Metagenomics reveals combined effects of microplastics and antibiotics on microbial community structure and function in coastal sediments

Microplastics and antibiotics are emerging pollutants in marine environments, yet their combined effects on coastal sediments remain poorly understood. This study examined the impacts of microplastics and antibiotics on sediment properties and microbial communities through a 60-day laboratory simulation. Results showed that microplastics significantly reduced carbon, nitrogen, and phosphorus levels in sediments, while both antibiotics and combined pollution decreased phosphorus content. Combined pollution also increased NH-N concentration. Enzyme activity analysis revealed that microplastics elevated alkaline phosphatase activity, antibiotics increased fluorescein diacetate (FDA) hydrolase activity but decreased urease activity, and their combination further enhanced FDA hydrolase activity. Metagenomics analysis demonstrated that the presence of microplastics and antibiotics altered microbial community structure and metabolic functions. The dominant phylum Pseudomonadota (42.62 %-56.24 %) showed reduced abundance under combined pollution. Antibiotics significantly increased resistance gene abundance, while combined pollution led to selective enrichment of these genes. Both pollutants inhibited ammonia assimilation, and antibiotics also suppressed dissimilatory nitrate reduction. Conversely, combined pollution promoted nitrification and nitrogen fixation. While microplastics and antibiotics inhibited methane synthesis, combined pollution increased methane production via elevated mttB and hdrA genes. Antibiotics also reduced methane-oxidizing bacteria and genes, suppressing methane oxidation. These findings provide crucial insights into the ecological impacts of microplastics and antibiotics on coastal sediments, offering a theoretical basis for future marine pollution management strategies.