Temporal hormetic response of soil microbes to cadmium: A metagenomic perspective

The hormetic response of microbes to cadmium (Cd) is often observed in soil, but the mechanisms are unclear. In this study, we proposed a novel perspective of hormesis that successfully explained the temporal hermetic response of soil enzymes and microbes, and the variation of soil physicochemical properties. Several soil enzymatic and microbial activities were stimulated by 0.5 mg·kg^-1 exogenous Cd, but inhibited at higher Cd dosages. The phenomena suggested the hormetic response to 0.5 mg·kg^-1 Cd was highly generalizable concerning soil enzymes and microbial activity. However, the response disappeared after incubation for >10 days. Soil respiration was also initially enhanced by exogenous Cd and decreased after consumption of labile soil organic matter. The metagenomic results revealed Cd stimulation of genes involved in labile soil organic matter decomposition. Additionally, Cd enriched the antioxidant enzymatic activity and abundances of the corresponding marker genes, rather than genes involved in the efflux-mediated heavy metal resistance. The microbes enhanced their primary metabolism to make up the energy gaps, with hormesis evident. The hormetic response disappeared after the labile compounds in soil were exhausted. Overall, this study illustrates the dose-dependence and temporal variation of stimulants and provides a novel and feasible strategy for the study of Cd in soil microorganisms.