Relationships among protozoa, bacteria and fungi in polycyclic aromatic hydrocarbon-contaminated soils

Soil bacterial and fungal communities play key roles in the degradation of organic contaminants, and their structure and function are regulated by bottom-up and top-down factors. Microbial ecological effects of polycyclic aromatic hydrocarbons (PAHs) and trophic interactions among protozoa and bacteria/fungi in PAH-polluted soils have yet to be determined. We investigated the trophic interactions and structure of the microbiome in PAH-contaminated wasteland and farmland soils. The results indicated that the total concentration of the 16 PAHs (∑PAHs) was significantly correlated with the Shannon index, NMDS1 and the relative abundances of bacteria, fungi and protozoa (e.g., Pseudofungi) in the microbiome. Structural equation modelling and linear fitting demonstrated cascading relationships among PAHs, protozoan and bacterial/fungal communities in terms of abundance and diversity. Notably, individual PAHs were significantly correlated with microbe-grazing protozoa at the genus level, and the abundances of these organisms were significantly correlated with those of PAH-degrading bacteria and fungi. Bipartite networks and linear fitting indicated that protozoa indirectly modulate PAH degradation by regulating PAH-degrading bacterial and fungal communities. Therefore, protozoa might be involved in regulating the microbial degradation of PAHs by predation in contaminated soil.