The development of nanomaterials for various purposes has led to the diversification of their fields of application but has also raised questions regarding the environmental safety of releasing NPs into the environment. In the biotechnology of microalgae and cyanobacteria, NPs can serve as biotechnological tools to control specific processes. In the field of biosecurity, these microorganisms can be used as models for NP toxicity. Among the metal nanoparticles, gold and silver nanoparticles stand out due to their multiple functionalities, unique properties, and wide practical applications. The effect of 10 nm - sized Au and Ag nanoparticles stabilized in citrate has been studied on cyanobacterium A. platensis (spirulina) and microalga P. cruentum. Both cultures were grown in media with nanoparticles in different concentrations. It was found that AgNPs reduced the biomass production of A. platensis and altered its quality. In the case of P. cruentum culture, AgNPs provided a significant increase in the content of phycobiliproteins and lipids, while reducing the carbohydrate content. The effect of AuNPs resulted in increased phycobiliprotein and lipid content in A. platensis and P. cruentum biomass. In P. cruentum culture, AuNPs caused a significant reduction in protein content. In both cultures, nanoparticles induced an increase in the oxidative stress marker - malondialdehyde, and a decrease in the antioxidant activity. These microorganisms can be used as test objects to identify the toxic effects of NPs. At the same time, NPs can serve as tools for directing certain biotechnological processes in the cultivation technologies of spirulina and porphyridium.