The two faces of nanomaterials: A quantification of hormesis in algae and plants

The rapid progress in nanotechnology has dramatically promoted the application of engineered nanomaterials in numerous sectors. The wide application of nanomaterials and the potential accumulation in the environment sparked interest in studying the effects of nanomaterials on algae and plants. Hormesis is a dose response phenomenon characterized by a biphasic dose response with a low dose stimulation and a high dose inhibition. This paper quantifies for the first time nanomaterial-induced hormesis in algae and plants. Five hundred hormetic concentration-response relationships were mined from the published literature. The median maximum stimulatory response (MAX) was 123%, and commonly below 200%, of control response. It was also lower in algae than in plants, and occurred commonly at concentrations <100 mg L^-1. The no-observed-adverse-effect-level (NOAEL) to MAX ratio was 2.4 for algae and 1.7 for plants, and the two distributions differed significantly. Ag nanoparticles induced higher MAX than TiO₂ and ZnO nanoparticles. The MAX varied upon nanomaterial application methods, growth stage of application (seed versus vegetative), type of endpoint and time window. While nanomaterial size did not affect significantly the MAX, sizes ≤50 nm appeared to have lower NOAEL:MAX ratio than sizes ≥100 nm, suggesting higher risks from incorrect application. The mechanisms underlying nanomaterial-induced hormetic concentration responses are discussed. This paper provides a strong foundation for enhancing research protocols of studies on nanomaterial effects on algae and plants as well as for incorporating hormesis into the risk assessment practices.