Microplastics and nanoplastics are emerging pollutants, widespread both in marine and in freshwater environments. Cyanobacteria are also ubiquitous in water and play a vital role in natural ecosystems, using photosynthesis to produce oxygen. Using photography, fluorescence microscopy and cryogenic and scanning electron microscopy (cryo-SEM, SEM) we investigated the physicochemical response of one of the most predominant seawater cyanobacteria (Synechococcus elongatus, PCC 7002) and freshwater cyanobacteria (S. elongatus Nageli PCC 7942) when exposed to 10 mu m diameter polystyrene (microPS) and 100 nm diameter polystyrene (nanoPS) particles. Marine and freshwater cyanobacteria formed aggregates with the nanoPS, bound together by extracellular polymeric substances (EPS), and these aggregates sedimented. The aggregates were larger, and the sedimentation was more rapid for the marine system. Aggregate morphologies were qualitatively different for the microPS samples, with the bacteria linking up a small number of particles, all held together by EPS. There was no sedimentation in these samples. The cyanobacteria remained alive after exposure to the particles. The particle size- and salt concentration-dependent response of cyanobacteria to these anthropogenic stressors is an important factor to consider for a proper understanding of the fate of the particles as well as the bacteria.