Do co-solvents used in exposure studies equally perturb the metabolic profile of Daphnia magna?

Dissolution methods such as co-solvents are used to solubilize insoluble compounds in exposure experiments. Several exposure studies have followed the guidelines from the Organization for Economic Co-operation and Development where co-solvents are applied at 0.01% v/v of the total exposure volume. Although no observable apical endpoint abnormalities were reported following these guidelines, little is known about the molecular-level impacts of co-solvents used in exposure studies. A targeted metabolomics approach using liquid chromatography coupled with triple quadrupole mass spectrometry was used to assess Daphnia magna responses to four commonly used co-solvents, including acetone (ACT), acetonitrile (ACN), methanol (MeOH), and dimethyl sulfoxide (DMSO), at three different levels (0.01%, 0.05%, and 0.1% v/v) over 48 hr. Based on the observed metabolic disruptions, exposure to MeOH and DMSO induced higher metabolic perturbations in amino acid levels and associated biochemical pathways in comparison to ACT and ACN exposures. However, as with mixtures, when co-solvents are combined with the pollutants under investigation, there is a possibility for additive, synergistic, or antagonistic interactions. Hence, to examine the possible impairments in co-solvent and pollutant mixtures, ACT and ACN applied at 0.01% v/v were chosen to be tested with phenanthridine (PN). Daphnia magna exposure to PN dissolved in ACT had less disruptions; in contrast to PN prepared in ACN, which triggered a higher degree of antagonism in the D. magna metabolic profile. Consequently, exposing D. magna to ACT applied at 0.01% v/v resulted in the lowest metabolic perturbation in both parts of this study, suggesting that it is the least disruptive co-solvent for molecular-level exposure studies involving D. magna.