Competition effects for copper between soil, soil solution and yeast in a bio assay for Folsomia candida willem

We investigated the accumulation of copper (Cu) by the springtail Folsomia candida Willem, if exposed to Cu-contaminated sandy soil with yeast as a food source. Commonly, the dissolved and the easily desorbed Cu fractions are assumed to be available for uptake, and as both fractions depend on pH, a pH dependency of copper uptake and accumulation is expected. In recent studies with springtails this dependency was not observed. To explain this, we show that both the adsorption of copper by yeast and by soil is indeed pH dependent; however, these dependencies differ. Addition of yeast as a food source to copper-contaminated soil leads to competition for copper by yeast and soil that suppresses the pH dependency of copper adsorption by yeast. This may cause a pH dependency not to be observed in copper accumulation by springtails if they predominantly feed on yeast in bioassays. We conclude that the addition of artificial food sources in bioassays may affect the cause-effect relationships that are investigated. A combination of (soil) chemical experimentation and modeling and ecotoxicological studies may help in identifying such bias and, therefore, with interpreting bioassays.