Toxicity of ZnO/TiO2 -conjugated carbon-based nanohybrids on the coastal marine alga Thalassiosira pseudonana.

Abstract

Increasing consumption of metal-oxide nanoparticles (NPs) and carbon-based nanomaterials has caused significant concern about their potential hazards in aquatic environments. The release of NPs into aquatic environments could result in adsorption of NPs on microorganisms. While metal-oxide NP-conjugated carbon-based nanohybrids (NHs) may exhibit enhanced toxicity toward microorganisms due to their large surface area and the generation of reactive oxygen species (ROS), there is a lack of information regarding the ecotoxicological effects of NHs on marine diatom algae, which are an indicator of marine pollution. Moreover, there is scant information on toxicity mechanisms of NHs on aquatic organisms. In this study, four NHs (ie, ZnO-conjugated graphene oxide [GO], ZnO-conjugated carbon nanotubes [CNTs], TiO2 -conjugated GO, and TiO2 -conjugated CNT) that were synthesized by a hydrothermal method were investigated for their toxicity effects on a Thalassiosira pseudonana marine diatom. The in vitro cellular viability and ROS formation employed at the concentration ranges of 50 and 100 mg/L of NHs over 72 hours revealed that ZnO-GO had the most negative effect on T. pseudonana. The primary mechanism identified was the generation of ROS and GO-induced dispersion that caused electrostatic repulsion, preventing aggregation, and an increase in surface areas of NHs. In contrast to GO-induced dispersion, large aggregates were observed in ZnO/TiO2 -conjugated CNT-based NHs. The scanning electron microscopy images suggest that NHs covered algae cells and interacted with them (shading effects); this reduced light availability for photosynthesis. Detailed in vitro toxicity effects and mechanisms that cause high adverse acute toxicity on T. pseudonana were unveiled; this implied concerns about potential hazards of these mechanisms in aquatic ecosystems.