The ecotoxicity of single-layer molybdenum disulfide nanosheets on freshwater microalgae Selenastrum capricornutum

Molybdenum disulfide (MoS2) nanomaterials have attracted considerable attention due to their unique physicochemical properties and promising applications, and subsequent risks to the environment and human health. However, the toxicological investigations of MoS2 nanosheets in aquatic environments are still limited. Herein, their aquatic toxic effects and involved mechanisms were explored using a freshwater green microalgae model, Selenastrum capricornutum. Several endpoints, including algal growth, photosynthetic pigment and macromolecular substances synthesis, were investigated after exposure to 0.02-50.0 mg/L MoS2 nanosheets. Furthermore, oxidative stress, membrane damage, lipid oxidation, and secretion of extracellular polymeric substances were elucidated as well. The results showed that single-layer MoS2 (SLMoS2) nanosheets at low concentrations (<= 1.0 mg/L) stimulated the synthesis of chlorophyll and primary macromolecules, and they slightly promoted the growth of S. capricornutum. In contrast, higher concentrations of SLMoS2 (> 1.0 mg/L) inhibited algae growth through ROS-mediated membrane destruction and oxidative damage. Surprisingly, SLMoS2 accelerated extracellular polymeric substances (EPS) secretion and changed their fluorescent components, thus influencing algal growth. Taken together, MoS2 nanomaterials exerted bidirectional effects on S. capricornutum at the cellular, histological and organismal levels. These results provide considerable support regarding the adverse effects of MoS2 nanomaterials on freshwater microalgae.