Microplastics enter the aquatic environment in the form of smaller plastic particles or as a result of the degradation and processing of plastic materials used in everyday life. Due to environmental risks associated with the persistence of plastics, biodegradable alternatives such as polylactic acid (PLA) and poly(3-hydroxybutyrate) (P3HB) are increasingly being investigated. The aim of this study was to compare and examine the effects of microplastics from three polymers with different degrees of biodegradability: non-biodegradable polyethylene terephthalate (PET), poorly biodegradable PLA and readily biodegradable P3HB on the selected freshwater organisms Daphnia magna (water flea) and Lemna minor (duckweed). To better mimic environmental conditions, the organisms were combined in a microcosm system and exposed for 21 days to two concentrations of microplastics (10 mg/L and 100 mg/L), with the particle size being smaller than 80 µm. In the duckweed, we monitored the effect on chlorophyll a content, root length and specific growth rate, while in water flea we monitored reproduction, survival and potential accumulation of microplastics in the organism. The most pronounced effects were observed in the presence of biodegradable microplastics P3HB. At a lower concentration (10 mg/L), we observed a stimulatory effect on the reproduction of water flea, while at a higher concentration (100 mg/L) the exposed organisms were completely immobile. In duckweed, at a lower concentration (10 mg/L), we observed a mild stimulatory effect on growth, while at a higher concentration the opposite effect was observed. No pronounced effects were detected in presence of PLA and PET at both concentrations, but microscopic analyses showed the accumulation of microplastic particles in water flea. The results show that microplastics can affect aquatic organisms both directly and indirectly through changes in the environment, with the responses of organisms depending on the type of polymer, its properties and concentration. The research highlights the importance of using microcosms in assessing environmental impacts, as it requires a more realistic simulation of what happens when microplastics are present in the environment. Further studies examining the long-term effects of microplastics, especially biodegradable ones, would significantly contribute to understanding the degradation processes, biological responses and their impact on the freshwater environment.
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