Daphnia magna as an Alternative Model for (Simultaneous) Bioaccumulation and Chronic Toxicity Assessment─Controlled...

Testing the bioaccumulation and chronic toxicity of (highly) hydrophobic compounds is extremely challenging, but crucial for hazard assessment. Fish are used as a model organism in these tests, but have many limitations, including a long time to reach steady-state, difficulty in maintaining constant exposure, and ethical concerns. We developed a method for the (simultaneous) assessment of chronic reproductive toxicity and bioaccumulation using Daphnia magna as a model organism. As test chemicals, we selected heterocyclic polyaromatic hydrocarbons (heterocyclic PAHs), which are often persistent and show high acute aquatic toxicity, raising concerns about their long-term effects. In this study, we developed a robust passive dosing method to maintain constant exposure in chronic toxicity and bioaccumulation tests of four heterocyclic PAHs in Daphnia magna. Passive dosing maintained stable exposure concentrations in the ng to μg L^-1 range, even after reusing disks up to three times. All chemicals were toxic to Daphnia magna with EC₁₀ values between 0.1 and 15 μg L^-1. Bioaccumulation tests showed that steady-state was not reached, and the uptake rate constant (k₁) could not be reliably determined due to complex exposure routes (both via water and diet). However, depuration rates in Daphnia magna were about 2 orders of magnitude higher than in fish, which is advantageous in the assessment of highly hydrophobic compounds. We propose to use the depuration rate constant (k₂), which is independent of the uptake route, as an indicator of bioaccumulation potential. The k₂ thresholds for Daphnia magna were estimated to identify (very) bioaccumulative compounds by correlating k₂ values with bioconcentration factors (BCFs) for Daphnia magna and applying fish BCF thresholds. We suggest that a Daphnia magna bioaccumulation test can be used as a screening tool to trigger further bioaccumulation testing in fish, as it offers higher throughput, is more ethical, and reaches steady-state faster. However, further validation with reference test protocols and substances is essential.