The thermal regime modifies the response of aquatic keystone species Daphnia to microplastics: Evidence from population fitness, accumulation, histopathological analysis and candidate gene expression

Changes in characteristics and risk of freshwater microplastics under global warming

Microplastics present a significant threat to freshwater ecosystems. However, the impact of global warming on their characteristics and associated risks remains uncertain. This study collected 2793 sample sites from literature and datasets to create a new risk assessment and rank methodology, known as the Multi-characteristics Potential Ecological Risk Index (MPERI), which incorporates various microplastic characteristics, such as concentration, size distribution, color, shape, and polymer diversity. Using regression random forest models (RRF), this study predicted that a 10 °C increase would raise microplastic concentration from 12,465.34 ± 68,603.87 to 13,387.17 ± 60,692.96 particles/m3. The percentage of small-size microplastics initially decreased (from 69.10 % to 68.72 %) and then increased (from 68.72 % to 68.78 %), while the diversity of color, shape, and polymer decreased by 0.29 %, 3.24 %, and 0.17 %, respectively. Furthermore, global warming could increase the rank of microplastic risks from high (405.25 ± 528.9) to dangerous (535.37 ± 582.03) based on the MPERI method. Most countries would experience an increase in risk values, with Indonesia and Vietnam transitioning from low to medium risk, and China and Malaysia transitioning from high to dangerous risk. The feature importance assessment of the RRF model indicated that concentration was the most influential variable in determining the change in risk values. While other microplastic characteristics had a lesser impact compared to concentration, they still influenced the risk ranking. This study highlights the role of global warming in shaping microplastic risks.

Local thermal adaption mediates the sensitivity of Daphnia magna to nanoplastics under global warming scenarios

The toxicity of nanoplastics at environmentally relevant concentrations has received widespread attention in the context of global warming. Despite numerous studies on the impact of mean temperature (MT), the effects of daily temperature fluctuations (DTFs) on the ecotoxicity of nanoplastics remains largely unexplored. Moreover, the role of evolutionary adaptation in assessing long-term ecological risks is unclear. Here, we investigated the effects of polystyrene nanoplastics (5 μg L-1) on Daphnia magna under varying MT (20 °C and 24 °C) and DTFs (0 °C, 5 °C, and 10 °C). Capitalizing on a space-for-time substitution approach, we further assessed how local thermal adaptation affect the sensitivity of Daphnia to nanoplastics under global warming. Our results indicated that nanoplastics exposure in general reduced heartbeat rate, thoracic limb activity and feeding rate, and increased CytP450, ETS activity and Hgb concentrations. Higher MT and DTFs enhanced these effects. Notably, clones originating from their respective sites performed better under their native temperature conditions, indicating local thermal adaptation. Warm-adapted low-latitude D. magna showed stronger nanoplastics-induced increases in CytP450, ETS activity and Hgb concentrations under local MT 24 °C, while cold-adapted high-latitude D. magna showed stronger nanoplastics-induced decreases in heartbeat rate, thoracic limb activity and feeding rate under high MT than under low MT.

The analysis of inducible family members in the water flea Daphnia magna led to the identification of an uncharacterized lineage of heat shock protein 70

To explore the function and evolutionary relationships of inducible heat shock protein 70 (Hsp70) in Daphnia magna, cDNAs of four Hsp70 family members (DmaHsp70, DmaHsp70-2, DmaHsp70-12, DmaHsp70-14) were cloned. While all DmaHsp70s possess three function domains, it is noteworthy that only DmaHsp70 ends with a "EEVD" motif. Phylogenetic analysis indicates that the Hsp70-12 lineage is distanced from the rest, and therefore it is an uncharacterized lineage of Hsp70. The differences in isoelectric point and 3-dimensional (3D) conformation of the N-terminal nucleotide binding domain (NBD) of DmaHsp70s further support the theory. DmaHsp70s exhibit varied motif distribution patterns and the logo sequences of motifs have diverse signature characteristics, indicating that different mechanisms are involved in the regulation of ATP binding and hydrolysis for the DmaHsp70s. Protein-protein network together with the predicted subcellular locations of DmaHsp70s suggest that they likely fulfill distinct roles in cells. The transcription of four DmaHsp70s were changed during the recovery stage after thermal stress or oxidative stress. But the expression pattern of them were dissimilar. Collectively, these results collectively elucidated the identification of a previously uncharacterizedHsp70 lineage in animal and extended our understanding of the Hsp70 family.

Warming temperatures exacerbate effects of microplastics in a widespread zooplankton species

The emergence of microplastics as a global contaminant of concern has coincided with climate change induced temperature warming in aquatic ecosystems. Warmer temperatures have been previously demonstrated to increase the toxicity of certain contaminants, but it is currently unclear if microplastics are similarly affected by temperature. As aquatic organisms simultaneously face microplastic pollution and both increasing and variable temperatures, understanding how temperature affects microplastic toxicity is pertinent in this era of human-induced global change. In this study, we investigate the effects of environmentally relevant microplastic exposure to Daphnia pulex survival, reproduction, and growth at three different temperatures. To simulate an environmentally relevant exposure scenario, we created microplastics with physicochemical characteristics often detected in nature, and exposed organisms to concentrations close to values reported in inland waters and 1-2 orders of magnitude higher. The three temperatures tested in this experiment included 12 °C, 20 °C, and 24 °C, to simulate cool/springtime, current, and warming scenarios. We found the highest concentration of microplastics significantly impacted survival and total offspring compared to the control at 20 °C and 24 °C, but not at 12 °C. The adverse effect of high microplastic concentrations on total offspring at warmer temperatures was driven by the high mortality of the juveniles. We observed no effect of microplastics on time to first reproduction or average growth rate at any temperature. Warmer temperatures exacerbated microplastic toxicity, although only for concentrations of microplastics not currently observed in nature, but these concentrations are possible in pollution hotspots, through pulses pollution events or future worsening environmental contamination. The results of our study illustrate the continued need to further investigate climate change related co-stressors such as warming temperatures in microplastic and pollution ecology, through environmentally realistic exposure scenarios.

Ecotoxicity of the fluoroquinolone antibiotic delafloxacin to the water flea Simocephalus vetulus and its offspring under the influence of calcium modulation

Delafloxacin (DFX), one of the latest additions to the fluoroquinolone antibiotics, is gaining heightened recognition in human therapy due to its potential antibacterial efficacy in a wide range of applications. Concerns have arisen regarding its presence in the environment and its potential interactions with multivalent metals, such as calcium (Ca). The present study investigated the trans- and multigenerational effects of environmentally projected concentrations of DFX (100-400 μg DFX L-1) on individual- and population-level responses of parental S. vetulus (F0) and its descendants (F1) under normal (26 mg L-1) and high (78 mg L-1) Ca conditions. Exposure of the F0 generation to DFX under the normal Ca condition resulted in reduced juvenile body length (JBL), increased age-specific survival rate (lx), indicating prolonged developmental time, reduced age-specific fecundity rate (mx), and decreased population growth rate (rm). Under the high Ca condition, JBL, mx, and rm were adversely affected. Transgenerational effects of DFX existed, as F1 individuals exhibited persistent suppressions in at least one endpoint under both Ca conditions even after being transferred to a clear medium. Continuous exposure of the F1 generation to DFX had negative impacts on JBL, mx, and rm under the normal Ca condition, and on JBL and rm under the high Ca condition. However, cumulative effects were not observed, suggesting the potential development of tolerance to DFX in the F1 organisms. These findings suggest that DFX is a harmful compound for the non-target model organism S. vetulus and reveal a potential antagonism between DFX and Ca. Nevertheless, the interaction between other (fluoro)quinolones and Ca remains unclear, necessitating further research to establish this phenomenon more comprehensively, including understanding the interaction mechanism in ecotoxicological contexts.

Differential interference effects of thermal pollution on the induced defense of different body-sized cladocerans

Climate warming influences the biological activities of aquatic organisms, including feeding, growth, and reproduction, thereby affecting predator-prey interactions. This study explored the variation in thermal sensitivity of anti-predator responses in two cladoceran species with varying body sizes, Daphnia pulex and Ceriodaphnia cornuta. These species were cultured with or without the fish (Rhodeus ocellatus) kairomone at temperatures of 15, 20, 25, and 30 °C for 15 days. Results revealed that cladocerans of different body sizes exhibited varying responses to fish kairomones in aspects such as individual size, first-brood neonate size, total offspring number, average brood size, growth rate, and reproductive effort. Notably, low temperature differently affected defense responses in cladocerans of different body sizes. Both high and low temperatures moderated the intensity of the kairomone-induced response on body size at maturity. Additionally, low temperature reversed the reducing effect of fish kairomone on the total offspring number, average brood size, and reproductive effort in D. pulex. Conversely, it enhanced the increasing effect of fish kairomone on these parameters in C. cornuta. These results suggest that inducible anti-predator responses in cladocerans are modifiable by temperature. The differential effects of fish kairomones on various cladocerans under temperature influence offer crucial insights for predicting changes in predator-prey interactions within freshwater ecosystems under future climate conditions.

Combined effects of nanoplastics and elevated temperature in the freshwater water flea Daphnia magna

Global warming and nanoplastics (NPs) are critical global issues. Among NPs, one of the most hazardous types of plastics, polystyrene (PS), poses ecotoxicological threats to several freshwater organisms. The degree of toxicity of PS-NPs is strongly influenced by various environmental factors. This study illustrates the combined effects of temperature and PS-NPs on the water flea Daphnia magna. The sensitivity of D. magna to PS-NPs was tested under control (23 °C) and elevated temperatures (28 °C). As a result, increased temperatures influenced the uptake and accumulation of PS-NPs. Co-exposure to both higher temperatures and PS-NPs resulted in a drastic decrease in reproductive performance. The level of oxidative stress was found to have increased in a temperature-dependent manner. Oxidative stress was stimulated by both stressors, leading to increased levels of reactive oxygen species and antioxidant enzyme activity supported by upregulation of antioxidant enzyme-related genes under combined PS-NPs exposure and elevated temperature. In the imbalanced status of intracellular redox, activation of the p38 mitogen-activated protein kinase signaling pathway was induced by exposure to PS-NPs at high temperatures, which supported the decline of the reproductive capacity of D. magna. Therefore, our results suggest that PS-NPs exposure along with an increase in temperature significantly affects physiological processes triggered by damage from oxidative stress, leading to severely inhibited reproduction of D. magna.

Microplastic ingestion in aquatic and soil biota: A comprehensive review of laboratory studies on edible size and intake pattern

Microplastic contamination is ubiquitous and can be transferred through the food chain to humans. However, studies on microplastic size have mainly focused on large animals with a body length >20 mm. To address this gap, we conducted a comprehensive review of 169 laboratory studies to determine the edible size of microplastics for macrofauna and flora in aquatic and soil biota. Our findings indicate that microplastics with a size of <300 μm and 1 μm, respectively, are edible for these organisms, which are positioned at the base of the food chain. We also analyzed intake and depuration patterns and identified factors affecting microplastic ingestion. Our study fills an important knowledge gap by identifying the range of microplastic sizes that can enter the food chain and be transferred to humans. The study findings have strong implications for the ecological risk assessment of microplastics and suggest a starting point for mitigating this threat.

Thermal stress-stimulated ZnO toxicity inhibits reproduction of freshwater crustacean Daphnia magna

Elevated temperatures due to climate change pose a variety of environmental risks to the freshwater ecosystem. At the same time, zinc oxide (ZnO) has become widely used and has entered the freshwater environment. As thermal stress may potentially impact the physicochemical properties of ZnO, its toxicity to freshwater organisms in the face of global warming is poorly understood. The potential effects on reproductive performances, including oogenesis, are of particular concern. In this study, we investigate the reproductive performances and related mRNA abundance of the zooplankton Daphnia magna under conditions of ZnO exposure and heat stress. The results revealed that ZnO and elevated temperature delayed maturity and juvenile production of D. magna. Histological observations indicated that oogenesis was inhibited, and the number and size of oocytes were reduced in the condition of ZnO exposure under heat stress. Eventual offspring in the same treatment exhibited decreased numbers, size, and quality. Congenital juvenile anomalies were increased, such as deformed eye, and impaired antenna and tail spine. Moreover, both ZnO and elevated temperature treatments inhibited expression levels of reproduction-related genes (vtg, EcR and VMO1) and induced the dmrt93b gene involved in the production of male offspring. Furthermore, we found that D. magna tried to cope with ZnO and thermal stress by upregulating hsp90, HIF-1α and HIF-1β. ZnO and heat stress inhibited the reproductive capacity of D. magna, produced deleterious effects on reproduction-associated physiological pathways, and damaged reproductive outcomes.

Combined impact of elevated temperature and zinc oxide nanoparticles on physiological stress and recovery responses of Scylla serrata

Global climate change is the major cause behind unexpected fluctuations in temperature. In recent years, application of nanotechnology also has become widespread and nanomaterials are constantly being released into aquatic environments, posing a potential risk to various organisms and ecosystems. The lack of detailed understanding of how multiple stressors work, and how they differ from single stressors, impede to assess their combined effect on aquatic organisms and ecosystems. The prime aim of the current investigation is to decipher the toxicity of ZnO-NP after simultaneous exposure to a global environmental stressor, elevated temperature for 14 days, followed by a 7 days recovery period, on the eco-physiological responses of mud crab Scylla serrata collected from Sundarbans. Physiological energetics such as ingestion, assimilation, absorption, respiration, and excretion rates were measured to determine the Scope for growth (SfG). Additionally, we assessed various biomarkers from different levels of biological organisation (antioxidant, detoxification defence mechanisms, and lipid peroxidation levels) of the species. Combined stress attenuated the SfG in crabs which deteriorated further in the recovery phase. Oxidative stress also exacerbated under coalesced stress condition. Recovery was not observed in crabs with increased lipid peroxidation level under combined stress conditions. Elevated temperature disturbed the energy budget of crabs as mirrored by diminished energy left for compensatory actions under added metal stress, ultimately sensitizing the animals to ZnO NP pollutants. The current results advocate future ocean temperature to aggravate the impact of metal NP pollution and induce oxidative damage in S. serrata.

Freshwater Lacustrine Zooplankton and Microplastic: An Issue to Be Still Explored

Lakes are essentially interlinked to humans as they provide water for drinking, agriculture, industrial and domestic purposes. The upsurge of plastic usage, its persistence, and potential detrimental effects on organisms cause impacts on the trophic food web of freshwater ecosystems; this issue, however, still needs to be explored. Zooplankton worldwide is commonly studied as an indicator of environmental risk in aquatic ecosystems for several pollutants. The aim of the review is to link the existing knowledge of microplastic pollution in zooplankton to assess the potential risks linked to these organisms which are at the first level of the lacustrine trophic web. A database search was conducted through the main databases to gather the relevant literature over the course of time. The sensitivity of zooplankton organisms is evident from laboratory studies, whereas several knowledge gaps exist in the understanding of mechanisms causing toxicity. This review also highlights insufficient data on field studies hampering the understanding of the pollution extent in lakes, as well as unclear trends on ecosystem-level cascading effects of microplastics (MPs) and mechanisms of toxicity (especially in combination with other pollutants). Therefore, this review provides insight into understanding the overlooked issues of microplastic in lake ecosystems to gain an accurate ecological risk assessment.

Ecotoxicological, ecophysiological, and mechanistic studies on zinc oxide (ZnO) toxicity in freshwater environment

The world has faced climate change that affects hydrology and thermal systems in the aquatic environment resulting in temperature changes, which directly affect the aquatic ecosystem. Elevated water temperature influences the physico-chemical properties of chemicals in freshwater ecosystems leading to disturbing living organisms. Owing to the industrial revolution, the mass production of zinc oxide (ZnO) has been led to contaminated environments, and therefore, the toxicological effects of ZnO become more concerning under climate change scenarios. A comprehensive understanding of its toxicity influenced by main factors driven by climate change is indispensable. This review summarized the detrimental effects of ZnO with a single ZnO exposure and combined it with key climate change-associated factors in many aspects (i.e., oxidative stress, energy reserves, behavior and life history traits). Moreover, this review tried to point out ZnO kinetic behavior and corresponding mechanisms which pose a problem of observed detrimental effects correlated with the alteration of elevated temperature.

Elevated temperature enhanced lethal and sublethal acute toxicity of polyethylene microplastic fragments in Daphnia magna

Microplastic (MP) pollution poses a growing concern in freshwater ecosystems, which are further threatened by global warming. Thus, this study investigated the effect of elevated temperature (25 °C) on acute toxicity of polyethylene MP fragments to Daphnia magna over a 48 h period. At the reference temperature (20 °C), MP fragments (41.88 ± 5.71 µm) induced over 70 times higher lethal toxicity than that induced by MP beads (44.50 ± 2.50 µm), with median effective concentrations (EC50) of 3.89 and 275.89 mg L-1, respectively. Elevated temperature significantly increased (p < 0.05) the lethal (EC50 = 1.88 mg L-1) and sublethal (lipid peroxidation and total antioxidant capacity) toxicity in D. magna exposed to MP fragments compared to those at the reference temperature. Additionally, the elevated temperature led to a significant increase (p < 0.05) in the bioconcentration of MP fragments in D. magna. Overall, the present study increases understanding for the ecological risk assessment of microplastics under global warming, highlights that elevated temperature can be seriously increased bioconcentration of MP fragments, leading to increased acute toxicity in D. magna.

Disturbances in growth, oxidative stress, energy reserves and the expressions of related genes in Daphnia magna after exposure to ZnO under thermal stress

The toxicological effects of metal contamination are influenced by the ambient temperature. Therefore, global warming affects the toxicity of metal contamination in aquatic ecosystems. ZnO is widely used as a catalyst in many industries, and causes contamination in aquatic ecosystems. Here, we investigated the effects of ZnO concentration under elevated temperature by observing growth, oxidative stress, energy reserves and related gene expression in exposed Daphnia magna. Body length and growth rate increased in neonates exposed to ZnO for 2 days but decreased at 9 and 21 days under elevated temperature. ZnO concentration and elevated temperature induced oxidative stress in mature D. magna by reducing superoxide dismutase (SOD) activity and increasing malondialdehyde (MDA) levels. In contrast, juveniles were unaffected. Carbohydrate, protein and caloric contents were reduced throughout development in D. magna treated with ZnO and elevated temperature in all exposure periods (2, 9 and 21 days). However, lipid content also decreased in mature D. magna treated with ZnO cultured under elevated temperature, while that of juveniles showed an increase in lipid content. Therefore, energy was perhaps allocated to physiological processes for detoxification and homeostasis. Moreover, expression patterns of genes related to physiological processes changed under elevated temperature and ZnO exposure. Taken together, our results highlight that the combination of temperature and ZnO concentration induced toxicity in D. magna. This conclusion was confirmed by the Integrated Biological Response (IBR) index. This study shows that changes in biological levels of organization could be used to monitor environmental change using D. magna as a bioindicator.

Impact of atrazine on the dynamic response of Daphnia pulex populations to fish predation risk

Herbicide pollution is persistent, which not only has a negative impact on individual organisms, but also may alter population dynamics and stability of interspecific relationships. Cladocerans, an important part of zooplankton, are often simultaneously exposed to environmental pollutants and predation risk in the aquatic environment. To evaluate the combined effects of atrazine and fish predation risk on the population traits of cladocerans, we exposed Daphnia pulex to different concentrations of atrazine (0, 0.05, 0.10, and 1.0 mg L−1) with or without fish (Rhodeus ocellatus) kairomone, recorded the key population traits, and fitted Gaussian model to population dynamics. Results showed that fish kairomone increased the population density at the end of the experiment and resting eggs production, and tended to decrease the total biomass and the average dry weight per individual of D. pulex. Atrazine reduced the total biomass, the average dry weight per individual, and resting eggs production of D. pulex populations. Atrazine also decreased the population density at the end of the experiment of D. pulex in fish kairomone treatment, and attenuated the promoting effect of fish kairomone on resting eggs production and the reduction of the total biomass. The findings highlighted the importance of considering the combined impact of environmental pollutants and predation risks on zooplankton populations.

Warmer water, high light intensity, lithium and microplastics: Dangerous environmental combinations to zooplankton and Global Health?

Nowadays there is a high concern about the combined effects of global warming and emerging environmental contaminants with significant increasing trends of use, such as lithium (Li) and microplastics (MPs), both on wildlife and human health. Therefore, the effects of high light intensity (26,000 lx) or warmer water temperature (25 °C) on the long-term toxicity of Li and mixtures of Li and MPs (Li-MPs mixtures) were investigated using model populations of the freshwater zooplankton species Daphnia magna. Three 21-day bioassays were done in the laboratory at the following water temperatures and light intensities: (i) 20 °C/10830 lx; (ii) 20 °C/26000 lx (high light intensity); (iii) 25 °C/10830 lx (warmer temperature). Based on the 21-day EC50s on reproduction, high light intensity increased the reproductive toxicity of Li and Li-MPs mixtures by ~1.3 fold; warmer temperature increased the toxicity of Li by ~1.2 fold, and the toxicity of Li-MPs mixtures by ~1.4 fold based on the concentration of Li, and by ~2 fold based on the concentrations of MPs. At high light intensity, Li (0.04 mg/L) and Li-MPs mixtures (0.04 Li + 0.09 MPs mg/L) reduced the population fitness by 32 % and 41 %, respectively. Warmer temperature, Li (0.05 mg/L) and Li-MPs mixtures (0.05 Li + 0.09 MPs mg/L) reduced it by 63 % and 71 %, respectively. At warmer temperature or high light intensity, higher concentrations of Li and Li-MPs mixtures lead to population extinction. Based on the population growth rate and using data of bioassays with MPs alone done simultaneously, Li and MPs interactions were antagonistic or synergistic depending on the scenario. High light intensity and chemical stress generally acted synergistically. Warmer temperature and chemical stress always acted synergistically. These findings highlight the threats of long-term exposure to Li and Li-MPs mixtures to freshwater zooplankton and Global Health in a warmer world.

Joint effects of microplastics and ZnO nanoparticles on the life history parameters of rotifers and the ability of rotifers to eliminate harmful phaeocystis

The rising concentration of microplastics and nanoparticles coexisting simultaneously in marine may bring joint harm to zooplankton. Rotifer is an important functional group of marine zooplankton, which plays an important role in the energy flow of marine ecosystem. To evaluate the comprehensive effects of nano-sized microplastics and metal oxide nanoparticles on life history parameters of rotifers and population dynamics of rotifers during eliminating harmful algae Phaeocystis, we exposed rotifers Brachionus plicatilis to the multiple combinations of different concentrations of nanoplastics and ZnO nanoparticles. Results showed that rotifer maturation time was prolonged and the total offspring was decreased significantly with rising ZnO nanoparticles and microplastics concentrations, and microplastics and ZnO nanoparticles had significant interaction, which brought more serious joint deleterious effects on survival, development, and reproduction. At the population level, ZnO nanoparticles exacerbated the delayed effect of microplastics on the elimination of Phaeocystis by rotifers, although eventually rotifers also completely eliminated Phaeocystis in the closed system. This study provided new insights into revealing the comprehensive impact of microplastics and ZnO nanoparticles on zooplankton not only from the perspective of life history parameters of rotifers but also from the perspective of population dynamics of rotifers controlling harmful algae, which is of great significance to understand the impact of mixed pollutants on marine ecosystem.

Ecotoxicology of microplastics in Daphnia: A review focusing on microplastic properties and multiscale attributes of Daphnia

The ubiquitous presence of microplastics in aquatic environments is considered a global threat to aquatic organisms. Species of the genus Daphnia provide an important link between aquatic primary producers and consumers of higher trophic levels; furthermore, these organisms exhibit high sensitivity to various environmental pollutants. Hence, the biological effects of microplastics on Daphnia species are well documented. This paper reviews the latest research regarding the ecotoxicological effects of microplastics on Daphnia, including the: 1) responses of individual, population, and community attributes of Daphnia to microplastics; 2) influence of the physical and chemical properties of microplastics; and 3) joint toxicity of microplastics and other pollutants on responses of Daphnia. Our literature review found that the published literature does not provide sufficient evidence to reveal the risks of microplastics at the population and community levels. Furthermore, we emphasized that high-level analysis has more general implications for understanding how individual-level research can reveal the ecological hazards of microplastics on Daphnia. Based on this review, we suggest avenues for future research, including microplastic toxicology studies based on both omics-based and community-level methods, especially the latter.

Microplastics increases the heat tolerance of Daphnia magna under global warming via hormetic effects

The ecological risk assessment of microplastics under global warming receives increasing attention. Yet, such studies mostly focused on increased mean temperatures (MT), ignoring another key component of global warming, namely daily temperature fluctuations (DTF). Moreover, we know next to nothing about the combined effects of multigenerational exposure to microplastics and warming. In this study, Daphnia magna was exposed to an environmentally relevant concentration of polystyrene microplastics (5 μg L-1) under six thermal conditions (MT: 20 ℃, 24 ℃; DTF: 0 ℃, 5 ℃, 10 ℃) over two generations to investigate the interactive effects of microplastics and global warming. Results showed that microplastics had no effects on Daphnia at standard thermal conditions (constant 20 °C). Yet, microplastics increased the fecundity, heat tolerance, amount of energy storage, net energy budget and cytochrome P450 activity, and decreased the energy consumption when tested under an increased MT or DTF, indicating a hormesis effect induced by microplastics under warming. The unexpected increase in heat tolerance upon exposure to microplastics could be partly explained by the reduced energy consumption and/or increased energy availability. Overall, the present study highlighted the importance of including DTF and multigenerational exposure to improve the ecological risk assessment of microplastics under global warming.

ZnO nanoparticles interfere with top-down effect of the protozoan paramecium on removing microcystis

Under intensive human activity, sewage discharge causes eutrophication-driven cyanobacteria blooms as well as nanomaterial pollution. In biological control of harmful cyanobacteria, top-down effect of protozoan has great potentials for removing cyanobacterial populations, degrading cyanotoxins, and improving phytoplankton community. ZnO nanoparticles as a kind of emerging contaminants have attracted increasing attention because of wide application and their high bio-toxicity effects on reducing the ingestion of aquatic animals including Paramecium, thereby possibly disturbing top-down control of cyanobacteria. Therefore, this study investigated the effects of ZnO nanoparticles at environmental-relevant concentrations on the protozoan Paramecium removing toxic Microcystis. Results showed Paramecium effectively eliminated all the Microcystis, despite exposure to ZnO nanoparticles. However, their ingestion rate was significantly reduced at more than 0.1 mg L^-1 ZnO nanoparticles, thereby delaying Microcystis removal. Nevertheless, at 0.1 mg L^-1 ZnO nanoparticles, the time to Microcystis extinction decreased compared to the group without ZnO nanoparticles, because Microcystis populations were reduced under this circumstance, while ingestion rate of Paramecium was unaffected. Furthermore, ZnO nanoparticles obviously accumulated in food vacuoles of Paramecium, and the size of nanoparticles aggregates and zinc concentrations in Paramecium were increased with ZnO nanoparticles concentrations. At the end of experiment, these food vacuoles were not dissipated. Overall, these findings suggest that ZnO nanoparticles impair protozoan top-down effects through reducing Microcystis and ingestion rate as well as disturbing functions of their digestive organelles, and highlight the need to consider the interfering effects of environmental pollutants on cyanobacterial removal efficiency by protozoans in natural waters.

Disentangling Diet- and Medium-Associated Microbes in Shaping Daphnia Gut Microbiome

Host genotype and environment are considered crucial factors in shaping Daphnia gut microbiome composition. Among the environmental factors, diet is an important factor that regulates Daphnia microbiome. Most of the studies only focused on the use of axenic diet and non-sterile medium to investigate their effects on Daphnia microbiome. However, in natural environment, Daphnia diets such as phytoplankton are associated with microbes and could affect Daphnia microbiome composition and fitness, but remain relatively poorly understood compared to that of axenic diet. To test this, we cultured two Daphnia magna genotypes (genotype-1 and genotype-2) in sterile medium and fed with axenic diet. To check the effects of algal diet-associated microbes versus free water-related microbes, Daphnia were respectively inoculated with three different inoculums: medium microbial inoculum, diet-associated microbial inoculum, and medium and diet-mixed microbial inoculum. Daphnia were cultured for 3 weeks and their gut microbiome and life history traits were recorded. Results showed that Daphnia inoculated with medium microbial inoculum were dominated by Comamonadaceae in both genotypes. In Daphnia inoculated with mixed inoculum, genotype-1 microbiome was highly changed, whereas genotype-2 microbiome was slightly altered. Daphnia inoculated with diet microbial inoculum has almost the same microbiome in both genotypes. The total number of neonates and body size were significantly reduced in Daphnia inoculated with diet microbial inoculum regardless of genotype compared to all other treatments. Overall, this study shows that the microbiome of Daphnia is flexible and varies with genotype and diet- and medium-associated microbes, but not every bacteria is beneficial to Daphnia, and only symbionts can increase Daphnia performance.

Increased food availability reducing the harmful effects of microplastics strongly depends on the size of microplastics

Shallow lakes and ponds, providing essential ecological and environmental services, are simultaneously disrupted by various pollutants of emerging concern (PECs). As a group of PECs, microplastics (MPs) ubiquitously found in freshwater are toxic to a huge variety of organisms. However, the consequence of secondary factors such as food quantity determining MPs toxicity, and the corresponding water safety risks await assessment is still poorly understood. Accordingly, we investigated how MPs across three particle sizes (10, 1 and 0.07 µm) interacted with food abundance to affect survival, reproduction and population performance in the waterflea Daphnia magna. Across multiple population traits, we found that MPs toxicity on Daphnia population performance was attenuated by higher food quantity, but this attenuation size was strongly dependent on MPs size. Path analysis results showed population growth rate was mainly constrained by reduced survival rather than fecundity. Furthermore, the additive null model revealed that the interactive effects of food abundance and MPs were predominately recognized as synergism and trait dependency. The present findings underscore the importance of considering the complexity of interactions that can occur in the wild, when assessing the effects of plastics pollution on population dynamics of the basic trophic level in lakes and ponds.

Combined toxicity of polystyrene microplastics and ammonium perfluorooctanoate to Daphnia magna: Mediation of intestinal blockage

Microplastics (MPs) have worldwide accumulated in aquatic environments and coexisted with various water contaminants including perfluorinated compounds (PFCs) that are frequently detected. The adverse effects of individual MPs or PFCs on aquatic organisms have been extensively reported; however, the combined toxicity of MPs and PFCs remains unknown. This study evaluated the combined toxicity of MPs [pristine and aged polystyrene (PS)] and a PFC [ammonium perfluorooctanoate (APFO)] to Daphnia magna under different concentration ratios by three classic methods: toxicity unit, additive index, and mixed toxicity index. The adsorption kinetics of APFO on MPs, aggregation of MPs in exposure medium, MP gut fullness of daphnids, intestinal histology, and lipid peroxidation were analyzed to reveal the mechanism underlying the combined toxicity. Our results showed that the combined toxic modes varied with the concentration ratios of MPs to APFO (antagonism at 4:1 and 1:4, synergism at 3:1, 1:2, and 1:3, and partial addition/antagonism at 2:1 and 1:1 for pristine PS + APFO; antagonism at all ratios except partial addition/antagonism at 3:1 and 1:3 for aged PS + APFO), which could be attributed to the alteration of MP aggregation and thus MP gut fullness in the daphnids. The combined toxicity was further confirmed to occur in the daphnid's gut, which was reflected in physiological and biochemical responses mediated by intestinal blockage. Observable intestinal damages under co-exposures at μg•L^-1 levels indicated the risks from future long-term exposure to MPs and PFCs in aquatic environments. This work demonstrates the necessity of assessing combined toxicity with different concentration ratios and provides new insights into the potential risks of MPs in aquatic environments.

A method for measuring the emissions of in situ agricultural plastic film microplastics by ultraviolet and mechanical abrasion

Agricultural plastic film (APF) is widely used in modern agriculture. Under natural environmental conditions, the structure, surface properties and mechanical properties of APFs change because of sunlight, wind and other factors and gradually break into debris, resulting in the generation of microplastics (MPs). Studies have reported that the MPs concentration in soil is positively correlated with the use intensity and duration of APFs. Unfortunately, to the best of our knowledge, no method to measure the emissions of in situ APFs has been developed. In this study, the effects of mechanical abrasion driven by wind on MPs fragmentation by polyethylene (PE) and polyvinyl chloride (PVC) APFs with the increase of exposure time were investigated. Meanwhile, based on the release rate model of PS fragmented MPs under natural sunlight, a modified model to quantify the effect of ultraviolet (UV) radiation exposure duration on the production of APF fragmented MPs was developed. Based on these models, the amount of MPs produced from APFs in farmland in China was estimated. The national annual MPs mass emissions from APFs in agricultural soil were approximately 5 × 10⁴ to 6.8 × 10⁴ tons in 2018 due to wind and 6.5 × 10³ tons due to sunlight, and the total emission level due to both wind and sunlight was 5.1 × 10⁴ to 7.0 × 10⁴ tons. Compared with that of wind, the contribution of UV radiation to MPs emission is smaller. Our estimates are comparable to data reported in previous studies, indicating that our models have good practical applications and are of great significance for predicting MPs production from APFs in farmland.

Understanding host-microbiome-environment interactions: Insights from Daphnia as a model organism

Microbes perform a variety of vital functions that are essential for healthy ecosystems, ranging from nutrient recycling, antibiotic production and waste decomposition. In many animals, microbes become an integral part by establishing diverse communities collectively termed as "microbiome/s". Microbiomes defend their hosts against pathogens and provide essential nutrients necessary for their growth and reproduction. The microbiome is a polygenic trait that is dependent on host genotype and environmental variables. However, the alteration of microbiomes by stressful condition and their recovery is still poorly understood. Despite rapid growth in host-associated microbiome studies, very little is known about how they can shape ecological processes. Here, we review current knowledge on the microbiome of Daphnia, its role in fitness, alteration by different stressors, and the ecological and evolutionary aspects of host microbiome interactions. We further discuss how variation in Daphnia physiology, life history traits, and microbiome interactive responses to biotic and abiotic factors could impact patterns of microbial diversity in the total environment, which drives ecosystem function in many freshwater environments. Our literature review provides evidence that microbiome is essential for Daphnia growth, reproduction and tolerance against stressors. Though the core and flexible microbiome of Daphnia is still debatable, it is clear that the Daphnia microbiome is highly dependent on interactions among host genotype, diet and the environment. Different environmental factors alter the microbiome composition and diversity of Daphnia and reduce their fitness. These interactions could have important implications in shaping microbial patterns and their recycling as Daphnia are keystone species in freshwater ecosystem. This review provides a framework for studying these complex relationships to gain a better understanding of the ecological and evolutionary roles of the microbiome.

Warming, temperature fluctuations and thermal evolution change the effects of microplastics at an environmentally relevant concentration

Microplastics are sometimes considered not harmful at environmentally relevant concentrations. Yet, such studies were conducted under standard thermal conditions and thereby ignored the impacts of higher mean temperatures (MT), and especially daily temperature fluctuations (DTF) under global warming. Moreover, an evolutionary perspective may further benefit the future risk assessment of microplastics under global warming. Here, we investigated the effects of two generations of exposure to an environmentally relevant concentration of polystyrene microplastics (5 μg L^-1) under six thermal conditions (2 MT × 3 DTF) on the life history, physiology, and behaviour of Daphnia magna. To assess the impact of thermal evolution we thereby compared Daphnia populations from high and low latitudes. At the standard ecotoxic thermal conditions (constant 20 °C) microplastics almost had no effect except for a slight reduction of the heartbeat rate. Yet, at the challenging thermal conditions (higher MT and/or DTF), microplastics affected each tested variable and caused an earlier maturation, a higher fecundity and intrinsic growth rate, a decreased heartbeat rate, and an increased swimming speed. These effects may be partly explained by hormesis and/or an adaptive response to stress in Daphnia. Moreover, exposure to microplastics at the higher mean temperature increased the fecundity and intrinsic growth rate of cold-adapted high-latitude Daphnia, but not of the warm-adapted low-latitude Daphnia, suggesting that thermal evolution in high-latitude Daphnia may buffer the effects of microplastics under future warming. Our results highlight the critical importance of DTF and thermal evolution for a more realistic risk assessment of microplastics under global warming.