Distinct toxicological characteristics and mechanisms of Hg 2+ and MeHg in Tetrahymena under low concentration exposure.

Inorganic divalent mercury complexes (Hg ²⁺ ) and monomethylmercury complexes (MeHg) are the main mercury species in aquatic systems and their toxicity to aquatic organisms is of great concern. Tetrahymena is a type of unicellular eukaryotic protozoa located at the bottom of food chain that plays a fundamental role in the biomagnification of mercury. In this work, the dynamic accumulation properties, toxicological characteristics and mechanisms of Hg ²⁺ and MeHg in five Tetrahymena species were evaluated in detail. The results showed that both Hg ²⁺ and MeHg were ingested and exhibited inhibitory effects on the proliferation or survival of Tetrahymena species. However, the ingestion rate of MeHg was significantly higher than that of Hg ²⁺ . The mechanisms responsible for the toxicity of MeHg and Hg ²⁺ were different, although both chemicals altered mitochondrial membrane potential (MMP). MeHg disrupted the integrity of membranes while Hg ²⁺ had detrimental effects on Tetrahymena as a result of the increased generation of reactive oxygen species (ROS). In addition, the five Tetrahymena species showed different capacities in accumulating Hg ²⁺ and MeHg, with T. corlissi exhibiting the highest accumulations. The study also found significant growth-promoting effect on T. corlissi under low concentration exposure (0.003 and 0.01μg Hg/mL (15 and 50nM)), suggesting different effect and mechanism that should be more closely examined when assessing the bioaccumulation and toxicity of mercury in aquatic ecosystems.
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