae, which are important primary producers in marine ecosystems and form the basis of the marine food web. While research on the potential environmental impacts of nanoplastics has increased in recent years, the ecotoxicological effects of nanoplastics on marine microalgae have been rarely studied. Our AlgaNanoPlast project aims to investigate the effects of polystyrene nanoplastics (PS NPs) on the nanostructural, nanomechanical and physiological responses of microalgae. We will investigate the effects of PS NPs (both pristine and artificially aged) with different surface charges and sizes on two selected marine microalgae species: the diatom Cylindrotheca closterium and the green alga Dunaliella tertiolecta. The response of the selected microalgae to nanoplastics will be studied at both the population and single-cell level. This will include monitoring growth inhibition, changes in the nanostructural and nanomechanical properties of the cells, measuring the response to oxidative stress and assessing the effects on photosynthetic activity. Given the crucial role of microalgae in the marine food web and ecosystem functioning, the results of the AlgaNanoPlast project will contribute significantly to the understanding of the ecological impact of nanoplastics on the entire marine ecosystem.