Impact of microplastics and polycyclic aromatic hydrocarbons on microbial community in marine sediments: A case study of the Bohai Sea.

Microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) are ubiquitous marine contaminants. While MPs adsorb PAHs, altering their behavior, the combined effects on sediment microorganisms remain unclear. This study employed a micro-ecosystem experimental approach to investigate the effects of MPs (represented by polystyrene, Ps) and PAHs (represented by phenanthrene, Phe) and their combined exposure (CoPhe) on microbial community composition, biodiversity, assembly processes, and RMT-based networks in marine sediments. Results indicated that individual Ps or Phe additions did not significantly alter microbial diversity, composition, or assembly patterns. Conversely, combined exposure significantly reduced microbial diversity, shifted community composition, and markedly affected key functional taxa, including PAH-degrading bacteria (Porticoccus, Sulfurovum, Desulfobulbus, and Clostridia) and nitrogen-metabolizing bacteria (Alphaproteobacteria and Desulfobacteria). Co-occurrence network analysis revealed that the Ps treatment exhibited the highest number of nodes and edges, reflecting greater community complexity, whereas the Phe and CoPhe treatments showed no marked differences. Moreover, CoPhe networks exhibited a lower proportion of positive correlations (66.14%) than Ps (85.14%) and Phe (79.67%), indicating that the combined toxicity of Ps and Phe may disrupt microbial synergistic metabolism and functional complementarity, thereby potentially increasing competitive or antagonistic interactions under modified niche and resource conditions. Analysis of assembly mechanisms revealed that stochastic processes predominantly governed microbial taxa assembly, with intensified dispersal limitation under co-exposure. This study investigates the combined effects of MPs and PAHs on microbial communities, extending beyond conventional single-pollutant studies and filling research gaps regarding microbial composition, co-occurrence networks, and assembly mechanisms in high-salinity, low-hydrodynamic marine sediment environments.