Oscillapeptin J, a new grazer toxin of the freshwater cyanobacterium Planktothrix rubescens

Metabolic activity of Planktothrix rubescens and its consequences on oxygen dynamics in laboratory experiment: A stable isotope study

Fluctuations in dissolved oxygen (DO) contents in natural waters can become intense during cyanobacteria blooms. In a reconnaissance study, we investigated DO concentrations and stable isotope dynamics during a laboratory experiment with the cyanobacterium Planktothrix rubescens in order to obtain insights into primary production under specific conditions. This observation was extended to sub-daily timescales with alternating light and dark phases. Dissolved oxygen concentrations and its isotopes (δ18ODO) ranged from 0.02 to 0.06 mmol · L-1 and from +9.6‰ to +23.4‰. The δ18ODO proved to be more sensitive than concentration measurements in response to metabolic variation and registered earlier shifts to dominance by respiration. Oxygen (O2) contents in the headspace and its isotopes (δ18OO2) ranged from 2.62 to 3.20 mmol · L-1 and from +9.8‰ to +21.9‰. Headspace samples showed less fluctuations in concentration and isotope trends because aquatic processes were hardly able to alter signals once the gas had reached the headspace. Headspace δ18OO2 values were corrected for gas-water equilibration and were determined to be higher than the mean δ18OH2O of -8.7‰. This finding suggests that counteracting respiration was important even during the highest photosynthetic activity. Additionally, headspace analyses led to the definition of a fractionation factor for respiration (αR) of this cyanobacterium with a value of 0.980. This value confirms the one commonly used for cyanobacteria. Our findings may become important for the management of water bodies where decreases in DO are caused by cyanobacteria.

Vertical distribution and seasonal dynamics of planktonic cyanobacteria communities in a water column of deep mesotrophic Lake Geneva

Background

Temperate subalpine lakes recovering from eutrophication in central Europe are experiencing harmful blooms due to the proliferation of Planktothrix rubescens, a potentially toxic cyanobacteria. To optimize the management of cyanobacteria blooms there is the need to better comprehend the combination of factors influencing the diversity and dominance of cyanobacteria and their impact on the lake's ecology. The goal of this study was to characterize the diversity and seasonal dynamics of cyanobacteria communities found in a water column of Lake Geneva, as well as the associated changes on bacterioplankton abundance and composition.

Methods

We used 16S rRNA amplicon high throughput sequencing on more than 200 water samples collected from surface to 100 meters deep monthly over 18 months. Bacterioplankton abundance was determined by quantitative PCR and PICRUSt predictions were used to explore the functional pathways present in the community and to calculate functional diversity indices.

Results

The obtained results confirmed that the most dominant cyanobacteria in Lake Geneva during autumn and winter was Planktothrix (corresponding to P. rubescens). Our data also showed an unexpectedly high relative abundance of picocyanobacterial genus Cyanobium, particularly during summertime. Multidimensional scaling of Bray Curtis dissimilarity revealed that the dominance of P. rubescens was coincident with a shift in the bacterioplankton community composition and a significant decline in bacterioplankton abundance, as well as a temporary reduction in the taxonomic and PICRUSt2 predicted functional diversity.

Conclusion

Overall, this study expands our fundamental understanding of the seasonal dynamics of cyanobacteria communities along a vertical column in Lake Geneva and the ecology of P. rubescens, ultimately contributing to improve our preparedness against the potential occurrence of toxic blooms in the largest lake of western Europe.

Structural Diversity and Biological Activity of Cyanopeptolins Produced by Nostoc edaphicum CCNP1411

Cyanopeptolins (CPs) are one of the most commonly occurring class of cyanobacterial nonribosomal peptides. For the majority of these compounds, protease inhibition has been reported. In the current work, the structural diversity of cyanopeptolins produced by Nostoc edaphicum CCNP1411 was explored. As a result, 93 CPs, including 79 new variants, were detected and structurally characterized based on their mass fragmentation spectra. CPs isolated in higher amounts were additionally characterized by NMR. To the best of our knowledge, this is the highest number of cyanopeptides found in one strain. The biological assays performed with the 34 isolated CPs confirmed the significance of the amino acid located between Thr and the unique 3-amino-6-hydroxy-2-piperidone (Ahp) on the activity of the compounds against serine protease and HeLa cancer cells.

Deep Cyanobacteria Layers: An Overlooked Aspect of Managing Risks of Cyanobacteria

The risk of human exposure to cyanotoxins is partially influenced by the location of toxin-producing cyanobacteria in waterbodies. Cyanotoxin production can occur throughout the water column, with deep water production representing a potential public health concern, specifically for drinking water supplies. Deep cyanobacteria layers are often unreported, and it remains to be seen if lower incident rates reflect an uncommon phenomenon or a monitoring bias. Here, we examine Sunfish Lake, Ontario, Canada as a case study lake with a known deep cyanobacteria layer. Cyanotoxin and other bioactive metabolite screening revealed that the deep cyanobacteria layer was toxigenic [0.03 μg L^-1 microcystins (max) and 2.5 μg L^-1 anabaenopeptins (max)]. The deep layer was predominantly composed of Planktothrix isothrix (exhibiting a lower cyanotoxin cell quota), with Planktothrix rubescens (exhibiting a higher cyanotoxin cell quota) found at background levels. The co-occurrence of multiple toxigenic Planktothrix species underscores the importance of routine surveillance for prompt identification leading to early intervention. For instance, microcystin concentrations in Sunfish Lake are currently below national drinking water thresholds, but shifting environmental conditions (e.g., in response to climate change or nutrient modification) could fashion an environment favoring P. rubescens, creating a scenario of greater cyanotoxin production. Future work should monitor the entire water column to help build predictive capacities for identifying waterbodies at elevated risk of developing deep cyanobacteria layers to safeguard drinking water supplies.

Toxic effects of a cyanobacterial strain on Chironomus riparius larvae in a multistress environment

Cyanobacteria and their toxic metabolites present a global threat to water habitats, but their impact on aquatic organisms in a multistress environment has been poorly investigated. Here we present the results of a survey on the effects of the toxic cyanobacterial strain Trichormus variabilis (heterotypic synonym Anabaena variabilis), and its toxic metabolite, cyanotoxin microcystin-LR, on Chironomus riparius larvae in a multistress environment. An environmentally relevant concentration of microcystin-LR (0.01 mg/L) caused an increase in larvae mortality in an acute toxicity test, which became greater in the presence of environmental stressors (NO₃^-, NH4⁺, PO₄^3- and Cd²⁺), pointing to an additive effect of these agents. Chronic exposure of C. riparius larvae to the microcystin-LR producing strain of T. variabilis in a multistress environment led to a reduction in the larval mass and hemoglobin concentration, and it induced DNA damage in larval somatic cells. The results revealed the additive effect of microcystin-LR in combination with all three tested stressors (NO₃^-, NH4⁺, PO₄^3-), and the deleterious effect of chronic exposure of C. riparius larvae to the microcystin-LR producing T. variabilis in a multistress environment. However, the present study further emphasizes the importance of investigating interactions between stressors and cyanotoxins, and their effect on aquatic organisms.

Novel high throughput sequencing - fluorometric approach demonstrates Microcystis blooms across western Lake Erie are promoted by grazing resistance and nutrient enhanced growth

Cyanobacterial harmful algal blooms (CHABs) are a global public health threat. While CHABs are often promoted by nutrients, an important and often overlooked influence on bloom dynamics is zooplankton grazing. In the present study, zooplankton grazing and nutrient enrichment experiments were combined with next generation sequencing and fluorometric analyses to quantify differential grazing and nutrient effects on specific cyanobacterial genera across the western basin of Lake Erie. Grazing by two different sized daphnids, Daphnia magna and Daphnia pulex, was compared to protozooplankton grazing effects assessed via a dilution approach at sites within the Maumee and Sandusky Bays where Planktothrix, Microcystis, Synechococcus, and Dolichospermum were the dominant genera. Daphnid grazing significantly reduced Synechococcus net growth rates at most sites as well as Planktothrix net growth in Sandusky Bay and Dolichospermum in Maumee Bay. Dilution resulted in significant growth increase of Synechococcus at half of the sites and Planktothrix at most sites evidencing substantial grazing pressure by the protozooplankton community on these genera. In contrast, Microcystis populations were largely unaffected by daphnids and protozooplankton grazing but benefitted from nutrient enrichment more than other CHAB genera. When diatoms were present in moderate abundance, grazing rates by daphnids on diatoms were significantly greater than grazing rates on cyanobacteria. The novel approach used in this study established differences in grazing pressure and nutrient effects on differing taxa and revealed that, while many taxa were grazed by multiple classes of zooplankton (e.g. Planktothrix, Synechococcus, Dolichospermum, diatoms), the lack of grazing pressure on Microcystis coupled with nutrient-enhanced growth in western Lake Erie promotes the occurrence of CHABs of this genus.

Cyanobacterial Toxins and Cyanopeptide Transformation Kinetics by Singlet Oxygen and pH-Dependence in Sunlit Surface Waters

To assess the risks associated with cyanobacterial blooms, the persistence and fate processes of cyanotoxins and other bioactive cyanobacterial metabolites need to be evaluated. Here, we investigated the reaction with photochemically produced singlet oxygen (¹O₂) for 30 cyanopeptides synthesized by Dolichospermum flos aquae, including 9 anabaenopeptins, 18 microcystins, 2 cyanopeptolins, and 1 cyclamide. All compounds were stable in UVA light alone but in the presence of a photosensitizer we observed compound-specific degradation. A strong pH effect on the decay was observed for 18 cyanopeptides that all contained tyrosine or structurally related moieties. We can attribute this effect to the reaction with ¹O₂ and triplet sensitizer that preferentially react with the deprotonated form of tyrosine moieties. The contribution of ¹O₂ to indirect phototransformation ranged from 12 to 39% and second-order rate constants for 9 tyrosine-containing cyanopeptides were assessed. Including the pH dependence of the reaction and system-independent second-order rate constants with ¹O₂ will improve the estimation of half-lives for multiclass cyanopeptide in surface waters. Our data further indicates that naturally occurring triplet sensitizers are likely to oxidize deprotonated tyrosine moieties of cyanopeptides and the specific reactivity and its pH dependence needs to be investigated in future studies.

The Red Harmful Plague in Times of Climate Change: Blooms of the Cyanobacterium Planktothrix rubescens Triggered by Stratification Dynamics and Irradiance

Planktothrix rubescens is a harmful planktonic cyanobacterium, forming concentrated metalimnetic populations in deep oligo- and mesotrophic lakes, even after successful restoration. In Lake Zurich (Switzerland), P. rubescens emerged as a keystone species with annual mass developments since the 1970s. Its success was partly attributed to effects of lake warming, such as changes in thermal stratification and seasonal deep mixing. However, recent observations based on a biweekly monitoring campaign (2009-2020) revealed two massive breakdowns and striking seasonal oscillations of the population. Here, we disentangle positive from negative consequences of secular lake warming and annual variations in weather conditions on P. rubescens dynamics: (i) despite the high survival rates of overwintering populations (up to 25%) during three consecutive winters (2014-2016) of incomplete deep convective mixing, cyanobacterial regrowth during the following stratified season was moderate and not overshooting a distinct standing stock threshold. Moreover, we recorded a negative trend for annual population maxima and total population size, pointing to a potential nutrient limitation after a series of incomplete winter mixing. Thus, the predication of steadily increasing blooms of P. rubescens could not be confirmed for the last decade. (ii) The seasonal reestablishment of P. rubescens was strongly coupled with a timely formation of a stable metalimnion structure, where the first positive net growth in the following productive summer season was observed. The trigger for the vertical positioning of filaments within the metalimnion was irradiance and not maximal water column stability. Repetitive disruptions of the vernal metalimnion owing to unstable weather conditions, as in spring 2019, went in parallel with a massive breakdown of the standing stock and marginal regrowth during thermal stratification. (iii) Driven by light intensity, P. rubescens was entrained into the turbulent epilimnion in autumn, followed by a second peak in population growth. Thus, the typical bimodal growth pattern was still intact during the last decade. Our long-term study highlights the finely tuned interplay between climate-induced changes and variability of thermal stratification dynamics and physiological traits of P. rubescens, determining its survival in a mesotrophic temperate lake.

Warming mitigates the enhancement effect of elevated air CO2 on anti-grazer morphological defense in Scenedesmus obliquus

Atmospheric CO₂ and temperature are increasing, which will have substantial impacts on interactions among organisms. While each stressor in isolation has been studied extensively, there has been less focus on their combined effects on the interspecies interaction. In order to reveal how warming and elevated CO₂ interact on the induced defense of phytoplankton, we investigated the combined influences of elevated CO₂ (750 ppm vs 390 ppm) and high temperature (28 °C and 31 °C vs 25 °C) on grazer Daphnia-induced morphological defense in Scenedesmus obliquus. Results showed that S. obliquus formed big-sized colonies (e.g., four- and eight-celled colonies) as response to Daphnia infochemicals, resulting in an increase in the number of cells per particle. Elevated CO₂ further decreased the proportion of unicells from >40% in the populations growing at 390 ppm CO₂ without Daphnia filtrate to <7% in the populations growing at 750 ppm CO₂ with Daphnia filtrate, with the formation of more than 90% colonies, thus enhancing this morphological defense in S. obliquus. However, under elevated CO₂, increasing temperature up to 31 °C remarkably increased the four-celled colonies by at least 159% but decreased the eight-celled colonies by 37% compared with 25 °C. As a result, the maximum cells per particle were significantly decreased to the 390 ppm CO₂-grown level at high temperature. The time to reach the maximum cells per particle was also shortened by high temperature under elevated CO₂. These results suggest that high temperature has an overwhelming inhibitory effect on the enhanced anti-grazer defense by elevated CO₂, which provides significant implications for forecasting the predator-prey interaction changes in freshwater ecosystem under future climate regimes.

The sensitivity of multiple ecotoxicological assays for evaluating Microcystis aeruginosa cellular algal organic matter and contribution of cyanotoxins to the toxicity

Secondary metabolites of cyanobacteria and algae released during algal blooms often exhibit toxic effects, but only a small number of the metabolites are the subject of routine analytical screenings. Alternatively, ecotoxicological assays offer a better representation of the overall negative effects. The aim of this work was to compare multiple assays in their sensitivity towards cellular algal organic matter (COM) of the toxin-producing cyanobacterium Microcystis aeruginosa. Multiple endpoints were investigated: mortality, growth inhibition, bioluminescence inhibition, genotoxicity, endocrine-disrupting effects, oxidative stress, and the induction of ethoxyresorufin-O-deethylase (EROD). Three rainbow trout (Oncorhynchus mykiss) cell lines as well as representatives of bacteria, yeasts, algae, vascular plants, and crustaceans were employed, and the results were expressed per mg of dissolved organic carbon (DOC) in the COM. M. aeruginosa COM was toxic to the RTgill-W1, RTG-2, and RTL-W1 cell lines (EC₅₀ values ranging from 0.48 ± 0.02 to 1.9 ± 0.1 mg_DOC/L), to the crustacean Thamnocephalus platyurus (LC₅₀ = 20 ± 1 mg_DOC/L), and to Lepidium sativum (IC₅₀ = 241 ± 13 mg_DOC/L). In contrast, no effect was observed for bacteria and yeasts, and the growth of the alga Desmodesmus subspicatus was even stimulated. No genotoxicity, endocrine-disrupting effects or increase in oxidative stress or EROD activity was detected. The content of six microcystins (MC-LR, MC-RR, MC-YR, MC-LY, MC-LW, and MC-LF), anatoxin-a, cylindrospermopsin, and nodularin in the M. aeruginosa COM was determined by liquid chromatography-tandem mass spectrometry. An artificially prepared mixture of the detected cyanotoxins in the corresponding concentrations did not induce response in the O. mykiss cell lines and T. platyurus, suggesting that other cyanobacterial metabolites are responsible for the toxicity of M. aeruginosa.

From dolastatin 13 to cyanopeptolins, micropeptins, and lyngbyastatins: the chemical biology of Ahp-cyclodepsipeptides

Covering: 1989 up to 2019 Ahp-cyclodepsipeptides (also known as Ahp-containing cyclodepsipeptides, cyanopeptolins, micropeptins, microginines, and lyngbyastatins, and by many other names) are a family of non-ribosomal peptide synthesis (NRPS)-derived natural products with potent serine protease inhibitory properties. Here, we review their isolation and structural elucidation from natural sources as well as studies of their biosynthesis, molecular mode of action, and use in drug discovery efforts. Accordingly, this summary aims to provide a comprehensive overview of the current state-of-the-art Ahp-cyclodepsipeptide research.

Luciferase reporter assay for small-molecule inhibitors of MIR92b-3p function: Screening cyanopeptolins produced by Nostoc from the Baltic Sea

We developed a cell sensor that detects the liver cancer-specific microRNA MIR92b-3p, involved in hepatocellular carcinoma development and hepatitis C virus infection. To validate our small-molecule screen that employs a Huh7 human hepatoma cell line stably transfected with a pmirGLO vector containing dual luciferase reporters, we used i) a MIR92b-3p antisense or a MIR92b-3p mimicking agent (concentrations from 0.1 pM to 100 nM), ii) expression of XIST, a long non-coding RNA that is a cellular target of MIR92b, and iii) ectopic expression of Luc2 luciferase. This reporter system was used to test four cyanopeptolins from a de novo library of peptides that were isolated from the Baltic Sea cyanobacteria Nostoc edaphicum strain CCNP1411. Exposure of the Huh7-pmirGLO-MIR92b-3p cells to increasing concentrations (from 10 nM to 100 μM) of the cyanopeptolins and microcystin-LR (MC-LR; a treatment control) did not lead to a dose-dependent restoration of the luciferase signal. Instead, when the reporter cells were treated with MC-LR, the luciferase signal decreased markedly, most likely due to non-target, toxic effects of MC-LR on the cells. Although the first use of this reporter system to screen selected Nostoc peptides did not identify inhibitors of MIR92b, this method provides a means to identify functional miRNA regulators and could be readily extended to other compounds.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Natural Products from Cyanobacteria: Focus on Beneficial Activities

Cyanobacteria are photosynthetic microorganisms that colonize diverse environments worldwide, ranging from ocean to freshwaters, soils, and extreme environments. Their adaptation capacities and the diversity of natural products that they synthesize, support cyanobacterial success in colonization of their respective ecological niches. Although cyanobacteria are well-known for their toxin production and their relative deleterious consequences, they also produce a large variety of molecules that exhibit beneficial properties with high potential in various fields (e.g., a synthetic analog of dolastatin 10 is used against Hodgkin's lymphoma). The present review focuses on the beneficial activities of cyanobacterial molecules described so far. Based on an analysis of 670 papers, it appears that more than 90 genera of cyanobacteria have been observed to produce compounds with potentially beneficial activities in which most of them belong to the orders Oscillatoriales, Nostocales, Chroococcales, and Synechococcales. The rest of the cyanobacterial orders (i.e., Pleurocapsales, Chroococcidiopsales, and Gloeobacterales) remain poorly explored in terms of their molecular diversity and relative bioactivity. The diverse cyanobacterial metabolites possessing beneficial bioactivities belong to 10 different chemical classes (alkaloids, depsipeptides, lipopeptides, macrolides/lactones, peptides, terpenes, polysaccharides, lipids, polyketides, and others) that exhibit 14 major kinds of bioactivity. However, no direct relationship between the chemical class and the respective bioactivity of these molecules has been demonstrated. We further selected and specifically described 47 molecule families according to their respective bioactivities and their potential uses in pharmacology, cosmetology, agriculture, or other specific fields of interest. With this up-to-date review, we attempt to present new perspectives for the rational discovery of novel cyanobacterial metabolites with beneficial bioactivity.

Cyanobacterial peptides beyond microcystins - A review on co-occurrence, toxicity, and challenges for risk assessment

Cyanobacterial bloom events that produce natural toxins occur in freshwaters across the globe, yet the potential risk of many cyanobacterial metabolites remains mostly unknown. Only microcystins, one class of cyanopeptides, have been studied intensively and the wealth of evidence regarding exposure concentrations and toxicity led to their inclusion in risk management frameworks for water quality. However, cyanobacteria produce an incredible diversity of hundreds of cyanopeptides beyond the class of microcystins. The question arises, whether the other cyanopeptides are in fact of no human and ecological concern or whether these compounds merely received (too) little attention thus far. Current observations suggest that an assessment of their (eco)toxicological risk is indeed relevant: First, other cyanopeptides, including cyanopeptolins and anabaenopeptins, can occur just as frequently and at similar nanomolar concentrations as microcystins in surface waters. Second, cyanopeptolins, anabaenopeptins, aeruginosins and microginins inhibit proteases in the nanomolar range, in contrast to protein phosphatase inhibition by microcystins. Cyanopeptolins, aeruginosins, and aerucyclamide also show toxicity against grazers in the micromolar range comparable to microcystins. The key challenge for a comprehensive risk assessment of cyanopeptides remains their large structural diversity, lack of reference standards, and high analytical requirements for identification and quantification. One way forward would be a prevalence study to identify the priority candidates of tentatively abundant, persistent, and toxic cyanopeptides to make comprehensive risk assessments more manageable.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Isolation and Structure Elucidation of Secondary Metabolites from a Microcystis sp. Bloom Material Collected in Southern Israel

The hydrophilic extract of Microcystis sp. bloom material collected from Bror Hayil Reservoir in southern Israel afforded four new metabolites, (2 S,3 S)-3-hydeoxy-1,4-diphenylbutan-2-yl-acetate, aeruginosins BH604, BH462A and BH462B, and two known metabolites cyanopeptolins S and SS. The planar structure of 1–4 was established by analyses of their 1D and 2D NMR data and mass spectrometric data. The absolute configurations of the chiral centers of 1 were established by Mosher method and analysis of the coupling constants between H-2 and H-3, and those of 2–4 by Merfay's method and advanced Merfay's method and chiral HPLC. The compounds do not inhibit the serine proteases trypsin and thrombin.

Cyanopeptolins with Trypsin and Chymotrypsin Inhibitory Activity from the Cyanobacterium Nostoc edaphicum CCNP1411

Cyanopeptolins (CPs) are one of the most frequently occurring cyanobacterial peptides, many of which are inhibitors of serine proteases. Some CP variants are also acutely toxic to aquatic organisms, especially small crustaceans. In this study, thirteen CPs, including twelve new variants, were detected in the cyanobacterium Nostoc edaphicum CCNP1411 isolated from the Gulf of Gdańsk (southern Baltic Sea). Structural elucidation was performed by tandem mass spectrometry with verification by NMR for CP962 and CP985. Trypsin and chymotrypsin inhibition assays confirmed the significance of the residue adjacent to 3-amino-6-hydroxy-2-piperidone (Ahp) for the activity of the peptides. Arginine-containing CPs (CPs-Arg²) inhibited trypsin at low IC₅₀ values (0.24⁻0.26 &micro;M) and showed mild activity against chymotrypsin (IC₅₀ 3.1⁻3.8 &micro;M), while tyrosine-containing CPs (CPs-Tyr²) were selectively and potently active against chymotrypsin (IC₅₀ 0.26 &micro;M). No degradation of the peptides was observed during the enzyme assays. Neither of the CPs were active against thrombin, elastase or protein phosphatase 1. Two CPs (CP962 and CP985) had no cytotoxic effects on MCF-7 breast cancer cells. Strong and selective activity of the new cyanopeptolin variants makes them potential candidates for the development of drugs against metabolic disorders and other diseases.

Inhibitors of Serine Proteases from a Microcystis sp. Bloom Material Collected from Timurim Reservoir, Israel

Two new natural products, micropeptin TR1058 (1) and aeruginosin TR642 (2), were isolated from the hydrophilic extract of bloom material of Microcystis sp. collected from the Timurim water reservoir in Israel. The structures of compounds 1 and 2 were determined using 1D and 2D NMR spectroscopy and HR ESI MS and MS/MS techniques. Micropeptin TR1058 (1) was extremely unstable under the isolation conditions, and several degradation products were identified. NMR analysis of aeruginosin TR642 (2) revealed a mixture of eight isomers, and elucidation of its structure was challenging. Aeruginosin TR642 contains a 4,5-didehydroaraginal subunit that has not been described before. Micropeptin TR1058 (1) inhibited chymotrypsin with an IC₅₀ of 6.78 µM, and aeruginosin TR642 (2) inhibited trypsin and thrombin with inhibition concentration (IC₅₀) values of 3.80 and 0.85 µM, respectively. The structures and biological activities of the new compounds are discussed.

Gender-Specific Toxicological Effects of Chronic Exposure to Pure Microcystin-LR or Complex Microcystis aeruginosa Extracts on Adult Medaka Fish

Cyanobacterial blooms often occur in freshwater lakes and constitute a potential health risk to human populations, as well as to other organisms. However, their overall and specific implications for the health of aquatic organisms that are chronically and environmentally exposed to cyanobacteria producing hepatotoxins, such as microcystins (MCs), together with other bioactive compounds have still not been clearly established and remain difficult to assess. The medaka fish was chosen as the experimental aquatic model for studying the cellular and molecular toxicological effects on the liver after chronic exposures (28 days) to environmentally relevant concentrations of pure MC-LR, complex extracts of MC producing or nonproducing cyanobacterial biomasses, and of a Microcystis aeruginosa natural bloom. Our results showed a higher susceptibility of females to the different treatments compared to males at both the cellular and the molecular levels. Although hepatocyte lysis increased with MC-containing treatments, lysis always appeared more severe in the liver of females compare to males, and the glycogen cellular reserves also appeared to decrease more in the liver of females compared to those in the males. Proteomic investigations reveal divergent responses between males and females exposed to all treatments, especially for proteins involved in metabolic and homeostasis processes. Our observations also highlighted the dysregulation of proteins involved in oogenesis in female livers. These results suggest that fish populations exposed to cyanobacteria blooms may potentially face several ecotoxicological issues.

Role of toxic and bioactive secondary metabolites in colonization and bloom formation by filamentous cyanobacteria Planktothrix

Bloom-forming cyanobacteria Planktothrix agardhii and P. rubescens are regularly involved in the occurrence of cyanotoxin in lakes and reservoirs. Besides microcystins (MCs), which inhibit eukaryotic protein phosphatase 1 and 2A, several families of bioactive peptides are produced, thereby resulting in impressive secondary metabolite structural diversity. This review will focus on the current knowledge of the phylogeny, morphology, and ecophysiological adaptations of Planktothrix as well as the toxins and bioactive peptides produced. The relatively well studied ecophysiological adaptations (buoyancy, shade tolerance, nutrient storage capacity) can partly explain the invasiveness of this group of cyanobacteria that bloom within short periods (weeks to months). The more recent elucidation of the genetic basis of toxin and bioactive peptide synthesis paved the way for investigating its regulation both in the laboratory using cell cultures as well as under field conditions. The high frequency of several toxin and bioactive peptide synthesis genes observed within P. agardhii and P. rubescens, but not for other Planktothrix species (e.g. P. pseudagardhii), suggests a potential functional linkage between bioactive peptide production and the colonization potential and possible dominance in habitats. It is hypothesized that, through toxin and bioactive peptide production, Planktothrix act as a niche constructor at the ecosystem scale, possibly resulting in an even higher ability to monopolize resources, positive feedback loops, and resilience under stable environmental conditions. Thus, refocusing harmful algal bloom management by integrating ecological and phylogenetic factors acting on toxin and bioactive peptide synthesis gene distribution and concentrations could increase the predictability of the risks originating from Planktothrix blooms.

Nodularia spumigena peptides--accumulation and effect on aquatic invertebrates

Thus far, the negative effects of Nodularia spumigena blooms on aquatic organisms have been mainly attributed to the production of the hepatotoxic nodularin (NOD). In the current work, the accumulation of other N. spumigena metabolites in blue mussels and crustaceans, and their effect on Thamnocephalus platyurus and Artemia franciscana, were examined. The liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses provided evidence that both blue mussels collected after a cyanobacterial bloom in the Baltic Sea and the crustaceans exposed under laboratory conditions to N. spumigena extract accumulated the cyclic anabaenopeptins (APs). In the crustaceans, the linear peptides, spumigins (SPUs) and aeruginosins (AERs), were additionally detected. Exposure of T. platyurus and A. franciscana to N. spumigena extract confirmed the negative effect of nodularin on the organisms. However, high numbers of dead crustaceans were also recorded in the nodularin-free fraction, which contained protease inhibitors classified to spumigins and aeruginosins. These findings indicate that cyanobacterial toxicity to aquatic organisms is a complex phenomenon and the induced effects can be attributed to diverse metabolites, not only to the known hepatotoxins.

Structural Characterization of New Peptide Variants Produced by Cyanobacteria from the Brazilian Atlantic Coastal Forest Using Liquid Chromatography Coupled to Quadrupole Time-of-Flight Tandem Mass Spectrometry

Cyanobacteria from underexplored and extreme habitats are attracting increasing attention in the search for new bioactive substances. However, cyanobacterial communities from tropical and subtropical regions are still largely unknown, especially with respect to metabolite production. Among the structurally diverse secondary metabolites produced by these organisms, peptides are by far the most frequently described structures. In this work, liquid chromatography/electrospray ionization coupled to high resolution quadrupole time-of-flight tandem mass spectrometry with positive ion detection was applied to study the peptide profile of a group of cyanobacteria isolated from the Southeastern Brazilian coastal forest. A total of 38 peptides belonging to three different families (anabaenopeptins, aeruginosins, and cyanopeptolins) were detected in the extracts. Of the 38 peptides, 37 were detected here for the first time. New structural features were proposed based on mass accuracy data and isotopic patterns derived from full scan and MS/MS spectra. Interestingly, of the 40 surveyed strains only nine were confirmed to be peptide producers; all of these strains belonged to the order Nostocales (three Nostoc sp., two Desmonostoc sp. and four Brasilonema sp.).

New prenylated aeruginosin, microphycin, anabaenopeptin and micropeptin analogues from a Microcystis bloom material collected in Kibbutz Kfar Blum, Israel

Thirteen new and eighteen known natural products were isolated from a bloom material of an assembly of various Microcystis spp. collected in November, 2008, from a commercial fishpond near Kibbutz Kfar Blum, the Jordan Valley, Israel. The new natural products included the prenylated aeruginosin KB676 (1), microphycin KB921 (2), anabaenopeptins KB906 (3) and KB899 (4) and micropeptins KB928 (5), KB956 (6), KB970A (7), KB970B (8), KB984 (9), KB970C (10), KB1048 (11), KB992 (12) and KB1046 (13). Their structures were elucidated primarily by interpretation of their 1D and 2D nuclear magnetic resonance spectra and high-resolution mass spectrometry. Marfey's and chiral-phase high performance liquid chromatography methods were used to determine the absolute configurations of their chiral centers. Aeruginosin KB676 (1) contains the rare (2S,3aS,6S,7aS)-Choi and is the first prenylated aeruginosin derivative described in the literature. Compounds 1 and 5-11 inhibited trypsin with sub-μM IC50s, while Compounds 11-13 inhibited chymotrypsin with sub-μM IC50s. The structures and biological activities of the new natural products and our procedures of dereplication are described.

Chemical compounds toxic to invertebrates isolated from marine cyanobacteria of potential relevance to the agricultural industry

In spite of advances in invertebrate pest management, the agricultural industry is suffering from impeded pest control exacerbated by global climate changes that have altered rain patterns to favour opportunistic breeding. Thus, novel naturally derived chemical compounds toxic to both terrestrial and aquatic invertebrates are of interest, as potential pesticides. In this regard, marine cyanobacterium-derived metabolites that are toxic to both terrestrial and aquatic invertebrates continue to be a promising, but neglected, source of potential pesticides. A PubMed query combined with hand-curation of the information from retrieved articles allowed for the identification of 36 cyanobacteria-derived chemical compounds experimentally confirmed as being toxic to invertebrates. These compounds are discussed in this review.

The toxicity and enzyme activity of a chlorine and sulfate containing aeruginosin isolated from a non-microcystin-producing Planktothrix strain

The toxicity of six different Planktothrix strains was examined in acute toxicity assays with the crustacean Thamnocephalus platyurus. The presence of toxicity in two strains could be explained by the occurrence of microcystins. The other four Planktothrix strains were not able to produce microcystins due to different mutations in the microcystin synthetase (mcy) gene cluster. In these strains, toxicity was attributed to the presence of chlorine and sulfate containing compounds. The main representative, called aeruginosin 828A, of such a compound in the Planktothrix strain 91/1 was isolated, and structure elucidation by 2D-NMR and MS methods revealed the presence of phenyllactic acid (Pla), chloroleucine (Cleu), 2-carboxy-6-(4'-sulfo-xylosyl)-octahydroindole (Choi), and 3-aminoethyl-1-N-amidino-Δ-3-pyrroline (Aeap) residues. Aeruginosin 828A was found to be toxic for Thamnocephalus platyurus with a LC₅₀ value of 22.4 µM, which is only slightly higher than the toxicity found for microcystins. Additionally, very potent inhibition values for thrombin (IC₅₀ = 21.8 nM) and for trypsin (IC₅₀ = 112 nM) have been determined for aeruginosin 828A. These data support the hypothesis that aeruginosins containing chlorine and sulfate groups, which were found in microcystin-deficient Planktothrix strains, can be considered as another class of toxins.

Secondary metabolite gene expression and interplay of bacterial functions in a tropical freshwater cyanobacterial bloom

Cyanobacterial harmful algal blooms (cyanoHABs) appear to be increasing in frequency on a global scale. The Cyanobacteria in blooms can produce toxic secondary metabolites that make freshwater dangerous for drinking and recreation. To characterize microbial activities in a cyanoHAB, transcripts from a eutrophic freshwater reservoir in Singapore were sequenced for six samples collected over one day-night period. Transcripts from the Cyanobacterium Microcystis dominated all samples and were accompanied by at least 533 genera primarily from the Cyanobacteria, Proteobacteria, Bacteroidetes and Actinobacteria. Within the Microcystis population, abundant transcripts were from genes for buoyancy, photosynthesis and synthesis of the toxin microviridin, suggesting that these are necessary for competitive dominance in the Reservoir. During the day, Microcystis transcripts were enriched in photosynthesis and energy metabolism while at night enriched pathways included DNA replication and repair and toxin biosynthesis. Microcystis was the dominant source of transcripts from polyketide and non-ribosomal peptide synthase (PKS and NRPS, respectively) gene clusters. Unexpectedly, expression of all PKS/NRPS gene clusters, including for the toxins microcystin and aeruginosin, occurred throughout the day-night cycle. The most highly expressed PKS/NRPS gene cluster from Microcystis is not associated with any known product. The four most abundant phyla in the reservoir were enriched in different functions, including photosynthesis (Cyanobacteria), breakdown of complex organic molecules (Proteobacteria), glycan metabolism (Bacteroidetes) and breakdown of plant carbohydrates, such as cellobiose (Actinobacteria). These results provide the first estimate of secondary metabolite gene expression, functional partitioning and functional interplay in a freshwater cyanoHAB.

Ahp cyclodepsipeptides: the impact of the Ahp residue on the "canonical inhibition" of S1 serine proteases

S1 serine proteases are by far the largest and most diverse family of proteases encoded in the human genome. Although recent decades have seen an enormous increase in our knowledge, the biological functions of most of these proteases remain to be elucidated. Chemical inhibitors have proven to be versatile tools for studying the functions of proteases, but this approach is hampered by the limited availability of inhibitor scaffold structures with the potential to allow rapid discovery of selective, noncovalent small-molecule protease inhibitors. The natural product class of Ahp cyclodepsipeptides is an unusual class of small-molecule canonical inhibitors; the incorporation of protease cleavage sequences into their molecular scaffolds enables the design of specific small-molecule inhibitors that simultaneously target the S and S' subsites of the protease through noncovalent mechanisms. Their synthesis is tedious, however, so in this study we have investigated the relevance of the Ahp moiety for achieving potent inhibition. We found that although the Ahp residue plays an important role in inhibition potency, appropriate replacement with β-hydroxy amino acids results in structurally less complex derivatives that inhibit serine proteases in the low micromolar range.

Putative antiparasite defensive system involving ribosomal and nonribosomal oligopeptides in cyanobacteria of the genus Planktothrix

Parasitic chytrid fungi can inflict significant mortality on cyanobacteria but frequently fail to keep cyanobacterial dominance and bloom formation in check. Our study tested whether oligopeptide production, a common feature in many cyanobacteria, can be a defensive mechanism against chytrid parasitism. The study employed the cyanobacterial strain Planktothrix NIVA-CYA126/8 and its mutants with knockout mutations for microcystins, anabaenopeptins, and microviridins, major oligopeptide classes to be found in NIVA-CYA126/8. Four chytrid strains were used as parasite models. They are obligate parasites of Planktothrix and are unable to exploit alternative food sources. All chytrid strains were less virulent to the NIVA-CYA126/8 wild type than to at least one of its oligopeptide knockout mutants. One chytrid strain even failed to infect the wild type, while exhibiting considerable virulence to all mutants. It is therefore evident that producing microcystins, microviridins, and/or anabaenopeptins can reduce the virulence of chytrids to Planktothrix, thereby increasing the host's chance of survival. Microcystins and anabaenopeptins are nonribosomal oligopeptides, while microviridins are produced ribosomally, suggesting that Planktothrix resists chytrids by relying on metabolites that are produced via distinct biosynthetic pathways. Chytrids, on the other hand, can adapt to the oligopeptides produced by Planktothrix in different ways. This setting most likely results in an evolutionary arms race, which would probably lead to Planktothrix and chytrid population structures that closely resemble those actually found in nature. In summary, the findings of the present study suggest oligopeptide production in Planktothrix to be part of a defensive mechanism against chytrid parasitism.

Case studies of the synthesis of bioactive cyclodepsipeptide natural products

Cyclodepsipeptide natural products often display intriguing biological activities that along with their complex molecular scaffolds, makes them interesting targets for chemical synthesis. Although cyclodepsipeptides feature highly diverse chemical structures, their synthesis is often associated with similar synthetic challenges such as the establishment of a suitable macrocyclization methodology. This review therefore compiles case studies of synthetic approaches to different bioactive cyclodepsipeptide natural products, thereby illustrating obstacles of cyclodepsipeptide synthesis as well as their overcomings.

Gene flow, recombination, and selection in cyanobacteria: population structure of geographically related Planktothrix freshwater strains

Several Planktothrix strains, each producing a distinct oligopeptide profile, have been shown to coexist within Lake Steinsfjorden (Norway). Using nonribosomal peptide synthetase (NRPS) genes as markers, it has been shown that the Planktothrix community comprises distinct genetic variants displaying differences in bloom dynamics, suggesting a Planktothrix subpopulation structure. Here, we investigate the Planktothrix variants inhabiting four lakes in southeast of Norway utilizing both NRPS and non-NRPS genes. Phylogenetic analyses showed similar topologies for both NRPS and non-NRPS genes, and the lakes appear to have similar structuring of Planktothrix genetic variants. The structure of distinct variants was also supported by very low genetic diversity within variants compared to the between-variant diversity. Incongruent topologies and split decomposition revealed recombination events between Planktothrix variants. In several strains the gene variants seem to be a result of recombination. Both NRPS and non-NRPS genes are dominated by purifying selection; however, sites subjected to positive selection were also detected. The presence of similar and well-separated Planktothrix variants with low internal genetic diversity indicates gene flow within Planktothrix populations. Further, the low genetic diversity found between lakes (similar range as within lakes) indicates gene flow also between Planktothrix populations and suggests recent, or recurrent, dispersals. Our data also indicate that recombination has resulted in new genetic variants. Stability within variants and the development of new variants are likely to be influenced by selection patterns and within-variant homologous recombination.

Inducible tolerance to dietary protease inhibitors in Daphnia magna

Daphnia has been shown to acquire tolerance to cyanobacterial toxins within an animals' lifetime and to transfer this tolerance to the next generation. Here we used a strain of the cyanobacterium Microcystis aeruginosa, which contained two chymotrypsin inhibitors (BN920 and CP954), the green alga Scenedesmus obliquus as reference food and a clone of D. magna to investigate the physiological mechanism of acquired tolerance to these cyanobacterial toxins. The intracellular concentrations of CP954 and BN920 were 1550 and 120 μmol l–1, respectively. When food suspensions of the green alga contained &gt;60% M. aeruginosa, growth rates of D. magna were reduced. When grown on the green alga, three chymotrypsins ranging in mass from 16 to 22 kDa were distinguished in D. magna. Purified BN920 and CP954 specifically inhibited D. magna chymotrypsins. Feeding with encapsulated BN920 resulted in growth depression in D. magna and replacement of the chymotrypsins by three chymotrypsins with smaller molecular mass. With just 20% M. aeruginosa, the same changes in the chymotrypsin pattern as with the pure inhibitor were observed. IC50 values for inhibition of chymotrypsins of D. magna growing on the green alga were 5.4 nmol l–1 (BN920) and 7.4 nmol l–1 (CP954). When D. magna was grown on 20% M. aeruginosa, 2.2-fold higher IC50 values were observed. This indicated that increased tolerance to these dietary inhibitors was acquired within an animal's lifetime by remodelling the digestive chymotrypsins, which in turn serves as an intra-generational defence against these cyanobacterial inhibitors. This mechanism might be relevant for the transfer of tolerance to the next generation through maternal effects.

Effects of a toxic cyanobacterial bloom (Planktothrix agardhii) on fish: insights from histopathological and quantitative proteomic assessments following the oral exposure of medaka fish (Oryzias latipes)

Cyanobacterial toxic blooms often occur in freshwater lakes and constitute a potential health risk to human populations, as well as to fish and other aquatic organisms. Microcystin-LR (the cyanotoxin most commonly detected in the freshwater environment) is a potent hepatotoxin, deregulating the kinase pathway by inhibiting phosphatases 1 and 2A. Although toxicological effects have been clearly linked to the in vitro exposure of fish to purified microcystins, cyanotoxins are produced by the cyanobacteria together with numerous other potentially toxic molecules, and their overall and specific implications for the health of fish have still not been clearly established and remain puzzlingly difficult to assess. The medaka fish (Oryzias latipes) was chosen as an in vitro model for studying the effects of a cyanobacterial bloom on liver protein contents using a gel free quantitative approach, iTRAQ, in addition to pathology examinations on histological preparations. Fish were gavaged with 5 μL cyanobacterial extracts (Planktothrix agardhii) from a natural bloom (La Grande Paroisse, France) containing 2.5 μg equiv. MC-LR. 2h after exposure, the fish were sacrificed and livers were collected for analysis. Histological observations indicate that hepatocytes present glycogen storage loss, and cellular damages, together with immunological localization of MCs. Using a proteomic approach, 304 proteins were identified in the fish livers, 147 of them with a high degree of identification confidence. Fifteen of these proteins were statistically significantly different from those of controls (gavaged with water only). Overall, these protein regulation discrepancies clearly indicate that oxidative stress and lipid regulation had occurred in the livers of the exposed medaka fish. In contrast to previous pure microcystin-LR gavage experiments, marked induction of vitellogenin 1 protein was observed for the first time with a cyanobacterial extract. This finding was confirmed by ELISA quantification of vitellogenin liver content, suggesting that the Planktothrix bloom extract had induced the occurrence of an endocrine-disrupting effect.

Chemodiversity in freshwater and terrestrial cyanobacteria - a source for drug discovery

Cyanobacteria are considered a promising source for new pharmaceutical lead compounds and a large number of chemically diverse and bioactive metabolites have been obtained from cyanobacteria over the last few decades. This review highlights the structural diversity of natural products from freshwater and terrestrial cyanobacteria. The review is divided into three areas: cytotoxic metabolites, protease inhibitors, and antimicrobial metabolites. The first section discusses the potent cytotoxins cryptophycin and tolytoxin. The second section covers protease inhibitors from freshwater and terrestrial cyanobacteria and is divided in five subsections according to structural class: aeruginosins, cyanopeptolins, microviridins, anabaenopeptins, and microginins. Structure activity relationships are discussed within each protease inhibitor class. The third section, antimicrobial metabolites from freshwater and terrestrial cyanobacteria, is divided by chemical class in three subsections: alkaloids, peptides and terpenoids. These examples emphasize the structural diversity and drug development potential of natural products from freshwater and terrestrial cyanobacteria.

Possible implications of chytrid parasitism for population subdivision in freshwater cyanobacteria of the genus Planktothrix

Populations of the cyanobacterium Planktothrix comprise multiple coexisting oligopeptide chemotypes that can behave differently in nature. We tested whether this population subdivision can, in principle, be driven by parasitic chytrid fungi, which are almost neglected agents of Planktothrix mortality. Two chytrid strains, Chy-Lys2009 and Chy-Kol2008, were isolated from Planktothrix-dominated lakes in Norway. The two strains shared 98.2% and 86.2% of their 28S and internal transcribe spacer rRNA gene sequences, respectively. A phylogenetic analysis placed them in the order Rhizophydiales family Angulomycetaceae. Chy-Lys2009 and Chy-Kol2008 could completely lyse Planktothrix cultures within days, while they failed to infect other filamentous cyanobacteria. The effect on Planktothrix was chemotype dependent, and both chytrid strains showed distinct chemotype preferences. These findings identify chytrid fungi infecting Planktothrix as highly potent and specialized parasites which may exert strong selective pressure on their hosts. According to established hypotheses on host-parasite coevolution, parasitism with the above properties may result in subdivision of Planktothrix populations into coexisting chemotypes and periodic shifts in the relative Planktothrix chemotype composition. These predictions are in agreement with field observations. Moreover, a genetic analysis verified the co-occurrence of Chy-Lys2009 and Chy-Kol2008 or related chytrid strains along with distinct Planktothrix chemotypes in at least one water body. Our findings are consistent with a scenario where chytrid parasitism is one driving force of Planktothrix population subdivision, which in turn leads to polymorphism in parasitic chytrid fungi. Future studies should test the validity of this scenario under field conditions.

Solid phase total synthesis of the 3-amino-6-hydroxy-2-piperidone (Ahp) cyclodepsipeptide and protease inhibitor Symplocamide A

The solid phase total synthesis of the marine cyanobacterial Ahp-cyclodepsipeptide Symplocamide A is reported as a model for a general route for the synthesis of tailor-made non-covalent serine protease inhibitors.

Multiple toxin production in the cyanobacterium microcystis: isolation of the toxic protease inhibitor cyanopeptolin 1020

The isolation and structure of cyanopeptolin 1020 (hexanoic acid-Glu-N[-O-Thr-Arg-Ahp-Phe-N-Me-Tyr-Val-]) from a Microcystis strain is reported. Very potent picomolar trypsin inhibition (IC(50) = 670 pM) and low nanomolar values against human kallikrein (4.5 nM) and factor XIa (3.9 nM) have been determined for cyanopeptolin 1020. For plasmin and chymotrypsin, low micromolar concentrations were necessary for 50% inhibition. Cyanopeptolin 1020 was found to be toxic against the freshwater crustacean Thamnocephalus platyurus (LC(50) = 8.8 microM), which is in the same range as some of the well-known microcystins. These data support the hypothesis that cyanopeptolins can be considered as a second class of toxins in addition to the well-established microcystins in Microcystis.

Micropeptins from an Israeli fishpond water bloom of the cyanobacterium Microcystis sp

Seven new natural products, micropeptin MZ845 (1), micropeptin MZ859 (2), micropeptin MZ939A (3), micropeptin MZ925 (4), micropeptin MZ939B (5), micropeptin MZ1019 (6), and micropeptin MZ771 (7), as well as two known micropeptins, cyanopeptolin S (8) and cyanopeptolin SS (9), were isolated from the hydrophilic extract of the cyanobacterium Microcystis sp. that was collected from a fishpond in Kibbutz Ma'ayan Tzvi, Israel, in July 2006. The structures of the pure natural products were elucidated using spectroscopic methods, including UV, 1D and 2D NMR, and MS techniques. The absolute configuration of the chiral centers of the compounds was determined using Marfey's method for HPLC. The inhibitory activity of the compounds was determined for the serine proteases: trypsin, chymotrypsin, thrombin, and elastase. These micropeptins inhibited trypsin with IC(50)'s that varied between 0.6 and 24.2 microM. The SAR of these micropeptins is discussed.

Automated quantification and sizing of unbranched filamentous cyanobacteria by model-based object-oriented image analysis

Quantification and sizing of filamentous cyanobacteria in environmental samples or cultures are time-consuming and are often performed by using manual or semiautomated microscopic analysis. Automation of conventional image analysis is difficult because filaments may exhibit great variations in length and patchy autofluorescence. Moreover, individual filaments frequently cross each other in microscopic preparations, as deduced by modeling. This paper describes a novel approach based on object-oriented image analysis to simultaneously determine (i) filament number, (ii) individual filament lengths, and (iii) the cumulative filament length of unbranched cyanobacterial morphotypes in fluorescent microscope images in a fully automated high-throughput manner. Special emphasis was placed on correct detection of overlapping objects by image analysis and on appropriate coverage of filament length distribution by using large composite images. The method was validated with a data set for Planktothrix rubescens from field samples and was compared with manual filament tracing, the line intercept method, and the Utermöhl counting approach. The computer program described allows batch processing of large images from any appropriate source and annotation of detected filaments. It requires no user interaction, is available free, and thus might be a useful tool for basic research and drinking water quality control.

Comparison of cyanopeptolin genes in Planktothrix, Microcystis, and Anabaena strains: evidence for independent evolution within each genus

The major cyclic peptide cyanopeptolin 1138, produced by Planktothrix strain NIVA CYA 116, was characterized and shown to be structurally very close to the earlier-characterized oscillapeptin E. A cyanopeptolin gene cluster likely to encode the corresponding peptide synthetase was sequenced from the same strain. The 30-kb oci gene cluster contains two novel domains previously not detected in nonribosomal peptide synthetase gene clusters (a putative glyceric acid-activating domain and a sulfotransferase domain), in addition to seven nonribosomal peptide synthetase modules. Unlike in two previously described cyanopeptolin gene clusters from Anabaena and Microcystis, a halogenase gene is not present. The three cyanopeptolin gene clusters show similar gene and domain arrangements, while the binding pocket signatures deduced from the adenylation domain sequences and the additional tailoring domains vary. This suggests loss and gain of tailoring domains within each genus, after the diversification of the three clades, as major events leading to the present diversity. The ABC transporter genes associated with the cyanopeptolin gene clusters form a monophyletic clade and accordingly are likely to have evolved as part of the functional unit. Phylogenetic analyses of adenylation and condensation domains, including domains from cyanopeptolins and microcystins, show a closer similarity between the Planktothrix and Microcystis cyanopeptolin domains than between these and the Anabaena domain. No clear evidence of recombination between cyanopeptolins and microcystins could be detected. There were no strong indications of horizontal gene transfer of cyanopeptolin gene sequences across the three genera, supporting independent evolution within each genus.

Planktocyclin, a cyclooctapeptide protease inhibitor produced by the freshwater cyanobacterium Planktothrix rubescens

The freshwater cyanobacterium Planktothrix rubescens produces the cyclooctapeptide cyclo(Pro-Gly-Leu-Val-Met-Phe-Gly-Val). The chemical structure is new. This homodetic cyclic octapeptide was named planktocyclin ( 1). It consists solely of proteinogenic l-amino acids and is a strong inhibitor of mammalian trypsin and alpha-chymotrypsin and a moderately active inhibitor of human recombinant caspase-8. Mass spectrometric and 2D-NMR spectroscopic data allowed the determination of its structure. Synthetic planktocyclin was identical to the natural product.

Oral toxicity of the microcystin-containing cyanobacterium Planktothrix rubescens in European whitefish (Coregonus lavaretus)

The microcystin-producing cyanobacterium Planktothrix is one of the most widespread genera amongst toxin producing cyanobacteria in European lakes. In particular, the metalimnic blooms of Planktothrix rubescens have been associated with growing problems in the professional freshwater fishery as a decrease in yearly yields in the important coregonids fishery often coincides with the appearance of P. rubescens. P. rubescens is a cyanobacterial species known to produce toxic compounds, e.g. microcystins. Although microcystins have been reported to affect fish health, behaviour, development and growth and have also been associated with feral fish kills, there is currently no specific information on the effects of toxic Planktothrix filaments in fish and especially coregonids. Therefore, the aim of this study was to investigate the effects of an environmentally relevant dose of P. rubescens filaments orally applied to coregonids and to discuss the findings in the context of microcystin toxicity previously reported in carp and trout. A single dose of P. rubescens culture, at a density of 80,000 cells per 120 microl, was applied to coregonids thus corresponding to 0.6-0.9 microg microcystin-LR(equiv.)/kg body weight. Behavioural changes and opercular beat rates, growth, hepatosomatic index, condition and plasma glucose were determined. Liver, kidney, gill and the gastrointestinal tract were assessed histopathologically and immunohistologically. Exposed fish showed behavioural changes, increased opercular beat rates and elevated plasma glucose levels, possibly representing a physiological stress response. Histopathological alterations in liver, gastrointestinal tract and kidney, also immunopositive for microcystin suggested causality of tissue damage and the in situ presence of microcystins. The observed combination of stress and organ damage may explain the frequently reduced weight and thus the fitness noted in coregonids subjected to regular occurrences of stratified and dispersed P. rubescens blooms, e.g. in lake Ammersee, Bavaria, Germany.

Identification of peptide metabolites of Microcystis (Cyanobacteria) that inhibit trypsin-like activity in planktonic herbivorous Daphnia (Cladocera)

Cyanobacteria are recognized as producers of a broad variety of bioactive metabolites. Among these, the peptides synthesized by the non-ribosomal peptide synthetase pathway occur in high structural variability. One class of cyanobacterial peptides, the cyanopeptolins or micropeptins, have been shown to be strong inhibitors of vertebrate serine proteases, like trypsin. In the present study we screened extracts of ten strains of the unicellular cyanobacterium Microcystis sp. for their potential to inhibit trypsin-like activity in the planktonic crustacea Daphnia, the main herbivores in freshwater ecosystem. Respective standardized IC(50)'s varied for nearly two orders of magnitude. In HPLC fractions we could identify mainly cyanopeptolins as active compounds by MALDI-TOF mass spectrometry. Cyanopeptolins were found in 22 structural variants with 13 variants produced by one strain alone. Peptides of the microviridin class were moderately active while no activity was evident for microginins and microcystins. Among the cyanopeptolins only those were active that had an arginine or lysine residue N-terminal to the modified amino acid 3-amino-6-hydroxy-piperidone. Structural variants that had a tyrosine residue at this particular position did not inhibit trypsin-like activity. The highly variable composition of the side chain of cyanopeptolins had no marked effect on the activity. Among the six cyanobacterial strains we tested intensively two did not produce any cyanopeptolins and were accordingly less active as crude extracts. The present study underlines the potential importance of the biochemistry of cyanobacteria for the feeding ecology of a planktonic herbivore.

Frequency of inhibitors of daphnid trypsin in the widely distributed cyanobacterial genus Planktothrix

Recent findings showed that inhibition of the digestive enzyme trypsin by cyanobacterial metabolites can result in the death of the microcrustacean Daphnia. Compounds that are active against daphnid trypsin can therefore be considered as potentially toxic to Daphnia. Here we reported on the frequency of such compounds in the widely distributed cyanobacterial genus Planktothrix. Of the 89 Planktothrix strains analysed, about 70% produced inhibitors of daphnid trypsin. The strains tested positive represented three common Planktothrix species and were isolated from diverse localities and geographical regions. Our findings suggest therefore that inhibitors of daphnid trypsin are common in Planktothrix and maybe other cyanobacterial genera. These compounds should therefore be considered in future studies on the chemical ecology of cyanobacteria.

Characterization of proteases in guts of Daphnia magna and their inhibition by Microcystis aeruginosa PCC 7806

Many cyanobacteria produce peptides that inhibit mammalian proteases. The hypothesis that inhibitors of mammalian proteases produced by cyanobacteria also interfere with digestive proteases of natural cladoceran grazers was tested by comparing the effects of cyanobacterial protease inhibitors on digestive proteases from Daphnia magna and on commercially available bovine proteases. The major digestive proteases of D. magna are trypsins and chymotrypsins, which differ from those of bovine origin in substrate specificity and susceptibility to synthetic inhibitors. An extract from Microcystis aeruginosa strain PCC 7806 inhibited both types of D. magna proteases. Subsequent fractionation of the extract by high-performance liquid chromatography indicated that several inhibitors are produced by M. aeruginosa that differ in their specificity for the trypsins and chymotrypsins of D. magna. Two fractions differed in their inhibitory effect on proteases of D. magna and bovine origin; therefore, assessment of the impact of cyanobacterial protease inhibitors on natural communities requires the use of digestive proteases from ecologically relevant grazers.