Plastic degradation in Lake Geneva: Influence of depth, seasonal shifts, and bacterial community dynamics

Aquatic ecosystems suffer disproportionately from plastic pollution given that they integrate material from terrestrial watersheds. Most studies on microbial colonisation and degradation of plastics have focused on marine environments, leaving a knowledge gap for freshwaters. Our study explores the possible degradation and the role of bacterial community composition of plastics in Lake Geneva. We exposed polyethylene terephthalate (PET) and low-density polyethylene (LDPE) for 45 weeks to environmental lake gradients that change with depth and season. The substrates were suspended at 2 and 30 m depth, resulting in strikingly different environmental conditions for biofilm development, including light (PAR), temperature, and nutrient availability. We monitored the bacterial colonisation using 16S rRNA sequencing and assessed the abundance of the alkane hydrolase gene (alkB) to evaluate the potential ability of the biofilm to degrade PET and LDPE. Additionally, we analysed plastic surface modifications through spectroscopy, contact angle measurements and microscopy. We found that the PET surface showed no degradation after 45 weeks in the lake, at either depth. The LDPE surface at 2 m exhibited a decrease in hydrophobicity, but no evidence of oxidation or degradation was found. In contrast, the LDPE surface at 30 m displayed oxidation, a decrease in hydrophobicity, and porous cavities. In addition, we observed an increase in the alkane alkB gene abundance in the biofilm, with the development of plastic-degrading taxa in the community. Our results underline the complexity of plastic degradation in aquatic ecosystems; not only does the type of plastic have an effect, so do the spatio-temporal variable environmental lake conditions and the biofilm community. The multifactorial nature of these processes complicates predictions on the fate of plastics in the environment.