Effects of Biofouling on the Sinking Behavior of Microplastics in Aquatic Environments

Abstract

Microplastics (MPs) feature a range of characteristics such as size, shape, polymer type and associated density, and additive composition, which shape their sinking behavior. In addition, a number of processes affect MPs when they enter aquatic environments, causing physicochemical changes of the surface and microbial colonization. Together with the environmental conditions present in receiving water bodies, these traits modify sinking velocities, resulting in complex vertical MP dynamics in different habitats and at regional and global scales. This review gives a summary of recent literature describing MP surface degradation and the subsequent succession from early to mature biofilms. Biofilm formation is often followed by settling of multicellular eukaryotes on large enough MPs, further increasing density and sinking, but reversing processes such as defouling and ascent need to also be considered. Several modeling approaches have come a long way in explaining these complex processes, providing useful insight into MP sinking behaviors at different spatial and temporal scales, but they remain imperfect, as the diversity in environmental determining factors is high and changes over time and with environmental conditions. Biological interactions with MPs, especially uptake into and transfer within aquatic food webs override sinking patterns of individual MP particles. The review suggests that neither the ocean surface nor seafloor sediments reveal total MP input into aquatic environments, with implications for monitoring and risk assessments.