Comparative transcriptome analysis reveals the response mechanism of the typical marine ciliate Euplotes vannus to heavy metal cadmium exposure

Abstract

Cadmium (Cd) is one of the major heavy metal pollutants. High concentrations of Cd in surface sediments have led to serious damage to marine ecosystems and threatened the health of marine organisms. Euplotes vannus, a marine epiphytic ciliate, is susceptible to Cd in sediments as a model unicellular eukaryote without the cell wall. This study used Euplotes vannus as a test organism to investigate the biotoxicity of Cd and the response mechanisms of marine microorganisms to Cd. The results showed that Euplotes vannus showed high tolerance to Cd²⁺ (24 h-LC₅₀ = 38.75 mg/L). We found that Cd inhibited the population growth of E. vannus in a dose-dependent manner. This may be due to the cellular Cd²⁺ uptake leads to a series of negative cellular effects, including DNA damage, protein damage and mitochondrial disruption. These oxidative damages led to a rapidly increased activities of four antioxidant enzymes (SOD, CAT, POD and GSH-PX) in Euplotes vannus. Furthermore, transcriptome analysis indicated that transporter proteins could be involved in Cd²⁺ efflux, and the expression of genes for DNA repair, energy metabolism, and protein synthesis was also significantly changed. These play a crucial role in stabilizing cellular gene expression, providing energy support and maintaining cell growth. In addition, we found that ribosome biogenesis, glutathione metabolism and mitochondrial biogenesis pathways were significantly enriched. The differential expression of these genes indicates that Cd exposure induces a wide range of post-transcriptional regulatory mechanisms in Euplotes vannus to enhance tolerance to Cd.