β-Ionone causes endocrine disruption, hyperpigmentation and hypoactivity in zebrafish early life stages

Identification of 2, 4-di-tert-butylphenol from Microcystis lysate after bloom control and its potential risks to aquatic ecosystems

With the increasing concern of cyanobacterial blooms, numerous techniques have been developed to mitigate these environmental nuisances. During bloom control, the allelopathic effects of compounds released from cyanobacterial cells are considered as secondary hazards. In this study, the findings indicated that Microcystis lysate inhibited algal proliferation and disrupted the development of zebrafish embryos. Then, allelochemicals in Microcystis lysate were identified using gas chromatography-mass spectrometry, with 2, 4-di-tert-butylphenol (2, 4-DTBP) being the only identified phenol, which was selected for further study. The results showed that 2,4-DTBP caused oxidative damages, disrupted metabolic activity, and suppressed photosynthetic activity, consequently impeding the growth of Microcystis aeruginosa (M. aeruginosa) and Chlorella pyrenoidosa (C. pyrenoidosa). Moreover, it enhanced the interspecies competitive advantages of M. aeruginosa by increasing phosphate uptake rate. Furthermore, at a concentration of 2 mg L-1, 2, 4-DTBP negatively affected the development of zebrafish embryos, manifesting in mortality, malformation, and hatching delay. Therefore, the investigation identified 2, 4-DTBP as a potential allelochemical within Microcystis lysate. Although the effective concentration for freshwater algae and zebrafish embryos was higher than that found in Microcystis lysate, it highlighted the need for careful monitoring of aquatic ecosystem health during cyanobacterial bloom mitigation.

Effects of 2,4,6-Trichloroanisole on the morphological development and motility of zebrafish

2,4,6-Trichloroanisole (2,4,6-TCA), a compound with a characteristic earthy odor, is a common source of odorous pollutants in drinking water and wine. However, research on its biological toxicity is limited. In this study, we used zebrafish as an indicator model to investigate the effects of 2,4,6-TCA exposure on morphological development, oxidative stress, apoptosis, heart rate, blood flow, and motility. We found that exposure to 2,4,6-TCA resulted in significant spinal, tail, and cardiac deformities in zebrafish larvae and promoted a pronounced oxidative stress response and extensive cell apoptosis, notably in the digestive tract, head, spine, and heart, ultimately leading to significant reductions in zebrafish heart rate, blood flow, and motility. Moreover, these effects became more pronounced with an increase in the concentration of 2,4,6-TCA to which the zebrafish were exposed. Furthermore, qPCR analysis revealed that exposure to 2,4,6-TCA promoted significant changes in the expression levels of genes associated with oxidative stress, apoptosis, cardiac development, and the nervous system, particularly key genes (p53, apaf1, casp9, and casp3) in the mitochondrial apoptotic pathway, which were significantly upregulated. Similarly, we detected significant upregulation of ache gene expression. These findings indicated that exposure to 2,4,6-TCA resulted in the accumulation of reactive oxygen species in zebrafish, induced strong oxidative stress responses, and triggered lipid peroxidation and extensive cell apoptosis. Cellular apoptosis, which mitochondrial signaling pathways may mediate, has been found to lead to malformations in zebrafish embryos, resulting in significant reductions in cardiac function and motility. To our knowledge, this is the first systematic assessment of the toxicity of 2,4,6-TCA, and our findings provide an important reference for risk assessment and early warning of 2,4,6-TCA exposure.

β-ionone inhibits the grazing of Daphnia sinensis by reducing the activity of acetylcholinesterase

β-ionone is a volatile metabolite of Microcystis aeruginosa that is toxic to aquatic organisms. Using Daphnia sinensis as model, our present study found that β-ionone could significantly reduce heart rate and feeding rate, and induce intestinal emptying. Transcriptomic analysis showed that β-ionone could significantly inhibit the expression of acetylcholinesterase (AchE) mRNA, while metabolomics further revealed that β-ionone could significantly increase the level of acetylcholine (Ach) in D. sinensis. These results indicated that β-ionone might act as an AchE inhibitor, resulting in an increase in Ach levels. To test this hypothesis, both in vivo and in vitro experiments demonstrated that β-ionone could significantly reduce AchE activity. Furthermore, the inhibitory effects of β-ionone on heart rate and feeding rate could be blocked by the M-type Ach receptor (mAchR) blocker. These findings confirm that β-ionone is a novel AchE inhibitor. β-ionone could inhibit the activity of AchE, which in turn resulted in an increase of Ach in D. sinensis. Consequently, elevated levels of Ach could suppress the heart rate and feeding rate of D. sinensis by activating the mAchR, while concurrently accelerating the rate of intestinal emptying by stimulating intestinal peristalsis, thereby obstructing the digestion of algae within the intestinal tract.

Co-Occurrence of Taste and Odor Compounds and Cyanotoxins in Cyanobacterial Blooms: Emerging Risks to Human Health?

Cyanobacteria commonly form large blooms in waterbodies; they can produce cyanotoxins, with toxic effects on humans and animals, and volatile compounds, causing bad tastes and odors (T&O) at naturally occurring low concentrations. Notwithstanding the large amount of literature on either cyanotoxins or T&O, no review has focused on them at the same time. The present review critically evaluates the recent literature on cyanotoxins and T&O compounds (geosmin, 2-methylisoborneol, β-ionone and β-cyclocitral) to identify research gaps on harmful exposure of humans and animals to both metabolite classes. T&O and cyanotoxins production can be due to the same or common to different cyanobacterial species/strains, with the additional possibility of T&O production by non-cyanobacterial species. The few environmental studies on the co-occurrence of these two groups of metabolites are not sufficient to understand if and how they can co-vary, or influence each other, perhaps stimulating cyanotoxin production. Therefore, T&Os cannot reliably serve as early warning surrogates for cyanotoxins. The scarce data on T&O toxicity seem to indicate a low health risk (but the inhalation of β-cyclocitral deserves more study). However, no data are available on the effects of combined exposure to mixtures of cyanotoxins and T&O compounds and to combinations of T&O compounds; therefore, whether the co-occurrence of cyanotoxins and T&O compounds is a health issue remains an open question.