The combined effects of microplastics and their additives on mangrove system: From the sinks to the sources of carbon

Mangrove ecosystems, a type of blue carbon ecosystems (BCEs), are vital to the global carbon cycle. However, the combined effects of microplastics (MPs) and plastic additives on carbon sequestration (CS) in mangroves remain unclear. Here, we comprehensively review the sources, occurrence, and environmental behaviors of MPs and representative plastic additives in mangrove ecosystems, including flame retardants, such as polybrominated diphenyl ethers (PBDEs), and plasticizers, such as phthalate esters (PAEs). Mangrove ecosystems have a complex influence on the behaviors of MPs and additives. Under the action of natural and unnatural factors, these pollutants exhibit complex behaviors including migration, interception, deposition and transformation, that are closely linked to those of particulate carbon, particularly carbon sequestration processes. MPs and additives hinder the CS function of mangroves by harming the growth of flora and fauna, influencing microbial nitrogen and sulfur cycles, and enhancing the degradation of organic matter in the sediment. The increasing accumulation and widespread occurrence of MPs and additives will greatly influence the carbon cycle. Future work is encouraged on systematic investigation of new alternatives to plastics and additives, and research methods to uncover the impact mechanisms of MPs and additives on BCEs. The developments of management measures and engineering technologies are also required to enhance pollutant control and mangrove CS.