Lead toxicity to earthworms (Eisenia fetida) and their accumulation mechanisms in soil

Lead (Pb) is a cumulative toxic metal. however, the absorption mechanism and accumulation strategy of Pb by earthworms remain unclear. This study investigated the toxic effects of Pb on Eisenia fetida after 28 days of exposure to Pb-spiked soil, as well as the absorption pathways and accumulation sites of Pb in earthworms. The results indicated that as Pb concentrations increased, Pb content in earthworms gradually rose, leading to reduced survival rate and body weight. Additionally, damage to tissues and organs, such as the body wall and gut, was observed. Earthworm activity increased the bioavailability of Pb in the soil. The primary pathway for Pb absorption in earthworms was through ingestion of the reducible and oxidizable fractions of Pb present in the soil. Pb staining results indicated that chloragogenous tissue is the primary accumulation site for Pb in earthworms. The distribution pattern of Pb in subcellular fractions showed a hierarchy: granules > cellular debris > cellular cytosol, suggesting that earthworms detoxify Pb by sequestering it into less bioavailable forms. Structural equation models demonstrated that the oxidizable and reducible fractions of Pb in soil significantly influenced the concentration and subcellular distribution of Pb in earthworms, serving as important indicators of Pb bioavailability. This study elucidated the absorption pathways and accumulation sites of Pb in earthworms, providing a theoretical basis for future research on Pb detoxification mechanisms in these organisms.