Long-term study of the bacterial colonization of polypropylene microplastics in a freshwater lake by optical and molecular methods

Bacterial colonization and the associated bacterial community on microplastic surfaces in freshwater might alter the physiochemical properties of the plastic surfaces. In our study, a long-term investigation of these potential alterations was performed, as biofilm development needs a more extended period to form, and it could change in connection with the environmental conditions or the location. In this study, our formerly published[56], self-developed plastic colonizer method was used for continuous monitoring of plastic associated bacterial community. The same colonizers, filled with commercially available polypropylene plastic straws cut into less than 5 mm small pieces (microplastic particles), were submerged in a freshwater lake in Hungary (Vácszentlászló), but in this study these sampling tools were kept in the freshwater environment for a much longer period. Time of incubation was fourteen months, started from April 2019 and last until June 2020. Samples from plastic colonizers and a lake water were collected every month (first was taken in May 2019) during this study. The waterborne and polypropylene-associated bacterial communities of these samples were analyzed by Illumina 16S rRNA gene amplicon sequencing. The plastic-associated bacterial community was described at different taxonomic levels. In the first month, the water-associated waterborne community was dominated by Proteobacteria and Cyanobacteria. Proteobacteria followed by Bacteroidetes, and Actinobacteria dominated the polypropylene-associated community. Over time, the bacterial composition on the polypropylene (PP) surface, in terms of phylum level, was altered, eventually becoming distinct from the bacterial community found in the lake water. In addition, Raman microscopy, scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were used to study the physiochemical changes of the incubated microplastic surfaces due to bacterial colonization. The physical changes were dominant, with the relatively little ageing impact on the chemical composition. Surface morphology and composition of the plastic were studied as they could significantly influence the physical properties of microplastics, the microbiota, and environmental interactions that affect plastisphere colonization. General water analyses of the lake water was also the part of the research, and it was conducted four different time points during the study. Some of the measured components (mostly in August and October of 2019) exceeded the Hungarian governmental threshold for water quality.