Cytotoxicity of aluminium oxide nanoparticles towards fresh water algal isolate at low exposure concentrations

The growing commercial applications had brought aluminium oxide nanoparticles under toxicologists’ purview. In the present study, the cytotoxicity of two different sized aluminium oxide nanoparticles (ANP1, mean hydrodynamic diameter 82.6 ± 22 nm and ANP2, mean hydrodynamic diameter 246.9 ± 39 nm) towards freshwater algal isolate Chlorella ellipsoids at low exposure levels (≤1 μg/mL) using sterile lake water as the test medium was assessed. The dissolution of alumina nanoparticles and consequent contribution towards toxicity remained largely unexplored owing to its presumed insoluble nature. Herein, the leached Al3+ ion mediated toxicity has been studied along with direct particulate toxicity to bring out the dynamics of toxicity through colloidal stability, biochemical, spectroscopic and microscopic analyses. The mean hydrodynamic diameter increased with time both for ANP1 [82.6 ± 22 nm (0 h) to 246.3 ± 59 nm (24 h), to 1204 ± 140 nm (72 h)] and ANP2 [246.9 ± 39 nm (0 h) to 368.28 ± 48 nm (24 h), to 1225.96 ± 186 nm (72 h)] signifying decreased relative abundance of submicron sized particles (<1000 nm). The detailed cytotoxicity assays showed a significant reduction in the viability dependent on dose and exposure. A significant increase in ROS and LDH levels were noted for both ANPs at 1 μg/mL concentration. The zeta potential and FT-IR analyses suggested surface chemical interaction between nanoparticles and algal cells. The substantial morphological changes and cell wall damage were confirmed through microscopic analyses (SEM, TEM, and CLSM). At 72 h, significant Al3+ ion release in the test medium [0.092 μg/mL for ANP1, and 0.19 μg/mL for ANP2] was noted, and the resulting suspension containing leached ions caused significant cytotoxicity, revealing a substantial ionic contribution. This study indicates that both the nano-size and ionic dissolution play a significant role in the cytotoxicity of ANPs towards freshwater algae, and the exposure period largely determines the prevalent mode of nano-toxicity.

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► Cytotoxicity of different size alumina NP towards freshwater algae.
► Low test concentrations of NPs.
► Contribution of nano-size and ion dissolution.
► Mode of toxicity depends on exposure period.