Inhibition to crucial enzymes in the lethal effects of the dinoflagellate Karenia mikimotoi on the rotifer Brachionus plicatilis

Metabolic pathways of methylmercury in rotifer Brachionus plicatilis

Methylmercury (MeHg) readily accumulates in aquatic organisms while transferring and amplifying in the aquatic food chains. This study firstly explores the in vivo accumulation sites and metabolic regulation of MeHg in the rotifer Brachionus plicatilis by aggregation-induced emission fluorogen (AIEgen) and metabolomics. Fluorescent image analysis by AIEgen showed that MeHg in B. plicatilis mainly occured in the ciliary corona, esophagus, mastax, stomach and intestine in the direct absorption group. In the other group, where B. plicatilis were indirectly supplied with MeHg via food intake, the accumulation of MeHg in the rotifer occurred in the ciliary corona, various digestive organs, and the pedal gland. However, the MeHg accumulated in the rotifer is difficult to metabolize outside the body. Metabolomics analysis showed that the significant enrichment of ABC transporters was induced by the direct exposure of rotifers to dissolved MeHg. In contrast, exposure of rotifers to MeHg via food intake appeared to influence carbon, galactose, alanine, aspartate and glutamate metabolisms. Besides, the disturbed biological pathways such as histidine metabolism, beta-alanine metabolism and pantothenate and CoA biosynthesis in rotifers may be associated with L-aspartic acid upregulation in the feeding group. The significant enrichment of ABC transporters and carbon metabolism in rotifers may be related to the accumulation of MeHg in the intestine of rotifers. In both pathways of MeHg exposure, the arginine biosynthesis and metabolism of rotifers were disturbed, which may support the hypothesis that rotifers produce more energy to resist MeHg toxicity. This study provides new insight into the accumulation and toxicity mechanisms of MeHg on marine invertebrates from the macro and micro perspectives.

Toxic dinoflagellate Karenia mikimotoi induces apoptosis in Neuro-2a cells through an oxidative stress-mediated mitochondrial pathway

The dinoflagellate Karenia mikimotoi is a toxic bloom-forming species that threatens aquaculture and public health worldwide. Previous studies showed that K. mikimotoi induces neurotoxicity; however, the underlying mechanism is poorly understood. In this study, three neural cell lines were used to investigate the potential neurotoxicity of K. mikimotoi. The tested cells were exposed to a ruptured cell solution (RCS) of K. mikimotoi at different concentrations (0.5 × 105, 1.0 × 105, 2.0 × 105, 4.0 × 105, and 6 × 105 cells mL-1) for 24 h, and the RCS decreased cell viabilities and promoted Neuro-2a (N2A) cell apoptosis in a dose-dependent manner. The underlying mechanism was further investigated in N2A cells. At the biochemical level, the RCS stimulated reactive oxygen species (ROS) and malondialdehyde (MDA) formation, decreased SOD activity, and reduced mitochondrial membrane potential (MMP). At the gene level, the moderate RCS treatment (2.0 × 105 cells mL-1) upregulated antioxidant response genes (e.g., nrf-2, HO-1, NQO-1, and cat) to alleviate RCS-induced oxidative stress, while the high RCS treatment (4.0 × 105 cells mL-1) downregulated these genes, thereby aggravating oxidative stress. Meanwhile, apoptosis-related genes (e.g., p53, caspase 3, and bax2) were significantly upregulated and the anti-apoptotic gene bcl2 was suppressed after RCS treatment. Western blotting results for Caspase 3, Bax2 and Bcl2 were consistent with the mRNA trends. These results revealed that K. mikimotoi RCS can induce neural cell apoptosis via the oxidative stress-mediated mitochondrial pathway, providing novel insights into the neurotoxicity of K. mikimotoi.

Dinoflagellate Karenia mikimotoi on the growth performance, antioxidative responses, and physiological activities of the rotifer Brachionus plicatilis

The harmful dinoflagellate Karenia mikimotoi is responsible for the mortality of aquatic animals. However, the mechanism behind these toxic effects has not been fully determined. Herein, the toxic effects of K. mikimotoi on the growth performance, antioxidative responses, physiological activities, and energetic substance contents of rotifer Brachionus plicatilis were assessed. Rotifers were exposed to Nannochloropsis salina (Eustigmatophyceae), K. mikimotoi, and a mixture of N. salina and K. mikimotoi with biomass ratio proportions of 3:1, 1:1, and 1:3, respectively. Results indicated that K. mikimotoi negatively affected the population growth, survival, and specific growth rates of rotifers within 24 h. The level of reactive oxygen species (ROS), the content of malondialdehyde, and the activity of amylase increased. However, the total antioxidant capacity level, pepsase, cellulase, alkaline phosphatase, xanthine oxidase, and lactate dehydrogenase activities, and glycogen and protein contents decreased with increasing proportions of K. mikimotoi. The mixture of 50% N. salina and 50% K. mikimotoi promoted the increase in glutamic-pyruvic transaminase activity and triglyceride content. These findings underscore ROS-mediated antioxidative responses, physiological responses, and energetic substance content changes in B. plicatilis work together to affect population dynamics inhibition of rotifers by K. mikimotoi.

Impacts of a bacterial algicide on metabolic pathways in Chlorella vulgaris

Chlorella is a dominant species during harmful algal blooms (HABs) worldwide, which bring about great environmental problems and are also a serious threat to drinking water safety. Application of bacterial algicides is a promising way to control HABs. However, the identified bacterial algicides against Chlorella and the understanding of their effects on algal metabolism are very limited. Here, we isolated a novel bacterium Microbacterium paraoxydans strain M1 that has significant algicidal activities against Chlorella vulgaris (algicidal rate 64.38 %, at 120 h). Atrazine-desethyl (AD) was then identified from strain M1 as an effective bacterial algicide, with inhibition or algae-lysing concentration values (EC50) of 1.64 μg/mL and 1.38 μg/mL, at 72 h and 120 h, respectively. LAD (2 μg/mL AD) or HAD (20 μg/mL AD) causes morphology alteration and ultrastructure damage, chlorophyll a reduction, gene expression regulation (for example, psbA, 0.05 fold at 24 h, 2.97 fold at 72 h, and 0.23 fold of the control in HAD), oxidative stress, lipid oxidation (MDA, 2.09 and 3.08 fold of the control in LAD and HAD, respectively, at 120 h) and DNA damage (average percentage of tail DNA 6.23 % at 120 h in HAD, slight damage: 5∼20 %) in the algal cells. The impacts of AD on algal metabolites and metabolic pathways, as well as the algal response to the adverse effects were investigated. The results revealed that amino acids, amines, glycosides and urea decreased significantly compared to the control after 24 h exposure to AD (p < 0.05). The main up-regulated metabolic pathways implied metabonomic resistance and defense against osmotic pressure, oxidative stress, photosynthesis inhibition or partial cellular structure damage, such as phenylalanine metabolism, arginine biosynthesis. The down-regulated glycine, serine and threonine metabolism is a major lead in the algicidal mechanism according to the value of pathway impact. The down-regulated glycine, and serine are responsible for the downregulation of glyoxylate and dicarboxylate metabolism, aminoacyl-tRNA biosynthesis, glutathione metabolism, and sulfur metabolism, which strengthen the algae-lysing effect. It is the first time to highlight the pivotal role of glycine, serine and threonine metabolism in algicidal activities, which provided a new perspective for understanding the mechanism of bacterial algicides exerting on algal cells at the metabolic level.

Reduced Fitness and Elevated Oxidative Stress in the Marine Copepod Tigriopus japonicus Exposed to the Toxic Dinoflagellate Karenia mikimotoi

Blooms of the toxic dinoflagellate Karenia mikimotoi cause devastation to marine life, including declines of fitness and population recruitment. However, little is known about the effects of them on benthic copepods. Here, we assessed the acute and chronic effects of K. mikimotoi on the marine benthic copepod Tigriopus japonicus. Results showed that adult females maintained high survival (>85%) throughout 14-d incubation, but time-dependent reduction of survival was detected in the highest K. mikimotoi concentration, and nauplii and copepodites were more vulnerable compared to adults. Ingestion of K. mikimotoi depressed the grazing of copepods but significantly induced the generation of reactive oxygen species (ROS), total antioxidant capacity, activities of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), and acetylcholinesterase. Under sublethal concentrations for two generations, K. mikimotoi reduced the fitness of copepods by prolonging development time and decreasing successful development rate, egg production, and the number of clutches. Our findings suggest that the bloom of K. mikimotoi may threaten copepod population recruitment, and its adverse effects are associated with oxidative stress.

Assessing the Effect of Modified Clay on the Toxicity of Karenia mikimotoi Using Marine Medaka (Oryzias melastigma) as a Model Organism

Blooms of the toxic dinoflagellate Karenia mikimotoi could threaten the survival of marine life, and modified clay (MC) is considered a promising method for the control of harmful algal blooms. Here, using marine medaka as the model organism, the toxicity of K. mikimotoi before and after MC disposal was investigated. The results showed that only a certain density of intact K. mikimotoi cells could cause obvious damage to fish gills and lead to rapid death. A systematic analysis of morphology, physiology, and molecular biology parameters revealed that the fish gills exhibited structural damage, oxidative damage, osmotic regulation impairment, immune response activation, and signal transduction enhancement. MC can flocculate K. mikimotoi rapidly in water and reduce its toxicity by reducing the density of intact algae cells and hemolytic toxicity. The results indicate that MC is an effective and safe method for controlling K. mikimotoi blooms.

Toxic effects, mechanisms, and ecological impacts of harmful algal blooms in China

Over the last 30 years, harmful algal blooms (HABs) have occurred frequently in the coastal waters of China, resulting in financial losses of over 5.9 billion yuan (about 0.87 billion US dollars) due to massive fish and shellfish mortalities and negative impacts on tourism. To better understand HABs in China, herein we summarized bloom events with massive fish/shellfish mortalities and/or economic losses. Our results suggest that the diversity of HAB species has increased over the last 30 years, with the main causative species shifting from the raphidophyte Chattonella marina and dinoflagellates Gymnodinium spp. to various other species, including the dinoflagellates Karenia mikimotoi and Prorocentrum donghaiense, the haptophyte Phaeocystis globosa, and the pelagophyte Aureococcus anophagefferens. In addition, new types of HABs, such as macroalgal blooms, emerged with severe ecological impacts. We also reviewed the toxic effects, mechanisms, and ecological impacts of common HAB causative species in China. Analysis of the toxic effects of three types of harmful algae (toxin-producing, fish killing, and ecosystem disruptive algae) on marine organisms commonly found in China at different trophic levels revealed that HABs often had toxic effects on multiple organisms in addition to fish or shellfish, with species-specific impacts. Common mechanisms of intoxication include shifting environmental parameters, shellfish poisoning, reactive oxygen species, and haemolytic/cytotoxic toxins. The main mechanism appears to vary with the type of HAB species, and for some notorious algae such as K. mikimotoi and C. marina, further investigations are needed to identify their intoxication mechanism.

Progress on the usage of the rotifer Brachionus plicatilis in marine ecotoxicology: A review

The rotifer, Brachionus plicatilis, is a widely used model species in marine ecotoxicology for evaluating pollutions, toxins, and harmful algae. In this paper, the marine ecotoxicology of Brachionus plicatilis was reviewed, including toxicity measurements of harmful algae species and environmental stresses. In addition, marine pollution involving pesticides, heavy metals, drugs, petroleum, and petrochemicals were addressed. Methods for measuring toxicity were also discussed. The standard acute lethal assay and the chronic population dynamics test were indicated as common methods of toxicity evaluating using B. plicatilis. Research on other biomarkers, such as behaviour, enzyme activity, or gene expression, are also reported here, with potential applications for fast detection or the scientific exploration of underlying molecular mechanisms. It is suggested that the methods selected should reflect the experimental purpose. Additionally, series assays should be conducted for comprehensive evaluation of ecotoxicity as well as to elucidate the correct mechanisms. Genetic methods, such as transcriptomics, were suggested as useful tools for exploring the toxicity mechanism using the rotifer B. plicatilis.