Direct Evidence of Microplastic-Mediated Microbial Migration Across the River-Sea Transition via a Novel Field-Laboratory Coupled Approach

Large amounts of microplastics (MPs) are transported annually from river into the ocean. Biofilm-covered MPs, termed as the "plastisphere", may mediate microbial transfer. Previous studies have mostly focused on the evolution of the plastisphere itself, covering field experiments and its transformation during migration. Direct evidence for their impact on marine communities is still limited. To address this, we combined field and laboratory experiments to directly evaluate the effects of MPs on marine microbial communities along the river-sea shift. MPs were incubated for 0, 28, and 140 days in freshwater. They were then transferred to a laboratory-simulated marine micro-ecosystem constructed with a fresh seawater microbiome to allow the microbial communities to acclimate, and then further incubated in the laboratory for 1, 3, and 7 days. Microbial community dynamics were examined using metagenomic analysis. Long-term incubated plastispheres (140 days) rapidly shifted marine community structure toward plastisphere-like composition as early as Day 1. However, this overall structural change faded by Day 7. Interestingly, the presence of 28-day and 140-day plastispheres led to a consistent increase in microbial species diversity and a higher number of antibiotic resistance genes (ARGs) and virulence factors (VFs), this effect persisted through Day 7. Additionally, salt-tolerant, potentially pathogenic bacteria were also detected, reflecting the as carrier roles of plastispheres. This study provides direct evidence that plastispheres mediate microbial transfer, thereby enhancing diversity and spreading ARGs and VFs, contributing to a better understanding of the potential ecological and environmental risks of microplastics.