Subtle biogeochemical consequences of biodegradable and conventional microplastics in estuarine blue carbon systems

Blue carbon ecosystems act as critical sinks for microplastics (MPs), yet field-based evidence of their biogeochemical consequences remains scarce. In this study, we conducted in situ exposures of estuarine mangroves to environmentally relevant concentrations of polypropylene (PP) and polylactic acid (PLA) MPs for 30 and 100 days. Metagenomic analyses revealed stable microbial community composition across treatments. However, PLA exposure transiently induced significant functional disturbances in carbon, nitrogen, and phosphorus cycling processes. These responses also increased the sediment total organic carbon by 52.8 % and available phosphorus by 86.5 % under PLA exposure. Simultaneously, enhanced carbon decomposition and inhibited methane oxidation potentially exacerbate greenhouse gas emissions. PP amendments triggered no comparable functional shifts. Intriguingly, long-term exposure indicated microbial functional resilience and restored metabolic pathways, while PLA exposure remained associated with a 54.9 % increase in sediment available phosphorus. Two-dimensional diffusive gradients in thin films further suggested that PP may have restructured redox gradients, while PLA-driven acidification likely mobilized phosphorus, increasing labile phosphorus concentrations by 98.6 % and 64.4 %, respectively. These findings demonstrate that ostensibly low-impact MPs can subtly disrupt biogeochemical networks. Notably, biodegradable MPs pose unexpected long-term risks of phosphorus leakage, underscoring the need to consider conventional and biodegradable plastics in sustainable coastal ecosystem management.