Understanding the formation and influence of soil-typical eco-coronas on microplastics through laboratory and field incubation experiments

Microplastics (MPs), ubiquitous in terrestrial environments, are usually covered by an eco-corona (EC) under natural conditions. When mistakenly ingested by soil organisms, MPs could harm their development, reproduction, and survival rates. The EC, a natural layer on MP surfaces, contains organic matter like carbohydrates, proteins, DNA, and compounds like humic and fulvic acids. It strongly influences the transport and behaviour of MPs, by altering their surface properties, thereby affecting their adsorption efficiency. While limited research on EC formation on MP in aquatic environments exists, our understanding of typical ECs in soils is limited. Therefore, the present study aims to evaluate the identification, formation, and variation of EC on MPs in floodplain soils, and the physico-chemical changes induced on MP surfaces under different incubation conditions. Polystyrene MPs (600-1000 microns) were incubated with soil samples in cylindrical chambers (mesh size 500 microns) for 4, 8, and 16 weeks in both field (Northern floodplains in Cologne, Germany) and laboratory settings. Laboratory-incubated samples were controlled for temperature and moisture, while the field incubations were left to natural conditions. Additional soil parameters including pH, CN content, grain size, and elemental composition were also measured. After each incubation period, MPs were extracted manually and were analyzed, employing 16S-V4 and ITS1 for metabarcoding and sequencing for attached ECs (bacteria and fungi), ATR-FTIR spectroscopy for polymer-level analysis, and the SEM imaging for visual inspection along with EDS for identifying potential heavy metal attachments on MPs. While the degree of change in the lab samples stayed low, the DNA results in the field samples demonstrated that various bacterial communities formed on MP surfaces during the incubation periods. This communal change could be attributed to the variation in the environmental condition of the incubations. Both incubation settings resulted in intricate fungal structures on MP surfaces, which were also visible during SEM imaging. Potential attachments of heavy metals like Ti, Mn, Zr, Th, and Ag were also identified on incubated MP surfaces. Our findings help uncover the influence of soil organisms on environmental MPs and clarify the formation of EC in soil ecosystems, providing insights into the ecological impacts of MPs.