Microplastics and heavy metals alter blue carbon sequestration in coastal ecosystems: A climate–carbon nexus perspective

Blue carbon ecosystems (BCEs), including mangroves, seagrass meadows, and salt marshes, are critical natural carbon sinks that contribute significantly to climate mitigation through long-term carbon sequestration in biomass and sediments. However, increasing anthropogenic pressures, particularly microplastic and heavy-metal contamination, pose emerging threats to their carbon storage capacity. This review synthesizes current knowledge of how these pollutants influence carbon cycling in BCEs, focusing on their effects on primary production, sediment properties, and microbial-mediated biogeochemical pathways. Microplastics alter sediment structure, redox conditions, and microbial community composition, often enhancing decomposition and greenhouse gas emissions. Heavy metals, while suppressing microbial activity and promoting short-term carbon preservation through reduced decomposition, simultaneously inhibit plant productivity, thereby limiting carbon inputs. Their combined interactions further amplify ecological stress through increased toxicity, altered bioavailability, and disruption of ecosystem functioning. We propose a climate–carbon–pollution nexus framework that integrates these interacting stressors to better understand feedback between pollution and carbon sequestration. The review highlights critical knowledge gaps and emphasizes the need for integrated, multi-stressor approaches to accurately assess and manage the climate mitigation potential of blue carbon ecosystems.