Microplastics and heavy metals reshape mangrove rhizosphere microbiomes and compromise carbon fixation potential

Abstract

Mangroves with developed root systems at the land-sea junction act as sinks for microplastics (MPs). Heavy metals (HMs), persistent contaminants in coastal ecosystems, coexist with MPs globally. However, the complex effects of MP-HM pollution on mangrove ecosystems remain unclear. Herein, the combined effects of various MPs (polypropylene (PP), polyethylene (PE), polyamide (PA)) and 10 HM (Cr, Cu, Pb, Zn, Cd, Mn, Co, Hg, As, and Ni) on the rhizosphere microecology of the mangrove plant Avicennia. marina (Forssk.) Vierh. (A. marina) were determined. The combined pollution caused by MPs-HMs changes the rhizosphere microbial community structure of A. marina and decreases species richness and diversity. The relative abundance of dominant bacteria (Firmicutes) in the PP-HM exposure (PPz) group increased by 58 %, and that of ε-Proteobacteria decreased by 49 %, compared with those in the heavy metal exposure (Z) group. PP and PA in the sediment enriched Bacillales (12 %) and Sphingomonales (3.6 %), respectively. The following were the distinct responses to the combined MP-HM pollution by the rhizosphere bacterial community: (1) sensitivity, which changed rapidly within 7 d of exposure, (2) specificity, differential enrichment of bacterial communities under MP vs HM stress, and (3) persistent alterations in functional bacteria following exposure. MPs in the sediment changed the bacterial community structure of the rhizosphere sediment by selective enrichment of the microbial taxa with specific plastic degradation functions. Thus, the long-term coexistence of MPs-HMs and microbes in mangrove wetlands may subtly change the biogeochemical cycling processes in coastal ecosystems. This study indicates the ecological effects of MPs-HMs in the rhizosphere environment, which provides theoretical support and scientific basis for the study of biogeochemical cycles.

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