Toxicity Assessment and Bioremediation of Deep Eutectic Solvents by Haloferax mediterranei: A Step toward Sustainable Circular Chemistry.

Understanding the toxicity of deep eutectic solvents (DESs) remains a major obstacle to large-scale applications. Existing toxicity studies show inconsistent results due to the choice of different test organisms and methods, synergistic effects between DES components, and their interactions with culture media. This study introduces the haloarchaeon Haloferax mediterranei as a novel model to assess both the toxicity and biodegradability of acetylcholine chloride (AcChCl) and choline chloride (ChCl) based DESs. Unlike other models that may not accurately reflect the environmental risks posed by halide-rich DES residues, H. mediterranei is an extremophile naturally adapted to high-salt and high-halide environments. DES concentrations of up to 300 mM were well tolerated. AcChCl-based DESs inhibited growth, likely via medium acidification due to some DES components hydrolysis, ChCl: acetamide has partial effects depending on acetamide concentration, and ChCl: ethylene glycol shows no toxicity. The haloarchaeon metabolizes specific DES components, reducing environmental impact. Urea and AcChCl: urea (100 mM) serve as nitrogen sources, while AcChCl-based DESs are consumed as carbon sources, likely due to the presence of acetate. H. mediterranei's metabolic versatility and high tolerance to toxic compounds position it as a promising candidate for sustainable bioremediation, advancing circular chemistry, and responsible DES waste management.