Variation of Tolerance to Isothiazolinones Among Daphnia pulex Clones

Isothiazolinones are a family of broad-spectrum biocides widely used in industry and consumer products. Chloro- and methyl-isothiazolinones (CMIT and MIT) are documented as strong irritants, yet they are still used in a wide variety of applications, including cosmetics, cleansers, hygiene products and various industrial applications. The subsequent substantial release of these molecules from urban sources into freshwater environments, and their potential impacts on aquatic species, have nevertheless received little attention so far, with few studies reporting on the toxicity of either CMIT or MIT to non-target organisms. The present work addresses this current knowledge gap by evaluating CMIT/MIT (3:1) and MIT acute toxicity to Daphnia pulex (Cladocera), the two formulations most commonly used by manufacturers. Additionally, genetic diversity is known to be a major component of variability in phenotypic responses, although it is largely overlooked in typical toxicity tests. Subsequently the potential range of responses inherent to genetic diversity is rarely considered. Therefore, to account for intraspecific variation in sensitivity, the design involved eight clonal lines of D. pulex stemming from distinct natural populations or commercial strains. Clones exhibited strong variation in their responses, with lethal concentrations (LC50) ranging from 0.10 to 1.84 mg/L for the mixture CMIT/MIT, and from 0.68 to 2.84 mg/L for MIT alone. These intraspecific ranges of LC50 challenge the use of single clones of daphnids in standard ecotoxicological tests and the predictions based on their results. The present study brings new evidence that assessing ecological risk of chemicals while ignoring genotype diversity is neither ecologically relevant, nor a representative evaluation of the diversity of potential adverse outcomes.