Evaluating additive versus interactive effects of copper and cadmium on Daphnia pulex life history

Abstract

A key challenge of standard ecotoxicological risk assessment is to predict the sub-lethal risk of multiple contaminants on aquatic organisms. Our study assessed the sub-lethal mixture toxicity of copper (Cu) and cadmium (Cd) on Daphnia pulex and included manipulations of food level and assessment of three genotypes. We investigated the interaction between essential (Cu) and non-essential (Cd) metals on ingestion rate, reproduction, maturation time, size at maturity and somatic growth rate of three D. pulex genotypes, over 21 days and under standard and high food conditions. We explored the potential interaction of the metals on ingestion and life history by implementing a response surface experimental design combining control and two levels of Cu and Cd and their combinations. Overall, both metals reduced ingestion rates, reduced reproduction, delayed maturation, reduced body size at maturity and lowered somatic growth rate. Our results further indicated pervasive interactions between the metals; numerous instances where the effects of each metal were nonlinear; the effect of a metal varied by D. pulex food levels (ingestion rate and size at maturity), and the effect of a metal varied by genotypes (reproduction). Apart from the maturation time and somatic growth rate, our results suggest that life history traits are affected in non-additive ways by three factors that are often discussed and rarely estimated together: mixtures of metals, genotypes and resource levels. Our data that are derived from exposing daphnids to two metals highlight how metals interact with each other and the context of food resource and genetic variation. While interactions make it harder to generate predictions, and ultimately water quality regulations about the effects of metals, those detected in this study appear to be tractable.

Copper mediates life history responses of Daphnia pulex to predation threat

“…Frontiers in Ecology Evolution 12 frontiersin.org found to have a profound effect on life history traits, in line with previous findings (De Schamphelaere et al, 2007;Agra et al, 2011;Sadeq and Beckerman, 2019;Sadeq and Beckerman, 2020): exposure of animals to a gradient of copper concentrations alone resulted in a strong reduction of size at maturity and somatic growth rate, in addition to substantial increases in the age at maturity. Our data further indicate that an exposure to high levels of copper reduced the chances of survival until maturity.…”

“…The full model to detect non-additive effects and characterize the shape of the response surface takes the form of response ~ PredationRisk + PredationRisk 2 + Cu + Cu 2 + PredationRisk*Cu + (1|Genotype) where (1|Genotype) represents the random effect of genotype. The fixed effects, response surface component is a standard statistical model with applications in ecology and ecotoxicology that can detect planar and several non-linear "response" surfaces (e.g., peaks, troughs and valleys) driven by two covariates (Jonker et al, 2005;Scholes et al, 2005;Sadeq and Beckerman, 2020). The covariates are modeled as polynomial terms such that the effects of predation risk and Cu can be linear or non-linear and this is combined with an explicit interaction term, which together with the non-linear terms, allows for various shapes of an interaction surface to emerge if justified by the data.…”

The microbiome mediates the interaction between predation and heavy metals

“…For predation exposure, concentrated chemical cues were extracted using the procedure of (see Beckerman et al, 2010;Carter et al, 2017;Dennis et al, 2011;Hammill et al, 2008;Lind et al, 2015;Tollrian, 1995) and added to treatment media at a concentration of 1µl / ml. Nominal and realised concentrations of Cu were strongly correlated (ICP-MS; r2 = 0.99, F = 1.91, p < 0.002; performed in a separate specialist chemistry laboratory at the University of Sheffield; see Sadeq and Beckerman, 2019b).…”

“…Theory and empirical data indicate that sub-lethal concentrations of Copper influence foraging and assimilation of nutritional resources which can lead to as increased metabolic cost and reduced energy acquisition. Copper's impact on digestive physiology is strongly predicted by classic life-history theory where reduced energy intake translates into delayed maturity at a smaller size, reduced reproduction and slower somatic growth rates Bui et al, 2016;De Schamphelaere et al, 2007;Sadeq and Beckerman, 2019b).…”

Complex Cd–Pb Multigenerational Exposure Improves the Growth and Food Utilization of the Cutworm Spodoptera litura

“…In addition, some studies showed that the development of insect heavy metal tolerance probably led to the increase in their tolerance to other stresses (e.g., pesticides) [29,40,41], thus the measures of pest management in the farmland with mild or moderate heavy metal pollution should be changed. In this study, we only examined the effects of complex Cd-Pb exposure on herbivorous insects under standardized conditions that were beneficial to a testing environment but not always reflective of natural communities [64]. Indeed, heavy metal pollution in the field is the coexistence of multiple heavy metals, and their impacts on organisms were more diversified and complicated [4,5,29].…”