Rapid Adaptation of a Daphnia magna Population to Metal Stress Is Associated with Heterozygote Excess.

Although natural populations can harbor evolutionary potential to adapt genetically to chemical stress, it is often thought that natural selection leads to a general reduction of genetic diversity and involves costs. Here, a 10 week microevolution experiment was conducted with a genetically diverse and representative sample of one natural Daphnia magna population that was exposed to copper and zinc. Both Cu- and Zn-selected populations developed a significantly higher metal tolerance (i.e., genetic adaptation), indicated by higher reproduction probabilities of clonal lines in Cu and Zn exposures than observed for the original and control populations. The complete recovery of the population densities after 10 weeks of Zn selection (following an initial decrease of 74%) illustrates an example of evolutionary rescue. Microsatellite genotyping revealed a decrease in clonal diversity but no change in allelic richness, and showed an excess in heterozygosity in the Cu- and Zn-selected populations compared to the control and original populations. The excess heterozygosity in metal-selected populations that we observed has important consequences for risk assessment, as it contributes to the maintenance of a higher allelic diversity under multigenerational chemical exposure. This study is, to our knowledge, the first report of an increase in heterozygosity following multigenerational exposure to metal stress, despite a decline in clonal diversity. In a follow-up study with the Zn-selected populations, we observed no effect of Zn selection on the tolerance to heat and cyanobacteria. However, we observed higher tolerance to Cd in the Zn-selected than in the original and control populations if the 20% effective concentration of Cd was considered (cross-tolerance). Our results suggest only limited costs of adaptation but future research is needed to evaluate the adaptive potential of metal-selected populations to novel stressors and to determine to what extent increased heterozygosity is preserved after genetic recombination following periods of sexual reproduction.

Temperature and food concentration have limited influence on the mixture toxicity of copper and Microcystis aeruginosa to Daphnia magna.

Standard ecotoxicity tests are conducted under constant and favorable experimental conditions. In natural communities, however, the toxicity of chemicals may be influenced by abiotic and biotic environmental factors. Firstly, the authors examined the influence of temperature and total food concentration on the nature of the combined effects of copper (Cu) and the cyanobacterium Microcystis aeruginosa to Daphnia magna (i.e., whether the combined effects deviated from noninteraction). Secondly, the authors investigated the relative influence of the percentage of M. aeruginosa in the diet, temperature, and total food concentration on chronic Cu toxicity to D. magna. The nature of the combined effects between Cu and M. aeruginosa (i.e., synergism according to the independent action reference model and noninteraction according to concentration addition reference model) was not affected by temperature and total food concentration. In line with other studies, the concentration addition reference model gave rise to more protective predictions of mixture toxicity than the independent action reference model, thus confirming the former model's suitability as a conservative scenario for evaluating mixture toxicity of Cu and M. aeruginosa under the temperature and food concentrations tested. Further, the 21-d median effective concentration for Cu based on reproduction varied between 20 µg/L and 100 µg/L, and the results indicate that the percentage of M. aeruginosa explained 76% of the variance in the Cu median effective concentration for reproduction, whereas the effects of temperature and total food were limited (together explaining 11% of the variance). The present study suggests that environmental risk assessment of Cu should consider specific situations where harmful M. aeruginosa blooms can co-occur with elevated Cu exposure.

Mixture toxicity of nickel and zinc to Daphnia magna is noninteractive at low effect sizes but becomes synergistic at high effect sizes.

To incorporate metal mixture toxicity effects into risk-assessment procedures, more information is needed about combined and interactive effects of metal mixtures during chronic exposure. The authors investigated the toxicity of binary Ni-Zn mixtures in 2 independent full-factorial experiments using standard chronic (21-d) Daphnia magna reproduction toxicity tests. Global statistical analysis (i.e., when considering all investigated mixture treatments simultaneously) showed noninteractive effects according to the concentration addition model and significant synergistic effects according to the independent action model. However, treatment-specific statistical analysis revealed that both occurrence and type of interactive effect were dependent on the effect size at which Ni and Zn were combined in the mixture. Only noninteractive or weakly antagonistic effects occurred in mixture treatments in which each of the individual metals produced only weak adverse effects on its own (i.e., ≤20% reduction of reproductive performance). On the other side of the spectrum, synergistic mixture effects occurred in all mixture treatments where both metals already caused a > 20% (for independent action) and a > 40% (for concentration addition) effect on reproduction on their own. Because low effect sizes are the most relevant in most regulatory frameworks, the authors' data suggest that the concentration addition and independent action mixture toxicity models can both serve as conservative models for predicting effects of Ni-Zn mixtures. The present study highlights the importance of investigating metal mixture toxicity at low effect sizes and warns against extrapolating conclusions about metal mixture interactions from high to low effect sizes.

The effect of temperature on the sensitivity of Daphnia magna to cyanobacteria is genus dependent.

In the present study, the authors investigated the effects of 6 different genera of cyanobacteria on multiple endpoints of Daphnia magna in a 21-d life table experiment conducted at 3 different temperatures (15 °C, 19 °C, and 23 °C). The specific aims were to test if the effect of temperature on Daphnia's sensitivity to cyanobacteria differed among different cyanobacteria and if the rank order from most to least harmful cyanobacteria to Daphnia reproduction changed or remained the same across the studied temperature range. Overall, the authors observed a decrease in harmful effects on reproduction with increasing temperature for Microcystis, Nodularia, and Aphanizomenon, and an increase in harmful effects with increasing temperature for Anabaena and Oscillatoria. No effect of temperature was observed on Daphnia sensitivity to Cylindrospermopsis. Harmful effects of Microcystis and Nodularia on reproduction appear to be mirrored by a decrease in length. On the other hand, harmful effects of Anabaena, Aphanizomenon, and Oscillatoria on reproduction were correlated with a decrease in intrinsic rate of natural increase, which was matched by a later onset of reproduction in exposures to Oscillatoria. In addition, the results suggest that the cyanobacteria rank order of harmfulness may change with temperature. Higher temperatures may increase the sensitivity of D. magna to the presence of some cyanobacteria (Anabaena and Oscillatoria) in their diet, whereas the harmful effects of others (Microcystis, Nodularia, and Aphanizomenon) may be reduced by higher temperatures.

Are interactive effects of harmful algal blooms and copper pollution a concern for water quality management?

Toxicity of mixtures of stressors is one of the major challenges in water quality management. Yet until now risk assessment focuses almost exclusively on the effect characterization of individual stressors. An important concern is the potential interactive effects of cyanobacteria, sometimes referred to as harmful algal blooms, with chemical stressors. Here, we evaluated the response of two clones of the freshwater cladoceran Daphnia magna to the combined effects of five cyanobacteria and copper. The latter remains the most commonly applied chemical algaecide and is also often detected in eutrophic run-offs that promote harmful algal blooms. Because the different cyanobacteria studied here have known modes of action that are similar, as well as dissimilar compared to the known modes of actions of copper, we based our assessment on two widely used reference models, i.e. the Concentration Addition (CA) model for similarly acting stressors and the Independent Action (IA) model for dissimilarly acting stressors. We highlight four major findings. First, the conclusions drawn on the interaction type (non-interaction vs. synergism or antagonism) between either of the five cyanobacteria species and copper were the same for both D. magna clones. Second, the interaction type differed between the Microcystis + copper mixture (non-interaction according to CA and synergism according to IA) and the four other cyanobacteria + copper mixtures (antagonism according to CA and non-interaction according to IA). Third, both reference models provided reasonable predictions for all observed mixture toxicities. Fourth, we consistently obtained different results with the IA reference model compared to the CA model. More specifically, mixtures of Cu and Microcystis were synergistic with IA whereas non-interaction was observed with CA, while the remaining four cyanobacteria + copper combinations all displayed non-interaction with IA and antagonism with CA. Despite the IA reference model providing a marginally better fit to the data in general, the CA reference model delivered more conservative predictions for mixture toxicity of cyanobacteria + copper in all cases compared to the IA reference model. Thus, the CA model could serve as a conservative model to account for mixture toxicity of cyanobacteria and copper in water quality management, as it gives rise to conservative predictions of mixed stressor toxicity at sub-lethal effect levels in D. magna. Finally, and in accordance with other studies of cyanobacteria + chemical mixtures, we did not detect any strong synergistic effects of copper and cyanobacteria mixtures on D. magna. Consequently, based on our study with the model freshwater zooplankton species Daphnia, interactive effects of harmful algal blooms and copper pollution appear to be of limited concern for water quality management.