Viability of gut microbes as a complementary earthworm biomarker of metal exposure

Earthworms are standard species used in soil ecotoxicology to evaluate the adverse effects of soil contaminants. This study proposes the assessment of the viability of earthworm gut microbes as an indicator in a site-specific test of soil toxicity. Using slow centrifugation, the microbial community was extracted from the guts of earthworms that had been exposed to copper (Cu)- or nickel (Ni)-contaminated soil. Microbial cell viability was assessed using calcein acetoxymethyl ester staining and flow cytometric analysis. We confirmed a metal concentration-dependent decrease in the cell viability of the gut microbial community. The general endpoints, including survival, abnormalities, coelomocyte activity, and metal bioaccumulation, showed a metal concentration-dependent response, and were strongly associated with gut microbial viability in the Ni-exposure group. In contrast, the general endpoints in the Cu-exposure group were significantly different from those in the former group, because the soil penetration rate of the earthworms was very low on the Cu-contaminated soil. Our results indicated that the gut microbial community viability assay holds potential for assessing the toxicity of soil to field worms by simply and rapidly monitoring the viability of the earthworm gut microbial community.