Ecotoxicological biotests are conducted to evaluate effects on non-target organisms under standardised conditions. Usually, those protocols direct testing at a single neutral pH. However, natural conditions vary greatly and, in aquatic environments, pH values between 6 and 8 usually prevail. Depending on hydrogeological characteristics and anthropogenic impact, surface waters can even shift towards clearly both acidic and alkaline conditions with pH values between 4.5 and 10. Results obtained from toxicity tests conducted at a neutral pH may thus not apply to such divergent conditions. This is the case particularly for ionisable substances that are known to change their electric charge depending on the pH of the surrounding matrix. To exert their toxicity, uptake of the compound into the organism is prerequisite, thus, neutral or ionic species have to pass through a biomembrane. Whereas neutral species enter cells by passive diffusion along a concentration gradient, ions have to overcome the electrostatic barrier of the membrane. To pursue the investigation of a potential over- or underestimation of toxicity of ionisable substances due to shifts in pH, PHION, a joint project of the Animal Physiological Ecology group with the University of Athens, which is funded by the German Federal Environment Agency (UBA) will systematically generate data on fish embryo toxicity and chemical accumulation and aims at developing models that allow extrapolation of the toxicity of ionisable substances for different pH.
