Effects of environmental changes in the water column on microbial communities on microplastic surfaces during ocean entry

Microplastics (MPs) act as mobile substrates for microbial colonization during their transport from land to sea, forming unique microbial reservoirs with potential ecological implications. Understanding the dynamics of these surface-associated communities is essential for assessing the environmental fate and risks of MPs. However, how shifts in aquatic environmental conditions influence the assembly and succession of MP biofilm communities remains unclear. In this study, water samples were collected from river, estuarine, and marine environments (the Jiuzhou River in Zhanjiang, China, and the Beibu Gulf region), and a laboratory based microcosm culture system was established to investigate changes in microplastic communities in response to shifts in the aquatic environment. It is worth noting that in marine environments, the number of taxonomic units exhibiting significant differences in abundance on MPs surfaces is higher than in river environments. Notably, the number of taxa exhibiting significant differential abundance on MPs surfaces was higher in marine than in riverine environments. Additionally, a rapid decrease in MPs-surface biomass was observed within 3 h following transitions from riverine to estuarine and from estuarine to marine conditions, suggesting that cross-environmental transport may temporarily inhibit biofilm biomass accumulation. This study indicates that changes in the aquatic environment and variations in salinity gradients may influence the colonization of microplastics by microorganisms, thereby deepening our understanding of the ecological role of microplastics in dynamic aquatic systems.