Microplastic pollution threatens mangrove carbon sequestration capacity

Microplastics are a pervasive environmental pollutant, altering microbial communities and disrupting global biogeochemical cycles. Mangrove forests, critical blue carbon habitats, are significant sinks for microplastic accumulation, yet they also cycle large amounts of methane, a potent greenhouse gas. The effect of plastic pollution on methane dynamics in these vital habitats remains, however, poorly understood. Here we show that microplastic pollution in mangrove soils is linked to an increased potential for methane production by favouring methanogenic archaea. Through a nationwide survey of Chinese mangroves, we found that microplastic concentrations were higher (6516 ± 1725 particles kg^-1) in surface soils (0-20 cm) and exhibited stronger association with methane-cycling microbes (four linkage pathways), compared to concentrations (2246 ± 497 particles kg^-1) and two linkage pathways in deeper soils (20-40 cm). Microplastics in topsoil were correlated with more complex microbial networks, consisting of 150 nodes and 237 links, relative to 113 nodes and 196 links in deeper soils. Furthermore, we directly linked elevated microplastic pollution in surface soils to secondary industry output, which positively correlated with the methanogens-to-methanotrophs gene ratio, establishing a clear anthropogenic driver for this shift. These findings reveal a critical, previously unrecognized mechanism by which industrial plastic pollution may compromise the net carbon sequestration capacity of mangrove ecosystems. Mitigating microplastic discharge is therefore not only a waste management issue but is also essential for preserving the climate-regulating function of these crucial habitats amid global conservation efforts.