Dissolved organic matter and aluminum oxide nanoparticles synergistically cause cellular responses in freshwater microalgae.

This study investigated the impact of dissolved organic matters (DOM) on the ecological toxicity of aluminum oxide nanoparticles (Al2 O3 NPs) at a relatively low exposure concentration (1 mg L-1 ). The unicellular green alga Scenedesmus obliquus was exposed to Al2 O3 NP suspensions in the presence of DOM (fulvic acid) at various concentrations (1, 10, and 40 mg L-1 ). The results show that the presence of DOM elevated the growth inhibition toxicity of Al2 O3 NPs towards S. obliquus in a dose-dependent manner. Moreover, the combination of DOM at 40 mg L-1 and Al2 O3 NPs resulted in a synergistic effect. The relative contribution of Al-ions released from Al2 O3 NPs to toxicity was lower than 5%, indicating that the presence of the particles instead of the dissolved ions in the suspensions was the major toxicity sources, regardless of the presence of DOM. Furthermore, DOM at 10 and 40 mg L-1 and Al2 O3 NPs synergistically induced the upregulation of intercellular reactive oxygen species levels and superoxide dismutase activities. Analysis of the plasma malondialdehyde concentrations and the observation of superficial structures of S. obliquus indicated that the mixtures of DOM and Al2 O3 NPs showed no significant effect on membrane lipid peroxidation damage. In addition, the presence of both DOM and Al2 O3 NPs contributed to an enhancement in both the mitochondrial membrane potential and the cell membrane permeability (CMP) in S. obliquus. In particular, Al2 O3 NPs in the presence of 10 and 40 mg L-1 DOM caused a greater increase in CMP compared to Al2 O3 NPs and DOM alone treatments. In conclusion, these findings suggest that DOM at high concentrations and Al2 O3 NPs synergistically interrupted cell membrane functions and triggered subsequent growth inhibition toxicity.