Chronic exposure of the freshwater alga Pseudokirchneriella subcapitata to five oxide nanoparticles: Hazard assessment and cytotoxicity mechanisms.

The increasing use of nanoparticles (NPs) unavoidably enhances their unintended introduction into the aquatic systems, raising concerns about their nanosafety. This work aims to assess the toxicity of five oxide NPs (Al ₂ O ₃ , Mn ₃ O ₄ , In ₂ O ₃ , SiO ₂ and SnO ₂ ) using the freshwater alga Pseudokirchneriella subcapitata as a primary producer of ecological relevance. These NPs, in OECD medium, were poorly soluble and unstable (displayed low zeta potential values and presented the tendency to agglomerate). Using the algal growth inhibition assay and taking into account the respective 72 h-EC ₅₀ values, it was possible to categorize the NPs as: toxic (Al ₂ O ₃ and SnO ₂ ); harmful (Mn ₃ O ₄ and SiO ₂ ) and non-toxic/non-classified (In ₂ O ₃ ). The toxic effects were mainly due to the NPs, except for SnO ₂ which toxicity can mainly be attributed to the Sn ions leached from the NPs. A mechanistic study was undertaken using different physiological endpoints (cell membrane integrity, metabolic activity, photosynthetic efficiency and intracellular ROS accumulation). It was observed that Al ₂ O _3, Mn ₃ O ₄ and SiO ₂ induced an algistatic effect (growth inhibition without loss of membrane integrity) most likely as a consequence of the cumulative effect of adverse outcomes: i) reduction of the photosynthetic efficiency of the photosystem II (Ф _PSII ); ii) intracellular ROS accumulation and iii) loss of metabolic activity. SnO ₂ NPs also provoked an algistatic effect probably as a consequence of the reduction of Ф _PSII since no modification of intracellular ROS levels and metabolic activity were observed. Altogether, the results here presented allowed to categorize the toxicity of the five NPs and shed light on the mechanisms behind NPs toxicity in the green alga P. subcapitata.
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