There is an increasing concern about the potential occurrence of nanoplastics as a result of degradation processes suffered by microplastics in the environment. Biodegradable plastics are emerging as an alternative to conventional plastics mainly because they undergo rapid biotic and/or abiotic hydrolytic degradation. In this work we investigated the most relevant physicochemical features and the biological effects of nanoplastics released from microplastics, made of the biodegradable polymer polycaprolactone (PCL), that underwent artificial abiotic degradation up to one year under environmentally representative conditions. The physicochemical characterization of the nanoplastics obtained from the abiotic degradation of microplastics was performed by DLS (size distribution), ELS (surface net charge or ζ-potential), NTA (number of particles per unit volume), FTIR (chemical bonds), MALDI TOFF (polymer chain length distribution), DRX and DSC (crystallinity) and SEM (morphology). Base on the outcomes of these techniques, a mathematical model has been developed in order to estimate the potential occurrence of micro- nano-plastics and free oligomers on the aquatic environment. The ecotoxicological effects of released nanoplastics after two weeks in aqueous suspension was also assessed using two cyanobacteria: the filamentous cyanobacterium Anabaena sp. PCC7120 and the unicellular Synechococcus sp PCC7942. After abiotic incubation of the microplastics in aqueous suspension, the supernatant containing released nanoplastics, resulted to be toxic towards both microorganisms. Nanoplastics induced a significant decrease in the growth as well as several physiological alterations, studied by flow cytometry, of the two photosynthetic organisms. However, after removing the nanoplastics by ultrafiltration, only a mild toxicity remained, affecting few physiological parameters which are probably more sensitive to the leachates. In conclusion, nanoplastics released from biodegradable microplastics exhibited toxicity towards aquatic photosynthetic organisms. Our findings indicated that biodegradable plastics may not be as harmless as previously suggested.