Microplastic in surface waters of urban rivers: concentration, sources, and associated bacterial assemblages

Abstract

Abstract. The ecological dynamics of microplastic (<5 mm) are well documented in marine ecosystems, but the sources, abundance, and ecological role of microplastic in rivers are unknown and likely to be substantial. Microplastic fibers (e.g., synthetic fabrics) and pellets (e.g., abrasives in personal care products) are abundant in wastewater treatment plant (WWTP) effluent, and can serve as a point source of microplastic in rivers. The buoyancy, hydrophobic surface, and long transport distance of microplastic make it a novel substrate for the selection and dispersal of unique microbial assemblages. We measured microplastic concentration and bacterial assemblage composition on microplastic and natural surfaces upstream and downstream of WWTP effluent sites at nine rivers in Illinois, United States. Microplastic concentration was higher downstream of WWTP effluent outfall sites in all but two rivers. Pellets, fibers, and fragments were the dominant microplastic types, and polymers were identified as polypropylene, polyethylene, and polystyrene. Mean microplastic flux was 1,338,757 pieces per day, although the flux was highly variable among nine sites (min = 15,520 per day, max = 4,721,709 per day). High-throughput sequencing of 16S rRNA genes showed bacterial assemblage composition was significantly different among microplastic, seston, and water column substrates. Microplastic bacterial assemblages had lower taxon richness, diversity, and evenness than those on other substrates, and microplastic selected for taxa that may degrade plastic polymers (e.g., Pseudomonas) and those representing common human intestinal pathogens (e.g., Arcobacter). Effluent from WWTPs in rivers is an important component of the global plastic "life cycle," and microplastic serves as a novel substrate that selects and transports distinct bacterial assemblages in urban rivers. Rates of microplastic deposition, consumption by stream biota, and the metabolic capacity of microplastic biofilms in rivers are unknown and merit further research.

Evidence for selective bacterial community structuring on microplastics

“…Betaproteobacteria and against Actinobacteria and Cytophagia on the plastics agrees well with other reports on bacterial communities recovered from microplastics in lotic environments (McCormick et al, 2014(McCormick et al, , 2016. Among the individual substrates, however, no substantial differences were detected, probably because of the low replication and high within-treatment variability, reflecting the influence of stochastic processes and founder effects.…”

Microplastic Contamination in Karst Groundwater Systems

“…Using the same collection method, McNeish et al () found microplastic concentrations in Lake Michigan tributaries were 2.9 to 89.6 particles/L. Microplastic particles isolated from rivers in the Chicago, Illinois region included common polymer types such as polyethylene, polypropylene, and polystyrene (McCormick et al ). We acknowledge the limited success in identifying the polymer types of particles reported as microplastic.…”

“…Discarded plastic debris and microplastic contamination is widespread in surface-water environments and aquatic ecosystems worldwide (Gregory and Ryan 1997;Thompson et al 2004; Barnes et al 2009), including the open ocean, freshwater (streams and lakes), and soils (Law and Thompson 2014;Faure et al 2015;McCormick et al 2016;Rillig and Bonkowski 2018). Microplastics are contaminants of emerging concern and consist of plastic particles smaller than 5 mm (Moore 2008).…”

Trophic transfer of microplastics in aquatic ecosystems: Identifying critical research needs

“…Upward of 300 000 microplastics/m 3 have been found in wastewater effluent from urbanized rivers in coastal areas (Dris et al 2015). It is therefore very likely that organisms that either live in, or frequent urbanized coastal areas are at potential risk from microplastic exposures (McCormick et al 2016), with coastal food webs being possibly more susceptible to the trophic transfer of microplastics than those in other habitats (Browne et al 2010).…”