Biofilm-mediated interactions between plastics and radiocesium in coastal environments

A ubiquitous distribution of plastic debris has been reported in aquatic and terrestrial environments; however, the interactions between plastics and radionuclides and the radioactivity of environmental plastics remain largely unknown. Here, we characterize biofilms developing on the surface of plastic debris to explore the role of plastic-associated biofilms as an interaction medium between plastics and radiocesium (¹³⁷Cs) in the environment. Biofilm samples were extracted from plastics (1-50 mm in size) collected from two contrasting coastal areas in Japan. The radioactivity of plastics was estimated based on the ¹³⁷Cs activity concentration of the biofilms and compared seasonally with surrounding environmental samples (i.e., sediment and sand). ¹³⁷Cs traces were detected in biofilms with activity concentrations of 21-1300 Bq·kg^-1 biofilm (dry weight), corresponding to 0.04-4.5 Bq·kg^-1 plastic (dry weight). Our results reveal the interaction between ¹³⁷Cs and plastics and provide evidence that organic and mineral components in biofilms are essential in ¹³⁷Cs retention in environmental plastics. Given the ubiquitous distribution of plastic debris in the environment, more attention should be directed to bioaccumulation and the radioecological impacts of plastic-associated radionuclides on ecosystems.