Toxic effect and mechanism of tris (1,3-dichloro-2-propyl)phosphate (TDCPP) on the marine alga Phaeodactylum tricornutum

Acute toxicity of salicylic acid and its derivatives on the diatom Phaeodactylum tricornutum: Physico-Biochemical and transcriptomic insights

Salicylate pollutants (SAs) poses a serious threat to marine ecosystems as emerging contaminants. However, the toxic effects of SAs on marine phytoplankton, as well as the potential mechanisms and their ecological risks linked with them, are remain largely unknown. In this study, we aimed to evaluate the toxic effects of salicylic acid (SA) and its 5-substituted derivatives (5-sSA) on the marine diatom Phaeodactylum tricornutum, as well as the potential molecular mechanism involved in the toxicity. Physiological assays conducted on P. tricornutum revealed significant changes in photosynthetic pigments, chlorophyll fluorescence parameters, and antioxidant enzyme activities. The results showed that exposure of P. tricornutum to SAs caused a significant decline in chlorophyll contents and damage to the photosystem II (PSII) core resulting in the decline of photosynthesis. Although the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were enhanced, oxidative damage occurred. Transcriptome analysis showed that a large number of differentially expresses genes (DEGs) were significantly enriched in metabolic pathways such as porphyrin metabolism, terpenoid backbone biosynthesis, and carbon fixation in photosynthetic organisms after SA and 5-BrSA treatments. In addition, key genes in transcriptomic metabolic pathways were further analyzed and validated using weighted correlation network analysis (WGCNA) and real-time fluorescence quantitative PCR (qPCR). Considering the above results, SAs mainly inhibit the processes of photosynthesis by repressing the expression of genes involved in secondary metabolite synthesis and photosynthetic carbon sequestration pathways, thus exerting toxic effects on algal cells. The results of the study will provide key data for understanding the ecological risk and toxicity mechanisms of SA pollutants.

Bioactivity assessment of organophosphate flame retardants via a dose-dependent yeast functional genomics approach

Organophosphate flame retardants (OPFRs) have been widely detected in multiple environment media and have many adverse effects with complex toxicity mechanisms. However, the early molecular responses to OPFRs have not been fully elucidated, thereby making it difficult to assess their risks accurately. In this work, we systematically explored the point of departure (POD) of biological pathways at genome-wide level perturbed by 14 OPFRs with three substituents (alkyl, halogen, and aryl) using a dose-dependent functional genomics approach in Saccharomyces cerevisiae at 24 h exposure. Firstly, our results demonstrated that the overall biological potency at gene level (PODDRG20) ranged from 0.013 to 35.079 μM for 14 OPFRs, especially the tributyl phosphate (TnBP) exhibited the strongest biological potency with the least PODDRG20. Secondly, we found that structural characteristics of carbon number and logKow were significantly negatively correlated with POD, and carbon number and logKow also significantly affected lipid metabolism associated processes. Thirdly, these early biological pathways of OPFRs toxification were found to be involved in lipid metabolism, oxidative stress, DNA damage, MAPK signaling pathway, and amino acid and carbohydrate metabolism, among which the lipid metabolism was the most sensitive molecular response perturbed by most OPFRs. More importantly, we identified one resistant mutant strain with knockout of ERG2 (YMR202W) gene participated in steroid biosynthesis pathway, which can serve as a key yeast strain of OPFRs toxification. Overall, our study demonstrated an effective platform for accurately assessing OPFRs risks and provided a basis for further green OPFRs development.

Repeated Exposure Enhanced Toxicity of Clarithromycin on Microcystis aeruginosa Versus Single Exposure through Photosynthesis, Oxidative Stress, and Energy Metabolism Shift

Antibiotics are being increasingly detected in aquatic environments, and their potential ecological risk is of great concern. However, most antibiotic toxicity studies involve single-exposure experiments. Herein, we studied the effects and mechanisms of repeated versus single clarithromycin (CLA) exposure on Microcystis aeruginosa. The 96 h effective concentration of CLA was 13.37 μg/L upon single exposure but it reduced to 6.90 μg/L upon repeated exposure. Single-exposure CLA inhibited algal photosynthesis by disrupting energy absorption, dissipation and trapping, reaction center activation, and electron transport, thereby inducing oxidative stress and ultrastructural damage. In addition, CLA upregulated glycolysis, pyruvate metabolism, and the tricarboxylic acid cycle. Repeated exposure caused stronger inhibition of algal growth via altering photosynthetic pigments, reaction center subunits biosynthesis, and electron transport, thereby inducing more substantial oxidative damage. Furthermore, repeated exposure reduced carbohydrate utilization by blocking the pentose phosphate pathway, consequently altering the characteristics of extracellular polymeric substances and eventually impairing the defense mechanisms of M. aeruginosa. Risk quotients calculated from repeated exposure were higher than 1, indicating significant ecological risks. This study elucidated the strong influence of repeated antibiotic exposure on algae, providing new insight into antibiotic risk assessment.

Exposure to tris (1,3-dichloro-2-propyl) phosphate affects the embryonic cardiac development of Oryzias melastigma

Tris (1,3-dichloro-2-propyl) phosphate (TDCPP) is a growing concern and may be a potential risk to marine environmental health due to its widespread usage and distribution. However, the toxic effects of TDCPP on cardiac development in marine fish have not been reported. In this study, Oryzias melastigma embryos were exposed to TDCPP at doses of 0, 0.04, 0.4, 4 and 40 μg/L from early embryogenesis to 10 days postfertilization (dpf). Then, the heart rate and sinus venosus-bulbus arteriosus (SV-BA) distance of the exposed embryos were measured at 5, 6, 8 and 10 dpf. Furthermore, alterations in the mRNA levels of the genes encoding cyclooxygenase-2 (COX-2), bone morphogenetic protein 4 (BMP4), fibroblast growth factor 8 (FGF8), and GATA-binding protein 4 (GATA4) were evaluated at 5, 6, 8 and 10 dpf. We found that the heart rate significantly increased in all TDCPP exposure groups at 10 dpf. The SV-BA distance significantly decreased in all TDCPP exposure groups at all developmental stages (except for the 0.4 μg/L group at 5 dpf and the 4 μg/L group at 10 dpf). The mRNA expression of COX-2 was downregulated at 5 dpf, BMP4 was downregulated at 5 and 6 dpf, FGF8 was downregulated at 5, 6 and 8 dpf, GATA4 was downregulated at 8 dpf, and GATA4 was upregulated at 10 dpf. These results indicate that the changes in heart rate and SV-BA distance might be accompanied by disturbances in the four genes involved in cardiac development. Our findings will help to illustrate the possible cardiac toxic effects of marine fish exposed to TDCPP.

The physiological effect of organophosphate flame retardants (OPFRs) on wheat (Triticum aestivum L.) seed germination and seedling growth under the presence of copper

This study investigated the physiological and biochemical impacts of organophosphate flame retardants (OPFRs) on wheat (Triticum aestivum L.) germination and growth performance in the presence and absence of copper. The study evaluated seed germination, growth, OPFRs concentrations, chlorophyll fluorescence index (F_v/F_m and F_v/F₀), and antioxidant enzyme activity. It also calculated the root accumulation of OPFRs and their root-stem translocation. At the germination stage, at a concentration of 20 μg·L^-1 OPFR exposure, wheat germination vigor, root, and shoot lengths were significantly decreased compared to the control. However, the addition of a high concentration of copper (60 mg·L^-1) decreased by 80%, 82%, and 87% in the seed germination vitality index and root and shoot elongation, respectively, compared to 20 μg·L^-1 of OPFR treatment. At the seedling stage, a concentration of 50 μg·L^-1 of OPFRs significantly decreased by 42% and 5.4% in wheat growth weight and the photochemical efficiency of photosystem II (Fv/Fm) compared to the control. However, the addition of a low concentration of copper (15 mg·L^-1) slightly enhanced the growth weight compared to the other two co-exposure treatments, but the results were not significant (p > 0.05). After 7 days of exposure, the activity of superoxide dismutase (SOD) and malondialdehyde (MDA) (indicates lipid peroxidation) content in wheat roots significantly increased compared to the control and was higher than in leaves. MDA contents in wheat roots and shoots were decreased by 18% and 6.5% when OPFRs were combined with low Cu treatment compared with single OPFRs treatment, but SOD activity was slightly improved. These results suggest that the co-exposure of copper and OPFRs enhances reactive oxygen species (ROS) production and oxidative stress tolerance. Seven OPFRs were detected in wheat roots and stems, with root concentration factors (RCFs) and translocation factors (TFs) ranging from 67 to 337 and 0.05 to 0.33, respectively, for the seven OPFRs in a single OPFR treatment. The addition of copper significantly increased OPFR accumulation in the root and aerial parts. In general, the addition of a low concentration of copper promoted wheat seedling elongation and biomass and did not significantly inhibit the germination process. OPFRs could mitigate the toxicity of low-concentration copper on wheat but had a weak detoxification effect on high-concentration copper. These results indicated that the combined toxicity of OPFRs and Cu had antagonistic effects on the early development and growth of wheat.

Triclosan toxicity in a model cyanobacterium (Anabaena flos-aquae): Growth, photosynthesis and transcriptomic response

Exposure to triclosan (TCS) has been reported to reduce photosynthetic pigments, suppress photosynthesis, and inhibit growth in both prokaryotic and eukaryotic algae including Anabaena flos-aquae (a model cyanobacterium). In particular, cyanobacteria are more sensitive to TCS toxicity compared to eukaryotic algae possibly due to the structural similarity to bacteria (target organisms); however, whether TCS exerts its toxicity to cyanobacteria by targeting signaling pathways of fatty acid biosynthesis as in bacteria remains virtually unknown, particularly at environmental exposure levels. With the complete genome sequence of A. flos-aquae presented in this study, the transcriptomic alterations and potential toxic mechanisms in A. flos-aquae under TCS stress were revealed. The growth, pigments and photosynthetic activity of A. flos-aquae were markedly suppressed following a 7-day TCS exposure at 0.5 µg/L but not 0.1 µg/L (both concentrations applied are environmentally relevant). The transcriptomic sequencing analysis showed that signaling pathways, such as biofilm formation - Pseudomonas aeruginosa, two-component system, starch and sucrose metabolism, and photosynthesis were closely related to the TCS-induced growth inhibition in the 0.5 µg/L TCS treatment. Photosynthesis systems and potentially two-component system were identified to be sensitive targets of TCS toxicity in A. flos-aquae. The present study provides novel insights on TCS toxicity at the transcriptomic level in A. flos-aquae.

Transcriptome analysis of changes in M. aeruginosa growth and microcystin production under low concentrations of ethinyl estradiol

Ethinyl estradiol (EE2) is a synthetic environmental estrogen with considerable estrogenic activity. It has been found to consequently pose a significant threat to the aquatic ecosystem. Harmful algal blooms are a major aquatic ecological issue. However, the relationship between EE2 and cyanobacterial bloom is mainly unknown. In this study, the physiological and molecular responses of Microcystis aeruginosa to EE2 exposure were investigated. A low level of EE2 (0.02 μg/L) significantly enhanced the growth of algal cells (P < 0.05), whereas higher concentrations of EE2 (0.2-200 μg/L) inhibited it. EE2 at doses ranging from 0.02 to 200 μg/L promoted the production of microcystins (MCs), with genes mcyABD playing a key role in the regulation of MC synthesis. The alterations of chlorophyll-a, carotenoid, and phycocyanin contents caused by EE2 showed the same trend as cell growth. At the molecular level, 200 μg/L EE2 significantly down-regulated genes in photosynthetic pigment synthesis, light harvesting, electron transfer, NADPH, and ATP generation. High concentrations of EE2 caused oxidative damage to algal cells on the 4th d. After 12d exposure, although there was no significant change in superoxide dismutase (SOD) content and no damage observed in membrane lipids, genes related to SOD and glutathione were changed. In addition, due to the down-regulation of pckA, PK, gltA, nrtA, pstS, etc., carbon fixation, glycolysis, TCA cycle, nitrogen and phosphorus metabolism were hindered by EE2 (200 μg/L). Gene fabG in fatty acid biosynthesis was significantly up-regulated, promoting energy storage in cells. These findings provide important clues to elucidate the effects and mechanisms of cyanobacterial blooms triggered by EE2 and help to effectively prevent and control cyanobacterial blooms.

New insights into the growth response of the macrophyte Vallisneria natans exposed to phosphite

Renewed interest in phosphite, an analog of phosphate, has increased due to its widespread distribution and increasing abundance in many waterbodies. However, up until recently very little is known about their ecological effects on aquatic organisms. Herein we studied the effects of phosphite via root and foliar exposure on the growth responses of the dominant pioneer macrophyte V. natans. Overall, both exposures of phosphite to V. natans resulted in significant reductions in the leaf length, root length, relative growth rate (RGR) and photosynthetic pigments, suggesting phosphite had an inhibitory effect on the plant growth. Our results further confirmed phosphite could induce the oxidative stresses in the V. natans cells, as indicated by the significantly increased intracellular enzyme activities i.e. superoxide dismutase activity (SOD) and malondialdehyde (MDA). Microscopic evidence also showed phosphite penetrated the cell membrane and destroyed membrane integrity under high phosphite stress. Besides, V. natans leaves exhibited intuitive deterioration symptoms, which seemed to be more sensitive to phosphite toxicity than roots. It is concluded that the increased abundance of phosphite in waterbodies cannot be utilized as a bioavailable P source but impose adverse physiological and metabolic limitations to plant growth, which should be receive more attention in the ecological risk assessment. Our result is necessary to build a comprehensive understanding of phosphite biogeochemical behaviors in aquatic ecosystems.

Biphasic effects of typical chlorinated organophosphorus flame retardants on Microcystis aeruginosa

The potential accumulation of chlorinated organophosphorus flame retardants (Cl-OPFRs) in aquatic environments sparked interest in studying the effects of Cl-OPFRs on cyanobacterial blooms. In this work, two common Cl-OPFRs, tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and tris(2-chloroethyl) phosphate (TCEP), induced dose-dependent biphasic effect on bloom-forming M. aeruginosa. The hormetic response to low-dose Cl-OPFRs was associated with the upregulation of the type I NADH dehydrogenase (NDH-1) complex and its mediated cyclic electron transfer (CET) pathway, as reflected by a transient post-illumination increase in chlorophyll fluorescence, the dark reduction of P700⁺ and the change of NDH-1-related gene expression. The increased CET activity and carotenoid content jointly reduced the intracellular ROS production, facilitating cyanobacterial growth. Conversely, a higher concentration of both Cl-OPFRs induced severe inhibition of growth and photosynthetic oxygen-evolving activity through an imbalance between PSII and PSI. Toxic-dose Cl-OPFRs inhibited state transition and fixed cells into the State I with a higher PSII/PSI ratio, as indicated by chlorophyll fluorescence induction, 77 K fluorescence emission spectra and photosystem stoichiometry. The elevated PSII/PSI ratio created an imbalance between the two photosystems and eventually lead to ROS overproduction, which generate adverse effects on cell growth. This work provides important insights into the hormetic mechanism of Cl-OPFRs on Microcystis aeruginosa and their potential roles in harmful cyanobacteria blooms.

Transcriptome and biochemical analyses of rainbow trout (Oncorhynchus mykiss) RTG-2 gonadal cells in response to BDE-47 stress indicates effects on cell proliferation

2,2',4,4'-Tetrabromodiphenyl ether (BDE-47) is a biotoxin of polybrominated diphenyl ether (PBDEs) frequently detected in the environment. Apoptosis and cell cycle arrest are important toxic phenomena of xenobiotics that inhibit cell proliferation. In this study, we investigated the effects of BDE-47 (5 μM, 10 μM, 20 μM, 40 μM) on cell viability, morphology, cell cycle and apoptosis. BDE-47 significantly decreased cell viability, and morphological alterations were observed. The significant increase in cells at G1 phase indicated the occurrence of G1 phase cell cycle arrest in RTG-2 cells. An acridine orange and ethidium bromide (AO/EB) staining assay was employed and revealed the induction of apoptosis in RTG-2 cells. The results indicated that BDE-47 exposure inhibits cell proliferation. Transcriptome analysis was applied for further evidence. A total of 1300 differentially expressed genes (DEGs) were identified in RTG-2 cells, among which 26 DEGs were associated with the cell cycle and apoptosis. Western blotting and qPCR analyses also showed the expression of cell cycle- and apoptosis-related proteins and genes. Mapping the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, p53, Tumor necrosis factor (TNF), Mitogen-activated protein kinase (MAPK), phosphatidylinositide 3-kinase-AKT (PI3K-AKT), and reaction oxygen species (ROS)-mediated signaling pathways were determined to be the major pathways involved in modulating the cell cycle and apoptosis. Since we demonstrated simultaneous ROS overproduction during BDE-47 exposure in a previous study, we speculated a possible explanation for the observation: BDE-47-induced ROS overproduction was the initiating signal, which activated cell cycle arrest and apoptosis and finally inhibited cell proliferation.

Effects of polystyrene and triphenyl phosphate on growth, photosynthesis and oxidative stress of Chaetoceros meülleri

The toxicity of microplastics to marine organisms has attracted much attention; however, studies of their effects on marine microalgae remain limited. Here, the effects of the single and combined toxicity of polystyrene (PS) and triphenyl phosphate (TPhP) on the cell growth, photosynthesis, and oxidative stress of Chaetoceros meülleri were investigated. PS inhibited growth of the algae cells and caused a dose-dependent effect on oxidative stress. The significantly high production of reactive oxygen species (ROS) induced severe cell membrane damage, as confirmed by high fluorescence polarization. However, there was no obvious decrease in chlorophyll a content, and 80 mg/L of PS significantly promoted chlorophyll a synthesis. The TPhP also inhibited cell growth, except at low concentrations (0.2-0.8 mg/L), which stimulated algae growth over 48 h. Moreover, no obvious decrease in chlorophyll a and maximal photochemical efficiency of PSII was found in the TPhP experimental groups except for 3.2 mg/L TPhP, where the rapid light curves showed a significantly reduced photosynthetic capacity of algae. In addition, TPhP caused high ROS levels at 96 h, resulting in cell membrane damage. Using the additive index and independent action methods, the combined toxic effects of PS and TPhP on the algae were evaluated as antagonistic; however, cell membrane damage caused by high ROS levels was still noticeable. This study has shown the potential toxicity of PS and TPhP to marine microalgae, and provided insights into the combined risk assessment of TPhP and microplastics in the marine environment.

Responses of the reproduction, population growth and metabolome of the marine rotifer Brachionus plicatilis to tributyl phosphate (TnBP)

The typical alkyl organophosphorus flame retardant tributyl phosphate (TnBP) can leak from common products into the marine environment, with potential negative effects on marine organisms. However, risk assessments for TnBP regarding zooplankton are lacking. In this study, a marine rotifer, Brachionus plicatilis, was used to analyze the effect of TnBP (0.1 μg/L, environmental concentration; 1 and 6 mg/L) on reproduction, population growth, oxidative stress, mitochondrial function and metabolomics. Mortality increased as the TnBP concentration rose; the 24-h LC50 value was 12.45 mg/L. All tested TnBP concentrations inhibited B. plicatilis population growth, with reproductive toxicity at the higher levels. Microstructural imaging showed ovary injury, the direct cause of reproductive toxicity. Despite elevated glutathione reductase activities, levels of reactive oxygen species and malonyldialdehyde increased under TnBP stress, indicating oxidative imbalance. TnBP induced mitochondrial malformation and activity suppression; the ROS scavenger N-acetylcysteine alleviated this inhibition, suggesting an internal connection. Nontargeted metabolomics revealed 398 and 583 differentially expressed metabolites in the 0.1 μg/L and 6 mg/L treatments relative to control, respectively, which were enriched in the pathways such as biosynthesis of amino acids, purine metabolism, aminoacyl-tRNA biosynthesis. According to metabolic pathway analysis, oxidative stress from purine degradation, mitochondrial dysfunction, disturbed lipid metabolism and elevated protein synthesis were jointly responsible for reproduction and population growth changes. This study echoes the results previously found in rotifer on trade-off among different life processes in response to environmental stress. Our systematic study uncovers the TnBP toxic mode of action.

Computational simulation associated with biological effects of alkyl organophosphate flame retardants with different carbon chain lengths on Chlorella pyrenoidosa

The environmental safety of flame retardants has attracted growing attention. Alkyl organophosphorus flame retardants (OPFRs) have been prevalently applied, but the potential risk and the structure effects of different alkyl chain lengths OPFRs on aquatic microalgae remain unknown. This study investigated the biological response of five alkyl-OPFRs to Chlorella pyrenoidosa by computational simulation together with biological approaches. The reduced docking energy had a significantly positive correlation (R² = 0.9) with the cell inhibition alongside the incremental chain length of alkyl-OPFRs. Molecular docking simulations suggested that the toxicity of alkyl-OPFRs would be highly correlated to their molecular structures. Coincidently, the reactive oxygen species, superoxide dismutase and malondialdehyde were triggered by 85%, 92% and 155% (based on the control group), after exposure to the longest chain length tributyl phosphate (TBP_C12), respectively. Furthermore, combining the ultrastructure scrutiny with the photosynthesis analysis, TBP_C12 was also found to significantly inhibit the chlorophyll biosynthesis (43%) and restrain the photosynthetic efficiency (26%) when compared with the control group. Overall, this is the first study to comprehensively reveal the biological effects of different alkyl-OPFRs on microalgae via the combination of computational simulation and cellular responses, providing a novel insight into targeted predicting the aquatic ecological risks of OPFRs.