Abstract
Thorium (Th) is a natural radioactive element present in the environment and has the potential to be used as a nuclear fuel. Relatively little is known about the influence and toxicity of Th in the environment. In the present study, the toxicity of Th to the green algae Chlorella pyrenoidosa (C. pyrenoidosa) was evaluated by algal growth inhibition, biochemical assays and morphologic observations. In the cultural medium (OECD TG 201), Th(NO3)4 was transformed to amorphous precipitation of Th(OH)4 due to hydrolysis. Th was toxic to C. pyrenoidosa, with a 96 h half maximum effective concentration (EC50) of 10.4 μM. Scanning electron microscopy shows that Th-containing aggregates were attached onto the surface of the algal cells, and transmission electron microscopy indicates the internalization of nano-sized Th precipitates and ultrastructural alterations of the algal cells. The heteroagglomeration between Th(OH)4 precipitation and alga cells and enhanced oxidative stress might play important roles in the toxicity of Th. To our knowledge, this is the first report of the toxicity of Th to algae with its chemical species in the exposure medium. This finding provides useful information on understanding the fate and toxicity of Th in the aquatic environment.
Highlights
Thorium (Th) is an actinide that occurs naturally as 232Th with a very long half-life of 1.4 × 1010 years
The 24 h and 48 h EC50 of ThO2 were 7.3 and 4.7 μM, respectively [13]. This suggests that more attention should be paid to the toxicity of Th species that are present in insoluble forms
This study investigated the toxicity of Th to C. pyrenoidosa on the basis of its chemical species in the cultural medium
Summary
Thorium (Th) is an actinide that occurs naturally as 232Th with a very long half-life of 1.4 × 1010 years. People will always be exposed to small amounts of Th through inhalation, ingestion and skin penetration, because Th is naturally ubiquitous in air, water, soil and biological materials. Individuals that work in the mining, milling or thorium industries may be exposed to more Th than usual. There is evidence that breathing in Th dust increases the risk of lung and pancreatic cancer. ThO2 was found mainly in reticuloendothelial systems such as liver, spleen and bone and has been associated with an increased incidence of liver disease [5]. Studies on the toxicity of Th at cellular and molecular levels have been carried out. It was considered to be a major advance in understanding the biological chemistry of radioactive actinides
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