Density-mediated freshwater plastisphere microbiomes preferentially degrade conventional rather than biodegradable microplastics

The escalating demand for plastics leads to ubiquitous microplastic pollution worldwide. Existing evidence suggests that biodegradable microplastics degrade faster than conventional microplastics in aquatic environments. Here, we demonstrate the greater biodegradability of conventional polypropylene (PP) over biodegradable polylactic acid (PLA) in freshwater based on field survey, mesocosm experiment, co-culture assay, and multi-omics analysis. The biodegradation rate is 3.3-fold higher for PP compared to PLA, and this difference is more pronounced between photoaged microplastics (5.7-fold). The unexpected superior biodegradability of PP is supported by a greater diversity of microplastic-degrading bacteria in PP biofilm (predominantly aerobes) than in PLA biofilm (mainly facultative and obligate anaerobes). The inferior biodegradability of PLA is attributed to microbial growth constraints in the plastisphere driven by oxic-to-hypoxic/anoxic transition, oxygen-containing functional group detachment from the polymer, and lactide accumulation during long-term biodegradation. Our findings reveal previously overlooked but important environmental fates and impacts of biodegradable plastics against increasing substitution of conventional plastics with biodegradable alternatives.