Inducible pesticide tolerance in Daphnia pulex influenced by resource availability

Zooplankton vulnerability to glyphosate exacerbated by global change

Anthropogenic activities generate a severe footprint at a global scale. Intensive agriculture is a global change driver that affects aquatic systems due to the discharge of pollutants. This situation can be modified or aggravated by other aspects, such as the disturbance history and other global change factors. Following our study line, it is necessary to evaluate how the disturbance history combined with temperature changes can affect the functioning of aquatic systems. The objectives of this study were divided into two phases. The objectives of phase 1 were to induce vulnerability in Daphnia magna populations through a disturbance history based on sublethal glyphosate concentration exposure under different temperature conditions (20 °C and 25 °C). In phase 2, vulnerability was assessed through the exposure to subsequent stressors (starvation, increased salinity and paracetamol) combined with changes in temperature. During the glyphosate exposure period in phase 1, differences were observed in the D. magna populations with respect to temperature, with lower abundance at 25 °C than at 20 °C. However, no differences were observed in abundance regarding glyphosate treatment. The results obtained in phase 2 with the new stressors combined with temperature changes in both directions, revealed stronger effects in vulnerable populations than in control populations. In addition, the temperature changes modulated the effects in the starvation and increased salinity tests. Agrochemical sublethal concentrations induce vulnerability in D. magna populations and inflicted temperature changes can act as a modulating factor for this vulnerability, showing the complexity in assessing the responses under the multiple scenarios associated with global change.

Sublethal exposure to agrochemicals impairs zooplankton ability to face future global change challenges

Intensive agriculture is characterized by the application of multiple stressors that damage aquatic systems. Currently, ecotoxicological evaluations are considered insufficient to understand the environmental risks of stressor mixtures and their consequences in ecosystems. In addition, future global change scenarios could alter the predicted effects of agrochemicals in aquatic systems based on single exposures, making it necessary to consider the history of environmental disturbances that may result in vulnerability to subsequent environmental changes. The objectives of this study were to induce disturbance histories by exposure to sublethal glyphosate concentrations in microcosms and to assess whether this disturbance results in vulnerable populations using Daphnia magna as the target species. Populations were considered vulnerable if their sensitivity to new stressors (inanition, temperature and salinity) was higher than that of undisturbed populations. To induce disturbance history, microcosm aquatic communities (two cladocerans and one microalgae) were exposed to two glyphosate sublethal concentrations (below the No Observed Effect Concentration, NOEC values for D. magna, 0.1 and 1 mg L^-1) in microcosms under controlled conditions in a culture chamber for 48 days. After this period, no significant differences were observed with respect to the control microcosms in the selected parameters (cladoceran abundance, microalgae cell abundance, microalgae colony formation, pH and dissolved oxygen). To test vulnerability, our target D. magna populations, which were previously exposed to different glyphosate treatments, were subjected to inanition, elevated temperature and salinity. Our results showed that D. magna populations with disturbance history performed worse in all the scenarios compared to the populations from undisturbed conditions. These results underscore the need to study how environmental disturbance history influences population responses to new and future stressors. Moreover, our findings raise concern regarding the sublethal effects of pesticides on aquatic populations.

Laboratory versus wild populations: the importance of population origin in aquatic ecotoxicology

The origin of the populations used in ecotoxicological bioassays (from nature (wild populations) or from cultures (laboratory populations)) could have a key influence on the sensitivity of the tested species to different toxicants. However, the available information on this subject is scarce. To assess the likely influence of the population origin (wild vs. laboratory) of species-genus on the toxicant tolerance, we performed a quantitative review of the ECOTOX database, from which we collected the effective concentrations for a wide range of compounds (metals and organics), endpoints, and exposure times. We found a general trend of lower sensitivity of wild populations to toxicants than laboratory populations, although sensitivity was dependent on species and toxicant groups. This suggests that the results of bioassays with laboratory populations may overestimate the toxicity of most of the compounds. Our study highlights the relevance of the origin of the populations in the determination of the sensitivity of species to toxicants. This study also warns about the biases in the species and toxicants used in ecotoxicology, which may lead to an underrepresentation of the biodiversity and the toxicological context of aquatic ecosystems.

Possible enzymatic mechanism underlying chemical tolerance and characteristics of tolerant population in Scapholeberis kingi

To determine the potential effects of pesticides on aquatic organisms inhabiting a realistic environment, we explored the characteristics and mechanisms of chemical tolerance in Scapholeberis kingi(Cladocera). We established a chemical-tolerant population via continuous exposure to pirimicarb, an acetylcholinesterase (AChE) inhibitor, and examined the effects of pirimicarb concentration on the intrinsic growth rates (r) of tolerant cladocerans. We also explored the association between r and feeding rate and tested the involvement of antioxidant enzymes [peroxidase (PO) and superoxide dismutase] and AChE in pirimicarb sensitivity. S. kingi was continuously exposed to lethal and sublethal pirimicarb concentrations (0, 2.5, 5, and 10 µg/L) for 15 generations, and changes (half maximal effective concentration at 48 h, 48 h-EC₅₀) in chemical sensitivity were investigated. In the F14 generation, the sensitivity of the 10 µg/L group was three times lower than that of the control group, suggesting the acquisition of chemical tolerance. Moreover, r was significantly and negatively correlated with 48 h-EC₅₀, suggesting a fitness cost for tolerance. Surprisingly, there was no significant correlation between r and feeding rate. There was a weak but significant positive correlation between each enzyme activity and the 48 h-EC₅₀ value (p < 0.05). Thus, oxidative stress regulation and enhanced AChE may be involved in the acquisition of chemical tolerance in cladocerans. These findings will help elucidate the characteristics and mechanisms of chemical tolerance in aquatic organisms.

Evolutionary mechanisms underpinning fitness response to multiple stressors in Daphnia

Multiple stressors linked to anthropogenic activities can influence how organisms adapt and evolve. So far, a consensus on how multiple stressors drive adaptive trajectories in natural populations has not been reached. Some meta-analysis reports show predominance of additive effects of stressors on ecological endpoints (e.g., fecundity, mortality), whereas others show synergistic effects more frequently. Moreover, it is unclear what mechanisms of adaptation underpin responses to complex environments. Here, we use populations of Daphnia magna resurrected from different times in the past to investigate mechanisms of adaptation to multiple stressors and to understand how historical exposure to environmental stress shapes adaptive responses of modern populations. Using common garden experiments on resurrected modern and historical populations, we investigate (1) whether exposure to one stress results in higher tolerance to a second stressor; (2) the mechanisms of adaptation underpinning long-term evolution to multistress (genetic evolution, plasticity, evolution of plasticity); and (3) the interaction effects of multiple stressors on fitness (synergism, antagonism, additivity). We measure the combined impact of different levels of resource availability (algae) and biocides on fitness-linked life-history traits and interpret these results in light of historical environmental exposures. We show that exposure to one stressor can alter tolerance to second stressors and that the interaction effect depends on the severity of either stressor. We also show that mechanisms of adaptation underpinning phenotypic evolution significantly differ in single-stress and multistress scenarios. These adaptive responses are driven largely by synergistic effects on fecundity and size at maturity, and additive effects on age at maturity. Exposure to multiple stressors shifts the trade-offs among fitness-linked life-history traits, with a stronger effect on Daphnia populations when low-resource availability and high biocide levels are experienced. Our study indicates that mitigation interventions based on single-stress analysis may not capture realistic threats.

Eutrophic status influences the impact of pesticide mixtures and predation on Daphnia pulex populations

Pesticides, nutrients, and ecological stressors such as competition or predation co-occur in freshwater ecosystems impacted by agriculture. The extent to which combinations of these stressors affect aquatic populations and the role of nutrients availability in modulating these responses requires further understanding. In this study, we assessed how pesticides affecting different taxonomic groups and predation influence the response of Daphnia pulex populations under different trophic conditions. An outdoor experiment was designed following a factorial design, with the insecticide chlorpyrifos, the herbicide diuron, and the predation by Notonecta sp. individuals as key stressors. The single impact of each of these stressors, and their binary and tertiary combinations, was evaluated on D. pulex abundance and population structure under mesotrophic and eutrophic conditions for 21 days. Data were analyzed using generalized linear mixed models estimated by means of a novel Bayesian shrinkage technique. Our study shows a significant influence of each of the evaluated stressors on D. pulex abundance; however, the impacts of the herbicide and predation were lower under eutrophic conditions as compared to the mesotrophic ones. We found that binary stressor interactions were generally additive in the mesotrophic scenario, except for the herbicide-predation combination, which resulted in synergistic effects. The impacts of the binary stressor combinations in the eutrophic scenario were classified as antagonistic, except for the insecticide-herbicide combination, which was additive. The tertiary interaction resulted in significant effects on some sampling dates; however, these were rather antagonistic and resembled the most important binary stressor combination in each trophic scenario. Our study shows that the impact of pesticides on freshwater populations depends on the predation pressure, and demonstrates that the combined effect of pesticides and ecological stressors is influenced by the food availability and organism fitness related to the trophic status of freshwater ecosystems.

Pesticide tolerance induced by a generalized stress response in wood frogs (Rana sylvatica)

Increasing evidence suggests that phenotypic plasticity can play a critical role in ecotoxicology. More specifically, induced pesticide tolerance, in which populations exposed to a contaminant show increased tolerance to the contaminants later, has been documented in multiple taxa. However, the physiological mechanisms of induced tolerance remain unclear. We hypothesized that induced pesticide tolerance is the result of a generalized stress response based on previous studies showing that both natural stressors and anthropogenic stressors can induce tolerance to pesticides. We tested this hypothesis by first exposing larval wood frogs (Rana sylvatica) to either an anthropogenic stressor (sublethal carbaryl concentration), a natural stressor (cues from a caged predator), or a simulated stressor via exogenous exposure to the stress hormone corticosterone (125 nM). We also included treatments that inhibited corticosterone synthesis with the compound metyrapone (MTP). We then exposed the larvae to a lethal carbaryl treatment to assess time to death. We found that prior exposure to 125 nM of exogenous CORT and predator cues induced tolerance to a lethal concentration of carbaryl through a slight delay in time to death. Pre-exposure to sublethal carbaryl, as well as MTP alone or in combination with predator cues, did not induce tolerance to the lethal carbaryl concentration relative to the ethanol vehicle control treatment. Our study provides evidence that pesticide tolerance can be induced by a generalized stress response both in the presence and absence (exogenous CORT) of specific cues and highlights the importance of considering physiological ecology and environmental context in ecotoxicology.

Evaluating the fitness consequences of plasticity in tolerance to pesticides

In a rapidly changing world, phenotypic plasticity may be a critical mechanism allowing populations to rapidly acclimate when faced with novel anthropogenic stressors. Theory predicts that if exposure to anthropogenic stress is heterogeneous, plasticity should be maintained as it allows organisms to avoid unnecessary expression of costly traits (i.e., phenotypic costs) when stressors are absent. Conversely, if exposure to stressors becomes constant, costs or limits of plasticity may lead to evolutionary trait canalization (i.e., genetic assimilation). While these concepts are well-established in theory, few studies have examined whether these factors explain patterns of plasticity in natural populations facing anthropogenic stress. Using wild populations of wood frogs that vary in plasticity in tolerance to pesticides, the goal of this study was to evaluate the environmental conditions under which plasticity is expected to be advantageous or detrimental. We found that when pesticides were absent, more plastic populations exhibited lower pesticide tolerance and were more fit than less plastic populations, likely avoiding the cost of expressing high tolerance when it was not necessary. Contrary to our predictions, when pesticides were present, more plastic populations were as fit as less plastic populations, showing no signs of costs or limits of plasticity. Amidst unprecedented global change, understanding the factors shaping the evolution of plasticity will become increasingly important.