Stability of microcystins from cyanobacteria: effect of light on decomposition and isomerization

Photocatalytic degradation of microcystins from a field-collected cyanobacterial assemblage by 3D printed TiO2 structures using artificial versus solar irradiation

Microcystins (MCs) from freshwater cyanobacteria cause adverse effects to humans and ecological receptors through multiple exposure routes requiring adaptable and diverse treatment technologies. Photocatalysis of MCs using TiO2 is a promising technology; however, TiO2 photocatalysts as unbound nanoparticles in suspension are impractical to deploy. 3D Printing (3DP) provides a means to immobilize TiO2, producing deployable photocatalyst structures with extensive geometric freedom. The objective of this proof-of-concept experiment was to incrementally increase the environmental complexity (e.g., broad-spectrum fluorescent lamps and outdoor solar; filtered cell lysate and algal assemblage as the source of MCs) while comparing photocatalysis rates of MCs by 3DP TiO2 structures using polylactic acid (PLA) as the binder. Degradation half-lives of MCs were shorter in TiO2 embedded in 3DP PLA relative to PLA-only controls with differences in half-lives ranging from 3.6 to 10h. The one exception was the outdoor solar and an algal assemblage, where significant differences could not be discerned due to the already rapid photolysis rates. Ultimately, photocatalysis rates (half-life = 1.9-11.6h) were comparable to those previously published for TiO2 3DP structures in a laboratory environment and TiO2 fixed-films (half-life = 2-13h) demonstrating feasibility of 3DP to immobilize TiO2 photocatalysts under a range of conditions. This is the first time that MC concentrations from a field-collected HAB were photocatalytically degraded in both solar simulated light and sunlight using a custom-made advanced photocatalytic nano-composite with enhanced performance through high surface area design enabled by 3D printing. These data inform future development of scalable, retrievable, and operationally flexible structures with immobilized TiO2.

Optimization of isolation and concentration of the common freshwater cyanobacterial toxins ATX-a, CYN and MC-LR using standard techniques, optimization of cyanobacteria growth

Some of the most commonly identified freshwater toxins are anatoxin-a (ATX-a), cylindrospermopsin (CYN), and microcystin-LR (MC-LR). The aim of this paper was to compare different methods of extracting and concentrating these cyanotoxins and check the impact of selected physical factors on the accumulation of biomass of Dolichospermum flos-aquae, Microcystis aeruginosa, and Raphidiopsis raciborskii. The effect of different cyanobacteria cultivation conditions on the amount of cyanotoxins synthesized showed no significant changes over time in the average concentration of all tested toxins in the medium compared to the control. Mixing cultures increases the intracellular content of ATX-a. Aerating also positively affects the concentration of MC-LR intracellularly. In order to optimize the solid phase extraction (SPE) process of toxins, the C18 phase or activated carbon was used. In general, higher toxin recoveries were achieved when using the C18 phase. The best result was achieved for ATX-a, 94% recovery with elution using methanol with 0.1% trifluoroacetic acid (TFA). For MC-LR, the best recovery was 59%, and for CYN 22%. The study evaluated the various methods to release cyanotoxins from cyanobacteria showed that: the highest ATX-a concentration (0.60 μg/mg d.w) was obtained using MilliQ water and microwave treatment for 10 to 15 seconds. For MC-LR, the highest extracted amount (6.73 μg/mg d.w) resulted from methanol treatment and boiling at 100°C for 15 minutes. CYN extraction was the most effective by using MilliQ water and alternative freezing/thawing (1.54 μg/mg d.w). In conclusion, changing the optimal parameters of cyanobacterial cultivation, only slightly affects the increase in biomass accumulation and synthesis of cyanobacterial toxins. In the case of ATX, the key is the use of the TFA additive in the SPE process. No single method has been identified as the ideal approach for isolating various intracellular cyanotoxins.

Aerosolized algal bloom toxins are not inert

Harmful algal blooms (HABs) are projected to become increasingly prevalent, extending over longer periods and wider geographic regions due to the warming surface ocean water and other environmental factors, including but not limited to nutrient concentrations and runoff for marine and freshwater environments. Incidents of respiratory distress linked to the inhalation of marine aerosols containing HAB toxins have been documented, though the risk is typically associated with the original toxins. However, aerosolized toxins in micrometer and submicrometer particles are vulnerable to atmospheric processing. This processing can potentially degrade HAB toxins and produce byproducts with varying potencies compared to the parent toxins. The inhalation of aerosolized HAB toxins, especially in conjunction with co-morbid factors such as exposure to air pollutants from increased commercial activities in ports, may represent a significant exposure pathway for a considerable portion of the global population. Understanding the chemistry behind the transformation of these toxins can enhance public protection by improving the existing HAB alert systems.

Identification of a homoarginine biosynthetic gene from a microcystin biosynthetic pathway in Fischerella sp. PCC 9339

Microcystins (MCs) are a family of chemically diverse toxins produced by numerous distantly related cyanobacteria. They are potent inhibitors of eukaryotic protein phosphatases 1 and 2A and are responsible for the toxicosis and death of wild and domestic animals around the world. Microcystins are synthesized on large enzyme complexes comprised of peptide synthetases, polyketide synthases, and additional modifying enzymes. Bioinformatic analysis identified the presence of an additional uncharacterized enzyme in the microcystin (mcy) biosynthetic gene cluster in Fischerella sp. PCC 9339, which we named McyK, that lacked a clearly defined role in the biosynthesis of microcystin. Further bioinformatic analysis suggested that McyK belongs to the inosamine-phosphate amidinotransferase family and could be involved in synthesizing homo amino acids. Quadrupole time-of-flight tandem mass spectrometry (Q-TOFMS/MS) analysis confirmed that Fischerella sp. PCC 9339 produces MC-Leucine2-Homoarginine4(MC-LHar) and [Aspartic acid3]MC-Leucine2-Homoarginine4 ([Asp3]MC-LHar) as the dominant chemical variants. We hypothesized that the McyK enzyme might be involved in the production of microcystin variants containing homoarginine (Har) in the strain. Heterologous expression of a codon-optimized mcyK gene in Escherichia coli confirmed that McyK is responsible for the synthesis of L-Har. These results confirm the production of MC-LHar, a novel microcystin chemical variant [Asp3]MC-LHar, and a new microcystin biosynthetic enzyme involved in supply of the rare homo-amino acid Har to the microcystin biosynthetic pathway in Fischerella sp. PCC 9339. This study provides new insights into the logic underpinning the biosynthesis of microcystin chemical variants and broadens our knowledge of structural diversity of the microcystin family of toxins.

Characterizing the Impact of Cyanobacterial Blooms on the Photoreactivity of Surface Waters from New York Lakes: A Combined Statewide Survey and Laboratory Investigation

Cyanobacterial blooms introduce autochthonous dissolved organic matter (DOM) into aquatic environments, but their impact on surface water photoreactivity has not been investigated through collaborative field sampling with comparative laboratory assessments. In this work, we quantified the apparent quantum yields (Φapp,RI) of reactive intermediates (RIs), including excited triplet states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), for whole water samples collected by citizen volunteers from more than 100 New York lakes. Multiple comparisons tests and orthogonal partial least-squares analysis identified the level of cyanobacterial chlorophyll a as a key factor in explaining the enhanced photoreactivity of whole water samples sourced from bloom-impacted lakes. Laboratory recultivation of bloom samples in bloom-free lake water demonstrated that apparent increases in Φapp,RI during cyanobacterial growth were likely driven by the production of photoreactive moieties through the heterotrophic transformation of freshly produced labile bloom exudates. Cyanobacterial proliferation also altered the energy distribution of 3DOM* and contributed to the accelerated transformation of protriptyline, a model organic micropollutant susceptible to photosensitized reactions, under simulated sunlight conditions. Overall, our study provides insights into the relationship between the photoreactivity of surface waters and the limnological characteristics and trophic state of lakes and highlights the relevance of cyanobacterial abundance in predicting the photoreactivity of bloom-impacted surface waters.

Comparative genomic analysis and functional investigations for MCs catabolism mechanisms and evolutionary dynamics of MCs-degrading bacteria in ecology

Microcystins (MCs) significantly threaten the ecosystem and public health. Biodegradation has emerged as a promising technology for removing MCs. Many MCs-degrading bacteria have been identified, including an indigenous bacterium Sphingopyxis sp. YF1 that could degrade MC-LR and Adda completely. Herein, we gained insight into the MCs biodegradation mechanisms and evolutionary dynamics of MCs-degrading bacteria, and revealed the toxic risks of the MCs degradation products. The biochemical characteristics and genetic repertoires of strain YF1 were explored. A comparative genomic analysis was performed on strain YF1 and six other MCs-degrading bacteria to investigate their functions. The degradation products were investigated, and the toxicity of the intermediates was analyzed through rigorous theoretical calculation. Strain YF1 might be a novel species that exhibited versatile substrate utilization capabilities. Many common genes and metabolic pathways were identified, shedding light on shared functions and catabolism in the MCs-degrading bacteria. The crucial genes involved in MCs catabolism mechanisms, including mlr and paa gene clusters, were identified successfully. These functional genes might experience horizontal gene transfer events, suggesting the evolutionary dynamics of these MCs-degrading bacteria in ecology. Moreover, the degradation products for MCs and Adda were summarized, and we found most of the intermediates exhibited lower toxicity to different organisms than the parent compound. These findings systematically revealed the MCs catabolism mechanisms and evolutionary dynamics of MCs-degrading bacteria. Consequently, this research contributed to the advancement of green biodegradation technology in aquatic ecology, which might protect human health from MCs.

Biodegradation of MC-LR and its key bioactive moiety Adda by Sphingopyxis sp. YF1: Comprehensive elucidation of the mechanisms and pathways

Microcystins (MCs) are harmful to the ecology and public health. Some bacteria can degrade MCs into Adda, but few can destroy Adda. Adda is the key bioactive moiety of MCs and mainly contributes to hepatotoxicity. We had previously isolated an indigenous novel bacterial strain named Sphingopyxis sp. YF1 that can efficiently degrade MCs and its key bioactive moiety Adda, but the mechanisms remained unknown. Here, the biodegradation mechanisms and pathways of Adda were systematically investigated using multi-omics analysis, mass spectrometry and heterologous expression. The transcriptomic and metabolomic profiles of strain YF1 during Adda degradation were revealed for the first time. Multi-omics analyses suggested that the fatty acid degradation pathway was enriched. Specifically, the expression of genes encoding aminotransferase, beta oxidation (β-oxidation) enzymes and phenylacetic acid (PAA) degradation enzymes were significantly up-regulated during Adda degradation. These enzymes were further proven to play important roles in the biodegradation of Adda. Simultaneously, some novel potential degradation products of Adda were identified successfully, including 7‑methoxy-4,6-dimethyl-8-phenyloca-2,4-dienoic acid (C₁₇H₂₂O₃), 2-methyl-3‑methoxy-4-phenylbutyric acid (C₁₂H₁₆O₃) and phenylacetic acid (PAA, C₈H₈O₂). In summary, the Adda was converted into PAA through aminotransferase and β-oxidation enzymes, then the PAA was further degraded by PAA degradation enzymes, and finally to CO₂ via the tricarboxylic acid cycle. This study comprehensively elucidated the novel MC-LR biodegradation mechanisms, especially the new enzymatic pathway of Adda degradation. These findings provide a new perspective on the applications of microbes in the MCs polluted environment.

The monthly variation tendency of microcystin-LR levels in the Huangpu River (China) by applications of ELISA and HPLC

In this study, the contents of microcystin-LR (MC-LR) of Microcystis aeruginosa cultures in the laboratory and natural water samples from the Huangpu River in different seasons were detected through enzyme-linked immunosorbent assay (ELISA) and high-performance liquid chromatography (HPLC), respectively. Excellent correlation between the two methods was obtained (R² > 0.99). ELISA was a reliable and simple method with high reproducibility (coefficient of variation < 25%) and satisfactory recovery for the monitoring of low levels of MC-LR. MC-LR concentrations in Huangpu River varied with the seasonal variation, which peaked in August with the temperature over 30 °C and then gradually declined with the decreasing temperature after August. The highest MC-LR concentration in the Huangpu River was below the WHO drinking water quality standard (1 µg/L). These results indicated that warm temperature accelerated the MC-LR synthesis and release, and it is necessary to regularly monitor the MC-LR levels, especially during the high algae period in summer. ELISA can be applied to detect the low levels of MC-LR in the field without complex treatment, avoiding the samples from denaturation and degradation during the transportation. Hence, ELISA is a better alternative of HPLC when HPLC is unavailable, especially when rapid testing is required in routine MC-LR analysis.

Microcystin Contamination and Toxicity: Implications for Agriculture and Public Health

Microcystins are natural hepatotoxic metabolites secreted by cyanobacteria in aquatic ecosystems. When present at elevated concentrations, microcystins can affect water quality aesthetics; contaminate drinking water reservoirs and recreational waters; disrupt normal ecosystem functioning; and cause health hazards to animals, plants, and humans. Animal and human exposures to microcystins generally result from ingesting contaminated drinking water or physically contacting tainted water. Much research has identified a multitude of liver problems from oral exposure to microcystins, varying from hepatocellular damage to primary liver cancer. Provisional guidelines for microcystins in drinking and recreational water have been established to prevent toxic exposures and protect public health. With increasing occurrences of eutrophication in freshwater systems, microcystin contamination in groundwater and surface waters is growing, posing threats to aquatic and terrestrial plants and agricultural soils used for crop production. These microcystins are often transferred to crops via irrigation with local sources of water, such as bloom-forming lakes and ponds. Microcystins can survive in high quantities in various parts of plants (roots, stems, and leaves) due to their high chemical stability and low molecular weight, increasing health risks for consumers of agricultural products. Studies have indicated potential health risks associated with contaminated fruits and vegetables sourced from irrigated water containing microcystins. This review considers the exposure risk to humans, plants, and the environment due to the presence of microcystins in local water reservoirs used for drinking and irrigation. Additional studies are needed to understand the specific health impacts associated with the consumption of microcystin-contaminated agricultural plants.

Multiple pathways for the anaerobic biodegradation of microcystin-LR in the enriched microbial communities from Lake Taihu

Anaerobic biodegradation is a non-negligible elimination approach for microcystin (MC) pollution and exhibits important bioremediation potential for environmental problems. However, the specific anaerobic MC-degrading mechanism remains unclear and few functional bacteria have been found. In this study, three microbial communities of sludges from different locations in Lake Taihu were collected and further enriched by microcystin-LR (MC-LR) under anaerobic conditions. MC-LR (1 mg/L) could be completely degraded by these enriched microbial communities under anaerobic conditions, but their degradation rates were significantly different. In addition, two different ring-opening sites of MC-LR in Ala-Leu and Arg-Adda were observed, and three new anaerobic degradation products were first identified, including two hexapeptides (MeAsp-Arg-Adda-Glu-Mdha-Ala and Adda-Glu-Mdha-Ala-Leu-MeAsp) and one end-product pentapeptide (Glu-Mdha-Ala-Leu-MeAsp). Based on the chemical structures and temporal trends of all detected degradation products, two novel anaerobic biodegradation pathways of MC-LR were proposed. Moreover, the MC-degrading genes mlrABC were not detected among all microbial communities, which suggested that some new MC-degrading mechanisms might exist under anaerobic conditions. Finally, through the comparison of microbial community structure, Gemmatimonas and Smithella were deduced as possible anaerobic MC-degrading bacteria. These findings strongly indicate that anaerobic biodegradation is an important method of self-repair in the natural environment and provides a potential removal strategy for MC pollution.

Removal of microcystins from water and primary treatment technologies - A comprehensive understanding based on bibliometric and content analysis, 1991-2020

Microcystins are a group of heptapeptide hepatotoxins produced by a variety of algae and are frequently detected in aquatic ecosystems, posing a global threat to ecological stability and human health. However, it is difficult to eliminate them completely and innocuously from water by conventional water treatment processes. This study comprehensively evaluated a total of 821 original articles retrieved from the Web of Science (1991-2020) about the removal of microcystins using bibliometric and content analysis to provide a qualitative and quantitative research landscape and a global view of research hotspots and future research directions. Furthermore, the primary and promising treatment technologies for microcystin pollution were also summarized and discussed. The results indicated an urgent practical demand to remediate microcystin pollution according to the increasing number of publications since 2005. China had the highest number of publications, whereas the United States was the core country in the international collaboration network. The Chinese Academy of Sciences and University of Cincinnati showed their leading positions considering article amounts and academic cooperation. Dionysiou DD contributed the most articles, and Carmichael WW had the highest number of co-citations. Three treatment technologies, including biodegradation, chemical oxidation and adsorption, were the major strategies to remediate the pollution of microcystins in water. In addition, the toxicity of toxins/their metabolites, degradation kinetics, and elimination mechanism were also important research contents. Bacterial degradation, photocatalytic degradation, and multiple-technologies approach have been identified with great potential and should be given more attention in future studies. This work summarizes the current research status on microcystin management, provides a valuable reference for researchers to identify potential opportunities for collaboration in related fields, and guides future research directions to inter-disciplinary and multi-perspective approaches.

Dynamics of the prokaryotic and eukaryotic microbial community during a cyanobacterial bloom

Toxic cyanobacterial blooms frequently develop in eutrophic freshwater bodies worldwide. Microcystis species produce microcystins (MCs) as a cyanotoxin. Certain bacteria that harbor the mlr gene cluster, especially mlrA, are capable of degrading MCs. However, MC-degrading bacteria may possess or lack mlr genes (mlr+ and mlr- genotypes, respectively). In this study, we investigated the genotype that predominantly contributes to biodegradation and cyanobacterial predator community structure with change in total MC concentration in an aquatic environment. The 2 genotypes coexisted but mlr+ predominated, as indicated by the negative correlation between mlrA gene copy abundance and total MC concentration. At the highest MC concentrations, predation pressure by Phyllopoda, Copepoda, and Monogononta (rotifers) was reduced; thus, MCs may be toxic to cyanobacterial predators. The results suggest that cooperation between MC-degrading bacteria and predators may reduce Microcystis abundance and MC concentration.

The Effects of Ferric Sulfate (Fe2(SO4)3) on the Removal of Cyanobacteria and Cyanotoxins: A Mesocosm Experiment

Cyanobacterial blooms are a global concern. Chemical coagulants are used in water treatment to remove contaminants from the water column and could potentially be used in lakes and reservoirs. The aims of this study was to: 1) assess the efficiency of ferric sulfate (Fe₂(SO₄)₃) coagulant in removing harmful cyanobacterial cells from lake water with cyanobacterial blooms on a short time scale, 2) determine whether some species of cyanobacteria can be selectively removed, and 3) determine the differential impact of coagulants on intra- and extra-cellular toxins. Our main results are: (i) more than 96% and 51% of total cyanobacterial cells were removed in mesocosms with applied doses of 35 mgFe/L and 20 mgFe/L, respectively. Significant differences in removing total cyanobacterial cells and several dominant cyanobacteria species were observed between the two applied doses; (ii) twelve microcystins, anatotoxin-a (ANA-a), cylindrospermopsin (CYN), anabaenopeptin A (APA) and anabaenopeptin B (APB) were identified. Ferric sulfate effectively removed the total intracellular microcystins (greater than 97% for both applied doses). Significant removal of extracellular toxins was not observed after coagulation with both doses. Indeed, the occasional increase in extracellular toxin concentration may be related to cells lysis during the coagulation process. No significant differential impact of dosages on intra- and extra-cellular toxin removal was observed which could be relevant to source water applications where optimal dosing is difficult to achieve.

Algicidal mechanism of Raoultella ornithinolytica against Microcystis aeruginosa: Antioxidant response, photosynthetic system damage and microcystin degradation

Water eutrophication caused by harmful algal blooms (HABs) occurs worldwide. It causes huge economic losses and has serious and potentially life-threatening effects on human health. In this study, the bacterium Raoultella sp. S1 with high algicidal efficiency against the harmful algae Microcystis aeruginosa was isolated from eutrophic water. The results showed that Raoultella sp. S1 initially flocculated the algae, causing the cells to sediment within 180 min and then secreted soluble algicidal substances that killed the algal cells completely within 72 h. The algicidal activity was stable across the temperature range -85.0 to 85.0 °C and across the pH range 3.00-11.00. Scanning electron microscopy (SEM) revealed the crumpling and fragmentation of cells algal cells during the flocculation and lysis stages. The antioxidant system was activated under conditions of oxidative stress, causing the increased antioxidant enzymes activities. Meanwhile, the oxidative stress response triggered by the algicidal substances markedly increased the malondialdehyde (MDA) and glutathione (GSH) content. We investigated the content of Chl-a and the relative expression levels of genes related to photosynthesis, verifying that the algicidal compounds attack the photosynthetic system by degrading the photosynthetic pigment and inhibiting the expression of key genes. Also, the results of photosynthetic efficiency and relative electric transport rate confirmed that the photosynthetic system in algal cells was severely damaged within 24 h. The algicidal effect of Raoultella sp. S1 against Microcystis aeruginosa was evaluated by analyzing the physiological response and photosynthetic system impairment of the algal cells. The concentration of microcystin-LR (MC-LR) slightly increased during the process of algal cells ruptured, and then decreased below its initial level due to the biodegradation of Raoultella sp. S1. To further investigate the algicidal mechanism of Raoultella sp. S1, the main components in the cell-free supernatant was analyzed by UHPLC-TOF-MS. Several low-molecular-weight organic acids might be responsible for the algicidal activity of Raoultella sp. S1. It is concluded that Raoultella sp. S1 has the potential to control Microcystis aeruginosa blooms.

Negative impacts of microcystin-LR and glyphosate on zebrafish intestine: Linked with gut microbiota and microRNAs?

Microcystin-LR (MC-LR) and glyphosate (GLY) have been classified as a Group 2B and Group 2A carcinogens for humans, respectively, and frequently found in aquatic ecosystems. However, data on the potential hazard of MC-LR and GLY exposure to the fish gut are relatively scarce. In the current study, a subacute toxicity test of zebrafish exposed to MC-LR (35 μg L-1) and GLY (3.5 mg L-1), either alone or in combination was performed for 21 d. The results showed that MC-LR or/and GLY treatment reduced the mRNA levels of tight junction genes (claudin-5, occludin, and zonula occludens-1) and altered the levels of diamine oxidase and D-lactic, indicating increased intestinal permeability in zebrafish. Furthermore, MC-LR and/or GLY treatment remarkably increased the levels of intestinal IL-1β and IL-8 but decreased the levels of IL-10 and TGF-β, indicating that MC-LR and/or GLY exposure induced an inflammatory response in the fish gut. MC-LR and/or GLY exposure also activated superoxide dismutase and catalase, generally upregulated the levels of p53, bax, bcl-2, caspase-3, and caspase-9, downregulated the levels of caspase-8 and caused notable histological injury in the fish gut. Moreover, MC-LR and/or GLY exposure also significantly altered the microbial community in the zebrafish gut and the expression of miRNAs (miR-146a, miR-155, miR-16, miR-21, and miR-223). Chronic exposure to MC-LR and/or GLY can induce intestinal damage in zebrafish, and this study is the first to demonstrate an altered gut microbiome and miRNAs in the zebrafish gut after MC-LR and GLY exposure.

A Brief Review of the Structure, Cytotoxicity, Synthesis, and Biodegradation of Microcystins

Harmful cyanobacterial blooms pose an environmental health hazard due to the release of water-soluble cyanotoxins. One of the most prevalent cyanotoxins in nature is microcystins (MCs), a class of cyclic heptapeptide hepatotoxins, and they are produced by several common cyanobacteria in aquatic environments. Once released from cyanobacterial cells, MCs are subjected to physical chemical and biological transformations in natural environments. MCs can also be taken up and accumulated in aquatic organisms and their grazers/predators and induce toxic effects in several organisms, including humans. This brief review aimed to summarize our current understanding on the chemical structure, exposure pathway, cytotoxicity, biosynthesis, and environmental transformation of microcystins.

Prevalence and persistence of microcystin in shoreline lake sediments and porewater, and associated potential for human health risk

Midlatitude waterbodies are experiencing increased cyanobacteria blooms that necessitate health advisories to protect waterbody users. Although surface waters may contain cyanotoxins such as microcystin (MC), at concentrations that pose potential public health risks, little is known about MC contamination of shoreline sediments. Based on growing evidence that lake and reservoir sediments can accumulate MCs, we hypothesized that shoreline sediments (i.e., recreational beaches) may accumulate MCs and thereby pose a potential health risk to recreational users even if people stay out of contaminated water. We sampled nearshore surface water, shoreline sediment, and porewater from seven Washington State, USA, lakes/reservoirs recreational beaches to determine MC presence/absence during or immediately following cyanobacteria blooms. We found MCs in shoreline sediments at all waterbodies using ELISA and LC-MS/MS. MC concentrations in shoreline sediments and porewaters persisted for 20 days following dissipation of cyanobacteria blooms when MC concentrations were near analytical reporting limits in corresponding surface waters. A human health risk assessment based on potential MC exposure through incidental ingestion of porewaters and sediments found, even when very high MC concentrations occur in surface waters (i.e., >11,000 μg/L), estimated ingestion doses are below MC World Health Organization tolerable daily intake and U.S. Environmental Protection Agency's risk reference dose. While our findings suggest MCs in Washington State recreational beaches in 2018 did not present a significant human health risk, future blooms with higher MC concentrations could pose human health risks via the shoreline sediment/porewater exposure pathway.

Algal Toxic Compounds and Their Aeroterrestrial, Airborne and other Extremophilic Producers with Attention to Soil and Plant Contamination: A Review

The review summarizes the available knowledge on toxins and their producers from rather disparate algal assemblages of aeroterrestrial, airborne and other versatile extreme environments (hot springs, deserts, ice, snow, caves, etc.) and on phycotoxins as contaminants of emergent concern in soil and plants. There is a growing body of evidence that algal toxins and their producers occur in all general types of extreme habitats, and cyanobacteria/cyanoprokaryotes dominate in most of them. Altogether, 55 toxigenic algal genera (47 cyanoprokaryotes) were enlisted, and our analysis showed that besides the “standard” toxins, routinely known from different waterbodies (microcystins, nodularins, anatoxins, saxitoxins, cylindrospermopsins, BMAA, etc.), they can produce some specific toxic compounds. Whether the toxic biomolecules are related with the harsh conditions on which algae have to thrive and what is their functional role may be answered by future studies. Therefore, we outline the gaps in knowledge and provide ideas for further research, considering, from one side, the health risk from phycotoxins on the background of the global warming and eutrophication and, from the other side, the current surge of interest which phycotoxins provoke due to their potential as novel compounds in medicine, pharmacy, cosmetics, bioremediation, agriculture and all aspects of biotechnological implications in human life.

Competitive interactions as a mechanism for chemical diversity maintenance in Nodularia spumigena

Nodularia spumigena is a bloom-forming diazotrophic cyanobacterium inhabiting brackish waters worldwide. This species produces non-ribosomal peptides (NRPs), including the hepatotoxin nodularin, often referred to as cyanotoxin. Several known classes of NRPs have various biological activities, although their modes of action are poorly understood. In the Baltic N. spumigena, there is a high NRP chemodiversity among strains, allowing their grouping in specific chemotypes and subgroups. Therefore, it is relevant to ask whether the NRP production is affected by intraspecific interactions between the co-existing strains. Using a novel approach that combines culture technique and liquid chromatography-tandem mass spectrometry for the NRP analysis, we examined N. spumigena strains under mono- and co-culture conditions. The test strains were selected to represent N. spumigena belonging to the same or different chemotype subgroups. In this setup, we observed physiological and metabolic responses in the test strains grown without cell contact. The changes in NRP levels to co-culture conditions were conserved within a chemotype subgroup but different between the subgroups. Our results suggest that intraspecific interactions may promote a chemical diversity in N. spumigena population, with higher NRP production compared to a single-strain population. Studying allelochemical signalling in this cyanobacterium is crucial for understanding toxicity mechanisms and plankton community interactions in the Baltic Sea and other aquatic systems experiencing regular blooms.

Overview of toxic cyanobacteria and cyanotoxins in Ibero-American freshwaters: Challenges for risk management and opportunities for removal by advanced technologies

The increasing occurrence of cyanobacterial blooms worldwide represents an important threat for both the environment and public health. In this context, the development of risk analysis and management tools as well as sustainable and cost-effective treatment processes is essential. The research project TALGENTOX, funded by the Ibero-American Science and Technology Program for Development (CYTED-2019), aims to address this ambitious challenge in countries with different environmental and social conditions within the Ibero-American context. It is based on a multidisciplinary approach that combines ecology, water management and technology fields, and includes research groups from Chile, Colombia, Mexico, Peru and Spain. In this review, the occurrence of toxic cyanobacteria and cyanotoxins in freshwaters from these countries are summarized. The presence of cyanotoxins has been confirmed in all countries but the information is still scarce and further monitoring is required. In this regard, remote sensing or metagenomics are good alternatives at reasonable cost. The risk management of freshwaters from those countries considering the most frequent uses (consumption and recreation) has been also evaluated. Only Spain and Peru include cyanotoxins in its drinking water legislation (only MC-LR) and thus, there is a need for regulatory improvements. The development of preventive strategies like diminishing nutrient loads to aquatic systems is also required. In the same line, corrective measures are urgently needed especially in drinking waters. Advanced Oxidation Processes (AOPs) have the potential to play a major role in this scenario as they are effective for the elimination of most cyanotoxins classes. The research on the field of AOPs is herein summarized considering the cost-effectiveness, environmental character and technical applicability of such technologies. Fenton-based processes and photocatalysis using solar irradiation or LED light represent very promising alternatives given their high cost-efficiency. Further research should focus on developing stable long-term operation systems, addressing their scale-up.

Photodegradation of cyanotoxins in surface waters

Cyanotoxin-producing harmful algal blooms (HABs) are a global occurrence and pose ecotoxicological threats to humans and animals alike. The presence of cyanotoxins can seriously harm or kill nearby wildlife and restrict a body of water's use as a drinking water supply and recreational site, making it imperative to fully understand their fate and transport in natural waters. Photodegradation contributes to the overall degradation of cyanotoxins in environmental systems, especially for those present in the photic zone of surface waters. This makes photochemical transformation mechanisms important factors to account for when assessing the persistence of cyanotoxins in environmental systems. This paper reviews current knowledge on the photodegradation rates and pathways of cyanotoxins that can occur over the course of HABs. Sensitized, or indirect, photolysis contributes to the degradation of all cyanotoxins addressed in this paper (anatoxins, cylindrospermopsins, domoic acids, microcystins, and nodularins), with hydroxyl radicals (•OH), excited triplet states formed from the absorption of light by dissolved organic matter (³DOM*), and photosynthetic pigment sensitized pathways being of primary interest. Direct photolysis pathways play a less significant role, but are still relevant for most of the cyanotoxins discussed in this paper.

Cytotoxic and immunological responses of fish leukocytes to nodularin exposure in vitro

Nodularin (NOD) is a cyclic peptide released by bloom-forming toxic cyanobacteria Nodularia spumigena commonly occurring in brackish waters throughout the world. Although its hepatotoxic effects are well known, other negative effects of NOD have not yet been completely elucidated. The present study aims were to evaluate and compare the cytotoxic and immunotoxic effects of the toxin on primary leukocytes (from head kidney [HK]) and stable fish leukocytes (carp leucocyte cell line [CLC] cells). The cells were incubated with the cyanotoxin at concentrations of 0.001, 0.01, 0.05, or 0.1 μg/ml. After 24 h of exposure, the concentrations ≥0.05 μg/ml of toxin resulted in cytotoxicity in the primary cells, while in CLC cells, the toxic effect was obtained only with the highest concentration. Similarly, depending on the concentration, exposure to NOD causes a significant inhibition of chemotaxis of the phagocytic abilities of primary leukocytes and a significant reduction in the proliferation of lymphocytes isolated from the HKs. Moreover, CLC cells and HK leukocytes incubated with this toxin at all the mentioned concentrations showed an increased production of reactive oxygen and nitrogen species. NOD also evidently influenced the expression of genes of cytokine TNF-α and IL-10 and, to a minor extent, IL-1β and TGF-β. Notably, the observed changes in the mRNA levels of cytokines in NOD-exposed cells were evident, but not clearly dose-dependent. Interestingly, NOD did not affect the production and release of IL-1β of the CLC cells. This study provides evidence that NOD may exert cytotoxicity and immune-toxicity effects depending on cell type and toxin concentration.

Cyanotoxins in drinking water supply reservoir (Legedadi, Central Ethiopia): implications for public health safety

Cyanobacterial blooms in drinking water supply affect its quality, which ultimately impacts ecosystem and public health. Thus, this cross-sectional study was conducted to perform a preliminary study on cyanotoxins via analysis of samples collected only once from two sites during the month of peak algal bloom and to subsequently prompt a comprehensive risk assessment in a major drinking water source, Legedadi Reservoir, of Addis Ababa, the capital city of Ethiopia. Samples were collected during peak algal bloom month (January 2018) from two sampling sites, near the dam (S1) and at the center of the reservoir (S2). Identification and enumeration of phytoplankton taxa were done and the measurement of common hepatotoxin (MCs and NOD) concentrations was conducted using liquid chromatography-tandem mass spectrometry. In the reservoir, cyanobacteria made up to 98% of total phytoplankton abundance, with Dolichospermum and Microcystis spp, dominating the phytoplankton community. In these first cyanotoxin analyses conducted for a drinking water supply source in Ethiopia, six major MC variants, namely MC-dmRR, MC-RR, MC-YR, MC-dmLR, MC-LR, and MC-LA, were detected in both algal seston and water samples. MC-LR was the most dominant MCs variant, while nodularin was not detected for both sampling sites. Extracellular total MC concentrations (μg L−1) of 453.89 and 61.63 and intracellular total MC concentrations (μg L−1) of 189.29 and 112.34 were recorded for samples from S1 and S2, respectively. The high concentrations of extracellular MCs, with MC-LR constituting the greatest proportion, indicate the extremely high potential public health risk for end-users.

Marine harmful algal blooms (HABs) in the United States: History, current status and future trends

Harmful algal blooms (HABs) are diverse phenomena involving multiple. species and classes of algae that occupy a broad range of habitats from lakes to oceans and produce a multiplicity of toxins or bioactive compounds that impact many different resources. Here, a review of the status of this complex array of marine HAB problems in the U.S. is presented, providing historical information and trends as well as future perspectives. The study relies on thirty years (1990-2019) of data in HAEDAT - the IOC-ICES-PICES Harmful Algal Event database, but also includes many other reports. At a qualitative level, the U.S. national HAB problem is far more extensive than was the case decades ago, with more toxic species and toxins to monitor, as well as a larger range of impacted resources and areas affected. Quantitatively, no significant trend is seen for paralytic shellfish toxin (PST) events over the study interval, though there is clear evidence of the expansion of the problem into new regions and the emergence of a species that produces PSTs in Florida - Pyrodinium bahamense. Amnesic shellfish toxin (AST) events have significantly increased in the U.S., with an overall pattern of frequent outbreaks on the West Coast, emerging, recurring outbreaks on the East Coast, and sporadic incidents in the Gulf of Mexico. Despite the long historical record of neurotoxic shellfish toxin (NST) events, no significant trend is observed over the past 30 years. The recent emergence of diarrhetic shellfish toxins (DSTs) in the U.S. began along the Gulf Coast in 2008 and expanded to the West and East Coasts, though no significant trend through time is seen since then. Ciguatoxin (CTX) events caused by Gambierdiscus dinoflagellates have long impacted tropical and subtropical locations in the U.S., but due to a lack of monitoring programs as well as under-reporting of illnesses, data on these events are not available for time series analysis. Geographic expansion of Gambierdiscus into temperate and non-endemic areas (e.g., northern Gulf of Mexico) is apparent, and fostered by ocean warming. HAB-related marine wildlife morbidity and mortality events appear to be increasing, with statistically significant increasing trends observed in marine mammal poisonings caused by ASTs along the coast of California and NSTs in Florida. Since their first occurrence in 1985 in New York, brown tides resulting from high-density blooms of Aureococcus have spread south to Delaware, Maryland, and Virginia, while those caused by Aureoumbra have spread from the Gulf Coast to the east coast of Florida. Blooms of Margalefidinium polykrikoides occurred in four locations in the U.S. from 1921-2001 but have appeared in more than 15  U.S. estuaries since then, with ocean warming implicated as a causative factor. Numerous blooms of toxic cyanobacteria have been documented in all 50  U.S. states and the transport of cyanotoxins from freshwater systems into marine coastal waters is a recently identified and potentially significant threat to public and ecosystem health. Taken together, there is a significant increasing trend in all HAB events in HAEDAT over the 30-year study interval. Part of this observed HAB expansion simply reflects a better realization of the true or historic scale of the problem, long obscured by inadequate monitoring. Other contributing factors include the dispersion of species to new areas, the discovery of new HAB poisoning syndromes or impacts, and the stimulatory effects of human activities like nutrient pollution, aquaculture expansion, and ocean warming, among others. One result of this multifaceted expansion is that many regions of the U.S. now face a daunting diversity of species and toxins, representing a significant and growing challenge to resource managers and public health officials in terms of toxins, regions, and time intervals to monitor, and necessitating new approaches to monitoring and management. Mobilization of funding and resources for research, monitoring and management of HABs requires accurate information on the scale and nature of the national problem. HAEDAT and other databases can be of great value in this regard but efforts are needed to expand and sustain the collection of data regionally and nationally.

A Mini-Review on Detection Methods of Microcystins

Cyanobacterial harmful algal blooms (CyanoHABs) produce microcystins (MCs) which are associated with animal and human hepatotoxicity. Over 270 variants of MC exist. MCs have been continually studied due of their toxic consequences. Monitoring water quality to assess the presence of MCs is of utmost importance although it is often difficult because CyanoHABs may generate multiple MC variants, and their low concentration in water. To effectively manage and control these toxins and prevent their health risks, sensitive, fast, and reliable methods capable of detecting MCs are required. This paper aims to review the three main analytical methods used to detect MCs ranging from biological (mouse bioassay), biochemical (protein phosphatase inhibition assay and enzyme linked immunosorbent assay), and chemical (high performance liquid chromatography, liquid chromatography-mass spectrometry, high performance capillary electrophoresis, and gas chromatography), as well as the newly emerging biosensor methods. In addition, the current state of these methods regarding their novel development and usage, as well as merits and limitations are presented. Finally, this paper also provides recommendations and future research directions towards method application and improvement.

A critical review of ionizing radiation technologies for the remediation of waters containing Microcystin-LR and M. aeruginosa

Harmful algal and cyanobacterial blooms pose threats to human and ecological health due to their release of hazardous toxins. Microcystin-LR (MC-LR), a potent hepatotoxin, is the most prevalent cyanotoxin found in freshwater blooms. Although produced by many species of cyanobacteria, Microcystis aeruginosa is most commonly associated with MC-LR production. These blooms are increasing in occurrence in lakes, ponds, and other surface waters and, therefore, require efficient treatment methods to be removed from water supplies. Ionizing radiation technologies offer promising approaches for the removal of organic pollutants in water, including cyanotoxins and cyanobacteria. Gamma irradiation for the degradation of cyano-bacteria and toxins is effective for overall MC-LR degradation as well as reducing cell concentrations. However, gamma irradiation technology involves use of radioactive isotopes and, therefore, may not feasible commercially from a security perspective. Electron beam (eBeam) irradiation technology, which relies on regular electricity to generate highly energetic electrons, is able to achieve the same results without the confounding challenges of radioactive isotopes and related security issues. In this critical review, the current state of the science concerning the remediation of MC-LR and M. aeruginosa with ionizing radiation technologies is presented and future necessary research is discussed.

The solar photo-Fenton process at neutral pH applied to microcystin-LR degradation: Fe2+, H2O2 and reaction matrix effects

Microcystins are a group of cyanotoxins with known hepatotoxic effects, and their presence in drinking water represents a public health concern all over the world. The main objective of this work was to evaluate the solar photo-Fenton process at near-neutral pH in the degradation of microcystin-LR (MC-LR) under conditions close to those found in bloom episodes, with a high concentration of cell debris and natural organic matter (NOM). The influence of experimental parameters such as Fe²⁺ and H₂O₂ concentrations, reaction matrix, and the presence of scavenger ions, as well as ecotoxicity before and after treatment, was also evaluated. The reaction matrix was obtained from Microcystis aeruginosa cultivated in ASM-1 medium (ACE1 and ACE2 extracts). H₂O₂ and Fe²⁺ concentrations were optimized by 2² factorial design with the central point in a bench-scale solar reactor, using ACE1 extract, and the improved condition was applied in a compound parabolic collector (CPC) reactor, for the ACE2, natural water (RVW) and natural water with M. aeruginosa crude extract (RVCE). Matrix effect assays indicated that radical scavengers present in the medium were responsible for the decrease in the mineralization rates. The solar photo-Fenton process in the CPC reactor achieved COD (75%) and MC-LR (70%) reduction after 120 min at pH = 7.8, [H₂O₂]/COD = 3.18 and [H₂O₂]/[Fe²⁺] = 10 for the ACE2 sample. When the same conditions were applied to the RVCE sample, the process removed 77% of DOC and up to 99% of MC-LR after 45 min of the reaction. Sinapis alba bioassays showed that there was no increase in ecotoxicity after the solar photo-Fenton treatment. These results demonstrate the potential of the solar photo-Fenton process at neutral pH as an additional step in the treatment of natural matrices contaminated with microcystins. In addition, the work reinforces the importance of bioassays in treatment process monitoring.

Microcystin Incidence in the Drinking Water of Mozambique: Challenges for Public Health Protection

Microcystins (MCs) are cyanotoxins produced mainly by freshwater cyanobacteria, which constitute a threat to public health due to their negative effects on humans, such as gastroenteritis and related diseases, including death. In Mozambique, where only 50% of the people have access to safe drinking water, this hepatotoxin is not monitored, and consequently, the population may be exposed to MCs. The few studies done in Maputo and Gaza provinces indicated the occurrence of MC-LR, -YR, and -RR at a concentration ranging from 6.83 to 7.78 µg·L-1, which are very high, around 7 times above than the maximum limit (1 µg·L-1) recommended by WHO. The potential MCs-producing in the studied sites are mainly Microcystis species. These data from Mozambique and from surrounding countries (South Africa, Lesotho, Botswana, Malawi, Zambia, and Tanzania) evidence the need to implement an operational monitoring program of MCs in order to reduce or avoid the possible cases of intoxications since the drinking water quality control tests recommended by the Ministry of Health do not include an MC test. To date, no data of water poisoning episodes recorded were associated with MCs presence in the water. However, this might be underestimated due to a lack of monitoring facilities and/or a lack of public health staff trained for recognizing symptoms of MCs intoxication since the presence of high MCs concentration was reported in Maputo and Gaza provinces.

A complete route for biodegradation of potentially carcinogenic cyanotoxin microcystin-LR in a novel indigenous bacterium

Microcystin-leucine-arginine (MC-LR), a cyclic potentially carcinogenic hepatotoxin, occurs frequently in aquatic habitats worldwide and seriously threatens ecosystem and public health. Limited effectiveness of physicochemical treatments to remove MC-LR from drinking water has led to a search for alternative cost-effective and environment friendly biodegradation strategies. Obtaining MC-degrading bacteria and understanding their MC-degrading mechanisms are outstanding challenges. Here, a novel indigenous bacterium named Sphingopyxis sp. YF1 with a high efficient capacity for MC-degradation was successfully isolated from eutrophic Lake Taihu. Through integrating mass spectrometer and multi-omics analyses accompanied by functional verification of certain genes and proteins, a complete MC-degradation pathway was firstly identified, in which MC-LR was sequentially degraded into linearized MC-LR, tetrapeptide, Adda, phenylacetic acid, and finally potential product CO₂. Some specific proteins such as microcystinase, linearized-microcystinase, tetrapeptidease and PAAase responsible for this pathway were identified. This study pioneeringly demonstrated that MC-LR can be completely degraded through natural remediation processes and revealed a significant potential for MC-LR biodegradation in both natural environment and engineered systems.

Harmful Algal Bloom Toxins in Aerosol Generated from Inland Lake Water

Harmful algal blooms (HABs) caused by cyanobacteria in freshwater environments produce toxins (e.g., microcystin) that are harmful to human and animal health. HAB frequency and intensity are increasing with greater nutrient runoff and a warming climate. Lake spray aerosol (LSA) released from freshwater lakes has been identified on lakeshores and after transport inland, including from lakes with HABs, but little is known about the potential for HAB toxins to be incorporated into LSA. In this study, freshwater samples were collected from two lakes in Michigan: Mona Lake during a severe HAB with microcystin concentrations (>200 μg/L) well above the Environmental Protection Agency (EPA) recommended "do not drink" level (1.6 μg/L) and Muskegon Lake without a HAB (<1 μg/L microcystin). Microcystin toxins were identified in freshwater, as well as aerosol particles generated in the laboratory from Mona Lake water by liquid chromatography-tandem mass spectrometry (LC-MS/MS) at atmospheric concentrations up to 50 ± 20 ng/m3. Enrichment of hydrophobic microcystin congeners (e.g., microcystin-LR) was observed in aerosol particles relative to bulk freshwater, while enrichment of hydrophilic microcystin (e.g., microcystin-RR) was lower. As HABs increase in a warming climate, understanding and quantifying the emissions of toxins into the atmosphere is crucial for evaluating the health consequences of HABs.

A Mini Review on Microcystins and Bacterial Degradation

Microcystins (MCs) classified as hepatotoxic and carcinogenic are the most commonly reported cyanobacterial toxins found in the environment. Microcystis sp. possessing a series of MC synthesis genes (mcyA-mcyJ) are well documented for their excessive abundance, numerous bloom occurrences and MC producing capacity. About 246 variants of MC which exert severe animal and human health hazards through the inhibition of protein phosphatases (PP1 and PP2A) have been characterized. To minimize and prevent MC health consequences, the World Health Organization proposed 1 µg/L MC guidelines for safe drinking water quality. Further the utilization of bacteria that represent a promising biological treatment approach to degrade and remove MC from water bodies without harming the environment has gained global attention. Thus the present review described toxic effects and bacterial degradation of MCs.

Analysis of Covalently Bound Microcystins in Sediments and Clam Tissue in the Sacramento-San Joaquin River Delta, California, USA

Harmful cyanobacterial blooms compromise human and environmental health, mainly due to the cyanotoxins they often produce. Microcystins (MCs) are the most commonly measured group of cyanotoxins and are hepatotoxic, neurotoxic, and cytotoxic. Due to MCs ability to covalently bind to proteins, quantification in complex matrices is difficult. To analyze bound and unbound MCs, analytical methods were optimized for analysis in sediment and clam tissues. A clean up step was incorporated to remove lipids, improving percent yield. This method was then applied to sediment and clam samples collected from the Sacramento-San Joaquin River Delta (Delta) in the spring and fall of 2017. Water samples were also tested for intracellular and extracellular MCs. These analyses were used to quantify the partitioning of MCs among sediment, clams, and water, and to examine whether MCs persist during non-summer months. Toxin analysis revealed that multiple sediment samples collected in the Delta were positive for MCs, with a majority of the positive samples from sites in the San Joaquin River, even while water samples from the same location were below detection limit. These data highlight the importance of analyzing MCs in complex matrices to accurately evaluate environmental risk.

Elimination of cyanobacteria and microcystins in irrigation water-effects of hydrogen peroxide treatment

Cyanobacterial blooms pose a risk to wild and domestic animals as well as humans due to the toxins they may produce. Humans may be subjected to cyanobacterial toxins through many routes, e.g., by consuming contaminated drinking water, fish, and crop plants or through recreational activities. In earlier studies, cyanobacterial cells have been shown to accumulate on leafy plants after spray irrigation with cyanobacteria-containing water, and microcystin (MC) has been detected in the plant root system after irrigation with MC-containing water. This paper reports a series of experiments where lysis of cyanobacteria in abstracted lake water was induced by the use of hydrogen peroxide and the fate of released MCs was followed. The hydrogen peroxide-treated water was then used for spray irrigation of cultivated spinach and possible toxin accumulation in the plants was monitored. The water abstracted from Lake Köyliönjärvi, SW Finland, contained fairly low concentrations of intracellular MC prior to the hydrogen peroxide treatment (0.04 μg L^-1 in July to 2.4 μg L^-1 in September 2014). Hydrogen peroxide at sufficient doses was able to lyse cyanobacteria efficiently but released MCs were still present even after the application of the highest hydrogen peroxide dose of 20 mg L^-1. No traces of MC were detected in the spinach leaves. The viability of moving phytoplankton and zooplankton was also monitored after the application of hydrogen peroxide. Hydrogen peroxide at 10 mg L^-1 or higher had a detrimental effect on the moving phytoplankton and zooplankton.

Simultaneous Microcystis algicidal and microcystin synthesis inhibition by a red pigment prodigiosin

Microcystis blooms and their secondary metabolites microcystins (MCs) occurred all over the world, which have damaged aquatic ecosystems and threatened public health. Techniques to reduce the Microcystis blooms and MCs are urgently needed. This study aimed to investigate the algicidal and inhibitory mechanisms of a red pigment prodigiosin (PG) against the growth and MC-producing abilities of Microcystis aeruginosa (M. aeruginosa). The numbers of Microcystis cells were counted under microscope. The expression of microcystin synthase B gene (mcyB) and concentrations of MCs were determined by quantitative real-time polymerase chain reaction (qRT-PCR) and enzyme linked immunosorbent assay (ELISA) methods, respectively. The inhibitory effects of PG against M. aeruginosa strain FACHB 905 with 50% algicidal concentration (LC₅₀) at 120 h was 0.12 μg/mL. When M. aeruginosa cells exposed to 0.08 μg/mL, 0.16 μg/mL, 0.32 μg/mL PG, the expression of mcyB of M. aeruginosa was down-regulated 4.36, 8.16 and 18.51 times lower than that of the control at 120 h. The concentrations of total MC (TMC) also were 1.66, 1.72 and 5.75 times lower than that of the control at 120 h. PG had high algicidal effects against M. aeruginosa, with the activities of superoxide dismutase (SOD) initially increased and then decreased after 72 h, the contents of malondialdehyde (MDA) increase, the expression of mcyB gene down-regulation, and MCs synthesis inhibition. This study was first to report the PG can simultaneously lyse Microcystis cells, down-regulate of mcyB expression and inhibit MCs production effectively probably due to oxidative stress, which indicated PG poses a great potential for regulating Microcystis blooms and MCs pollution in the environment.

Insight Into the Molecular Mechanisms for Microcystin Biodegradation in Lake Erie and Lake Taihu

Microcystins are potent hepatotoxins that are frequently detected in fresh water lakes plagued by toxic cyanobacteria. Microbial biodegradation has been referred to as the most important avenue for removal of microcystin from aquatic environments. The biochemical pathway most commonly associated with the degradation of microcystin is encoded by the mlrABCD (mlr) cassette. The ecological significance of this pathway remains unclear as no studies have examined the expression of these genes in natural environments. Six metatranscriptomes were generated from microcystin-producing Microcystis blooms and analyzed to assess the activity of this pathway in environmental samples. Seventy-eight samples were collected from Lake Erie, United States/Canada and Lake Tai (Taihu), China, and screened for the presence of mlr gene transcripts. Read mapping to the mlr cassette indicated transcripts for these genes were absent, with only 77 of the collective 3.7 billion reads mapping to any part of the mlr cassette. Analysis of the assembled metatranscriptomes supported this, with only distantly related sequences identified as mlrABC-like. These observations were made despite the presence of microcystin and over 500,000 reads mapping to the mcy cassette for microcystin production. Glutathione S-transferases and alkaline proteases have been previously hypothesized to be alternative pathways for microcystin biodegradation, and expression of these genes was detected across space and time in both lakes. While the activity of these alternative pathways needs to be experimentally confirmed, they may be individually or collectively more important than mlr genes in the natural environment. Importantly, the lack of mlr expression could indicate microcystin biodegradation was not occurring in the analyzed samples. This study raises interesting questions about the ubiquity, specificity and locality of microcystin biodegradation, and highlights the need for the characterization of relevant mechanisms in natural communities to understand the fate of microcystin in the environment and risk to public health.

A Data-Independent Methodology for the Structural Characterization of Microcystins and Anabaenopeptins Leading to the Identification of Four New Congeners

Toxin-producing cyanobacteria are responsible for the presence of hundreds of bioactive compounds in aquatic environments undergoing increasing eutrophication. The identification of cyanotoxins is still emerging, due to the great diversity of potential congeners, yet high-resolution mass spectrometry (HRMS) has the potential to deepen this knowledge in aquatic environments. In this study, high-throughput and sensitive on-line solid-phase extraction ultra-high performance liquid chromatography (SPE-UHPLC) coupled to HRMS was applied to a data-independent acquisition (DIA) workflow for the suspect screening of cyanopeptides, including microcystin and anabaenopeptin toxin classes. The unambiguous characterization of 11 uncommon cyanopeptides was possible using a characterization workflow through extensive analysis of fragmentation patterns. This method also allowed the characterization of four unknown cyanotoxins ([Leu1, Ser7] MC-HtyR, [Asp3]MC-RHar, AP731, and AP803). The quantification of 17 common cyanotoxins along with the semi-quantification of the characterized uncommon cyanopeptides resulted with the identification of 23 different cyanotoxins in 12 lakes in Canada, United Kingdom and France. The concentrations of the compounds varied between 39 and 41,000 ng L-1. To our knowledge, this is the first DIA method applied for the suspect screening of two families of cyanopeptides simultaneously. Moreover, this study shows the great diversity of cyanotoxins in lake water cyanobacterial blooms, a growing concern in aquatic systems.

Accumulation of Microcystin-LR in Grains of Two Rice Varieties (Oryza sativa L.) and a Leafy Vegetable, Ipomoea aquatica

The potential transfer of microcystin-LR (MC-LR) to humans via crop plants irrigated with MC-contaminated water is causing serious concern. In this study, two Oryza sativa variants, a hybrid (BG358), a traditional (Suwandel) variety, and a leafy green vegetable crop, Ipomoea aquatica, were exposed under laboratory conditions to natural blooms of Microcystis aeruginosa sampled from a hypereutrophic lake contaminated with MC-LR (3,197.37 ± 1.04 µg/L). Field samples of O. sativa and I. aquatica were collected from farmlands that had been irrigated from a reservoir, containing MC-LR (180 µg/L). MC-LR was quantified by high performance liquid chromatography followed by photodiode-array detection (HPLC-PDA). From the laboratory study, we calculated the potential human health exposure from BG358, Suwandel and I. aquatica as 2.84 ± 0.01, 0.22 ± 0.01, and 0.06 ± 0.01 µg/kg of body weight/day, respectively, whereas the potential health exposures from BG358, Suwandel and I. aquatica collected from the field were 0.10 ± 0.01, 0.009 ± 0.005, and 0.03 ± 0.01 µg/kg of body weight/day, respectively. In certain instances, the results exceeded the World Health Organization’s (WHO) tolerable daily intake of MC-LR, posing a potential health risk to humans. Thus, our results emphasize the importance of continuous screening programs for cyanotoxins in edible plants in the future to prevent the consumption of contaminated crops.

Treated rice husk as a recyclable sorbent for the removal of microcystins from water

Microcystins (MCs) appear during harmful algal blooms (HABs) in water sources worldwide, and represent a threat for humans and animals ingesting or inhaling MCs from the environment. Herein, treated rice husk (RH) was tested as a recyclable sorbent for removal of six MCs (MC-RR, MC-LR, MC-YR, MC-LA, MC-LF, and MC-LW) from water. RH was refluxed with hydrochloric acid and heated to 250 °C to produce the sorbent material. Twenty milligrams of treated RH removed >95% of the MCs from a 30 mL solution containing 25 μg/L of each MC. The adsorption of MCs onto RH follows the Freundlich isotherm model (R² ≥ 0.9612) and pseudo-second-order kinetics (R² ≥ 0.9996). More than 90% of MCs were removed within 5 min, and >95% were removed at equilibrium (in <40 min). Performance of the RH sorbent was evaluated by removing MCs from Lake Erie water collected during an algal bloom in 2017. The total concentration (extracellular plus intracellular) of six tested MCs in lake water ranged from 3.7 to 13,605.9 μg/L, and removal of MCs by treated RH ranged from 100.0% to 71.8%, respectively. The removal capacity of RH for the six MCs from the lake water sample containing 13,605.9 μg/L of MCs was 586 μg per g of treated RH. After being used to extract MCs, the RH was heated to 560 °C to produce silica nanoparticles. Therefore, treated RH enables rapid and efficient removal of MCs from water and it can be recycled for use as a raw material. Overall, treated RH can contribute to mitigation of environmental and health effects caused by MCs and reduce concerns for toxic waste disposal.

Susceptibility of the Algal Toxin Microcystin-LR to UV/Chlorine Process: Comparison with Chlorination

Microcystin-LR (MC-LR), an algal toxin (cyanotoxin) common in sources of drinking water, poses a major human health hazard due to its high toxicity. In this study, UV/chlorine was evaluated as a potentially practical and effective process for the degradation of MC-LR. Via mass spectrometry analysis, fewer chlorinated-MC-LR products were detected with UV/chlorine treatment than with chlorination, and a transformation pathway for MC-LR by UV/chlorine was proposed. Different degrees of rapid degradation of MC-LR were observed with varying pH (6-10.4), oxidant dosage (0.5-3 mg L^-1), natural organic matter (0-7 mg L^-1), and natural water sources. In contrast to the formation of primarily chloroform and dichloroacetic acid in deionized water where MC-LR serves as the only carbon source, additional chlorinated disinfection byproducts were produced when sand filtered natural water was used as a background matrix. The UV/chlorine treated samples also showed quantitatively less cytotoxicity in vitro in HepaRG human liver cell line tests than chlorination treated samples. Following 16 min (96 mJ cm^-2) of UV irradiation combined with 1.5 mg L^-1 chlorine treatment, the cell viability of the samples increased from 80% after exposure to 1 mg L^-1 MC-LR to 90%, while chlorination treatment evidenced no reduction in cytotoxicity with the same reaction time.

Detection of cyanotoxins (microcystins/nodularins) in livers from estuarine and coastal bottlenose dolphins (Tursiops truncatus) from Northeast Florida

Microcystins/Nodularins (MCs/NODs) are potent hepatotoxic cyanotoxins produced by harmful algal blooms (HABs) that occur frequently in the upper basin of the St. Johns River (SJR), Jacksonville, FL, USA. Areas downstream of bloom locations provide critical habitat for an estuarine population of bottlenose dolphins (Tursiops truncatus). Since 2010, approximately 30 of these dolphins have stranded and died within this impaired watershed; the cause of death was inconclusive for a majority of these individuals. For the current study, environmental exposure to MCs/NODs was investigated as a potential cause of dolphin mortality. Stranded dolphins from 2013 to 2017 were categorized into estuarine (n = 17) and coastal (n = 10) populations. Because estuarine dolphins inhabit areas with frequent or recurring cyanoblooms, they were considered as a comparatively high-risk group for cyanotoxin exposure in relation to coastal animals. All available liver samples from estuarine dolphins were tested regardless of stranding date, and samples from coastal individuals that stranded outside of the known cyanotoxin bloom season were assessed as controls. The MMPB (2-methyl-3-methoxy-4-phenylbutiric acid) technique was used to determine total (bound and free) concentrations of MCs/NODS in liver tissues. Free MCs/NODs extractions were conducted and analyzed using ELISA and LC-MS/MS on MMPB-positive samples to compare test results. MMPB testing resulted in low-level total MCs/NODs detection in some specimens. The Adda ELISA produced high test values that were not supported by concurrent LC-MS/MS analyses, indicative of false positives. Our results indicate that both estuarine and coastal dolphins are exposed to MCs/NODs, with potential toxic and immune health impacts.

Seasonal dynamics of microcystin-degrading bacteria and toxic cyanobacterial blooms: Interaction and influence of abiotic factors

Massive proliferations of cyanobacteria coexist and have different interactions with other microorganisms, including microcystin (MC)-degrading bacteria. Despite their relevance in the environment for the removal of MCs, this bacterial community has been scarcely studied. The influence of physicochemical factors and the seasonal dynamics of toxic cyanobacteria on the relative abundance and seasonal dynamics of the MC-degrading bacterial community with mlr genes (mlr⁺) were investigated during a two-year study at a water reservoir in central Spain. The capacity of the total bacterial community on the degradation of MCs during the whole period of study was also evaluated. The results showed that the relative abundance of mlr⁺ bacteria started to increase after the increase in the relative abundance of toxic cyanobacteria and MC concentrations in the water, indicating a related seasonal dynamic and an important interaction between the two communities. The correspondence of several peaks of mlr⁺ bacteria with decreases in the relative abundance of toxic cyanobacteria and vice versa may also suggest a possible antagonistic relationship that deserves an in-depth study. The lack of a significant relationship between the physicochemical factors and the temporal shifts of both MC producers and degraders also supports the notion that the interaction of the two communities is an important driver of their seasonal dynamics in nature. Regarding the capacity of the total bacterial community for the degradation of MCs, this capacity was only observed during the toxic cyanobacterial bloom episodes, highlighting the importance of the pre-exposure to MCs in the reservoir for triggering the MC biodegradation process.

Diversity and temporal shifts of the bacterial community associated with a toxic cyanobacterial bloom: An interplay between microcystin producers and degraders

The biodegradation of microcystins (MCs) by bacteria constitutes an important process in freshwater ecosystems to prevent the accumulation of toxins. However, little is known about the diversity and the seasonal dynamics of the bacterial community composition (BCC) involved in the degradation of MCs in nature. To explore these BCC shifts, high-throughput sequencing was used to analyse the 16S rRNA, mcyE and mlrA genes during a year in a freshwater reservoir with a toxic cyanobacterial bloom episode. The analysis of the mcyE and mlrA genes from water samples revealed the coexistence of different MC-producing and MC-degrading genotypes, respectively. The patchy temporal distribution of the mlrA genotypes (from the families Sphingomonadaceae and Xanthomonadaceae) suggests their dissimilar response to environmental conditions and the influence of other factors besides the MCs that may control their presence and relative abundance. During the maximum toxic cyanobacterial biomass and cell lysis, other bacterial taxa that lack mlr genes increased their relative abundance. Among these bacteria, those with a recognized role in the degradation of xenobiotic and other complex organic compounds (e.g., orders Myxococcales, Ellin6067, Spirobacillales and Cytophagales) were the most representative and suggest their possible involvement in the removal of MCs in the environment.

Current research scenario for microcystins biodegradation - A review on fundamental knowledge, application prospects and challenges

Microcystins (MCs) are common cyanotoxins produced by harmful cyanobacterial blooms (HCBs) and severely threaten human and ecosystems health. Biodegradation is an efficient and sustainable biological strategy for MCs removal. Many novel findings in fundamental knowledge and application potential of MC-biodegradation have been documented. Little effort has devoted to summarize and comment recent research progress on MC-biodegradation, and discuss the research problems and gaps. This review deals with current research scenario in aerobic and anaerobic biodegradation for MCs. Diverse organisms capable of degrading MCs are encapsulated. Enzymatic mechanisms and influence factors regulating aerobic and anaerobic MC-biodegradation are summarized and discussed, which are essential for assessing and reducing MC-risks during HCBs episodes. Also, we propose some ideas to solve the challenges and bottleneck problems in practical application of MC-biodegradation, and discuss research gaps and promising research methods which deserve special attention. This review may provide new insights on future direction of MC-biodegradation research, in order to further broaden its application prospects for bioremediation.

Effect of chlorination by-products on the quantitation of microcystins in finished drinking water

Microcystins are toxic peptides that can be produced by cyanobacteria in harmful algal blooms (HABs). Various analytical techniques have been developed to quantify microcystins in drinking water, including liquid chromatography tandem mass spectrometry (LC/MS/MS), enzyme linked immunosorbent assay (ELISA), and oxidative cleavage to produce 2-methyl-3-methoxy-4-phenylbutyric acid (MMPB) with detection by LC/MS/MS, the "MMPB method". Both the ELISA and MMPB methods quantify microcystins by detecting a portion of the molecule common to most microcystins. However, there is little research evaluating the effect of microcystin chlorination by-products potentially produced during drinking water treatment on analytical results. To evaluate this potential, chlorinated drinking water samples were fortified with various microcystin congeners in bench-scale studies. The samples were allowed to react, followed by a comparison of microcystin concentrations measured using the three methods. The congener-specific LC/MS/MS method selectively quantified microcystins and was not affected by the presence of chlorination by-products. The ELISA results were similar to those obtained by LC/MS/MS for most microcystin congeners, but results deviated for a particular microcystin containing a variable amino acid susceptible to oxidation. The concentrations measured by the MMPB method were at least five-fold higher than the concentrations of microcystin measured by the other methods and demonstrate that detection of MMPB does not necessarily correlate to intact microcystin toxins in finished drinking water.