The Effect of Microplastics on Microbial Succession at Impaired and Unimpaired Sites in a Riverine System

Background Studies into biofilms and interactions with anthropogenic substrates like microplastic polymers are predominantly represented in the literature concerning marine environments. Less is known about microplastics in riverine environments that feed the microplastic accumulation of marine environments, transporting potentially harmful or pathogenic organisms that have accumulated on the microplastics. Environmental nutrient loads, seasonality, and geography are all known to influence microbiome formation. This project compared the microbial diversity of biofilms that developed on microplastics to natural stone substrates in an impaired and unimpaired section of the Quinnipiac River Watershed. We evaluated microbial diversity and composition via 16S rRNA gene sequencing while monitoring total colony and fecal coliform colony counts using standard water monitoring methods. Results Total coliform colony counts were higher in the impaired Quinnipiac River than in unimpaired Honeypot Brook (W = 583, p = 0.037) and on the microplastic substrate than stone substrate (W = 1038, p = 0.022). Sequenced features to the class level were dominated by Alphaproteobacteria, Betaproteobacteria , and Gammaproteobacteria , comprising 75% of the community biome. Simpson’s Diversity indices indicated that within the two substrates, there was little variation in the features present. However, it was noted that microplastic alpha diversity trended slightly lower than the stone. Further analysis of common aquatic enteropathogens showed that the genera Citrobacter was significantly more abundant on the microplastics at both locations. Conclusions Our results indicate impaired waterbodies with a microplastic burden may retain greater fecal coliform bacterial loads than unimpaired waterbodies. Increased microplastic loads in compromised lotic systems may have an additive impact. Water quality remediation and careful monitoring are recommended to reduce this effect. Comparing this study with environmental community analysis could provide valuable insight into preferential surface attachment of bacteria onto microplastic.