Life history responses of Daphnia similoides simultaneously exposed to microcystin-LR and ammonia and their postexposure recovery

Dietary exposure of the water flea Daphnia galeata to microcystin-LR

Harmful substances like the cyanotoxin microcystin-leucine-arginine (MC-LR) are commonly found in eutrophic freshwater environments, posing risks to aquatic organisms. The water flea, Daphnia, is a well-established model organism for environmental toxicology research. Nevertheless, there is currently insufficient research on the genes that respond to MC-LR in Daphnia galeata. This study aimed to gain insights into the notable genes that react significantly to MC-LR. In this study, we generated an extensive RNA-Seq sequences isolated from the D. galeata HK strain, Han River in Korea. This strain was nourished with a diet of the green microalga Chlorella vulgaris and treated with pure MC-LR at a concentration of 36 ug/L. The transcriptome profile in response to the MC-LR treatment was obtained and 336 differentially expressed genes were subjected to Gene Ontology (GO) and euKaryotic Orthologous Groups of proteins analyses. GO enrichment analysis showed that chemical stimulus, amino sugar metabolic and catabolic process, oxidative stress, and detoxification were highly enriched, in reverse, proteolysis and fucosylation were underpresented. Detoxification process related genes such as peroxidase-like, chorion, and thyroid peroxidase-like were enriched for eliminating or neutralizing MC_LR from an organism's body. Furthermore, functional protein classification revealed an upregulation of lipid and inorganic ion transport processes, while amino acid and carbohydrate transport processes were found to be downregulated. These findings offer insights into how organisms respond to ecotoxic stimuli, providing valuable information for understanding adaptation or defense pathways.

Behavioral Variables to Assess the Toxicity of Unionized Ammonia in Aquatic Snails: Integrating Movement and Feeding Parameters

Behavioral endpoints are important parameters to assess the effects of toxicants on aquatic animals. These endpoints are useful in ecotoxicology because several toxicants modify the animal behavior, which may cause adverse effects at higher levels of ecological organization. However, for the development of new bioassays and for including the behavior in ecotoxicological risk assessment, the comparison of sensitivity between different behavioral endpoints is necessary. Additionally, some toxicants remain in aquatic environments for a few hours or days, which may lead to animal recovery after toxicant exposure. Our study aimed to assess the effect of unionized ammonia on the movement and feeding behaviors of the aquatic gastropod Potamopyrgus antipodarum (Tateidae, Mollusca) and its recovery after exposure. Four treatments were used: a control and three nominal concentrations of unionized ammonia (0.25, 0.5 and 1 mg N-NH₃/L). Each treatment was replicated eight times, with six animals in each replicate. Animals were exposed to unionized ammonia for 48 h (exposure period) and, subsequently, to control water for 144 h (post-exposure period). Two movement variables were monitored without food and five feeding behavioral variables were monitored in the presence of food. Some of the feeding behavioral variables showed higher sensitivity (LOEC = 0.25-0.5 mg N-NH₃/L) than the movement behavior variables monitored without food (LOEC = 1 mg N-NH₃/L). After exposure to unionized ammonia, animals showed a recovery of most behavioral endpoints. The inclusion of post-exposure period and feeding behaviors in bioassays may make studies more realistic, which is crucial for a proper ecotoxicological risk assessment.

Acute and chronic toxicity of microcystin-LR and phenanthrene alone or in combination to the cladoceran (Daphnia magna)

Hazardous substances, such as microcystin-LR (MC-LR) and phenanthrene (Phe) are ubiquitous co-contaminants in eutrophic freshwaters, which cause harms to aquatic organisms. However, the risks associated with the co-exposure of aquatic biota to these two chemicals in the environment have received little attention. In this study, the single and mixture toxic effects of MC-LR and Phe mixtures were investigated in Daphnia magna after acute and chronic exposure. Acute tests showed that the median effective concentrations (48 h) for MC-LR, Phe and their mixtures were 13.46, 0.57 and 8.84 mg/L, respectively. Mixture toxicity prediction results indicated that the independent action model was more applicable than the concentration addition model. Moreover, combination index method suggested that the mixture toxicity was concentration dependent. Synergism was elicited at low concentrations of MC-LR and Phe exposure (≤4.04 + 0.17 mg/L), whereas antagonistic or additive effects were induced at higher concentrations. The involved mechanism of antagonism was presumably attributable to the protective effects of detoxification genes activated by high concentrations of MC-LR in mixtures. Additionally, chronic results also showed that exposure to a MC-LR and Phe mixture at low concentrations (≤50 +2 μg/L) resulted in greater toxic effects on D. magna life history than either chemical acting alone. The significant inhibition on detoxification genes and increased accumulation of MC-LR could be accounted for their synergistic toxic effects on D. magna. Our findings revealed the exacerbated ecological hazard of MC-LR and Phe at environmental concentrations (≤50 +2 μg/L), and provided new insights to the potential toxic mechanisms of MC-LR and Phe in aquatic animals.

The inhibitory effect of mixotrophic Ochromonas gloeopara on the survival and reproduction of Daphnia similoides sinensis

Mixotrophs account for a high proportion (occasionally up to 80%) of the phytoplankton biomass. Chrysophyte is one major component of mixotrophs. Because of their possible toxicity and linkage between microbial community and higher trophic levels, the effect of mixotrophic golden algae on potential grazers received much attention. The present study investigated the effect of Ochromonas gloeopara at different proportions in diet (combined with Scenedesmus obliquus) on the life history of Daphnia similoides sinensis. Results showed that osmotrophically grown O. gloeopara in light produced fish toxins and hemolysins, and negatively influenced the survival and reproduction of D. similoides sinensis. The mortality of the cladoceran increased as the proportion of O. gloeopara in food increased. The D. similoides sinensis could not reproduce throughout the life when Ochromonas comprised above 35%. When fed foods containing 15% of Ochromonas, the time to first brood of D. similoides sinensis was prolonged, together with the reduced number of offspring in first brood and total number of broods. Replacement by 100% S. obliquus delayed the time to death, but did not improve the reproduction of Daphnia. The present study indicated the strong inhibitory effect of O. gloeopara on D. similoides sinensis, and underlined the importance of evaluating its ecological role in aquatic ecosystems.

Combined effects of ZnO nanoparticles and toxic Microcystis on life-history traits of Daphnia magna

Rise in cyanobacterial blooms and massive discharge of nanoparticles (NPs) in aquatic ecosystems cause zooplankton to be exposed in toxic food and NPs simultaneously, which may impact on zooplankton interactively. Therefore, the present study focused on assessing the combined effects of different ZnO NPs levels (0, 0.10, 0.15, 0.20 mg L^-1) and different proportions of toxic Microcystis (0%, 10%, 20%, 30%) in the food on a model zooplankton, Daphnia magna. The results showed that both toxic Microcystis and ZnO NPs significantly delayed the development of D. magna to maturation, but there was no significant interaction between the two factors on the times to maturation except the body length at maturation. Both ZnO NPs and toxic Microcystis also significantly decreased the number of neonates in the first brood, total offspring, and number of broods per female, and there was a significant interaction between ZnO NPs and food composition on the reproductive performance of D. magna. Specifically, presence of toxic Microcystis reduced the gap among the effects of different ZnO NPs concentrations on the reproductive performance of D. magna. When the ZnO NPs concentration was at 0.15 mg L^-1, the gap of the reproductive performance among different proportions of toxic Microcystis also tended to be narrow. Similar phenomenon also occurred in mortality. Such results suggested that low concentration of ZnO NPs and toxic Microcystis can mutually attenuate their harmful effects on D. magna, which has significantly implications in appropriately assessing the ecotoxicological effects of emerging pollutants in a complex food conditions.

Effects of Microcystis aeruginosa on the expression of nuclear receptor genes in Daphnia similoides sinensis

Nuclear receptor (NR) genes form a conserved superfamily, which is involved in organism metabolism, reproduction, development, homeostasis, and resource allocation. Microcystis aeruginosa can inhibit the growth and reproduction of Daphnia. However, whether M. aeruginosa can affect the expression of Daphnia NR genes is unknown. In total, 18 NRs were identified in this study based on previous Daphnia similoides sinensis transcriptome data. In treatments containing M. aeruginosa, the gene expression of the NR1 subfamily (E75a, E75b, HR3, HR96, NHR-1, HR97a, HR97g, and NHR97) and the NR2 subfamily (RXR, TLL, PNR, and SVP) were down-regulated 59% and 79%, respectively. In treatments containing M. aeruginosa, although the expression of 78% of the genes showed a similar trend in clones 1 and 2, the expression of 42% of the genes in clone 3 showed the opposite trend compared to clones 1 and 2, suggesting that the adaptability and molecular mechanism differ in individuals with different Microcystis tolerance genotypes.

Interactive effects of microcystin and ammonia on the reproductive performance and phenotypic traits of the rotifer Brachionus calyciflorus

Elevated microcystin-LR (MC-LR) and ammonia (NH₃-N) concentrations co-occur during the degradation of Microcystis blooms, and are toxic to aquatic organisms. The freshwater rotifer, Brachionus calyciflorus, was exposed to mixtures of MC-LR (0, 10, 30, and 100µgL^-1) and NH₃-N (0, 270, and 540µgL^-1) to assess the combined effects of the two toxicants on reproductive performance and phenotype traits. Single solutions of MC-LR (100µgL^-1) and NH₃-N (540µgL^-1) had negative effects on rotifer reproductive timing and fecundity. Pre- and post-reproductive periods fluctuated with MC-LR and NH₃-N concentrations, while reproductive period and total offspring per female were reduced in mixtures of MC-LR and NH₃-N (p < 0.05). Grazing rate of rotifers decreased with grazing time and concentrations of the two toxicants (p < 0.001). MC-LR in combination with NH₃-N had negative effects on swimming speed and body length but positively stimulated posterolateral spine development (p < 0.001). MC-LR and NH₃-N had synergetic interactive effects on pre-reproductive period, reproductive period, total offspring per female, grazing rate, swimming speed, and body length (p < 0.05). In contrast, these effects were antagonistic on post-reproductive period and posterolateral spine length (p > 0.05). These results indicate that MC-LR and NH₃-N act synergistically and antagonistically in causing toxicity to B. calyciflorus regarding reproductive performance and the formation of defensive phenotypes.