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Abstract

Plastic pollution is a global issue, yet our understanding of its impact on organisms and ecosystems remains limited. My PhD thesis explores the presence of microplastics (MP) in river ecosystems, their interaction with freshwater hyporheic biofilms and specifically, the response of hyporheic biofilms to polyethylene terephthalate (PET) pollution in the laboratory and natural settings.
The thesis begins with assessing MP pollution in Slovenian rivers and reviewing global data on bacterial biodegradation of plastic. In small-scale Slovenian rivers (500 km2 < catchment <800km2, river length < 45km), MP pollution increased in the direction of flow and land use intensification. The results showed a prevalence of fibres in the water column and fragments in the sediments. The most frequent polymer types reported were polyethylene (PE) and polypropylene (PP) in both river compartments. Variability in MP colouration, size and shapes indicated diverse sources of MP pollution.
As part of a systematic review of 145 scientific papers, a list was made of all known strains of bacteria capable of degrading plastic. The main findings indicate that research on plastic biodegradation predominantly centres around polyethylene (PE) and its variants. Moreover, regarding biodegradation experiments, studies suggest that plastics should undergo pre-treatment, such as UV irradiation, to simulate natural environmental conditions better. They also highlight the lack of standardization and reporting consistency and the need for long-term studies.
A preliminary study was conducted on biodegrading different plastic materials in a low-carbon environment using activated sludge from a municipal wastewater treatment plant (WWTP). After two months, scanning electron microscopy (SEM) confirmed the presence of different biofilms on different plastic materials. This study was then followed by an examination of the interactions between plastics as a substrate and freshwater hyporheic biofilms as colonisers in the natural environment. The study focused on the one-month colonization and one-year seasonal changes of biofilm responses exposed to PET fibres. Different methods, such as respiratory electron transport system activity (ETSA), total protein content (TPC) and community-level physiological profiling (CLPP), were employed to characterise microbial community activity, biomass and metabolism. The study was then repeated, using the same methods but extending the time to two years and broadening the geographical scope to include four geomorphologically distinct locations. This study confirmed the inhibitory effect of PET on microbial biofilms in the hyporheic zone.
Finally, the effects of three different water regimes - flow, stagnant and unsaturated - on hyporheic biofilms under simultaneous pollution with PET fibres were investigated. Significant inhibitory effects of water regimes and PET pollution were observed for bacterial abundance and microbial metabolism on a specific substrate (CLPP) but not on microbial biomass (TPC) or activity (ETSA).
The knowledge acquired during this work enhances our understanding of MP pollution at local and global scales and its significance for hyporheic biofilms, which play a pivotal role in global nutrient cycling within freshwater environments. The study also highlighted the spatial variability within the hyporheic zone as an important factor in hyporheic biofilm research.