Structural diversity of ionic liquids determines acute and chronic toxicity in Pseudokirchneriella subcapitata

Ionic liquids (ILs) have attracted increasing attention as versatile “green solvents” in industrial applications, yet their environmental risks remain insufficiently understood. This study evaluates the acute and chronic toxicity of 21 ILs with varying cationic and anionic structures on 21 ILs on the freshwater microalga Pseudokirchneriella subcapitata . These ILs were selected based on their structural diversity and potential use in membrane-based applications. A key novelty of this work lies in the application of Pulse Amplitude Modulated (PAM) chlorophyll fluorescence, a rapid and non-invasive tool that revealed early photosynthetic impairment long before visible growth inhibition occurred. Acute toxicity was assessed through chlorophyll fluorescence parameters, revealing concentration-dependent declines in photosynthetic performance. ILs containing pyrrolidinium, ammonium, and phosphonium cations showed consistently lower toxicity compared to imidazolium-based ILs. Chronic exposure further identified three ILs, [Chol][H 2 PO 4 ], [P 8,8,8,8 ][Br], and [P 6,6,6,14 ][DBS] as environmentally compatible up to 0.7 g·L −1 . Notably, [Chol][H 2 PO 4 ] combined low toxicity with potential growth-promoting effects, likely due to its biocompatible choline cation and phosphate anion. In contrast, phosphonium ILs with long alkyl chains showed reduced bioavailability, which may explain their limited impact on microalgal physiology. By integrating acute and chronic EC 50 values with structural considerations, this work advances the understanding of how IL structure influences microalgal toxicity and offers a basis for the design of safer ILs. The combined use of short- and long-term assays strengthens the environmental risk assessment of ILs in aquatic systems. • Pyrrolidinium, ammonium, and phosphonium cations show lower microalgae toxicity. • Fluorescence-based assays enable rapid toxicity assessment in microalgae cultures. • [Chol][H 2 PO 4 ], [P 8,8,8,8 ][Br], and [P 6,6,6,14 ][DBS]exhibit minimal impact. • Significant discrepancies observed between acute and chronic IL toxicity profiles.