New insights into the vertical distribution and microbial degradation of microplastics in urban river sediments

Sediments have been found to be one of the most important reservoirs for microplastics, providing abundant indigenous microbes. The processes involved in the distribution and degradation behavior of microplastics are complex. This study investigated the vertical distribution of microplastics (with the size < 5 mm) and the bacterial community assemblages colonizing microplastics in urban river sediments at a depth from 0 to 50 cm. The results showed that both microplastics and associated microbial communities presented vertical profiles in river sediments. The mean concentration of microplastics increased from the shallow layers to the deep layers of sediments, and smaller microplastic particles were dominant in deeper layers. A greater degradation of microplastics in deeper layers was confirmed by contact angle measurements, scanning electron microscopy and Fourier transform infrared spectroscopy-attenuated total reflectance analyses. Unlike the surrounding sediments, the whole bacterial communities on microplastics exhibited higher frequency of positive correlations in the bacterial co-occurrence network, which indicated a less stability of bacterial communities on microplastics. The indicative plastic-degrading bacteria with an average abundance of 4.33% was found in the surrounding sediments, while on the microplastics 21.37% was found. From shallow layers to deep layers, the indicative plastic-degrading bacteria significantly increased both in the abundance and in their betweenness centrality in the co-occurrence network, which suggested a potentially primary role of these bacteria in the degradation of microplastics in deep layers. This study provides new insight into the vertical distribution and the potential microbial degrading characteristics of microplastics in urban river sediments, which expanded our understanding of the fate of microplastics in aquatic environments. The observed results implied a great risk that microplastics might become smaller and more in deepened sediments and finally migrate into groundwater.