Microbial plankton uptake enhances the degradation of a biodegradable microplastic

The use of biodegradable plastics as an alternative to conventional non-degradable synthetic polymers is gaining market to reduce plastic pollution, however, their biodegradability is not unconditional. In this study, we hypothesized that planktonic protists (nanoflagellates and ciliates) increase the degradation of the biodegradable PLGA (poly(lactic-co-glycolic) acid) due to particle uptake. We conducted uptake and degradation experiments using PLGA microspheres of 4.9 ± 2.8 μm diameter and the microbial planktonic community from the Baltic Sea. We found that planktonic protists ingested PLGA of different sizes, with ciliates displaying higher clearance rates and ingesting larger particles compared to nanoflagellates. In addition, we observed a more pronounced decrease in PLGA concentration and particle size over time in the presence of seawater containing microbial plankton compared to a control with only ultrapure water, suggesting that the presence of these organisms increases the rate of degradation of PLGA in marine ecosystems. Altogether, these results indicate that microbial plankton enhances the degradation of biodegradable microplastics like PLGA, specifically through rapid uptake by planktonic protists. These findings highlight the role of particle ingestion by planktonic protists in the fate of the so-called biodegradable plastics when they enter aquatic ecosystems.