Effects of cadmium and pyrene on earthworm-associated bacterial communities: Unveiling new perspectives for soil pollution management.

Earthworms are considered to be excellent bioindicators of soil pollution. In recent years, there has been increasing interest in examining the effects of soil pollution on earthworm-associated microbiomes, with a particular focus on the gut microbiomes. However, relatively little effort has been invested in comprehensively investigating other microbiomes associated with earthworms and their responses to soil pollution. To fill this gap, we systematically studied the effects of Cd, pyrene, and combined pollution on the bacterial community in different vermicompartments, i.e., burrow wall, gut, and cast, in both epigeic Eisenia fetida and anecic Metaphire guillelmi, using a 2D-terraria incubator and high-throughput sequencing techniques. The results showed that bacterial alpha diversity followed the order of burrow wall > cast > gut, and this did not vary with soil pollution or earthworm ecotypes. Moreover, the dominant phyla in the vermicompartments were similar across different pollution treatments. Principal coordinate analysis (PCoA) revealed that the bacterial communities in different vermicompartments and ecotypes of earthworm were separated from each other, whereas they were grouped together in polluted treatments and unpolluted conditions. These results imply that even in polluted soil, vermicompartment and earthworm ecotypes remain the most significant factors affecting earthworm-associated microbiomes. However, the impacts of soil pollution on the bacterial composition in each vermicompartment were still evident. A comprehensive analysis revealed that the gut bacterial communities are more sensitive to soil contamination than casts and burrow wall in different ecotypes. Additionally, linear discriminant analysis of effect size (LefSe) identified several bacteria in Gemmatimonadota, the Firmicutes phylum in the burrow walls, and Patescibacteria (phyla) in the gut as potential biomarkers for pyrene contamination in soil. This research provides a comprehensive understanding of the effects of soil pollution on earthworm-associated microbiomes, thereby enhancing our understanding of earthworm ecotoxicology and soil pollution management.

Cadmium and pyrene in the soil modify the properties of earthworm-mediated soil.

Earthworms are pivotal in soil ecosystems due to their crucial role in shaping soil characteristics through casts and burrow walls. Previous research has predominantly focused on the direct impact of soil pollution on live earthworms, overlooking the subsequent effects on earthworm-mediated soil, such as casts and burrow walls. Using 2D-terraria as incubation containers and the geophagous earthworm species Metaphire guillelmi, this study assessed the change in various properties of earthworm-mediated soil in both uncontaminated soils and Cd- and Pye-contaminated soils. Overall, both Cd and Pye overall improved the ammonium nitrogen (NH4+-N), Olsen's phosphorus (Olsen-P) levels, and invertase and catalase activities while decreasing catalase activities in earthworm-mediated soil. They also fluctuating affected the pH, soil organic matter (SOM) content, soil urease, alkaline phosphatase activities, and microbial functional genes in the cast and burrow walls. These results indicated that earthworms remained crucial "ecosystem engineers" even in polluted soil. Additionally, differences were observed in the responses of properties between casts and burrow walls, showing unequal contributions of transit-through-gut and burrowing processes to soil modification. Specifically, transit-through-gut was found to have a more significant influence on soil NH4+-N and Olsen-P content compared to burrowing behavior. Regarding the pattern of microbial functional genes in earthworm-associated compartments, results revealed that they differed significantly in casts from those in bulk soil and burrow walls under unpolluted conditions, with pollution-enhancing disparities among compartments. Furthermore, NH4+-N and Olsen-P content, urease, and catalase activities in burrow walls and/or casts were identified as potential biomarkers for soil pollution, exhibiting a clear dose-effect relationship. Developing such biomarkers could address ethical concerns related to conventional earthworm biomarkers that require sacrificing earthworms. This study provides insights into the consequences of soil pollution on earthworm-mediated soil components, highlighting the importance of considering the indirect effects of contaminants on soil ecosystems.