Microbial degradation of microcystin in Florida's freshwaters

Microcystin removal by microbial communities from a coastal lagoon: Influence of abiotic factors, bacterioplankton composition and estimated functions

Toxic cyanobacterial blooms present a substantial risk to public health due to the production of secondary metabolites, notably microcystins (MCs). Microcystin-LR (MC-LR) is the most prevalent and toxic variant in freshwater. MCs resist conventional water treatment methods, persistently impacting water quality. This study focused on an oligohaline shallow lagoon historically affected by MC-producing cyanobacteria, aiming to identify bacteria capable of degrading MC and investigating the influence of environmental factors on this process. While isolated strains did not exhibit MC degradation, microbial assemblages directly sourced from lagoon water removed MC-LR within seven days at 25 ºC and pH 8.0. The associated bacterial community demonstrated an increased abundance of bacterial taxa assigned to Methylophilales, and also Rhodospirillales and Rhodocyclales to a lesser extent. However, elevated atmospheric temperatures (45 ºC) and acidification (pH 5.0 and 3.0) hindered MC-LR removal, indicating that extreme environmental changes could contribute to prolonged MC persistence in the water column. This study highlights the importance of considering environmental conditions in order to develop strategies to mitigate cyanotoxin contamination in aquatic ecosystems.

Bacterial community shifts induced by high concentration hydrogen peroxide treatment of Microcystis bloom in a mesocosm study

Hydrogen peroxide has gained popularity as an environmentally friendly treatment for cyanobacterial harmful algal blooms (cHABs) that takes advantage of oxidative stress sensitivity in cyanobacteria at controlled concentrations. Higher concentrations of hydrogen peroxide treatments may seem appealing for more severe cHABs but there is currently little understanding of the environmental impacts of this approach. Of specific concern is the associated microbial community, which may play key roles in the succession/recovery process post-treatment. To better understand impacts of a high concentration treatment on non-target microbial communities, we applied a hydrogen peroxide spray equating to a total volume concentration of 14 mM (473 mg/L, 0.04%) to 250 L mesocosms containing Microcystis bloom biomass, monitoring treatment and control mesocosms for 4 days. Cyanobacteria dominated control mesocosms throughout the experiment while treatment mesocosms experienced a 99% reduction, as determined by bacterial amplicon sequencing, and a 92% reduction in bacterial cell density within 1 day post-treatment. Only the bacterial community exhibited signs of regrowth, with a fold change of 9.2 bacterial cell density from day 1 to day 2. Recovery consisted of succession by Planctomycetota (47%) and Gammaproteobacteria (17%), which were likely resilient due to passive cell component compartmentalization and rapid upregulation of dnaK and groEL oxidative stress genes, respectively. The altered microbiome retained beneficial functionality of microcystin degradation through a currently recognized but unidentified pathway in Gammaproteobacteria, resulting in a 70% reduction coinciding with bacterial regrowth. There was also an 81% reduction of both total nitrogen and phosphorus, as compared to 91 and 93% in the control, respectively, due to high expressions of genes related to nitrogen (argH, carB, glts, glnA) and phosphorus (pntAB, phoB, pstSCB) cycling. Overall, we found a portion of the bacterial community was resilient to the high-concentration hydrogen peroxide treatment, resulting in Planctomycetota and Gammaproteobacteria dominance. This high-concentration treatment may be suitable to rapidly end cHABs which have already negatively impacted the aquatic environment rather than allow them to persist.

Review: Current understanding on biological filtration for the removal of microcystins

Harmful algal blooms (HABs) have become a global problem not only in aquatic habitats but also in public health and safety due to the production of cyanotoxins as their secondary metabolites. Among the various identified cyanotoxin groups, microcystins (MCs) are one of the most prevalent cyanotoxin detected during HABs. Different strategies including advanced physical and chemical treatment processes have been developed to mitigate the threat of cyanotoxins in water utilities, but these have revealed certain limitations in terms of high operational costs, low removal efficacy, and harmful by-products formation. Recently, biological filtration systems (BFS) have gained attention for safe drinking water production as they can treat various natural organic matter (NOM) and emerging contaminants through a highly efficient and environmentally sustainable process. However, limited attention has been given to understand the current research progress, research challenges, and knowledge gaps for the successful implementation of BFS for MC removal. Therefore, in this review, currently identified MC biodegradation pathways and MC-degrading microorganisms with their degradation rates are summarized, which may be pivotal for studying bioaugmented BFS to enhance the MC removal during HABs. Moreover, both laboratory and field studies on BFS for MC removal are reviewed, followed by a discussion of current challenges and future research needs for the practical application of BFS.

Effects of chronic exposure to microcystin-LR on life-history traits, intestinal microbiota and transcriptomic responses in Chironomus pallidivittatus

Microcystins (MCs) are the most widely distributed cyanobacterial toxins that can exert adverse effects on aquatic organisms, but aside from the study of the harmful effect of cyanobacterial blooms, little is known about the effect of released MCs on the growth and development of chironomid larvae. To assess the harmful effect and the toxic mechanism of MCs on midges, the life-history traits, intestinal microbiota, and transcriptome of Chironomus pallidivittatus were analyzed after chronic exposure to 30 μg/L of MC-LR. Exposure inhibited larvae body length by 35.61% and wet weight by 21.92%, increased emergence time of midges, damaged mitochondria in the intestine, promoted oxidative stress, dysregulated lipid metabolism of chironomid larvae, and increased detoxification enzymes glutathione S-transferase (GST) and superoxide dismutase (SOD) by 32.44% and 17.41%, respectively. Exposure also altered the diversity and abundance of the intestinal microbiota, favoring pathogenic and MC degradation bacteria. RNA sequencing identified 261 differentially expressed genes under MC-LR stress, suggesting that impairment of the peroxisome proliferator-activated receptor signaling pathway upregulated fatty acid biosynthesis and elongation to promote lipid accumulation. In addition, exposure-induced detoxification and antioxidant responses, indicating that the chironomid larvae had the potential ability to resist MC-LR. To our knowledge, this is the first time that lipid accumulation, oxidative stress, and detoxification have been studied in this organism at the environmentally relevant concentration of MC-LR; the information may assist in ecological risk assessment of cyanobacterial toxins and their effects on benthic organisms.

Optimization of Biodegradation Characteristics of Sphingopyxis sp. YF1 against Crude Microcystin-LR Using Response Surface Methodology

Sphingopyxis sp. YF1 has proven to be efficient in biodegrading microcystin (MC)-leucine (L) and arginine (R) (MC-LR); however, the optimal environmental factors to biodegrade the toxin have not been investigated. In this study, the biodegrading characteristics of strain YF1 against MC-LR were assessed under diverse environmental factors, including temperature (20, 30 or 40 °C), pH (5, 7 or 9) and MC-LR concentration (1, 3 or 5 µg/mL). Data obtained from the single-factor experiment indicated that MC-LR biodegradation by strain YF1 was temperature-, pH- and MC-LR-concentration-dependent, and the maximal biodegradation rate occurred at 5 µg/mL/h. Proposing Box-Behnken Design in response surface methodology, the influence of the three environmental factors on the biodegradation efficiency of MC-LR using strain YF1 was determined. A 17-run experiment was generated and carried out, including five replications performed at the center point. The ANOVA analysis demonstrated that the model was significant, and the model prediction of MC-LR biodegradation was also validated with the experimental data. The quadratic statistical model was established to predict the interactive effects of the environmental factors on MC-LR biodegradation efficiency and to optimize the controlling parameters. The optimal conditions for MC-LR biodegradation were observed at 30 °C, pH 7 and 3 µg/mL MC-LR, with a biodegradation efficiency of 100% after 60 min. The determination of the optimal environmental factors will help to unveil the detailed biodegradation mechanism of MC-LR by strain YF1 and to apply it into the practice of eliminating MC-LR from the environment.

Role of Rhizospheric Microbiota as a Bioremediation Tool for the Protection of Soil-Plant Systems from Microcystins Phytotoxicity and Mitigating Toxin-Related Health Risk

Frequent toxic cyanoblooms in eutrophic freshwaters produce various cyanotoxins such as the monocyclic heptapeptides microcystins (MCs), known as deleterious compounds to plant growth and human health. Recently, MCs are a recurrent worldwide sanitary problem in irrigation waters and farmland soils due to their transfer and accumulation in the edible tissues of vegetable produce. In such cases, studies about the persistence and removal of MCs in soil are scarce and not fully investigated. In this study, we carried out a greenhouse trial on two crop species: faba bean (Vicia faba var. Alfia 321) and common wheat (Triticum aestivum var. Achtar) that were grown in sterile (microorganism-free soil) and non-sterile (microorganism-rich soil) soils and subjected to MC-induced stress at 100 µg equivalent MC-LR L⁻¹. The experimentation aimed to assess the prominent role of native rhizospheric microbiota in mitigating the phytotoxic impact of MCs on plant growth and reducing their accumulation in both soils and plant tissues. Moreover, we attempted to evaluate the health risk related to the consumption of MC-polluted plants for humans and cattle by determining the estimated daily intake (EDI) and health risk quotient (RQ) of MCs in these plants. Biodegradation was liable to be the main removal pathway of the toxin in the soil; and therefore, bulk soil (unplanted soil), as well as rhizospheric soil (planted soil), were used in this experiment to evaluate the accumulation of MCs in the presence and absence of microorganisms (sterile and non-sterile soils). The data obtained in this study showed that MCs had no significant effects on growth indicators of faba bean and common wheat plants in non-sterile soil as compared to the control group. In contrast, plants grown in sterile soil showed a significant decrease in growth parameters as compared to the control. These results suggest that MCs were highly bioavailable to the plants, resulting in severe growth impairments in the absence of native rhizospheric microbiota. Likewise, MCs were more accumulated in sterile soil and more bioconcentrated in root and shoot tissues of plants grown within when compared to non-sterile soil. Thereby, the EDI of MCs in plants grown in sterile soil was more beyond the tolerable daily intake recommended for both humans and cattle. The risk level was more pronounced in plants from the sterile soil than those from the non-sterile one. These findings suggest that microbial activity, eventually MC-biodegradation, is a crucial bioremediation tool to remove and prevent MCs from entering the agricultural food chain.

Optimization of microcystin biodegradation by bacterial community YFMCD4 using response surface method

The increasing production of microcystin-LR (MC-LR) causing animal and human health issues is found in eutrophic water bodies, marine habitats and desert environments. The health threat posed by MC-LR has led to the establishment of World Health Organization's water guideline value of 1 μg/mL. Combating this has increased the search for cost-effective approach to degrade MC-LR. The study aimed to optimize the MC-degrading environmental factors of bacterial community YFMCD4. Response surface methodology (RSM) was employed to evaluate the influence of varying temperatures, pH and initial MC-LR concentration on the biodegradation efficiency of MC-LR by bacterial community YFMCD4. The optimal MC-LR biodegradation environmental factors were found to be 30 °C, pH 7 and 2 μg/mL initial MC-LR. The biodegradation rate reached 100% after 10 h. YFMCD4 mainly consisted of genera Alacligenes, Sphingobacterium and Pseudomonas using High-throughput pyrosequencing technology. The mlrA gene encoding MlrA enzyme considered most important for MC-LR biodegradation was obtained from YFMCD4. Data demonstrated that the bacterial structure and biodegradation efficiency of YFMCD4 varied with the change of environmental factors including temperature, pH and MC-LR concentrations. RSM is considered a good method to examine the optimal biodegradation environmental conditions for MC-LR. To date, RSM and High-throughput pyrosequencing technology are employed to optimize the biodegradation conditions (30 °C, pH 7 and 2 μg/mL initial MC-LR) and analyze the structure of bacterial community for the first time.

Florida’s Harmful Algal Bloom (HAB) Problem: Escalating Risks to Human, Environmental and Economic Health With Climate Change

Harmful Algal Blooms (HABs) pose unique risks to the citizens, stakeholders, visitors, environment and economy of the state of Florida. Florida has been historically subjected to reoccurring blooms of the toxic marine dinoflagellate Karenia brevis (C. C. Davis) G. Hansen & Moestrup since at least first contact with explorers in the 1500’s. However, ongoing immigration of more than 100,000 people year–1 into the state, elevated population densities in coastal areas with attendant rapid, often unregulated development, coastal eutrophication, and climate change impacts (e.g., increasing hurricane severity, increases in water temperature, ocean acidification and sea level rise) has likely increased the occurrence of other HABs, both freshwater and marine, within the state as well as the number of people impacted by these blooms. Currently, over 75 freshwater, estuarine, coastal and marine HAB species are routinely monitored by state agencies. While only blooms of K. brevis, the dinoflagellate Pyrodinium bahamense (Böhm) Steidinger, Tester, and Taylor and the diatom Pseudo-nitzschia spp. have resulted in closure of commercial shellfish beds, other HAB species, including freshwater and marine cyanobacteria, pose either imminent or unknown risks to human, environmental and economic health. HAB related human health risks can be classified into those related to consumption of contaminated shellfish and finfish, consumption of or contact with bloom or toxin contaminated water or exposure to aerosolized HAB toxins. While acute human illnesses resulting from consumption of brevetoxin-, saxitoxin-, and domoic acid-contaminated commercial shellfish have been minimized by effective monitoring and regulation, illnesses due to unregulated toxin exposures, e.g., ciguatoxins and cyanotoxins, are not well documented or understood. Aerosolized HAB toxins potentially impact the largest number of people within Florida. While short-term (days to weeks) impacts of aerosolized brevetoxin exposure are well documented (e.g., decreased respiratory function for at-risk subgroups such as asthmatics), little is known of longer term (>1 month) impacts of exposure or the risks posed by aerosolized cyanotoxin [e.g., microcystin, β-N-methylamino-L-alanine (BMAA)] exposure. Environmental risks of K. brevis blooms are the best studied of Florida HABs and include acute exposure impacts such as significant dies-offs of fish, marine mammals, seabirds and turtles, as well as negative impacts on larval and juvenile stages of many biota. When K. brevis blooms are present, brevetoxins can be found throughout the water column and are widespread in both pelagic and benthic biota. The presence of brevetoxins in living tissue of both fish and marine mammals suggests that food web transfer of these toxins is occurring, resulting in toxin transport beyond the spatial and temporal range of the bloom such that impacts of these toxins may occur in areas not regularly subjected to blooms. Climate change impacts, including temperature effects on cell metabolism, shifting ocean circulation patterns and changes in HAB species range and bloom duration, may exacerbate these dynamics. Secondary HAB related environmental impacts are also possible due to hypoxia and anoxia resulting from elevated bloom biomass and/or the decomposition of HAB related mortalities. Economic risks related to HABs in Florida are diverse and impact multiple stakeholder groups. Direct costs related to human health impacts (e.g., increased hospital visits) as well as recreational and commercial fisheries can be significant, especially with wide-spread sustained HABs. Recreational and tourism-based industries which sustain a significant portion of Florida’s economy are especially vulnerable to both direct (e.g., declines in coastal hotel occupancy rates and restaurant and recreational users) and indirect (e.g., negative publicity impacts, associated job losses) impacts from HABs. While risks related to K. brevis blooms are established, Florida also remains susceptible to future HABs due to large scale freshwater management practices, degrading water quality, potential transport of HABs between freshwater and marine systems and the state’s vulnerability to climate change impacts.

Impacts of Microcystins on Morphological and Physiological Parameters of Agricultural Plants: A Review

Cyanobacteria are a group of photosynthetic prokaryotes that pose a great concern in the aquatic environments related to contamination and poisoning of wild life and humans. Some species of cyanobacteria produce potent toxins such as microcystins (MCs), which are extremely aggressive to several organisms, including animals and humans. In order to protect human health and prevent human exposure to this type of organisms and toxins, regulatory limits for MCs in drinking water have been established in most countries. In this regard, the World Health Organization (WHO) proposed 1 µg MCs/L as the highest acceptable concentration in drinking water. However, regulatory limits were not defined in waters used in other applications/activities, constituting a potential threat to the environment and to human health. Indeed, water contaminated with MCs or other cyanotoxins is recurrently used in agriculture and for crop and food production. Several deleterious effects of MCs including a decrease in growth, tissue necrosis, inhibition of photosynthesis and metabolic changes have been reported in plants leading to the impairment of crop productivity and economic loss. Studies have also revealed significant accumulation of MCs in edible tissues and plant organs, which raise concerns related to food safety. This work aims to systematize and analyze the information generated by previous scientific studies, namely on the phytotoxicity and the impact of MCs especially on growth, photosynthesis and productivity of agricultural plants. Morphological and physiological parameters of agronomic interest are overviewed in detail in this work, with the aim to evaluate the putative impact of MCs under field conditions. Finally, concentration-dependent effects are highlighted, as these can assist in future guidelines for irrigation waters and establish regulatory limits for MCs.

Protective Role of Native Rhizospheric Soil Microbiota Against the Exposure to Microcystins Introduced into Soil-Plant System via Contaminated Irrigation Water and Health Risk Assessment

Microcystins (MCs) produced in eutrophic waters may decrease crop yield, enter food chains and threaten human and animal health. The main objective of this research was to highlight the role of rhizospheric soil microbiota to protect faba bean plants from MCs toxicity after chronic exposure. Faba bean seedlings were grown in pots containing agricultural soil, during 1 month under natural environmental conditions of Marrakech city in Morocco (March–April 2018) and exposed to cyanobacterial extracts containing up to 2.5 mg·L⁻¹ of total MCs. Three independent exposure experiments were performed (a) agricultural soil was maintained intact “exposure experiment 1”; (b) agricultural soil was sterilized “exposure experiment 2”; (c) agricultural soil was sterilized and inoculated with the rhizobia strain Rhizobium leguminosarum RhOF34 “exposure experiment 3”. Overall, data showed evidence of an increased sensitivity of faba bean plants, grown in sterilized soil, to MCs in comparison to those grown in intact and inoculated soils. The study revealed the growth inhibition of plant shoots in both exposure experiments 2 and 3 when treated with 2.5 mg·L⁻¹ of MCs. The results also showed that the estimated daily intake (EDI) of MCs, in sterilized soil, exceeded 2.18 and 1.16 times the reference concentrations (0.04 and 0.45 µg of microcysin-leucine arginine (MC-LR). Kg⁻¹ DW) established for humans and cattle respectively, which raises concerns about human food chain contamination.

Using easy-to-biodegrade co-substrate to eliminate microcystin toxic on electrochemically active bacteria and enhance bioelectricity generation from cyanobacteria biomass

Cyanobacterial biomass is a promising natural resource for power generation, through the reactions bio-catalyzed by electrochemically active bacteria (EAB). However, the major limitation is the involvement of Microcystin-LR (MC-LR) in inhibiting EAB activation. In this work, toxic M. aeruginosa biomass was employed as analyte of a microbial fuel cell (MFC), and sodium acetate was applied as easy-to-biodegrade co-substrate to alleviate the MC-LR stress on EAB survival. The running stability was continuously enhanced with the increment of co-substrate concentration. The sufficient co-substrate supply (6.0 mM) eliminated the negative effects of MC-LR on the cyanobacteria biomass fed-MFC performance; it contributed 12.7% extension on the electric cyclic terms and caused the productions of the power density which was comparable and even 3.8% higher than its corresponding control (MFC treated with acetate alone). The co-substrate addition also increased coulombic efficiency by 60.1%, microcystin-LR removal efficiency increased by 64.7%, and diversified the microbial community with more species able to biodegrade the MC-LR, bio-transforming the metabolites and EAB. Microcystin-degrading bacteria, such as Sphingopyxis sp., Burkholderia-Paraburkholderia, and Bacillus sp., were remarkably increased, and EAB, including Shewanella sp., Desulfovibrio desulfuricans, Aeromonas hydrophila, were also much more enriched in co-substrate use protocol. Therefore, this study verified a co-substrate strategy for simultaneously eliminating MC-LR toxin and enhancing bioelectricity generation from cyanobacterial biomass via an MFC.

Taxonomic and Genotypical Heterogeneity of Microcystin degrading Bacterioplankton in Western Lake Erie

Microcystins (MCs) are among the predominant cyanotoxins that are primarily degraded by heterotrophic bacteria in various freshwater environments, including Lake Erie, a Laurentian Great Lake. However, despite the prevalence of MCs in Lake Erie basins, our knowledge about the taxonomic diversity of local MC-degrading bacteria is largely limited. The current study obtained thirty-four MC-degrading bacterial pure isolates from Lake Erie surface water and characterized their taxonomical and phenotypic identities as well as their MC-degradation rates under different pH, temperature, availability of organic substrates and with other MC-degrading isolates. Obtained MC-degrading isolates included both Gram-positive (18 isolates of Actinobacteria and Firmicutes) and Gram-negative bacteria (16 isolates of Gamma-proteobacteria); and 7 of these isolates were motile, and 13 had the capacity to form biofilms. In general, MC-degradation rates of the isolates were impacted by temperature and pH but insensitive to the presence of cyanobacterial exudates. At the optimal temperature (30-35°C) and pH (7-8), individual isolates degraded MC-LR, the most abundant MC isomer, at an average of 0.20 µg/mL/hr. With additions of cyanobacterial exudates, only Pseudomonas sp. LEw-2029, a non-motile biofilm maker, showed increased MC degradation (0.25 µg/mL/hr). Five out of nine tested dual culture mixtures showed rises in MC degradation rates than their corresponding monocultures; the highest rate reached 0.40 µg/mL/hr for the pair LEw-(1132 + 2029). PCR amplification of mlrA genes yielded negative results for all isolates; subsequent enzyme assay-Mass Spectrum analysis identified no product associated with the mlr gene-based MC degradation pathway. Collectively, our results demonstrated that a diversity of indigenous Lake Erie bacteria can degrade MCs via a novel mlr-independent pathway. Obtained MC degraders, especially Pseudomonas sp. LEw-2029, may serve as candidates for the development of biological filters to remove cyanotoxins in water treatment systems.

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.

Effect of the immobilized microcystin-LR-degrading enzyme MlrA on nodularin degradation and its immunotoxicity study

In freshwater ecosystems with frequent cyanobacterial blooms, the cyanobacteria toxin pollution is becoming increasingly serious. Nodularin (NOD), which has strong biological toxicity, has emerged as a new pollutant and affects the normal growth, development and reproduction of aquatic organisms. However, little information is available regarding this toxin. In this study, a graphene oxide material modified by L-cysteine was synthesized and used to immobilize microcystin-LR (MC-LR)-degrading enzyme (MlrA) to form an immobilized enzyme nanocomposite, CysGO-MlrA. Free-MlrA was used as a control. The efficiency of NOD removal by CysGO-MlrA was investigated. Additionally, the effects of CysGO-MlrA and the NOD degradation product on zebrafish lymphocytes were detected to determine the biological toxicity of these two substances. The results showed the following: (1) There was no significant difference in the degradation efficiency of NOD between CysGO-MlrA and free-MlrA; the degradation rate of both was greater than 80% at 1 h (2) The degradation efficiency of the enzyme could retain greater than 81% of the initial degradation efficiency after the CysGO-MlrA had been reused 7 times. (3) CysGO-MlrA retained greater than 50% of its activity on the 8th day when preserved at 0 °C, while free-MlrA lost 50% of its activity on the 4th day. (4) CysGO-MlrA and the degradation product of NOD showed no obvious cytotoxicity to zebrafish lymphocytes. Therefore, CysGO-MlrA might be used as an efficient and ecologically safe degradation material for NOD.

Potential of biological approaches for cyanotoxin removal from drinking water: A review

Biological treatment of cyanotoxins has gained much importance in recent decades and holds a promise to work in coordination with various physicochemical treatments. In drinking water treatment plants (DWTPs), effective removal of cyanotoxins with reduced toxicity is a primary concern. Commonly used treatments, such as ozonation, chlorination or activated carbon, undergo significant changes in their operating conditions (mainly dosage) to counter the variation in different environmental parameters, such as pH, temperature, and high cyanotoxin concentration. Presence of metal ions, natural organic matter (NOM), and other chemicals demand higher dosage and hence affect the activation energy to efficiently break down the cyanotoxin molecule. Due to these higher dose requirements, the treatment leads to the formation of toxic metabolites at a concentration high enough to break the guideline values. Biological methods of cyanotoxin removal proceed via enzymatic pathway where the protein-encoding genes are often responsible for the compound breakdown into non-toxic metabolites. However, in contrast to the chemical treatment, the biological processes advance at a much slower kinetic rate, predominantly due to a longer onset period (high lag phase). In fact, more than 90% of the studies reported on the biological degradation of the cyanotoxins attribute the biodegradation to the bacterial suspension. This suspended growth limits the mass transfer kinetics due to the presence of metal ions, NOMs and, other oxidizable matter, which further prolongs the lag phase and makes biological process toxic-free, albeit less efficient. In this context, this review attempts to bring out the importance of the attached growth mechanism, in particular, the biofilm-based treatment approaches which can enhance the biodegradation rate.

Effects of Bacillus subtilis on the growth, colony maintenance, and attached bacterial community composition of colonial cyanobacteria

In freshwater aquaculture ponds, application of algicidal Bacillus is a promising way in the control of cyanobacterial blooms. To best understand Bacillus algicidal characters and mechanisms in the field, different-sized colonial cyanobacteria were isolated from an aquaculture pond, and the effects of B. subtilis on their growth, colony maintenance, and colony-attached bacterial community composition were investigated. The results showed that B. subtilis could inhibit the growth of colonial cyanobacteria. Bigger-sized colonies isolated from the field could spontaneously disintegrate into smaller-sized colonies in the laboratory. Algicidal B. subtilis could accelerate the disintegration of colonies and decrease colony size. B. subtilis not only decreased the colony-attached bacterial community diversity but also changed its composition. B. subtilis increased the relative abundances of some attached bacterial genera, including Pseudomonas, Shewanella, Bacillus, Shinella, Rhizobium, and Ensifer. These bacteria with algicidal, microcystin-degrading, and flocculating activities might be an important contributor to algicidal effects of B. subtilis on colonial cyanobacteria.

Nonalcoholic fatty liver disease alters microcystin-LR toxicokinetics and acute toxicity

Microcystin-LR (MCLR) is a cyanotoxin produced by blue-green algae that causes liver and kidney toxicities. MCLR toxicity is dependent on cellular uptake through the organic anion transporting polypeptide (OATP) transporters. Nonalcoholic fatty liver disease (NAFLD) progresses through multiple stages, alters expression of hepatic OATPs, and is associated with chronic kidney disease. The purpose of this study was to determine whether NAFLD increases systemic exposure to MCLR and influences acute liver and kidney toxicities. Rats were fed a control diet or two dietary models of NAFLD; methionine and choline deficient (MCD) or high fat/high cholesterol (HFHC). Two studies were performed in these groups: 1) a single dose intravenous toxicokinetic study (20 μg/kg), and 2) a single dose intraperitoneal toxicity study (60 μg/kg). Compared to control rats, plasma MCLR area under the concentration-time curve (AUC) in MCD rats doubled, whereas biliary clearance (Clbil) was unchanged; in contrast, plasma AUC in HFHC rats was unchanged, whereas Clbil approximately doubled. Less MCLR bound to PP2A was observed in the liver of MCD rats. This shift in exposure decreased the severity of liver pathology only in the MCD rats after a single toxic dose of MCLR (60 μg/kg). In contrast, the single toxic dose of MCLR increased hepatic inflammation, plasma cholesterol, proteinuria, and urinary KIM1 in HFHC rats more than MCLR exposed control rats. In conclusion, rodent models of NAFLD alter MCLR toxicokinetics and acute toxicity and may have implications for liver and kidney pathologies in NAFLD patients.

Further Understanding of Degradation Pathways of Microcystin-LR by an Indigenous Sphingopyxis sp. in Environmentally Relevant Pollution Concentrations

Microcystin-LR (MC-LR) is the most widely distributed microcystin (MC) that is hazardous to environmental safety and public health, due to high toxicity. Microbial degradation is regarded as an effective and environment-friendly method to remove it, however, the performance of MC-degrading bacteria in environmentally relevant pollution concentrations of MC-LR and the degradation pathways remain unclear. In this study, one autochthonous bacterium, Sphingopyxis sp. m6 which exhibited high MC-LR degradation ability, was isolated from Lake Taihu, and the degrading characteristics in environmentally relevant pollution concentrations were demonstrated. In addition, degradation products were identified by utilizing the full scan mode of UPLC-MS/MS. The data illustrated that strain m6 could decompose MC-LR (1–50 μg/L) completely within 4 h. The degradation rates were significantly affected by temperatures, pH and MC-LR concentrations. Moreover, except for the typical degradation products of MC-LR (linearized MC-LR, tetrapeptide, and Adda), there were 8 different products identified, namely, three tripeptides (Adda-Glu-Mdha, Glu-Mdha-Ala, and Leu-MeAsp-Arg), three dipeptides (Glu-Mdha, Mdha-Ala, and MeAsp-Arg) and two amino acids (Leu, and Arg). To our knowledge, this is the first report of Mdha-Ala, MeAsp-Arg, and Leu as MC-LR metabolites. This study expanded microbial degradation pathways of MC-LR, which lays a foundation for exploring degradation mechanisms and eliminating the pollution of microcystins (MCs).

Biodegradation of microcystin-LR using acclimatized bacteria isolated from different units of the drinking water treatment plant

Bacterial community isolated from different units of a Drinking Water Treatment Plant (DWTP) including pre-ozonation unit (POU), the effluent-sludge mixture of the sedimentation unit (ESSU) and top-sand layer water sample from the filtration unit (TSFU) were acclimatized separately in the microcystin-leucine arginine (MC-LR)-rich environment to evaluate MC-LR biodegradation. Maximum biodegradation efficiency of 97.2 ± 8.7% was achieved by the acclimatized-TSFU bacterial community followed by 72.1 ± 6.4% and 86.2 ± 7.3% by acclimatized-POU and acclimatized-ESSU bacterial community, respectively. Likewise, the non-acclimatized bacterial community showed similar biodegradation efficiency of 71.1 ± 7.37%, 86.7 ± 3.19% and 94.35 ± 10.63% for TSFU, ESSU and POU, respectively, when compared to the acclimatized ones. However, the biodegradation rate increased 1.5-folds for acclimatized versus non-acclimatized conditions. The mass spectrometry studies on MC-LR degradation depicted hydrolytic linearization of cyclic MC-LR along with the formation of small peptide fragments including Adda molecule that is linked to the reduced toxicity (qualitative toxicity analysis). This was further confirmed quantitatively by using Rhizobium meliloti as a bioindicator. The acclimatized-TSFU bacterial community comprised of novel MC-LR degrading strains, Chryseobacterium sp. and Pseudomonas fragi as confirmed by 16S rRNA sequencing.

Importance of bacterial biodegradation and detoxification processes of microcystins for environmental health

Microcystins (MC) the most frequently reported cyanobacterial harmful algal bloom toxins primarily found in some species of freshwater genera pose a serious threat to human and animal health. To reduce health risks associated with MC exposure it is important to remove these toxins found in drinking and recreational waterbodies. Since the physical and chemical water treatment methods are inefficient in completely degrading MC, alternative approaches to effectively detoxify MC have become the focus of global research. The aim of this review was to provide the current approach to cost-effective biological treatment methods which utilize bacteria to degrade MC without generation of harmful by-products. In addition, the catabolic pathways involved in MC-degradation involving proteins encoded mlr gene cluster, intermediate products and efficiencies of bacteria strain/bacteria community are presented and compared.

Removal of Microcystin-LR by a Novel Native Effective Bacterial Community Designated as YFMCD4 Isolated from Lake Taihu

Microcystin-LR (MC-LR) is the most toxic and frequently detected monocyclic heptapeptide hepatotoxin produced by cyanobacteria, which poses a great threat to the natural ecosystem and public health. It is very important to seek environment-friendly and cost-efficient methods to remove MC-LR in water. In this study, the MC-degrading capacities of a novel indigenous bacterial community designated as YFMCD4 and the influence of environmental factors including various temperatures, MC concentrations and pH on the MC-degrading activities were investigated utilizing high-performance liquid chromatography (HPLC). In addition, the MC-degrading mechanism of YFMCD4 was also studied using HPLC coupled with a mass spectrometry equipped with electrospray ionization interface (HPLC-ESI-MS). The data showed MC-LR was completely removed at the maximum rate of 0.5 µg/(mL·h) under the optimal condition by YFMCD4. Two pure bacterial strains Alcaligenes faecalis and Stenotrophomonas acidaminiohila were isolated from YFMCD4 degraded MC-LR at a slower rate. The MC-degrading rates of YFMCD4 were significantly affected by different temperatures, pH and MC-LR concentrations. Two intermediates of a tetrapeptide and Adda appeared in the degradation process. These results illustrate that the novel YFMCD4 is one of the highest effective MC-degrading bacterial community, which can completely remove MC-LR and possesses a significant potential to treat water bodies contaminated by MC-LR.

Microcystin-LR degradation utilizing a novel effective indigenous bacterial community YFMCD1 from Lake Taihu

Microcystins (MC) produced by species of cyanobacteria including Microcystis, Anabaena, and Aphanizomenon are a group of monocyclic hepatotoxins posing serious threat to public health. Microcystin-LR (MC-LR) is the most toxic and frequently encountered microcystin variant in the environment, and thus removal of this toxin using bacteria was shown to be a reliable, efficient, and cost-effective method that avoids utilization of chemicals that may produce potentially harmful by-products. The aim of this study was to determine whether a novel indigenous bacterial community designated YFMCD1 was effective in destroying MC. In addition, the influence of environmental factors such as temperature, MC concentration, and pH was examined on the effectiveness of YFMCD1 to degrade MC-LR. MC-degradation products were identified by high performance liquid chromatography coupled with an ultra-high resolution LTQ Orbitrap Velos Pro ETD mass spectrometry equipped with electrospray ionization interface (HPLC-ESI-MS). MC-LR underwent maximal degradation at rate of 0.5 µg/ml/hr with YFMCD1 containing Klebsiella sp. termed YFMCD1-1 or Stenotrophomonas sp. termed YFMCD1-2. Moreover, Adda (3-amino-9-methoxy-2, 6, 8-trimethyl-10-phenyldeca-4, 6-dienoic acid) is a constituent within the MC-LR molecule found to be responsible for biological activity expression and critical for MC-induced toxicity, which is also degraded by YFMCD1. The results showed that YFMCD1 effectively degraded MC-LR. The degradation rate was significantly affected by temperature, pH, and MC-LR concentrations. Data indicate that this bacterial community may prove beneficial in bioremediation of lakes containing MC.

Priming of microbial microcystin degradation in biomass-fed gravity driven membrane filtration biofilms

Gravity-driven membrane (GDM) filtration is a promising tool for low-cost decentralized drinking water production. The biofilms in GDM systems are able of removing harmful chemical components, particularly toxic cyanobacterial metabolites such as microcystins (MCs). This is relevant for the application of GDM filtration because anthropogenic nutrient input and climate change have led to an increase of toxic cyanobacterial blooms. However, removal of MCs in newly developing GDM biofilms is only established after a prolonged period of time. Since cyanobacterial blooms are transient phenomena, it is important to understand MC removal in mature biofilms with or without prior toxin exposure. In this study, the microbial community composition of GDM biofilms was investigated in systems fed with water from a lake with periodic blooms of MC-producing cyanobacteria. Two out of three experimental treatments were supplemented with dead biomass of a MC-containing cyanobacterial strain, or of a non-toxic mutant, respectively. Analysis of bacterial rRNA genes revealed that both biomass-amended treatments were significantly more similar to each other than to a non-supplemented control. Therefore, it was hypothesized that biofilms could potentially be 'primed' for rapid MC removal by prior addition of non-toxic biomass. A subsequent experiment showed that MC removal developed significantly faster in mature biofilms that were pre-fed with biomass from the mutant strain than in unamended controls, indicating that MC degradation was a facultative trait of bacterial populations in GDM biofilms. The significant enrichment of bacteria related to both aerobic and anaerobic MC degraders suggested that this process might have occurred in parallel in different microniches.

Assessment of the influence of key abiotic factors on the alternative microcystin degradation pathway(s) (mlr-): A detailed comparison with the mlr route (mlr+)

Cyanobacterial proliferation and toxin production in water bodies around the world have led to global concern about the control of these issues. Indigenous bacteria have been shown to degrade the cyanotoxin microcystin (MC) in natural environments. The mlr cluster has been widely used as a marker for microcystin biodegradation; however, recent studies have shown that alternative pathway(s) also contribute to the natural removal of MCs in the ecosystem. The main objective of this study is to provide initial insights concerning how key abiotic factors affect the rate of MC biodegradation via alternative pathway(s) and to provide a detailed comparison with the mlr⁺ pathway. Our results show that nutrient inputs and previous exposure to MCs trigger changes in the rate of MC degradation via alternative pathway(s), while temperature does not produce any significant change. Our results further indicate that the alternative pathway(s) may be less efficient at degrading MCs than the mlr⁺ pathway, suggesting the importance of microbial diversity in determining the half-life of MCs in the water column.

The biodegradation of microcystins in temperate freshwater bodies with previous cyanobacterial history

Cyanobacterial blooms and cyanotoxins occur in freshwater lakes and reservoirs all over the world. Bacterial degradation of microcystins (MC), hepatotoxins produced by several cyanobacterial species, has also been broadly documented. However, information regarding MC biodegradation in European water bodies is very limited. In this paper, the occurrence and identification of MC biodegradation products was documented for 21 European lakes and reservoirs, many of which have well-documented cyanobacterial bloom histories. Varying cyanobacterial abundance and taxonomical composition were documented and MC producers were found in all the analysed samples. Planktothrix agardhii was the most common cyanobacterial species and it formed mass occurrences in four lakes. MC biodegradation was observed in 86% of the samples (18 out of 21), and four products of dmMC-LR decomposition were detected by HPLC and LC-MS methods. The two main products were cyclic dmMC-LR with modifications in the Arg-Asp-Leu region; additionally one product was recognized as the tetrapeptide Adda-Glu-Mdha-Ala. The composition of the detected products suggested a new biochemical pathway of MC degradation. The results confirmed the hypothesis that microcystin biodegradation is a common phenomenon in central European waters and that it may occur by a mechanism which is different from the one previously reported. Such a finding implies the necessity to develop a more accurate methodology for screening bacteria with MC biodegradation ability. Furthermore, it warrants new basic and applied studies on the characterization and utilization of new MC-degrading strains and biodegradation pathways.

A Novel and Native Microcystin-Degrading Bacterium of Sphingopyxis sp. Isolated from Lake Taihu

A native, highly efficient microcystin-LR (MC-LR)-degrading bacterium named a7 was isolated from Lake Taihu and identified as Sphingopyxis sp. by 16S rDNA sequence analysis. The strain a7 could totally degrade MC-LR at a rate of 3.33 mg/(L·h), as detected by high-performance liquid chromatography (HPLC). The mlrA, mlrC, and mlrD genes were detected in the strain a7 by sequence analysis. Tetrapeptide and Adda—which are the middle metabolites of MC-LR—were analyzed via liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) during degradation. These metabolites were degraded completely, which suggested that the native Sphingopyxis sp. a7 was highly efficient in MC-LR degradation under bench conditions. Thus, strain a7 exhibited a significant potential application for bioremediation in water bodies contaminated by MC-LR produced by harmful cyanobacterial blooms.

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.

Discerning biodegradation and adsorption of microcystin-LR in a shallow semi-enclosed bay and bacterial community shifts in response to associated process

Hepatotoxic microcystins (MCs) produced by cyanobacteria pose serious risks to aquatic ecosystems and human health, to understand elimination pathways and mechanisms for MCs, especially in a shallow and semi-enclosed eutrophic area, is of great significance. This study succeed in discerning biodegradation and adsorption of microcystin-LR (MCLR) mediated by water and/or sediment in northern part of Meiliang Bay in Lake Taihu, China, and among the first to reveal the shifts of indigenous bacterial community composition in response to MCLR-biodegradation in sediment by Illumina high-throughput sequencing (HTS). Results confirmed that biodegradation predominantly governed MCLR elimination as compared to adsorption in study area. Through faster biodegradation with a rate of 49.21μgL(-1)d(-1), lake water contributed more to overall MCLR removal than sediment. Sediment also played indispensable role in MCLR removal via primarily biodegradation by indigenous community (a rate of 17.27μgL(-1)d(-1)) and secondarily adsorption (<20% of initial concentration). HTS analysis showed that indigenous community composition shifted with decreased phylogenetic diversity in response to sediment-mediated MCLR-biodegradation. Proteobacteria became predominant (39.34-86.78%) in overall composition after biodegradation, which was mostly contributed by sharp proliferation of β-proteobacteria (22.76-74.80%), and might closely link to MCLR-biodegradation in sediment. Moreover, the members of several genera belonging to α-proteobacteria, β-proteobacteria and γ-proteobacteria seemed to be key degraders because of their dominance or increasing population as MCLR degraded. This study expands understanding on natural elimination mechanism for MCs, and provides guidance to reduce MCs' biological risks and guarantee ecosystem safety in aquatic habitats.

Evaluation of the Destruction of the Harmful Cyanobacteria, Microcystis aeruginosa, with a Cavitation and Superoxide Generating Water Treatment Reactor

Cyanobacterial/Harmful Algal Blooms are a major issue for lakes and reservoirs throughout the U.S.A. An effective destructive technology could be useful to protect sensitive areas, such as areas near water intakes. The study presented in this article explored the use of a reactor called the KRIA Water Treatment System. The reactor focuses on the injection of superoxide (O2 (-)), which is generated electrochemically from the atmosphere, into the water body. In addition, the injection process generates a significant amount of cavitation. The treatment process was tested in 190-L reactors spiked with water from cyanobacterial contaminated lakes. The treatment was very effective at destroying the predominant species of cyanobacteria, Microcystis aeruginosa, organic matter, and decreasing chlorophyll concentration. Microcystin toxin concentrations were also reduced. Data suggest that cavitation alone was an effective treatment, but the addition of superoxide improved performance, particularly regarding removal of cyanobacteria and reduction of microcystin concentration.

Microcystin-tolerant Rhizobium protects plants and improves nitrogen assimilation in Vicia faba irrigated with microcystin-containing waters

Irrigation of crops with microcystins (MCs)-containing waters-due to cyanobacterial blooms-affects plant productivity and could be a way for these potent toxins entering the food chain. This study was performed to establish whether MC-tolerant rhizobia could benefit growth, nodulation, and nitrogen metabolism of faba bean plants irrigated with MC-containing waters. For that, three different rhizobial strains-with different sensitivity toward MCs-were used: RhOF96 (most MC-sensitive strain), RhOF125 (most MC-tolerant strain), or Vicz1.1 (reference strain). As a control, plants grown without rhizobia and fertilized by NH4NO3 were included in the study. MC exposure decreased roots (30-37 %) and shoots (up to 15 %) dry weights in un-inoculated plants, whereas inoculation with rhizobia protects plants toward the toxic effects of MCs. Nodulation and nitrogen content were significantly impaired by MCs, with the exception of plants inoculated with the most tolerant strain RhOF125. In order to deep into the effect of inoculation on nitrogen metabolism, the nitrogen assimilatory enzymes (glutamine synthetase (GS) and glutamate synthase (GOGAT)) were investigated: Fertilized plants showed decreased levels (15-30 %) of these enzymes, both in shoots and roots. By contrast, inoculated plants retained the levels of these enzymes in shoots and roots, as well as the levels of NADH-GOGAT activity in nodules. We conclude that the microcystin-tolerant Rhizobium protects faba bean plants and improves nitrogen assimilation when grown in the presence of MCs.

Bloom Dynamics of Cyanobacteria and Their Toxins: Environmental Health Impacts and Mitigation Strategies

Cyanobacteria are ecologically one of the most prolific groups of phototrophic prokaryotes in both marine and freshwater habitats. Both the beneficial and detrimental aspects of cyanobacteria are of considerable significance. They are important primary producers as well as an immense source of several secondary products, including an array of toxic compounds known as cyanotoxins. Abundant growth of cyanobacteria in freshwater, estuarine, and coastal ecosystems due to increased anthropogenic eutrophication and global climate change has created serious concern toward harmful bloom formation and surface water contamination all over the world. Cyanobacterial blooms and the accumulation of several cyanotoxins in water bodies pose severe ecological consequences with high risk to aquatic organisms and global public health. The proper management for mitigating the worldwide incidence of toxic cyanobacterial blooms is crucial for maintenance and sustainable development of functional ecosystems. Here, we emphasize the emerging information on the cyanobacterial bloom dynamics, toxicology of major groups of cyanotoxins, as well as a perspective and integrative approach to their management.

Identification of a new microcystin-degrading bacterium isolated from Lake Chaohu, China

A microcystin-LR (MC-LR)-degrading bacterium was isolated from Lake Chaohu, a eutrophic freshwater lake containing toxic cyanobacterial blooms. Based on the analysis of 16S rDNA gene sequence and physiobiochemical characteristics, the isolated strain, most likely belongs to the genus Bacillus with the highest sequence similarity value with Bacillus nanhaiencis strain K-W39 (JQ799091.1), was named B. nanhaiencis strain JZ-2013. The strain JZ-2013 could grow on mineral salt medium supplied with MC-LR as sole carbon and nitrogen sources. The optimal temperature and pH for strain JZ-2013 growth and MC-LR biodegradation were 30°C and 8.0, respectively. The MC-LR with the initial concentration of 15 mg/L could be consumed 80 % by strain JZ-2013 within 9 days. The existence of exogenous carbon and nitrogen sources could significantly increase the removal efficiency of MC-LR. The strain JZ-2013 can efficiently removed MC-LR of low concentration in real water sample.

Cyanobacterial toxin degrading bacteria: who are they?

Cyanobacteria are ubiquitous in nature and are both beneficial and detrimental to humans. Benefits include being food supplements and producing bioactive compounds, like antimicrobial and anticancer substances, while their detrimental effects are evident by toxin production, causing major ecological problems at the ecosystem level. To date, there are several ways to degrade or transform these toxins by chemical methods, while the biodegradation of these compounds is understudied. In this paper, we present a meta-analysis of the currently available 16S rRNA and mlrA (microcystinase) genes diversity of isolates known to degrade cyanobacterial toxins. The available data revealed that these bacteria belong primarily to the Proteobacteria, with several strains from the sphingomonads, and one from each of the Methylobacillus and Paucibacter genera. Other strains belonged to the genera Arthrobacter, Bacillus, and Lactobacillus. By combining the ecological knowledge on the distribution, abundance, and ecophysiology of the bacteria that cooccur with toxic cyanobacterial blooms and newly developed molecular approaches, it is possible not only to discover more strains with cyanobacterial toxin degradation abilities, but also to reveal the genes associated with the degradation of these toxins.

Microbial degradation of microcystins

Hepatotoxic microcystins that are produced by freshwater cyanobacteria pose a risk to public health. These compounds may be eliminated by enzymatic degradation. Here, we review the enzymatic pathways for the degradation of these hepatotoxins, some of which are newly discovered processes. The efficiencies of microcystin biodegradation pathways are documented in several papers and are compared here. Additionally, a comprehensive description of the microcystin enzymatic degradation scheme has been supplemented with a proposal for a new biodegradation pathway. Critical comments on less documented hypotheses are also included. The genetic aspects of biodegradation activity are discussed in detail. We also describe some methods that are useful for studying the biological decomposition of microcystins, including screening for microcystin degraders and detecting microcystin degradation products, with an emphasis on mass spectrometric methodology.