Influence of genetic and environmental factors on the tolerance of Daphnia magna Straus to essential and non-essential metals

The ultimate aim of ecotoxicological studies is to predict how natural populations respond to contaminant exposure. Thus, it is crucial to understand how genetic and environmental factors in the field modify responses measured in the laboratory. In the present study the authors determine the genetic and environmental components of variability in acute responses among four Daphnia magna clones exposed to both essential (Zn and Cu) and non-essential (Cd and U) metals in waters with varying water hardness. The authors postulate that genotype acute responses to physiologically non-essential metals may be more variable than responses to essential metals and that this may be explained by hypothesizing that acute responses to non-essential substances are subject to intermittent selection (since the substances may not always be present in biologically significant amounts), whereas responses to essential substances are subject to continuous directional selection (since essential substances by definition co-occur with biota in biologically significant amounts). D. magna clones were exposed to single-metal solutions of varying concentration at two or three levels of water hardness (soft, moderate–hard and hard) for periods ranging from 12–96 h (12 h increments). LC50 values for each metal×genotype×water hardness×exposure period combination were determined for (i) total metal concentration and (ii) the free hydrated metal ion concentration (predicted using geochemical speciation modeling) and compared using analysis of covariance with environment (water hardness) and genotype (clone) as fixed factors and exposure time as a covariate. The results showed that Zn–Cd were consistent, but Cu–U inconsistent, with the essentiality hypothesis. In addition, the small, or even non-existent, genotype-environment interaction effect on the inheritance of metal tolerance supports the argument that genetic variability in resistance to trace metals is inherited through single major genes. Implications of these results for the prediction of population tolerance ranges are discussed in relation to the problem of predicting metal speciation and the biological effects of metals in fresh waters with varying hardness.