Anthropic eutrophication leads to water quality degradation because it may cause the development of harmful cyanobacterial blooms, affecting aquatic biota and threatening human health. Because in the natural environment zooplankters are exposed continuously or intermittently to cyanotoxins in the water or through cyanobacterial consumption, this study aimed to assess the effects of the toxigenic Microcystis aeruginosa VU-5 by different ways of exposure in Daphnia curvirostris. The acute toxicity produced by the cells, the aqueous crude extract of cells (ACE), and the cell-free culture medium (CFM) were determined. The effect on the survival and reproduction of D. curvirostris under continuous and intermittent exposure was determined during 26 d. The LC50 was 407,000 cells mL-1; exposure to the ACE and CFM produced mortality lower than 20%. Daphnia survivorship and reproduction were significantly reduced. Continuous exposure to Microcystis cells caused 100% mortality on the fourth day. Exposure during 4 and 24 h in 48 h cycles produced adult mortality, and reproduction decreased as the exposure time and the Microcystis concentrations increased. The higher toxicity of cells than the ACE could mean that the toxin\'s absorption is higher in the digestive tract. The temporary exposure to Microcystis cells produced irreversible damage despite the recovery periods with microalgae as food. The form and the continuity in exposure to Microcystis produced adverse effects, warning about threats to the zooplankton during HCBs.

Cyanobacterial blooms are increasingly common during winters, especially when they are mild. The goal of this study was to determine the summer and winter phytoplankton community structure, cyanotoxin presence, and toxigenicity in a eutrophic lake susceptible to cyanobacterial blooms throughout the year, using classical microscopy, an analysis of toxic cyanometabolites, and an analysis of genes involved in biosynthesis of cyanotoxins. We also assessed whether cyanobacterial diversity in the studied lake has changed compared to what was reported in previous reports conducted several years ago. Moreover, the bloom-forming cyanobacterial strains were isolated from the lake and screened for cyanotoxin presence and toxigenicity. Cyanobacteria were the main component of the phytoplankton community in both sampling times, and, in particular, Oscillatoriales were predominant in both summer (Planktothrix/Limnothrix) and winter (Limnothrix) sampling. Compared to the winter community, the summer community was denser; richer in species; and contained alien and invasive Nostocales, including Sphaerospermopsis aphanizomenoides, Raphidiopsis raciborskii, and Raphidiopsis mediterranea. In both sampling times, the blooms contained toxigenic species with genetic determinants for the production of cylindrospermopsin and microcystins. Toxicological screening revealed the presence of microcystins in the lake in summer but no cyanotoxins in the winter period of sampling. However, several cyanobacterial strains isolated from the lake during winter and summer produced anabaenopeptins and microcystins. This study indicates that summer and winter blooms of cyanobacteria in the temperate zone can differ in biomass, structure, and toxicity, and that the toxic hazards associated with cyanobacterial blooms may potentially exist during winter.

Peracetic acid (PAA) shows potential for use in drinking water treatment as an alternative to prechlorination, such as for mussel control and disinfection by-product precursor destruction, though its impact as a preoxidant during cyanobacterial blooms remains underexplored. Here, Microcystis aeruginosa inactivation and microcystin-LR and -RR release and degradation using PAA were explored. The toxin degradation rates were found to be higher in alkaline conditions than in neutral and acidic conditions. However, all rates were significantly smaller than comparable rates when using free chlorine. The inactivation of M. aeruginosa cells using PAA was faster at acidic pH, showing immediate cell damage and subsequent cell death after 15-60 min of exposure to 10 mg/L PAA. In neutral and alkaline conditions, cell death occurred after a longer lag phase (3-6 h). During cell inactivation, microcystin-LR was released slowly, with <35% of the initial intracellular toxins measured in solution after 12 h of exposure to 10 mg/L PAA. Overall, PAA appears impractically slow for M. aeruginosa cell inactivation or microcystin-LR and -RR destruction in drinking water treatment, but this slow reactivity may also allow it to continue to be applied as a preoxidant for other purposes during cyanobacterial blooms without the risk of toxin release.

Spatiotemporal patterns and drivers of hepatotoxic microcystins (MC) were investigated in the Atchafalaya-Vermilion Bay System (AVBS), a subtropical, river-dominated estuary in Louisiana. Along with environmental data, monthly particulate MC (pMC) samples were examined over a two-year period (2016-2018), and biweekly pMC and dissolved MC (dMC) samples were examined over a five-month period in 2020. Solid phase adsorption toxin tracking (SPATT) samplers used to quantify time-integrated dMC concentrations were also deployed in 2020. Low, but detectable concentrations of pMC (≤0.033 μg L-1) and dMC (≤0.190 μg L-1) were found throughout the AVBS in 37.8 and 21.2 % of samples, respectively. Time integrative SPATT samplers detected dMC in nearly 100 % of the deployments, compared to dMC detections in 30.8 % of the discrete samples. This study documents widespread MC presence throughout the AVBS and while concentrations were low, knowledge gaps remain regarding the potential long-term impacts of sublethal MC exposure to estuarine organisms.

Harmful algal blooms and the toxins produced during these events are a human and environmental health concern worldwide. Saxitoxin and its derivatives are potent natural aquatic neurotoxins produced by certain freshwater cyanobacteria and marine algae species during these bloom events. Saxitoxins effects on human health are well studied, however its effects on aquatic biota are still largely unexplored. This work aims at evaluating the effects of a pulse acute exposure (24 h) of the model cladoceran Daphnia magna to 30 μg saxitoxin L-1, which corresponds to the safety guideline established by the World Health Organization (WHO) for these toxins in recreational freshwaters. Saxitoxin effects were assessed through a comprehensive array of biochemical (antioxidant enzymes activity and lipid peroxidation), genotoxicity (alkaline comet assay), neurotoxicity (total cholinesterases activity), behavioral (swimming patterns), physiological (feeding rate and heart rate), and epigenetic (total 5-mC DNA methylation) biomarkers. Exposure resulted in decreased feeding rate, heart rate, total cholinesterases activity and catalase activity. Contrarily, other antioxidant enzymes, namely glutathione-S-transferases and selenium-dependent Glutathione peroxidase had their activity increased, together with lipid peroxidation levels. The enhancement of the antioxidant enzymes was not sufficient to prevent oxidative damage, as underpinned by lipid peroxidation enhancement. Accordingly, average DNA damage level was significantly increased in STX-exposed daphnids. Total DNA 5-mC level was significantly decreased in exposed organisms. Results showed that even a short-term exposure to saxitoxin causes significant effects on critical molecular and cellular pathways and modulates swimming patterns in D. magna individuals. This study highlights sub-lethal effects caused by saxitoxin in D. magna, suggesting that these toxins may represent a marked challenge to their thriving even at a concentration deemed safe for humans by the WHO.

Microcystins (MCs) can be detected in various matrices in two forms: a freely extractable fraction and a total (free and covalently protein-bound) fraction. Although the majority of MCs analyses are limited to the free fraction, they do not allow the analysis of all MCs variants or protein-bound forms. Other methods, known as total MCs analysis methods, enable simultaneous analysis of all MCs variants, as well as bound forms, which may be a major form of toxin accumulation in organisms. Among these techniques, the chemical oxidation method (e.g. Lemieux) allows the detection of total forms of MC (and nodularins) by oxidizing the common part to all MC and nodularins, and analyzing the resultant MMPB product (2-methyl-3-methoxy-4-phenylbutyric acid). However, the execution of this method in the context of health monitoring is challenging due to the variability of the protocols, the recoveries obtained with these protocols, and the important matrix effects associated with the method. The objectives of this study were i) to optimize an existing protocol of chemical oxidation \"Lemieux1\" on fresh fish fillet matrices, ii) to compare two existing protocols (\"Lemieux1\" and \"Lemieux2\"), and iii) apply Lemieux oxidation to fish fillets and livers naturally contaminated with MCs-producing cyanobacteria and to freshwater mussels contaminated with MCs in laboratories. Optimization of the \"Lemieux1\" protocol, in particular in the oxidation and SPE (solid phase extraction) steps improved the method\'s yields on the fresh fish fillet matrix (from <5 % to around 40 %). Moreover, several quantification methods have been compared through various calibration techniques (solvent calibration curve, matrix-matched calibration curve, oxidized MC-LR calibration curve and also by testing the addition of d3-MMPB as an internal standard). Comparison with the \"Lemieux2\" protocol showed the best results on the same matrix, with yields of around 65 %. MMPB was analyzed using this \"Lemieux 2\" protocol, in livers of carps sampled during an episode of cyanobacteria proliferation, at concentrations ranging from 17.9 to 27.5 μg/kg MMPB and at concentrations ranging from 50 to 2890 μg/kg MMPB in freshwater mussels laboratory contaminated to MCs.

Cyanobacteria in water supplies are considered an emerging threat, as some species produce toxic metabolites, cyanotoxins, of which the most widespread and well-studied are microcystins. Consumption of contaminated water is a common exposure route to cyanotoxins, making the study of cyanobacteria in drinking waters a priority to protect public health. In drinking water treatment plants, pre-oxidation with chlorinated compounds is widely employed to inhibit cyanobacterial growth, although concerns on its efficacy in reducing cyanotoxin content exists. Additionally, the effects of chlorination on abundant but less-studied cyanometabolites (e.g. cyanopeptolins whose toxicity is still unclear) remain poorly investigated. Here, two chlorinated oxidants, sodium hypochlorite (NaClO) and chlorine dioxide (ClO2), were tested on the toxic cyanobacterium Microcystis aeruginosa, evaluating their effect on cell viability, toxin profile and content. Intra- and extracellular microcystins and other cyanometabolites, including their degradation products, were identified using an untargeted LC-HRMS approach. Both oxidants were able to inactivate M. aeruginosa cells at a low dose (0.5 mg L-1), and greatly reduced intracellular toxins content (>90%), regardless of the treatment time (1-3 h). Conversely, a two-fold increase of extracellular toxins after NaClO treatment emerged, suggesting a cellular damage. A novel metabolite named cyanopeptolin-type peptide-1029, was identified based on LC-HRMSn (n = 2, 3) evidence, and it was differently affected by the two oxidants. NaClO led to increase its extracellular concentration from 2 to 80-100 μg L-1, and ClO2 induced the formation of its oxidized derivative, cyanopeptolin-type peptide-1045. In conclusion, pre-oxidation treatments of raw water contaminated by toxic cyanobacteria may lead to increased cyanotoxin concentrations in drinking water and, depending on the chemical agent, its dose and treatment duration, also of oxidized metabolites. Since the effects of such metabolites on human health remain unknown, this issue should be handled with extreme caution by water security agencies involved in drinking water management.

Harmful cyanobacterial blooms will be more intense and frequent in the future, contaminating surface waters with cyanotoxins and posing a threat to communities heavily reliant on surface water usage for crop irrigation. Constructed wetlands (CWs) are proposed to ensure safe crop irrigation, but more research is needed before implementation. The present study operated 28 mesocosms in continuous mode mimicking horizontal sub-surface flow CWs. Mesocosms were fed with synthetic lake water and spiked periodically with two cyanotoxins, microcystin-LR (MC-LR) and cylindrospermopsin (CYN), at environmentally relevant cyanotoxins concentrations (10 μg L-1). The influence of various design factors, including plant species, porous media, and seasonality, was explored. The mesocosms achieved maximum MC-LR and CYN mass removal rates of 95 % and 98 %, respectively. CYN removal is reported for the first time in CWs mimicking horizontal sub-surface flow CWs. Planted mesocosms consistently outperformed unplanted mesocosms, with Phragmites australis exhibiting superior cyanotoxin mass removal compared to Juncus effusus. Considering evapotranspiration, J. effusus yielded the least cyanotoxin-concentrated effluent due to the lower water losses in comparison with P. australis. Using the P-kC* model, different scaling-up scenarios for future piloting were calculated and discussed. Additionally, bacterial community structure was analyzed through correlation matrices and differential taxa analyses, offering valuable insights into their removal of cyanotoxins. Nevertheless, attempts to validate microcystin-LR biotransformation via the known mlrA gene degradation pathway were unfruitful, indicating alternative enzymatic degradation pathways occurring in such complex CW systems. Further investigation into the precise molecular mechanisms of removal and the identification of transformation products is needed for the comprehensive understanding of cyanotoxin mitigation in CW. This study points towards the feasibility of horizontal sub-surface flow CWs to be employed to control cyanotoxins in irrigation or recreational waters.

Algal blooms can threaten human health if cyanotoxins such as microcystin are produced by cyanobacteria. Regularly monitoring microcystin concentrations in recreational waters to inform management action is a tool for protecting public health; however, monitoring cyanotoxins is resource- and time-intensive. Statistical models that identify waterbodies likely to produce microcystin can help guide monitoring efforts, but variability in bloom severity and cyanotoxin production among lakes and years makes prediction challenging. We evaluated the skill of a statistical classification model developed from water quality surveys in one season with low temporal replication but broad spatial coverage to predict if microcystin is likely to be detected in a lake in subsequent years. We used summertime monitoring data from 128 lakes in Iowa (USA) sampled between 2017 and 2021 to build and evaluate a predictive model of microcystin detection as a function of lake physical and chemical attributes, watershed characteristics, zooplankton abundance, and weather. The model built from 2017 data identified pH, total nutrient concentrations, and ecogeographic variables as the best predictors of microcystin detection in this population of lakes. We then applied the 2017 classification model to data collected in subsequent years and found that model skill declined but remained effective at predicting microcystin detection (area under the curve, AUC ≥ 0.7). We assessed if classification skill could be improved by assimilating the previous years\' monitoring data into the model, but model skill was only minimally enhanced. Overall, the classification model remained reliable under varying climatic conditions. Finally, we tested if early season observations could be combined with a trained model to provide early warning for late summer microcystin detection, but model skill was low in all years and below the AUC threshold for two years. The results of these modeling exercises support the application of correlative analyses built on single-season sampling data to monitoring decision-making, but similar investigations are needed in other regions to build further evidence for this approach in management application.