Microbial colonization and succession on polylactic acid microplastics (PLA MPs) in mangrove forests - the role of environmental conditions and plastic properties

The concerns about possible risks of biodegradable plastics have increased in recent years. In this study, two types of biodegradable polylactic acid (PLA) MPs, 604 (low molecular weight) and 801 (high molecular weight), were incubated in-situ in mangrove ecosystems, across four different environmental matrix - mangrove sediment, mangrove water, mangrove air and beach air for 90 days. The fluorescence staining combined with scanning electron microscopy (SEM) results revealed that microbial colonization (both algae and bacteria) tended to be in the areas of depressions and cavities on MPs, which presumably showed signs of microbial degradation on the surface of the plastics. Over the 90-day incubation period, microbial colonization and succession on the plastics was significantly influenced by both environmental conditions and the properties of the MPs. Microbial colonization on plastic samples in mangrove sediment progressed more rapidly than that in mangrove water. Correspondingly, microbial communities on plastics in sediment showed high similarity to those in the surrounding environment, whereas the opposite was observed in water. Environmental disturbances and nutrient availability in different matrices also led to distinct microbial succession pathways for the two types of MPs. In sediment, which provided the most stable and nutrient-rich environment, divergent succession patterns were observed between 604 and 801 PLA MPs. Conversely, in flowing water and air, where environmental pressures were higher, convergent succession patterns were found. It is worth noting that the relatively stable environmental conditions and limited nutrient sources in mangrove air resulted in the highest enrichment of potential PLA-degrading microorganisms on both types of PLA MPs. Our findings highlighted the critical role of environmental conditions and MP properties in shaping microbial colonization and succession on PLA MPs. These results provided valuable scientific insights into the environmental degradation processes and long-term ecological risks of biodegradable plastics in mangrove coastal ecosystems.