Negative impacts of zinc molybdate nanoparticles on multiple endpoints of a freshwater Chlorophyceae

Zinc molybdate (ZMO) is a nanocrystalline semiconductor that in the past few decades has attracted increasing attention due to its intrinsic properties. However, there is limiting information regarding the toxicity of this emerging compound related to the risks of its disposal and entry into aquatic environments. We evaluated ZMO nanoparticles (ZMONPs) toxicity to the microalga Raphidocelis subcapitata using a multiple parameter approach, to better understand their mechanisms of toxicity and action sites. We found that ZMONPs were highly toxic, significantly inhibiting algal growth with a 96h-IC50 of 0.27 ± 0.02 mg L^-1. The morphology of R. subcapitata was also affected in response to the NPs, with increased levels of cell granularity and size. A massive increase of the non-photochemical quenching was detected and reveals that the major effects of the ZMONPs exposure occurred on the photosynthetic machinery, inducing the activation of photoprotection mechanisms and reducing the maximum and effective quantum yields. ROS levels increased at the NPs highest concentrations (0.39 and 0.58 mg L^-1). Based on our results, we recommend caution on the use of ZMO and its disposal.