Presence of polyethylene terephthalate (PET) fibers in hyporheic zone alters colonization patterns and seasonal dynamics of biofilm metabolic functioning

Worldwide, the production of plastics is increasing, and plastic pollution in aquatic environments is a major global concern. Under natural conditions, plastic weathers to smaller pieces called microplastics (MP), which come in various shapes, with fibers often being the most common in freshwater sediments. The hyporheic zone, an ecotone between surface and groundwater, is important for the transport and fate of all MP particles. The main metabolic pathways in rivers take place in the hyporheic zone and are driven by a diverse microbial community. The objective of this study was to investigate in situ whether the presence of PET fibers in riverbed sediments affects patterns of colonization and the seasonal dynamics of microbial metabolic activities in the hyporheic zone. The effects of the presence of PET on microbial metabolism were evaluated in situ over a month (colonization study) and over a year (seasonal study) by measuring total protein content (TPC), and microbial respiration as respiratory electron transport system activity (ETSA) and by community-level physiological profiling (CLPP). Additionally, PET fibers were examined under a scanning electron microscope (SEM), and isotopic analysis (δC) of PET was performed after one year of exposure to field conditions. The findings demonstrated that during colonization and biofilm formation, and also over the seasons, the date had a large and significant impact on biofilm growth and activity, while PET presence slightly suppressed microbial biomass (TPC) and respiratory activity (ETSA). Overall microbial activity was repressed in the presence of PET fibers but there was a higher capacity for the utilization of complex synthetic polymer substrates (i.e., Tween 40) which have previously been linked to polluted environments. SEM micrographs showed diverse microbial communities adhering to PET fibers but little surface deterioration. Similarly, isotopic analysis suggested little deterioration of PET fibers after one year of in situ conditions. The study indicated that PET fibers present in riverbed sediments could have impacts on the metabolic functioning in rivers and thus affect their self-cleaning ability.