Herbicides interfere with antigrazer defenses in Scenedesmus obliquus

Grazing inhibition in Daphnia and Bosmina by colony formation of Desmodesmus subspicatus triggered by sodium octyl sulfate

There are growing concerns that several aquatic contaminants can indirectly alter biological interactions by inhibiting the adaptive phenotypic plasticity of organisms, even at nonlethal concentrations. In Scenedesmaceae, a family of green algae, many chemicals interfere with defensive colony formation against grazers (i.e., through induced or limited coloniality). Although several studies have demonstrated that the effects of coloniality can limit the feeding capacity of Daphnia spp., grazing inhibition in other zooplankton species is not well understood. In this study, we examined the influence of sodium octyl sulfate (SOS) on the growth and morphology of Desmodesmus subspicatus and on the feeding rates of three cladoceran species (Daphnia galeata, Bosmina longirostris, and Bosmina fatalis) feeding on SOS-induced colonies under factorial conditions of different food levels and grazer ages. SOS remarkably induced colony formation with no observed effect on growth in D. subspicatus. D. galeata and B. fatalis showed a remarkable reduction in feeding rates when they fed on colonial D. subspicatus, whereas no significant effect of the prey morphotype was found on the feeding rates of B. longirostris. Microscopic observations of algal morphology after being grazed showed that each species can consume colonial prey depending on food level and age. Comparisons of the inhibition ratio of feeding among the three cladocerans revealed that Daphnia was more sensitive to prey coloniality compared with Bosmina. Our findings provide specific insights into the effects of chemically interfered colony formation on population dynamics and community structures.

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.

The stress effect of atrazine on the inducible defense traits of Daphnia pulex in response to fish predation risk: Evidences from morphology, life history traits, and expression of the defense-related genes

Herbicide pollution is persistent, which not only has a negative impact on individual organisms, but also may endanger the interspecific relationship between predators and prey. Cladocerans, i.e. zooplankton that plays an important role in the energy flow and material circulation in freshwater ecosystem, usually develop induced defense in response to predation risk. We used atrazine, one of the most used herbicides in the world, and Daphnia pulex, a representative cladocerans, to test the possible interference effect of herbicides on the induced defensive traits of cladocerans in response to predator fish (Rhodeus ocellatus) kairomone, including morphological defense, life history strategies, and the expression of defense-related genes. Atrazine reduced the body size, spine size, growth rate, total offspring, and the relative reproductive output of D. pulex, which further affected the response strength of the morphological and life history defenses, i.e., atrazine significantly reduced the spine size, relative spine size, and fecundity of D. pulex in response to R. ocellatus kairomone. Exposure to atrazine affected the expression of defense-related genes, and we speculated that atrazine affected the signaling process in the induced anti-predation defense of cladocerans. Specially, fish kairomone attenuated the negative effects of high concentrations of atrazine on the life history traits of D. pulex. Our results will help to accurately assess the potential risk of artificial compounds in freshwater ecosystems from the perspective of interspecific relationships, and help to understand the impact of environmental changes on the inducible anti-predator defense of prey in aquatic ecosystems.

The response of life history defense of cladocerans under predation risk varies with the size and concentration of microplastics

Microplastics are an emerging and increasingly serious pollutant in freshwater environment, which have become a threat to freshwater organisms. However, whether microplastics interfere with the responses of organisms to their predators is still unclear. In this study, we investigated the effects of microplastics with tiny different particle size (diameter: 0.7 and 1 µm) on the anti-predation (Rhodeus ocellatus as the predator) defense responses of different body-sized cladocerans, Daphnia pulex and Moina macrocopa. Results showed that microplastics had a size-based inhibitory effect on the induced defense of both D. pulex and M. macrocopa. Specifically, 0.7 µm microplastics had stronger effects on reduced survival time, delayed maturation time, and decreased offspring numbers. In addition, the effects of microplastics also varied with different body-sized cladocerans, i.e. medium-sized cladoceran (D. pulex) were more sensitive than the small-sized one (M. macrocopa) regarding the maturation time. This study illustrated for the first time that the effect of microplastics on induced defense was related to cladoceran species and microplastics size, and further revealed the extensive negative effects of microplastics from the perspective of interspecific relationship.

Warming mitigates the enhancement effect of elevated air CO2 on anti-grazer morphological defense in Scenedesmus obliquus

Atmospheric CO₂ and temperature are increasing, which will have substantial impacts on interactions among organisms. While each stressor in isolation has been studied extensively, there has been less focus on their combined effects on the interspecies interaction. In order to reveal how warming and elevated CO₂ interact on the induced defense of phytoplankton, we investigated the combined influences of elevated CO₂ (750 ppm vs 390 ppm) and high temperature (28 °C and 31 °C vs 25 °C) on grazer Daphnia-induced morphological defense in Scenedesmus obliquus. Results showed that S. obliquus formed big-sized colonies (e.g., four- and eight-celled colonies) as response to Daphnia infochemicals, resulting in an increase in the number of cells per particle. Elevated CO₂ further decreased the proportion of unicells from >40% in the populations growing at 390 ppm CO₂ without Daphnia filtrate to <7% in the populations growing at 750 ppm CO₂ with Daphnia filtrate, with the formation of more than 90% colonies, thus enhancing this morphological defense in S. obliquus. However, under elevated CO₂, increasing temperature up to 31 °C remarkably increased the four-celled colonies by at least 159% but decreased the eight-celled colonies by 37% compared with 25 °C. As a result, the maximum cells per particle were significantly decreased to the 390 ppm CO₂-grown level at high temperature. The time to reach the maximum cells per particle was also shortened by high temperature under elevated CO₂. These results suggest that high temperature has an overwhelming inhibitory effect on the enhanced anti-grazer defense by elevated CO₂, which provides significant implications for forecasting the predator-prey interaction changes in freshwater ecosystem under future climate regimes.

Aquaculture-derived distribution, partitioning, migration, and transformation of atrazine and its metabolites in seawater, sediment, and organisms from a typical semi-closed mariculture bay

Atrazine (ATR) is one of the most commonly used herbicides that could directly impair the growth and health of organisms in mariculture areas and adversely affect human health through the food chain. This study investigated the contaminant occurrence, migration, and transformation of ATR and three of its chlorinated metabolites, namely deethylatrazine (DEA), deisopropylatrazine (DIA), and didealkylatrazine (DDA), in surface seawater, sediment, and aquatic organisms from the Xiangshan Harbor. ATR was detected in all samples, while DIA and DDA were only respectively detected in aquatic and seawater samples. The distribution of ATR and its metabolites presented different patterns depending on the geographic location and showed a higher level in the aquaculture area than that in the non-aquaculture area. The bioaccumulation of ATR in aquaculture organisms showed that benthic organisms, such as Ditrema, and Sinonovacula constricta (Sin), had increased levels. The ecological risks indicated that ATR posed medium or high risks to algae in the water phase of the study area. The microcosm experiment showed that the main fate of ATR in the simulated microenvironment was sedimentation, which followed the first-order kinetic equation. The ATR in the sediment could be enriched 3-5 times in Sin, and its major metabolites were DEA and DIA.

Antidepressant sertraline impairs the induced morphological defense of Ceriodaphnia cornuta in response to Chaoborus larvae kairomone

Antidepressants discharged into natural waters are likely to become a new type of endocrine pollutant, which may impact the interspecific relationship in aquatic ecosystem. Induced defense of cladocerans plays an important role in maintaining the balance of interspecific relationships between cladocerans and higher trophic levels. Here we studied the effects of antidepressant sertraline, a selective serotonin reuptake inhibitor, on the induced defensive traits of Ceriodaphnia cornuta in response to invertebrate predator Chaoborus larvae kairomone, including morphological defense and life history traits. We also conducted the predation experiments to check the selection rate of Chaoborus larvae during directly ingesting C. cornuta that were exposed to Chaoborus larvae kairomone at high concentration of sertraline. Results showed sertraline had an interference effect on the induced morphological defense of C. cornuta in response to Chaoborus larvae kairomone, i.e. the high concentration of sertraline (20 and 100 μg L^-1) significantly reduced the horns induction. However, the different concentrations of sertraline generally did not affect the life history traits of C. cornuta, regardless of presence or absence of Chaoborus larvae kairomone. The predation experiment demonstrated that the inhibition of sertraline on the induced morphological defense of C. cornuta can promote the feeding selective efficiency of Chaoborus larvae, and thus cause C. cornuta easily to be predated by Chaoborus larvae. Our results suggested that sertraline at the concentrations that are not direct harmful to life history traits of C. cornuta can seriously affect the predator-prey relationship, indicating that effects of pollutants on interspecific relationships should be considered comprehensively to avoid underestimating the potential risk of pollutants to ecosystems.

Long-term effects of in ovo exposure to an environmentally relevant dose of atrazine on the thyroid gland of Caiman latirostris

The increased incidence of human thyroid disorders, particularly in women, suggests that the exposure to endocrine-disrupting compounds (EDCs) together with sex-related factors could play a role in thyroid dysregulation. Since the herbicide atrazine (ATZ) is an environmental EDC suspected to behave as a thyroid disruptor, and Caiman latirostris is a crocodilian species highly sensitive to endocrine disruption that can be exposed to ATZ, this study aimed to describe the histoarchitecture and sexually dimorphic features of the thyroid gland of C. latirostris, and to determine the long-term effects of in ovo exposure to an environmentally relevant dose of ATZ (0.2 ppm) on its thyroid gland and growth. Control caimans showed no sexual dimorphisms. In contrast, ATZ-exposed caimans showed altered embryo growth but an unaltered temporal pattern of development and a sexually dimorphic response in the body condition index growth curves postnatally, which suggests a female-related increase in fat storage. Besides, both male and female exposed caimans showed increases in the size of the thyroid stromal compartment, content of interstitial collagen, and follicular hyperplasia, and decreases in the expression of androgen receptor in the follicular epithelium. ATZ-exposed females, but not males, also showed evidences of thyroid enlargement, colloid depletion, increased follicular epithelial height and increased presence of microfollicular structures. Our results demonstrate that prenatal exposure of caimans to ATZ causes thyroid disruption and that females were more vulnerable to ATZ than males. The effects were organizational and observed long after exposure ended. These findings alert on ATZ side-effects on the growth, metabolism, reproduction and development of non-target exposed organisms, particularly females.

Surfactants at environmentally relevant concentrations interfere the inducible defense of Scenedesmus obliquus and the implications for ecological risk assessment

The ecotoxicology of surfactants is attracting wide attention due to the rapidly expanding global application. As interspecific relationships play one of the central roles in structuring biological communities, it is necessary to take it into risk assessments on surfactants. With this aim, our study investigated the interference of three common surfactants on the inducible defense of a freshwater phytoplankton Scenedesmus obliquus. Nonlethal environmentally relevant concentrations (10 and 100 μg L^-1) of several surfactants were set up. Results showed that growth and photosynthetic efficiency of Scenedesmus were inhibited during first 96 h, but recovered in the later stage. Surfactants interfered inducible defense of Scenedesmus against Daphnia grazing, and the interference was related to chemical characteristics of surfactants. The anionic surfactant sodium dodecyl sulfate (SDS) enhanced the colony formation even without grazing cues, whereas fewer defensive colonies were formed under the effects of cationic surfactant benzalkonium bromide (BZK) and nonionic surfactant polyoxyethylene (40) nonylphenol ether (NPE). These findings highlighted the sensitivity of grazer-induced morphological defense of Scenedesmus to surfactants even at nonlethal concentrations, which potentially affects the energy and information flow between trophic levels. This study appeals for more attention to take interspecific relationships into consideration in assessing the potential ecological risk of pollutants.

UVB Radiation Suppresses Antigrazer Morphological Defense in Scenedesmus obliquus by Inhibiting Algal Growth and Carbohydrate-Regulated Gene Expression

Solar ultraviolet-B (UVB) radiation reaching the earth's surface is increasing due to stratospheric ozone depletion. How the elevated UVB affects the trophic interactions is critical for predicting the ecosystem functioning under this global-scale stressor. Usually, inducible defenses in phytoplankton stabilize community dynamics within aquatic environments. To assess the effects of elevated UVB on induced defense, we examined the changes in antigrazer colony formation in Scenedesmus obliquus under environmentally relevant UVB. S. obliquus exposed to Daphnia infochemicals consistently formed multicelled colonies, traits confirmed to be adaptive under predation risk. However, the suppressed photochemical activity and the metabolic cost from colony formation resulted in the severer reductions in algal growth by UVB under predation risk. The transcriptions of key enzyme-encoding genes, regulating the precursor synthesis during polysaccharide production, were also inhibited by UVB. Combination of the reduced production of daughter cells and the ability of daughter cells to remain attached, the antigrazing colony formation was interrupted, leading to the dominant morphs of algal population shifting from larger-sized colonies to smaller ones at raised UVB. The present study revealed that elevated UVB will not only reduce the phytoplankton growth but also increase their vulnerability to predation, probably leading to potential shifts in plankton food webs.

Design, Synthesis, and Biological Activity of Novel Diazabicyclo Derivatives as Safeners

Herbicide safeners selectively protect crops from herbicide damage without reducing the herbicidal efficiency on target weed species. The title compounds were designed by the intermediate derivatization approach and fragment splicing to exploit novel potential safeners. A total of 31 novel diazabicyclo derivatives were synthesized by the microwave-assistant method using isoxazole-4-carbonyl chloride and diazabicyclo derivatives. All synthetic compounds were confirmed by infrared, ¹H and ¹³C nuclear magnetic resonance, and high-resolution mass spectrometry. The bioassay results demonstrated that most of the title compounds could reduce the nicosulfuron phytotoxicity on maize. The glutathione S-transferase (GST) activity in vivo was assayed, and compound 4(S15) revealed an inspiring safener activity comparable to commercialized safeners isoxadifen-ethyl and BAS-145138. The molecular docking model exhibited that the competition at the active sites of target enzymes between compound 4(S15) and nicosulfuron was investigated with respect to herbicide detoxification. The current work not only provided a powerful supplement to the intermediate derivatization approach and fragment splicing in design pesticide bioactive molecules but also assisted safener development and optimization.

An overview of the potential impacts of atrazine in aquatic environments: Perspectives for tailored solutions based on nanotechnology

Atrazine is a pre- and post-emergence herbicide used to control weeds in many crops. It was introduced in the late 1950s, but its use has been controversial because of its high potential for environmental contamination. In agriculture, the implementation of sustainable practices can help in reducing the adverse effects atrazine. This review addresses aspects related to the impacts of atrazine in the environment, with focus on its effects on aquatic species, as well as the potential use of nanoencapsulation to decrease the impacts of atrazine. The application of atrazine leads to its dispersal beyond the immediate area, with possible contamination of soils, sediments, plantations, pastures, public supply reservoirs, groundwater, streams, lakes, rivers, seas, and even glaciers. In aquatic ecosystems, atrazine can alter the biota, consequently interfering in the food chains of many species, including benthic organisms. Nanoformulations loaded with atrazine have been developed as a way to reduce the adverse impacts of this herbicide in aquatic and terrestrial ecosystems. Ecotoxicological bioassays have shown that this nanoformulations can improve the targeted delivery of the active ingredient, resulting in decreased dosages to obtain the same effects as conventional formulations. However, more detailed analyses of the ecotoxicological potential of atrazine-based nanoherbicides need to be performed with representative species of different ecosystems.

High temperature promotes the inhibition effect of Zn2+ on inducible defense of Scenedesmus obliquus

Morphological defense is assumed to be an effective anti-grazer strategy in phytoplankton. Scenedesmus obliquus, a globally widespread freshwater chlorophyte, can form colonies in response to the infochemicals of herbivorous zooplankton and survive in coexistence with grazers. However, the inducible defense response is often disturbed by abiotic or biotic factors, especially under the increasing global warming and environmental pollution. In this study, two nonlethal environmental factors, namely, elevated temperature and environmentally relevant Zn²⁺ concentrations, decreased colony formation of S. obliquus induced by Daphnia grazing infochemicals. Elevated temperature (30 °C) reduced the inducible colony size and shortened the maintenance time of defensive colonies. Decreased colony size was detected with increased Zn²⁺ concentration. Colony formation was inhibited even at low Zn²⁺ concentration (0.131 μmol L^-1), which neither retarded growth nor affected photosynthesis. Warming promoted the inhibition effect of Zn²⁺ on inducible colony formation of S. obliquus. Warming also enhanced Zn²⁺ toxicity, which caused the growth rate of S. obliquus to be hindered by high Zn²⁺ concentrations at elevated temperature. Specially, S. obliquus which formed inducible colonies under the condition of Daphnia infochemicals had higher tolerance to Zn²⁺ toxicity and thus likely exerted protective effects against heavy metals. The results indicated the combined effects of global warming and heavy-metal pollution result in more severe impact on the inducible defense of S. obliquus.

Indole-3-acetic-acid-induced phenotypic plasticity in Desmodesmus algae

Phenotypic plasticity is the ability of a single genotype of an organism to exhibit variable phenotypes in response to fluctuating environments. It plays a crucial role in their evolutionary success. In natural environments, the importance of interactions between microalgae and other microorganisms is generally well appreciated, but the effects of these interactions on algal phenotypic plasticity has not been investigated. In this study, it revealed that indole-3-acetic acid (IAA), the most common naturally occurring plant hormone, can exert stimulatory at low concentrations and inhibitory effects at high concentrations on the growth of the green alga Desmodesmus. The morphological characteristics of Desmodesmus changed drastically under exposure to IAA compared with the algae in the control environment. The proportion of Desmodesmus unicells in monocultures increased with the IAA concentration, and these unicells exhibited less possibility of sedimentation than large cells. Furthermore, we discovered that lipid droplets accumulated in algal cells grown at a high IAA concentration. Results also demonstrated that the presence of algal competitor further stimulated inducible morphological changes in Desmodesmus populations. The relative abundance of competitors influenced the proportion of induced morphological changes. The results indicate that phenotypic plasticity in microalgae can be a response to fluctuating environments, in which algae optimize the cost–benefit ratio.

Potential impact of the herbicide 2,4-dichlorophenoxyacetic acid on human and ecosystems

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is applied directly to aquatic and conventional farming systems to control weeds, and is among the most widely distributed pollutants in the environment. Non-target organisms are exposed to 2,4-D via several ways, which could produce toxic effects depending on the dose, frequency of exposure, and the host factors that influence susceptibility and sensitivity. An increasing number of experimental evidences have shown concerns about its presence/detection in the environment, because several investigations have pointed out its potential lethal effects on non-target organisms. In this review, we critically evaluated the environmental fate and behavior of 2,4-D along with its eco-toxicological effects on aquatic, plants and human life to provide concise assessment in the light of recently published reports. The findings demonstrate that 2,4-D is present in a low concentration in surface water of regions where its usage is high. The highest concentrations of 2,4-D were detected in soil, air and surface water surrounded by crop fields, which suggest that mitigation strategies must be implanted locally to prevent the entry of 2,4-D into the environment. A general public may have frequent exposure to 2,4-D due to its wide applications at home lawns and public parks, etc. Various in vivo and in vitro investigations suggest that several species (or their organs) at different trophic levels are extremely sensitive to the 2,4-D exposure, which may explain variation in outcomes of reported investigations. However, implications for the prenatal exposure to 2,4-D remain unknown because 2,4-D-induced toxicity thresholds in organism have only been derived from juveniles or adults. In near future, introduction of 2,4-D resistant crops will increase its use in agriculture, which may cause relatively high and potentially unsafe residue levels in the environment. The recent findings indicate the urgent need to further explore fate, accumulation and its continuous low level exposure impacts on the environment to generate reliable database which is key in drafting new regulation and policies to protect the population from further exposure.