Accumulation of polyethylene microplastics in river biofilms and effect on the uptake, biotransformation and toxicity of the antimicrobial triclosan

The interaction of multiple stressors in freshwater ecosystems may lead to adverse effects on aquatic communities and their ecological functions. Microplastics (MPs) are a class of contaminants of emerging concern that can exert both direct and indirect ecotoxicological effects. A growing number of studies have investigated MPs-attached microbial communities, but the interaction between MPs and substrate-associated biofilm (i.e., on natural river substrates, such as stones and sediments) remains poorly studied. In this work, the combined effects of polyethylene MPs (PE-MPs) with a particle size of 10-45 μm (2 mg/L) and the antimicrobial triclosan (TCS) (20 μg/L) were investigated on river biofilms through a short-term exposure experiment (72 h). To the best of authors' knowledge, this is the first time that the combined effects of MPs and chemical contaminants in substrate-associated river biofilms were assessed. Different response parameters were evaluated, including (i) exposure assessment and ii) contaminants effects at different levels: bacterial community composition, antibiotic resistance, extracellular polymeric substances (EPS), photosynthetic efficiency (Yeff), and leucine aminopeptidase activity (LAPA). Triclosan was accumulated in river biofilms (1189-1513 ng/g dw) alongside its biotransformation product methyl-triclosan (20-29 ng/g dw). Also, PE-MPs were detected on biofilms (168-292 MP/cm2), but they had no significant influence on the bioaccumulation and biotransformation of TCS. A moderate shift in bacterial community composition was driven by TCS, regardless of PE-MPs co-exposure (e.g., increased relative abundance of Sphingomonadaceae family). Additionally, Yeff and EPS content were significantly disrupted in TCS-exposed biofilms. Therefore, the most remarkable effects on river biofilms were related to the antimicrobial TCS, whereas single PE-MPs exposure did not alter any of the evaluated parameters. These results demonstrate that biofilms might act as environmental sink of MPs. Although no interaction between PE-MPs and TCS was observed, the possible indirect impact of other MPs-adsorbed contaminants on biofilms should be further assessed.

This work was supported by the Spanish Inter-Ministerial Science and Technology Commission through the ‘PLAS-MED’ project (CICYT, CTM2017-89701-C3). JM Castaño-Ortiz acknowledges the predoctoral grant from AGAUR (2019 FI_B 00881REF). The authors also thank Dr. Vicenç Acuña, Dr. Anna Freixa, Dr. Wolfgang Gernjak, Dr. Alexandre Sànchez Melsió, Mr. Dean Velikov and Ms. Ioanna Karakatsanidou for their contribution during experimental work. The Gas Chromatograph coupled to a triple quadrupole mass spectrometer (GC-MS/MS) (Thermo Fisher Scientific) received support from the CERCAGINYS programme, funded by the Spanish Ministry of Science and Innovation. The authors acknowledge the support of the Economy and Knowledge Department of the Catalan Government through a Consolidated Research Group (ICRA-ENV-2021 SGR 01282).

Peer reviewed