Biodegradation of microplastic in freshwaters: A long‐lasting process affected by the lake microbiome

Abstract

Plastics have been produced for over a century, but definitive evidence of complete plastic biodegradation in different habitats, particularly freshwater ecosystems, is still missing. Using 13C‐labelled polyethylene microplastics (PE‐MP) and stable isotope analysis of produced gas and microbial membrane lipids, we determined the biodegradation rate and fate of carbon in PE‐MP in different freshwater types. The biodegradation rate in the humic‐lake waters was much higher (0.45% ± 0.21% per year) than in the clear‐lake waters (0.07% ± 0.06% per year) or the artificial freshwater medium (0.02% ± 0.02% per year). Complete biodegradation of PE‐MP was calculated to last 100–200 years in humic‐lake waters, 300–4000 years in clear‐lake waters, and 2000–20,000 years in the artificial freshwater medium. The concentration of 18:1ω7, characteristic phospholipid fatty acid in Alpha‐ and Gammaproteobacteria, was a predictor of faster biodegradation of PE. Uncultured Acetobacteraceae and Comamonadaceae among Alpha‐ and Gammaproteobacteria, respectively, were major bacteria related to the biodegradation of PE‐MP. Overall, it appears that microorganisms in humic lakes with naturally occurring refractory polymers are more adept at decomposing PE than those in other waters.

Decomposition rate and biochemical fate of carbon from natural polymers and microplastics in boreal lakes

“…Thus, against our hypothesis, fungi were not contributing to the decomposition of microplastics. However, decomposition rates of PP and PE were extremely slow in contrast to earlier studies ( Taipale et al, 2019 , 2022 ), and the assimilation of PP and PE carbon into PLFAs was likely inhibited by the presence of other easier carbon sources in lake water. Therefore, it is plausible that several freshwater bacteria can assimilate aliphatic microplastic carbon, but the high recalcitrance of microplastics and the access to easier natural carbon sources make it an unfavorable carbon source and it is therefore slowly decomposed.…”

“…In accordance with our hypothesis, the major fate of all studied substrates’ carbon that was decomposed was in microbial biomass, whereas mineralization was found to play only a minor role in the decomposition process of microplastics and lignin. Similar results were found for PE decomposition by the freshwater microbial community ( Taipale et al, 2022 ) but not for PS decomposition by mealworms which used only minor amounts for biomass formation ( Yang et al, 2015 ). Although the mineralization of substrate carbon covers only a small proportion of all carbon fate, measurement of δ 13 C-values from gas could be used as an indicator for decomposition, since it is relatively easy, fast, and cheap to measure in comparison to CSIA.…”

The second life of terrestrial and plastic carbon as nutritionally valuable food for aquatic consumers

“…Isotopic enrichment (Δ) of δ 13 C was calculated between treatments and control (only lake water), and thus, positive values (‰) show mineralization of 13 C-polyethylene into carbon dioxide. For the calculation of mineralization of added substrates, we used previously published equations (Taipale et al, 2022;Supplemental document).…”

“…Conversely, we have recently revealed trophic uptake and conversion of plastic carbon to essential omega-3 (ω-3) and omega-6 (ω-6) polyunsaturated fatty acids (PUFA) (Taipale et al, 2019). This demonstrated trophic upgrading of microplastic (MP) carbon was an important finding since the persistence of MP in marine and freshwater ecosystems is an increasing environmental concern due to the predicted growth of slowly degrading plastic waste (Borrelle et al, 2020;Taipale et al, 2022;Vesamäki et al, 2022). Despite the slow process, direct utilization of some plastic blends and their leachates can support microbial biomass, complementing the trophic transfer of biomolecules (Sheridan et al, 2022;Taipale et al, 2019Taipale et al, , 2022Vesamäki et al, 2022;Yoshida et al, 2016).…”

“…However, microbial decomposition of recalcitrant organic carbon is generally slow and related to certain taxa including Actinobacteria and Proteobacteria (Bugg et al, 2011;Taipale et al, 2022;Vesamäki et al, 2022). These two phyla differ in their resistance to flagellate and zooplankton grazing, and thus, the microbial community structure can influence the trophic transfer of organic carbon (Taipale et al, 2012;Tarao et al, 2009).…”