Acute toxic effects of ionic liquids on zebra mussel (Dreissena polymorpha) survival and feeding

Ionic liquids (ILs) are being designed as green alternatives to volatile organic solvents that are currently used in a wide range of industrial processes. While knowledge about the toxicity of various ILs to aquatic organisms has expanded in recent years, toxicity data remains limited to a few animal taxa. Furthermore, few studies have examined the non-lethal effects of ILs on aquatic organisms. We investigated how ILs affect mortality and feeding of the zebra mussel (Dreissena polymorpha), a sessile bivalve that is invasive in North America and much of Europe. Lethal effects of six ILs (butyl-, hexyl-, or octyl-chains on either a methylpyridinium or methylimidazolium cationic base) were studied with 96 h acute bioassays. Filtering behavior was examined in 2 h feeding trials of individual mussels fed Chlamydomonas reinhardtii and exposed to sublethal concentrations of the three imidazolium-based ILs. The ILs tested caused acute mortality over a wide range of concentrations (LC₅₀ = 21.4 to 1290 mg L⁻¹), ILs with longer alkyl chains were more toxic, and pyridinium- and imidazolium-based ILs had similar toxicities. When comparing toxicity to the same IL across organisms, zebra mussels exhibit some of the highest LC₅₀ values and would be among the most resistant aquatic organisms in the event of an IL release. Short-term exposure to any IL reduced zebra mussel feeding. For butyl- and hexyl-chain ILs, feeding was significantly reduced at the acute LC₅₀, whereas the octyl-chain IL reduced feeding at the acute LC₅. Reduced survival and feeding by zebra mussels in the presence of ILs could have substantial effects on other trophic levels, which could lead to changes to affected ecosystems that would not have been predicted solely from standard mortality bioassays. Based on our mortality data, ILs are no worse than currently used industrial solvents; however, results from our feeding experiments demonstrate the potential for significant effects at sublethal concentrations. Consequently, we suggest that more ecologically relevant endpoints should be incorporated into a thorough hazard assessment of ILs.