Lethal Toxicity of Thymus mastichina and Helichrysum italicum Essential Oils to Non-Target Aquatic Organisms: Tools to Screen Environmental Effects?

Essential oils (EOs) from Thymus mastichina (EO-thyme) and Helichrysum italicum (EO-curry) have wide commercial applications, but little is known about their ecotoxicity to aquatic life. We evaluated the lethal toxicity of both EOs toward standard freshwater (Daphnia. magna and Thamnocephalus platyurus) and saltwater (Artemia sp.) species. Dimethylsulfoxide was used as a solvent after establishing a maximum safe but effective concentration of 1% (v/v). EO-curry was significantly more toxic than EO-thyme (24-48 h LC50 values of 15.93-55.80 and of 84.78-153.0 mg L-1, respectively) for all species; sensitivity ratios ranged from threefold for D. magna (48 h) and Artemia sp. (24 h) to fivefold for T. platyurus (24 h). Artemia sp. was the least sensitive, and T. platyurus was the most sensitive species, although significantly more so than D. magna only to EO-curry. The second major compound in EO-thyme, beta-pinene (5%), is more toxic to aquatic life than major compound 1,8-cineole (62%), although 1,8-cineole facilitates penetration of other EO constituents into crustaceans' epidermis. Among the main compounds of EO-curry, only alpha-pinene (13%) is known to be toxic to aquatic organisms. However, minor compounds present in both EOs, like p-cymene (0.3-1.1%), also cause synergistic effects by enhancing the penetration of other EO constituents. Before any of these standard tests can be recommended for the ecotoxicity characterization and environmental management of EOs, their sensitivity to a wider range of EOs, at least from closely related families, needs to be assessed.