Irreversible Collective Migration of Cyanobacteria in Eutrophic Conditions

In response to natural or anthropocentric pollutions coupled to global climate changes, microorganisms from aquatic environments can suddenly accumulate on water surface. These dense suspensions, known as blooms, are harmful to ecosystems and significantly degrade the quality of water resources. In order to determine the physico-chemical parameters involved in their formation and quantitatively predict their appearance, we successfully reproduced irreversible cyanobacterial blooms in vitro. By combining chemical, biochemical and hydrodynamic evidences, we identify a mechanism, unrelated to the presence of internal gas vesicles, allowing the sudden collective upward migration in test tubes of several cyanobacterial strains (Microcystis aeruginosa PCC 7005, Microcystis aeruginosa PCC 7806 and Synechocystis sp. PCC 6803). The final state consists in a foamy layer of biomass at the air-liquid interface, in which micro-organisms remain alive for weeks, the medium lying below being almost completely depleted of cyanobacteria. These "laboratory blooms" start with the aggregation of cells at high ionic force in cyanobacterial strains that produce anionic extracellular polymeric substances (EPS). Under appropriate conditions of nutrients and light intensity, the high photosynthetic activity within cell clusters leads the dissolved oxygen (DO) to supersaturate and to nucleate into bubbles. Trapped within the EPS, these bubbles grow until their buoyancy pulls the biomass towards the free surface. By investigating a wide range of spatially homogeneous environmental conditions (illumination, salinity, cell and nutrient concentration) we identify species-dependent thresholds and timescales for bloom formation. We conclude on the relevance of such results for cyanobacterial bloom formation in the environment and we propose an efficient method for biomass harvesting in bioreactors.Comment: 16 Pages, 4 figure

Fluorescence in situ hybridization (FISH) of Microcystis strains producing microcystin using specific mRNA probes : supplementary materials

Supplementary data related to Zeller et al. 201

Fluorescence in situ hybridization (FISH) of Microcystis strains producing microcystin using specific mRNA probes : supplementary materials

Supplementary data related to Zeller et al. 201

Fluorescence in situ hybridization (FISH) of Microcystis strains producing microcystin using specific mRNA probes : supplementary materials

Supplementary data related to Zeller et al. 201

Evidence that biosynthesis of the neurotoxic alkaloids anatoxin-a and homoanatoxin-a in the cyanobacterium Oscillatoria PCC 6506 occurs on a modular polyketide synthase initiated by L-proline.

International audienceAnatoxin-a and homoanatoxin-a are potent neurotoxins produced by cyanobacteria such as Oscillatoria PCC 6506. Sequencing of the genome of this strain is underway, and we have identified a 29 kb DNA fragment containing a sequence called ks2 that we previously showed to be specific to Oscillatoria cyanobacteria producing anatoxin-a and homoanatoxin-a. Bioinformatic analysis of this 29 kb fragment revealed a cluster of genes, which were annotated. The function assigned to the products of eight contiguous genes, from anaA to anaH, provides a clue to the biosynthesis of anatoxin-a and homoanatoxin-a. Proline is first loaded on an acyl carrier protein and its five-membered cycle oxidized to the pyrroline oxidation state. This activated ring is then successively loaded on three polyketide synthase modules for elongation, reduction, cyclization, and methylation. The final step is the hydrolysis of the thioester with subsequent decarboxylation. GC-MS and NMR analyses of homoanatoxin-a produced by PCC 6506 using labeled precursors confirm that proline is very likely the starter of these polyketide synthases. Using specific PCR amplifications, we have also shown that the anaC, anaE, anaF, and anaG genes are always present in the genome of cyanobacteria producing anatoxin-a and homoanatoxin-a and absent in nonproducing strains. Histidine-tagged AnaC was purified to homogeneity and showed to catalyze the loading of proline on purified histidine-tagged AnaD that had been previously transformed into its holo form using the Bacillus subtilis Sfp phosphopantetheinyl transferase. All of these data provide strong evidence that we have successfully identified the gene cluster responsible for the production of anatoxin-a and homoanatoxin-a in Oscillatoria PCC 6506

The Genome Sequence of the Cyanobacterium Oscillatoria sp. PCC 6506 Reveals Several Gene Clusters Responsible for the Biosynthesis of Toxins and Secondary Metabolites▿

We report a draft sequence of the genome of Oscillatoria sp. PCC 6506, a cyanobacterium that produces anatoxin-a and homoanatoxin-a, two neurotoxins, and cylindrospermopsin, a cytotoxin. Beside the clusters of genes responsible for the biosynthesis of these toxins, we have found other clusters of genes likely involved in the biosynthesis of not-yet-identified secondary metabolites

Dihydroanatoxin-a Is Biosynthesized from Proline in Cylindrospermum stagnale PCC 7417: Isotopic Incorporation Experiments and Mass Spectrometry Analysis

International audienc

Different Genotypes of Anatoxin-Producing Cyanobacteria Coexist in the Tarn River, France▿ †

Repeated dog deaths occurred in 2002, 2003, and 2005 after the animals drank water from the shoreline of the Tarn River in southern France. Signs of intoxication indicated acute poisoning due to a neurotoxin. Floating scum and biofilms covering pebbles were collected in the summers of 2005 and 2006 from six different sites along 30 km from the border of this river. The cyanobacterial neurotoxic alkaloid anatoxin-a and/or its methyl homolog, homoanatoxin-a, was detected in the extracts of most samples examined by gas chromatography-mass spectrometry. Fifteen filamentous cyanobacteria of the order Oscillatoriales were isolated and displayed four distinct phenotypes based on morphological characteristics and pigmentation. Three of the phenotypes can be assigned to the genus Oscillatoria or Phormidium, depending on the taxonomic treatises (bacteriological/botanical) employed for identification. The fourth phenotype is typical of the genus Geitlerinema Anagnostidis 1989. Eight strains rendered axenic were analyzed for production of anatoxin-a and homoanatoxin-a, and all strains of Oscillatoria/Phormidium proved to be neurotoxic. The genetic relatedness of the new isolates was evaluated by comparison of the intergenic transcribed spacer sequences with those of six oscillatorian strains from the Pasteur Culture Collection of Cyanobacteria. These analyses showed that the neurotoxic representatives are composed of five different genotypes, three of which correspond to phenotypes isolated in this study. Our findings prove that neurotoxic oscillatorian cyanobacteria exist in the Tarn River and thus were most likely implicated in the reported dog poisonings. Furthermore, they reemphasize the importance of monitoring benthic cyanobacteria in aquatic environments to fully assess the health risks associated with these organisms

Biosynthesis of Cylindrospermopsin and 7-Epicylindrospermopsin in Oscillatoria sp. Strain PCC 6506: Identification of the cyr Gene Cluster and Toxin Analysis ▿

Cylindrospermopsin is a cytotoxin produced by Cylindrospermopsis raciborskii and other cyanobacteria that has been implicated in human intoxications. We report here the complete sequence of the gene cluster responsible for the biosynthesis of this toxin in Oscillatoria sp. strain PCC 6506. This cluster of genes was found to be homologous with that of C. raciborskii but with a different gene organization. Using an enzyme-linked immunosorbent assay and an optimized liquid chromatography analytical method coupled to tandem mass spectrometry, we detected 7-epicylindrospermopsin, cylindrospermopsin, and 7-deoxycylindrospermopsin in the culture medium of axenic Oscillatoria PCC 6506 at the following relative concentrations: 68.6%, 30.2%, and 1.2%, respectively. We measured the intracellular and extracellular concentrations, per mg of dried cells of Oscillatoria PCC 6506, of 7-epicylindrospermopsin (0.18 μg/mg and 0.29 μg/mg, respectively) and cylindrospermopsin (0.10 μg/mg and 0.11 μg/mg, respectively). We showed that these two toxins accumulated in the culture medium of Oscillatoria PCC 6506 but that the ratio (2.5 ± 0.3) was constant with 7-epicylindrospermopsin being the major metabolite. We also determined the concentrations of these toxins in culture media of other Oscillatoria strains, PCC 6407, PCC 6602, PCC 7926, and PCC 10702, and found that, except for PCC 6602, they all produced 7-epicylindrospermopsin and cylindrospermopsin, with the former being the major toxin, except for PCC 7926, which produced very little 7-epicylindrospermopsin. All the cylindrospermopsin producers studied gave a PCR product using specific primers for the amplification of the cyrJ gene from genomic DNA