Inhibition of several protein phosphatases by a non-covalently interacting microcystin and a novel cyanobacterial peptide, nostocyclin

Microcystins produced by cyanobacterial 'blooms' in reservoirs and lakes pose significant public health problems because they are highly toxic due to potent inhibition of protein serine/threonine phosphatases in the PPP family. A dehydrobutyrine (Dhb)-containing microcystin variant [Asp3, ADMAdda5, Dhb7]microcystin-HtyR isolated from Nostoc sp. was found to potently inhibit PP1, PP2A, PPP4 and PPP5 with IC50 values similar to those of microcystin-LR. However, in contrast to microcystin-LR, which forms a covalent bond with a cysteine residue in these protein phosphatases, Asp,ADMAdda,Dhb-microcystin-HtyR did not form any covalent interaction with PP2A. Since the LD50 for Asp,ADMAdda,Dhb-microcystin-HtyR was 100 microg kg(-1) compared to 50 microg kg(-1) for microcystin-LR, the data indicate that the non-covalent inhibition of protein phosphatases accounts for most of the harmful effects of microcystins in vivo. A 3-amino-6-hydroxy-2-piperidone containing cyclic peptide, nostocyclin, also isolated from Nostoc sp., was non-toxic and exhibited more than 500-fold less inhibitory potency towards PP1, PP2A, PPP4 and PPP5, consistent with the conclusion that potent inhibition of one or more these protein phosphatases underlies the toxicity of microcystins, both lacking and containing Dhb.     

Isolation and characterization of hepatotoxic microcystin homologs from the filamentous freshwater cyanobacterium Nostoc sp. strain 152

A strain of the filamentous cyanobacterium Nostoc sp. isolated from a lake in Finland was found to produce at least nine hepatotoxic peptides with chemical and toxicological properties similar to those of the hepatotoxic hepta- and pentapeptides produced by other cyanobacteria. Toxins were isolated and purified by high-performance liquid chromatography. Amounts available for five of the purified toxins (P6, P14, P15, P16, and P18) were adequate for high-performance liquid chromatography amino acid analysis and determination of molecular weight by fast-atom bombardment-mass spectrometry (FAB-MS). Quantities of three toxins (P14, P15, and P16) were adequate for further analysis by high-resolution FAB-MS, FAB-MS/MS, and 1H-nuclear magnetic resonance. Analysis showed that the toxins are new types of microcystin-LR homologs. Microcystin-LR contains equimolar amounts of D-alanine, L-leucine, D-erythro-beta-methylaspartic acid, L-arginine, ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid), D-glutamic acid, and N-methyldehydroalanine (molecular weight [MW], 994). Nostoc sp. strain 152 was found to produce the following microcystin-LR homologs: (i) P6 contains an extra methylene group most probably due to the presence of N-methyldehydrobutyrine instead of N-methyldehydroalanine (MW, 1,008); (ii) P14 is O-acetyl-O-demethyl ADDA-microcystin-LR (MW, 1,022); (iii) P15 is 3-demethyl-O-acetylADDA-homoarginine-microcystin-LR (MW, 1,036); (iv) P16 is 3-demethyl-O-acetyl-O-demethylADDA-microcystin-LR (MW, 1,008); (v) P18 is 3-demethyl-O-acetyl-O-demethylADDA-homoarginine-microcystin- LR (MW, 1,022). The toxicities of the new microcystin homologs were not significantly different from those of microcystin-LR or demethylmicrocystin-LR.     

Isolation and characterization of hepatotoxic microcystin homologs from the filamentous freshwater cyanobacterium Nostoc sp. strain 152.

A strain of the filamentous cyanobacterium Nostoc sp. isolated from a lake in Finland was found to produce at least nine hepatotoxic peptides with chemical and toxicological properties similar to those of the hepatotoxic hepta- and pentapeptides produced by other cyanobacteria. Toxins were isolated and purified by high-performance liquid chromatography. Amounts available for five of the purified toxins (P6, P14, P15, P16, and P18) were adequate for high-performance liquid chromatography amino acid analysis and determination of molecular weight by fast-atom bombardment-mass spectrometry (FAB-MS). Quantities of three toxins (P14, P15, and P16) were adequate for further analysis by high-resolution FAB-MS, FAB-MS/MS, and 1H-nuclear magnetic resonance. Analysis showed that the toxins are new types of microcystin-LR homologs. Microcystin-LR contains equimolar amounts of D-alanine, L-leucine, D-erythro-beta-methylaspartic acid, L-arginine, ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid), D-glutamic acid, and N-methyldehydroalanine (molecular weight [MW], 994). Nostoc sp. strain 152 was found to produce the following microcystin-LR homologs: (i) P6 contains an extra methylene group most probably due to the presence of N-methyldehydrobutyrine instead of N-methyldehydroalanine (MW, 1,008); (ii) P14 is O-acetyl-O-demethyl ADDA-microcystin-LR (MW, 1,022); (iii) P15 is 3-demethyl-O-acetylADDA-homoarginine-microcystin-LR (MW, 1,036); (iv) P16 is 3-demethyl-O-acetyl-O-demethylADDA-microcystin-LR (MW, 1,008); (v) P18 is 3-demethyl-O-acetyl-O-demethylADDA-homoarginine-microcystin- LR (MW, 1,022). The toxicities of the new microcystin homologs were not significantly different from those of microcystin-LR or demethylmicrocystin-LR.     

Widespread distribution and identification of eight novel microcystins in antarctic cyanobacterial mats

The microcystin (MC) content and cyanobacterial community structure of Antarctic microbial mat samples collected from 40 ponds, lakes, and hydroterrestrial environments were investigated. Samples were collected from Bratina Island and four of the Dry Valleys, Wright, Victoria, Miers, and Marshall. Enzyme-linked immunosorbent assays (ELISAs), liquid chromatography-mass spectrometry (LC-MS), and protein phosphatase 2A (PP-2A) inhibition assays resulted in the identification of low levels (1 to 16 mg/kg [dry weight]) of MCs in all samples. A plot of indicative potencies of MCs (PP-2A inhibition assay/ELISA ratio) versus total MCs (ELISA) showed a general decrease in potency, as total MC levels increased, and a clustering of values from discrete geographic locations. LC-tandem MS analysis on selected samples identified eight novel MC congeners. The low-energy collisional activation spectra were consistent with variants of [D-Asp(3)] MC-RR and [D-Asp(3)] MC-LR containing glycine [Gly(1)] rather than alanine and combinations of homoarginine [hAr(2)] or acetyldemethyl 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid (acetyldemethyl ADDA) [ADMAdda(5)] substitutions. Nostoc sp. was identified as a MC producer using PCR amplification of a region of the 16S rRNA gene and the aminotransferase domain of the mcyE gene. Automated ribosomal intergenic spacer analysis (ARISA) was undertaken to enable a comparison of cyanobacterial mat community structure from distant geographical locations. Two-dimensional multidimensional scaling ordination analysis of the ARISA data showed that in general, samples from the same geographic location tended to cluster together. ARISA also enabled the putative identification of the MC-producing Nostoc sp. from multiple samples.     

Direct evidence for production of microcystins by Anabaena strains from the Baltic Sea

Anabaena is a filamentous, N(2)-fixing, and morphologically diverse genus of cyanobacteria found in freshwater and brackish water environments worldwide. It contributes to the formation of toxic blooms in freshwater bodies through the production of a range of hepatotoxins or neurotoxins. In the Baltic Sea, Anabaena spp. form late summer blooms, together with Nodularia spumigena and Aphanizomenon flos-aquae. It has been long suspected that Baltic Sea Anabaena may produce microcystins. The presence of microcystins has been reported for the coastal regions of the Baltic proper, and a recent report also indicated the presence of the toxin in the open Gulf of Finland. However, at present there is no direct evidence linking Baltic Sea Anabaena spp. to microcystin production. Here we report on the isolation of microcystin-producing strains of the genus Anabaena in the open Gulf of Finland. The dominant microcystin variants produced by these strains included the highly toxic MCYST-LR as well as [d-Asp(3)]MCYST-LR, [d-Asp(3)]MCYST-HtyR, MCYST-HtyR, [d-Asp(3),Dha(7)]MCYST-HtyR, and [Dha(7)]MCYST-HtyR variants. Toxic strains were isolated from the coastal Gulf of Finland as well as from the easternmost open-sea sampling station, where there were lower salinities than at other stations. This result suggests that lower salinity may favor microcystin-producing Anabaena strains. Furthermore, we sequenced 16S rRNA genes and found evidence for pronounced genetic heterogeneity of the microcystin-producing Anabaena strains. Future studies should take into account the potential presence of microcystin-producing Anabaena sp. in the Gulf of Finland.     

THE ECOLOGY OF NOSTOC

Nostoc is a genus of filamentous cyanobacteria that can form macroscopic or microscopic colonies and is common in both terrestrial and aquatic habitats. Much of the success of Nostoc in terrestrial habitats is related to its ability to remain desiccated for months or years and fully recover metabolic activity within hours to days after re-hydration with liquid water . Nostoc can also withstand repeated cycles of freezing and thawing and, thus, is an important component of extreme terrestrial habitats in the Arctic and Antarctic. The ability to fix atmospheric N ₂ can provide an advantage in nitrogen-poor environments . Nostoc also has the ability to screen damaging ultraviolet light in terrestrial and shallow benthic habitats. The genus potentially could be important in paddy rice culture because it fixes nitrogen that may later be released and used by plants; it also may play a role in soil formation and may increase nitrogen input to natural aquatic and terrestrial ecosystems. The abilities to survive in terrestrial habitats and fix N ₂ are important in symbiotic interactions with fungi (lichens), liverworts, hornworts, mosses, ferns, cycads, and the angiosperm Gunnera. Nostoc is somewhat resistant to predation; this probably is related to production of large amounts of sheath material, synthesis of microcystin-like toxins by some strains, and formation of colonies that are too large for many algivores to consume. Some organisms can subsist on Nostoc, although it may not be a preferred food source. Lytic cyanophages also infect Nostoc, but little is known about population control of Nostoc in its natural environment, Late Precambrian fossils resembling Nostoc have been described, and Nostoc possibly has been an important component of many terrestrial and aquatic communities since that time .     

A spontaneous mutant of microcystin biosynthesis: genetic characterization and effect on Daphnia

Microcystis aeruginosa strain MRC is unique in its' possession of the mcyA-J gene cluster, which encodes microcystin synthetase, but its' inability to produce microcystins. M. aeruginosa strain MRD is genetically identical to MRC at numerous genomic loci examined, but produces a variety of microcystins, mainly with the amino acid tyrosine in the molecule. Zooplankton studies with Daphnia galeata and D. pulicaria , using the mutant (MRC) and its' wild type (MRD), showed for the first time that microcystins other than microcystin-LR can be responsible for the poisoning of Daphnia by Microcystis . Regardless of microcystin content, both Daphnia exhibited significantly reduced ingestion rates when fed with either strain of M. aeruginosa compared with the green alga Scenedesmus acutus . A disruption of the molting process in both Daphnia spp. was noted when these species were fed with MRC cells. Such symptoms on Daphnia have not been previously reported for cyanobacteria and may point to a bioactive compound, other than microcystin, which inhibits the hardening of protein–chitin complexes in Daphnia .     

Distribution of microcystin-producing and non-microcystin-producing Microcystis sp. in European freshwater bodies: detection of microcystins and microcystin genes in individual colonies

Microcystis is a well-known cyanobacterial genus frequently producing hepatotoxins named microcystins. Toxin production is encoded by microcystin genes (mcy). This study aims (i) to relate the mcy occurrence in individual colonies to the presence of microcystin, (ii) to assess whether morphological characteristics (morphospecies) are related to the occurrence of mcy genes, and (iii) to test whether there are geographical variations in morphospecies specificity and abundance of mcy genes. Individual colonies of nine different European countries were analysed by (1) morphological characteristics, (2) PCR to amplify a gene region within mcyA and mcyB indicative for microcystin biosynthesis, (3) matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) to detect microcystins. Almost one hundred percent of the colonies predicted to produce microcystins by PCR analysis were found to contain microcystins. A high similarity in microcystin variants in the different colonies selected from lakes across Europe was demonstrated. The different morphospecies varied in the frequency with which they contained mcy genes. Most colonies (>75%) of M. aeruginosa and M. botrys contained the mcy genes, whereas < or = 20% of the colonies identified as M. ichthyoblabe and M. viridis gave a PCR product of the mcy genes. No colonies of M. wesenbergii gave a PCR product of either mcy gene. In addition, a positive relationship was found between the size of the colony and the frequency of those containing the mcy genes. It is concluded that the analysis of morphospecies is indicative for microcystin production, although the quantitative analysis of microcystin concentrations in water remains indispensable for hazard control.     

Isolation and characterization of a variety of microcystins from seven strains of the cyanobacterial genus Anabaena.

Hepatotoxins (microcystins) from seven freshwater Anabaena strains originating from three different Finnish lakes and one lake in Norway were isolated by high-performance liquid chromatography and characterized by amino acid analysis and fast atom bombardment mass spectrometry. All strains produced three to seven different microcystins. A total of 17 different compounds were isolated, of which 8 were known microcystins. The known compounds identified from six strains were MCYST (microcystin)-LR, [D-Asp3]MCYST-LR, [Dha7]MCYST-LR, [D-Asp3,Dha7]MCYST-LR, MCYST-RR, [D-Asp3]MCYST-RR, [Dha7]MCYST-RR, and [D-Asp3,Dha7]MCYST-RR. With the exception of MCYST-LR and [D-Asp3]MCYST-LR, this is the first time that isolation of these toxins from Anabaena strains has been reported. Three of the strains produced one to three toxins as minor components which could not be identified. Anabaena sp. strain 66 produced four unidentified toxins. The other Anabaena strains always contained both MCYST-LR and MCYST-RR and/or their demethyl variants. Quantitative differences between toxins within and between strains were detected; at times MCYST-LR and at other times MCYST-RR or demethyl derivatives thereof were the most abundant toxins found in a strain.     

Isolation and characterization of a variety of microcystins from seven strains of the cyanobacterial genus Anabaena.

Hepatotoxins (microcystins) from seven freshwater Anabaena strains originating from three different Finnish lakes and one lake in Norway were isolated by high-performance liquid chromatography and characterized by amino acid analysis and fast atom bombardment mass spectrometry. All strains produced three to seven different microcystins. A total of 17 different compounds were isolated, of which 8 were known microcystins. The known compounds identified from six strains were MCYST (microcystin)-LR, [D-Asp3]MCYST-LR, [Dha7]MCYST-LR, [D-Asp3,Dha7]MCYST-LR, MCYST-RR, [D-Asp3]MCYST-RR, [Dha7]MCYST-RR, and [D-Asp3,Dha7]MCYST-RR. With the exception of MCYST-LR and [D-Asp3]MCYST-LR, this is the first time that isolation of these toxins from Anabaena strains has been reported. Three of the strains produced one to three toxins as minor components which could not be identified. Anabaena sp. strain 66 produced four unidentified toxins. The other Anabaena strains always contained both MCYST-LR and MCYST-RR and/or their demethyl variants. Quantitative differences between toxins within and between strains were detected; at times MCYST-LR and at other times MCYST-RR or demethyl derivatives thereof were the most abundant toxins found in a strain.     

D-Leu1]Microcystin-LR, from the cyanobacterium Microcystis RST 9501 and from a Microcystis bloom in the Patos Lagoon estuary, Brazil

[D-Leu1]Microcystin-LR was identified as the most abundant microcystin from a laboratory strain of the cyanobacterium Microcystis sp. isolated from a hepatotoxic Microcystis bloom from brackish waters in the Patos Lagoon estuary, southern Brazil. Toxicity of [D-Leu1]microcystin-LR, according to bioassay and protein phosphatase inhibition assay, was similar to that of the commonly-occurring microcystin-LR, which was not detectable in the Patos Lagoon laboratory isolate. This is the first report of a microcystin containing [D-Leu1] in the cyclic heptapeptide structure of these potent cyanobacterial toxins.     

Removal of microcystin-LR by strains of metabolically active probiotic bacteria

The ability of specific strains of probiotic bacteria to remove the cyanobacterial peptide toxin microcystin-LR from aqueous solutions was assessed. Lactobacillus rhamnosus strains GG and LC-705, Bifidobacterium longum 46, Bifidobacterium lactis 420 and Bifidobacterium lactis Bb12 were shown to be the most effective in toxin removal among 11 tested strains. The highest removal percentage of microcystin-LR was 58.1%, observed with B. lactis Bb12 (toxin concentration 100 microg L(-1), 10(10) CFU mL(-1), 37 degrees C, 24 h). Freshly cultured bacteria were shown to be more efficient in microcystin removal than lyophilized or nonviable bacteria. Removal of microcystin-LR was shown to be dependent on both temperature and bacterial concentration. It is concluded that some of the tested strains have good potential in removing microcystins from aqueous solutions.     

Effects of physicochemical variables and cyanobacterial extracts on the immunoassay of microcystin-LR by two ELISA kits

Two types of commercially available ELISA kits for the immunoassay of cyanobacterial microcystins were evaluated for potential interference effects due to methanol, salinity, pH, plasticware and cyanobacterial extract. Of the treatments examined, methanol had the greatest effect, giving false positive microcystin concentrations with increasing methanol concentrations up to 30% (v/v) compared with the negative calibrators of each kit. False positive microcystin results were also produced with increasing salinity up to full strength seawater. Decreases in microcystin-LR equivalents were observed when assaying purified microcystin-LR at pH values between 6.25 and 10. Aqueous microcystin-LR solutions in plastic microcentrifuge tubes after pipetting with disposable plastic tips had lower toxin concentrations than expected when analysed by ELISA. Indicated microcystin concentrations in cyanobacterial extracts varied between kit types and the choice of blanks used. Although ELISAs can be useful tools for the screening of water and cyanobacterial blooms for microcystins and nodularins, users should be aware that commercial kits can be susceptible to interference by commonly encountered environmental and laboratory conditions and materials.     

Investigation of microcystin removal from eutrophic surface water by aquatic vegetable bed

In southern China, many freshwater ecosystems, including lakes, rivers and reservoirs, are eutrophic. The nutrient loading coupled with year-round warm weather favors the growth of cyanobacteria, several of which can produce cyanotoxins, especially the potent liver toxins called microcystins, which are often detected in eutrophic drinking water sources. For purifying raw water used as source of drinking water treatment plants, an aquatic vegetable bed (AVB) experiment had been carried out in a hypertrophic waterfront of Lake Taihu, China, since October 2002. AVB was a simplification of the nutrient film technique (NFT) used to produce vegetables, which requires large quantities of water and nutrients. The average removal rates of total microcystin-RR and microcystin-LR were 63.0% and 66.7%, respectively. This study indicated that Ipomoea aquatica was able to absorb microcystins by using enzyme-linked immunosorbent assay (ELISA), and that the roots absorbed more toxins than leaves and stems. We used fluorescence in situ hybridization (FISH) to analyze the density of microcystin-degrading bacteria in AVB sediment. Two species of microcystin-degrading bacteria were detected, which indicated that microcystin bio-degradation processes did occur in AVB. Protozoa and metazoa were abundant in the rhizosphere. Aspidisca sp., Vorticella sp., Philodina sp., and Lecane sp. were the dominant species. The predation function of protozoa and metazoa had a positive effect on removal of cyanobacteria and microcystins.     

Isolation and identification of eight microcystins from thirteen Oscillatoria agardhii strains and structure of a new microcystin.

Microcystins (cyclic heptapeptide hepatotoxins), isolated from 13 freshwater Oscillatoria agardhii strains from eight different Finnish lakes by high-performance liquid chromatography, were characterized by amino acid analysis, fast atom bombardment mass spectrometry (FABMS), and tandem FABMS (FABMS/collisionary-induced dissociation/MS). All strains produced two to five different microcystins. In total, eight different compounds, of which five were known microcystins, were isolated. The known compounds identified were [D-Asp3]MCYST (microcystin)-LR, [Dha7]MCYST-LR, [D-Asp3]MCYST-RR, [Dha7]MCYST-RR, and [D-Asp3,Dha7]MCYST-RR. This is the first time that isolation of these toxins from Oscillatoria spp., with the exception of [D-Asp3]MCYST-RR, has been reported. Three of the strains produced a new microcystin, and the structure was assigned as [D-Asp3,Mser7]MCYST-RR. The structures of two new microcystins, produced as minor components by one Oscillatoria strain, could not be determined because of the small amounts isolated from the cells. Four strains produced [Dha7]MCYST-RR as the main toxin, but [D-Asp3]MCYST-RR was clearly the most abundant and most frequently occurring toxin among these isolates of O. agardhii.     

Immuno-crossreactivity and toxicity assessment of conjugation products of the cyanobacterial toxin, microcystin-LR

Immunoassays are increasingly used to investigate the production, properties and fates of the cyanobacterial hepatotoxic microcystins in vitro and in vivo. Responses of an ELISA immunoassay to microcystins have been determined using the authentic toxin antigen, microcystin-LR, and conjugation products between the toxin and glutathione, cysteine-glycine and cysteine. The antibodies against microcystin-LR crossreacted with the toxin conjugation products with similar affinities (96-112%) to that of microcystin-LR, when assayed at a concentration of 1 microg l(-1). Toxicity assessment of the conjugates, in comparison to microcystin-LR, indicated a reduction according to mouse bioassay. In vitro protein phosphatase inhibition assay indicated that the conjugates possessed approximately 3-9-fold lower toxicity than microcystin-LR.     

Group 3 sigma factor gene, sigJ, a key regulator of desiccation tolerance, regulates the synthesis of extracellular polysaccharide in cyanobacterium Anabaena sp. strain PCC 7120.

The changes in the expression of sigma factor genes during dehydration in terrestrial Nostoc HK-01 and aquatic Anabaena PCC 7120 were determined. The expression of the sigJ gene in terrestrial Nostoc HK-01, which is homologous to sigJ (alr0277) in aquatic Anabaena PCC 7120, was significantly induced in the mid-stage of dehydration. We constructed a higher-expressing transformant of the sigJ gene (HE0277) in Anabaena PCC 7120, and the transformant acquired desiccation tolerance. The results of Anabaena oligonucleotide microarray experiments showed that a comparatively large number of genes relating to polysaccharide biosynthesis were upregulated in the HE0277 cells. The extracellular polysaccharide released into the culture medium of the HE0277 cells was as much as 3.2-fold more than that released by the control cells. This strongly suggests that the group 3 sigma factor gene sigJ is fundamental and conducive to desiccation tolerance in these cyanobacteria.     

Comparison of the terrestrial cyanobacterium Leptolyngbya sp. NIES-2104 and the freshwater Leptolyngbya boryana PCC 6306 genomes

The cyanobacterial genus Leptolyngbya is widely distributed throughout terrestrial environments and freshwater. Because environmental factors, such as oxygen level, available water content, and light intensity, vary between soil surface and water bodies, terrestrial Leptolyngbya should have genomic differences with freshwater species to adapt to a land habitat. To study the genomic features of Leptolyngbya species, we determined the complete genome sequence of the terrestrial strain Leptolyngbya sp. NIES-2104 and compared it with that of the near-complete sequence of the freshwater Leptolyngbya boryana PCC 6306. The greatest differences between these two strains were the presence or absence of a nitrogen fixation gene cluster for anaerobic nitrogen fixation and several genes for tetrapyrrole synthesis, which can operate under micro-oxic conditions. These differences might reflect differences in oxygen levels where these strains live. Both strains have the genes for trehalose biosynthesis, but only Leptolyngbya sp. NIES-2104 has genetic capacity to produce a mycosporine-like amino acid, mycosporine-glycine. Mycosporine-glycine has an antioxidant action, which may contribute to adaptation to terrestrial conditions. These features of the genomes yielded additional insights into the classification and physiological characteristics of these strains.     

微囊藻毒素对水环境的影响研究进展

微囊藻毒素是富营养化淡水水体中最常见的藻类毒素,是湖泊蓝藻产生的一类肽类毒素,它的产生受到藻类的遗传和环境因素的共同影响。由于其毒性大,分布广,结构稳定,从而成为水环境中的潜在危害物质。有关微囊藻毒素性质、毒理毒性、在环境中的迁移、转化以及控制预防已成为关注热点。在总结国内外研究的基础上,综述了微囊藻毒素的性质、产生机理以及其与水环境、水生生物(水生植物、鱼类、无脊椎动物)间的相互作用,讨论了微囊藻毒素对水生生物的影响以及水生生物对微囊藻毒素的降解作用,为水体中微囊藻毒素的防治提供科学的依据。     

Metagenomic Identification of Bacterioplankton Taxa and Pathways Involved in Microcystin Degradation in Lake Erie

Cyanobacterial harmful blooms (CyanoHABs) that produce microcystins are appearing in an increasing number of freshwater ecosystems worldwide, damaging quality of water for use by human and aquatic life. Heterotrophic bacteria assemblages are thought to be important in transforming and detoxifying microcystins in natural environments. However, little is known about their taxonomic composition or pathways involved in the process. To address this knowledge gap, we compared the metagenomes of Lake Erie free-living bacterioplankton assemblages in laboratory microcosms amended with microcystins relative to unamended controls. A diverse array of bacterial phyla were responsive to elevated supply of microcystins, including Acidobacteria, Actinobacteria, Bacteroidetes, Planctomycetes, Proteobacteria of the alpha, beta, gamma, delta and epsilon subdivisions and Verrucomicrobia. At more detailed taxonomic levels, Methylophilales (mainly in genus Methylotenera) and Burkholderiales (mainly in genera Bordetella, Burkholderia, Cupriavidus, Polaromonas, Ralstonia, Polynucleobacter and Variovorax) of Betaproteobacteria were suggested to be more important in microcystin degradation than Sphingomonadales of Alphaproteobacteria. The latter taxa were previously thought to be major microcystin degraders. Homologs to known microcystin-degrading genes (mlr) were not overrepresented in microcystin-amended metagenomes, indicating that Lake Erie bacterioplankton might employ alternative genes and/or pathways in microcystin degradation. Genes for xenobiotic metabolism were overrepresented in microcystin-amended microcosms, suggesting they are important in bacterial degradation of microcystin, a phenomenon that has been identified previously only in eukaryotic systems.