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Influence of urban wastewaters and rainfall runoffs on community composition and function of river biofilms: a focus on nanoplastics.
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Scientists studied slimy microbial communities (biofilms) and mussels living in a river near Montreal to see how sewer overflows and wastewater affect their ability to deal with tiny plastic particles (nanoplastics). They found that pollution hotspots—especially areas hit by combined sewer overflows during storms—had the most nanoplastic contamination, but also boosted the microbes' and mussels' ability to break down plastic, suggesting nature has some built-in capacity to respond to plastic pollution. This matters because these mussels and biofilms sit at the base of the food chain, so understanding how they handle plastic p
Biofilms are critical mediators of contaminant fate in aquatic environments, acting as sites for the accumulation and potential degradation of plastic materials amongst other contaminants. However, the ecological impacts of urban pollution on freshwater biofilm composition and function remain poorly understood. In this study, we examined how anthropogenic contamination, including nanoplastics, alters biofilm communities in the Saint-Lawrence River (Québec, Canada). To test this, freshwater mussels and bare terracotta tiles for biofilm colonization were placed together in cages at three sites along the Saint-Lawrence River: a combined sewers and street runoffs site, a site downstream of a large city (2 million inhabitants), and a site 8 km downstream of a municipal effluent dispersion plume. The experiment involved two replicate cages per site and a 3-month exposure period. Biofilms were harvested at the end of the experiment to determine the levels of plastic-related contaminants (plastic nanoparticles), functional activity (esterase activity, lipids, oxidative stress, and plastic biodegradation capacity), and community composition by 16S rRNA sequencing. In parallel, the digestive gland of mussels was sampled to assess the microbiome's capacity to degrade plastics, oxidative stress, and heterotrophic bacterial load from undsinfected wastewaters. The data revealed that biofilms from the rainfall overflow site were most contaminated with nanoplastics, while those from the municipal effluent dispersion plume contained significantly more lipids. Biofilms from the overflow site also exhibited increased esterase activity and biodegradation index compared to the other sites. However, no signs of oxidative stress were observed in biofilms from the overflow site compared to those of the municipal effluent plume site. Microbial community composition showed only a marginal shift among sites (PERMANOVA, F = 1.69, p = 0.066), while differences in community dispersion were highly significant (PERMDISP, p < 0.001), reflecting increased heterogeneity at urban-impacted locations. Thus, urban pollution did not uniformly impair biofilm communities; instead, it resulted in site-specific ecological responses, including elevated oxidative stress at the municipal effluent plume site and substantial community heterogeneity and instability at the overflow site. Additionally, microbial taxonomic analysis of biofilms revealed an increased presence of bacterial species typically associated with the plastisphere and known to degrade plastics in aquatic environments. In mussels, the biodegradation index and bacterial load were significantly increased at the overflow and downstream effluent sites, respectively. In conclusion, both mussels and biofilms may represent critical compartments in plastic pollution dynamics in urban environments as evidenced by their increased capacity to degrade plastics.
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