Transcriptome alterations in zebrafish gill after exposure to different sizes of microplastics.

Most studies on microplastics (MPs) focused on gut, liver, and brain, and MPs toxicity was size-dependent, but less has been reported on gill. Here, zebrafish were exposed to three sizes of MPs (45-53 µm, 90-106 µm, and 250-300 µm). Next, comparative transcriptome analysis and determination of physiological indices were performed in zebrafish gills to elucidate the size-associated toxicity of MPs to fish gills. Compared with the control, 60, 344, and 802 differentially expressed genes (DEGs) were identified after exposure to 45-53 µm, 90-106 µm, and 250-300 µm MPs for 5 days, respectively. More DEGs in treatment with bigger MPs suggested that bigger MPs might induce more changes in zebrafish gills than smaller ones. These DEGs were significantly enriched in the FoxO signaling, cellular senescence, circadian rhythm and p53 signaling pathways. Besides, 90-106 µm and 250-300 µm MPs treatments inhibited the cell cycle and prevented the apoptosis. The GSH content significantly increased after MPs exposure, suggesting the induction of oxidative stress. AChE and Na+/K+-ATPase activities were significantly lowered in all MPs treatments than in the control, suggesting the inhibition of neurotransmission and ion regulation. These changes might negatively influence the normal functioning of gills, such as osmoregulation, ion regulation, and respiration.

The time-dependent variations of zebrafish intestine and gill after polyethylene microplastics exposure.

Microplastics (MPs) are common environmental contaminants that present a growing health concern due to their increasing presence in aquatic and human systems. However, the mechanisms behind MP effects on organisms are unclear. In this study, zebrafish (Danio rerio) were used as an in vivo model to investigate the potential risks and molecular mechanisms of the toxic effects of polyethylene MPs (45-53 µm). In the zebrafish intestine, 6, 5, and 186 genes showed differential expression after MP treatment for 1, 5, and 10 days, respectively. In the gills, 318, 92, and 484 genes showed differential expression after MP treatment for 1, 5, and 10 days, respectively. In both the intestine and the gills, Gene Ontology (GO) annotation showed that the main enriched terms were biological regulation, cellular process, metabolic process, cellular anatomical entity, and binding. KEGG enrichment analysis on DEGs revealed that the dominant pathways were carbohydrate metabolism and lipid metabolism, which were strongly influenced by MPs in the intestine. The dominant pathways in the gills were immune and lipid metabolism. The respiratory rate of gills, the activity of SOD and GSH in the intestine significantly increased after exposure to MPs compared with the control (p < 0.05), while the activity of SOD did not change in the gills. GSH activity was only significantly increased after MP exposure for 5 days. Also, the MDA content was not changed in the intestine but was significantly decreased in the gills after MP exposure. The activity of AChE significantly decreased only after MPs exposure for 5 days. Overall, these results indicated that MPs pollution significantly induced oxidative stress and neurotoxicity, increased respiratory rate, disturbed energy metabolism and stimulated immune function in fish, displaying an environmental risk of MPs to aquatic ecosystems.

Algal density affects the influences of polyethylene microplastics on the freshwater rotifer Brachionus calyciflorus.

Most previous researches focused on the toxicity of polystyrene microplastics (MPs) to marine organisms, but less on polyethylene MPs and freshwater zooplanktons. The present study aims to elucidate the toxicity of polyethylene (PE) MPs (diameter = 10-22 µm) to the typical freshwater rotifer Brachionus calyciflorus. Firstly, fluorescent microscope observation showed that rotifers could ingest PE MPs and accumulate them in their digestive tracts. Life-table experiments revealed that exposure to 0.5 × 103, 2.5 × 103, and 1.25 × 104 particles/mL PE MPs significantly reduced net reproductive rate and intrinsic rate of pollution increase of rotifers under algal densities (Scenedesmus obliquus) of 0.1 × 106, and 0.5 × 106 cells/mL, but no significant effects were observed under 2.5 × 106 cells/mL algal density. These results showed that PE MPs suppressed the reproduction of rotifer and this negative effect could be alleviated by increasing food supply. The swimming linear speed of rotifers significantly decreased with increasing MP concentrations. The activities of superoxide dismutase and Na+-K+-ATPase significantly decreased in treatments with high concentration of PE MPs under 0.1 × 106 cells/mL algal density, but did not change significantly in MP treatments under 0.5 × 106 and 2.5 × 106 cells/mL, compared to the control. Glutathione peroxidase activity significantly increased in treatments with 1.25 × 104 particles/mL and 2.5 × 103 particles/mL under 0.1 × 106 and 0.5 × 106 cells/mL algal density, respectively, but did not change significantly in all MP treatments under 2.5 × 106 cells/mL. Exposure to PE MPs might lower the gathering capacity of algae, induce oxidative stress, trigger cell membrane damages and disturb energy metabolism in rotifers, which can explain the PE MPs toxicity to rotifer reproduction.

Morphological differentiation of Brachionus calyciflorus caused by predation and coal ash pollution.

Different rotifer stains exhibited remarkably morphological differences which could not be eliminated under laboratory conditions. In the present study, we hypothesized that predation pressure and pollution might be two forces driving morphological differentiation of rotifer. To test this hypothesis, rotifers (Brachionus calyciflorus) belonging to two sibling species were collected from three special lakes (with coal ash pollution, high predation pressure or neither) and cultured for more than three months to investigate their potential differentiation in morphology. Twelve morphological parameters were measured and compared among three lakes at four food density (Scenedesmus obliquus). The results showed that most of the tested morphological parameters changed in response to food level and differed among three habitats. Rotifers from the habitat with high predation pressure evolved stable long posterior lateral spine and relatively small body size. Rotifers collected from the polluted habitat was of smaller body size, compared with those from ordinary habitat. Bigger eggs were laid by rotifers from polluted area or lake with high predation pressure, enabling newborns more resistant to pollution or predation, and thus ensuring the survival rate of newborns. Finally, we concluded that both predation and pollution could affect the morphological differentiation and evolution of rotifers.

Coal fly ash effluent affects the distributions of Brachionus calyciflorus sibling species.

Fly ash, a coal combustion residue of thermal power plants and a source of multiple pollutants, has been recognized as an environmental hazard all over the world. Although it is known that fly ash effluent affects density, diversity and distribution of rotifers in drainage systems and receiving water bodies, the effect of fly ash effluent on the distributions of highly similar rotifer species remains unknown. In this study, the mtDNA COI genes of 90 individuals in Brachionus calyciflorus complex from Lake Hui (as a fly ash discharge water pond) and other two neighboring lakes (Lake Fengming and Lake Tingtang) were sequenced and analyzed, and the responses in selected life table demographic parameters (life expectancy at hatching, net reproductive rate, intrinsic rate of population increase and proportion of sexual offspring) of different rotifer populations to fly ash effluent were investigated. Overall, 72 mtDNA haplotypes were defined, and were split into two clades by the phylogenetic trees. The divergence of COI gene sequences between the two clades ranged from 11.8% to17.8%, indicating the occurrence of two sibling species (sibling species I and sibling species II). Sibling species I distributed in all the three lakes, showing strong capabilities for dispersal and colonization, which were supported by its higher level of gene flow (2.60-4.04) between the populations from Lake Hui and each of the other two lakes, longer life expectancy at hatching (101.6-148.2 h), and higher net reproductive rate (4.4-16.4 offspring/female) and intrinsic rate of population increase (0.60-0.98/d) when cultured in aerated tap water and fly ash effluent. Sibling species II distributed in both Lake Tingtang and Lake Fengming, showing that its dispersal existed between the two lakes. Considering that the distance between Lake Hui and Lake Fengming is shorter than that between Lake Tingtang and Lake Fengming, sibling species II is able to disperse at least from Lake Fengming to Lake Hui. The restricted distribution of sibling species II in Lake Hui might be attributed to its lower intrinsic rate of population increase (0.34-0.39/d) when cultured in aerated tap water and fiy ash effluent, which might be further lowered by the lower algal food level and quality in Lake Hui.