The success of the bloom-forming cyanobacteria Planktothrix: Genotypes variability supports variable responses to light and temperature stress.

Cyanobacterial blooms can modify the dynamic of aquatic ecosystems and have harmful consequences for human activities. Moreover, cyanobacteria can produce a variety of cyanotoxins, including microcystins, but little is known about the role of environmental factors on the prevalence of microcystin producers in the cyanobacterial bloom dynamics. This study aimed to better understand the success of Planktothrix in various environments by unveiling the variety of strategies governing cell responses to sudden changes in light intensity and temperature. The cellular responses (photosynthesis, photoprotection, heat shock response and metabolites synthesis) of four Planktothrix strains to high-light or high-temperature were studied, focusing on how distinct ecotypes (red- or green-pigmented) and microcystin production capability affect cyanobacteria's ability to cope with such abiotic stimuli. Our results showed that high-light and high-temperature impact different cellular processes and that Planktothrix responses are heterogeneous, specific to each strain and thus, to genotype. The ability of cyanobacteria to cope with sudden increase in light intensity and temperature was not related to red- or green-pigmented ecotype or microcystin production capability. According to our results, microcystin producers do not cope better to high-light or high-temperature and microcystin content does not increase in response to such stresses.

Guidance Level for Brevetoxins in French Shellfish.

Brevetoxins (BTXs) are marine biotoxins responsible for neurotoxic shellfish poisoning (NSP) after ingestion of contaminated shellfish. NSP is characterized by neurological, gastrointestinal and/or cardiovascular symptoms. The main known producer of BTXs is the dinoflagellate Karenia brevis, but other microalgae are also suspected to synthesize BTX-like compounds. BTXs are currently not regulated in France and in Europe. In November 2018, they have been detected for the first time in France in mussels from a lagoon in the Corsica Island (Mediterranean Sea), as part of the network for monitoring the emergence of marine biotoxins in shellfish. To prevent health risks associated with the consumption of shellfish contaminated with BTXs in France, a working group was set up by the French Agency for Food, Environmental and Occupational Health & Safety (Anses). One of the aims of this working group was to propose a guidance level for the presence of BTXs in shellfish. Toxicological data were too limited to derive an acute oral reference dose (ARfD). Based on human case reports, we identified two lowest-observed-adverse-effect levels (LOAELs). A guidance level of 180 µg BTX-3 eq./kg shellfish meat is proposed, considering a protective default portion size of 400 g shellfish meat.

Development of a new extraction method based on high-intensity ultra-sonication to study RNA regulation of the filamentous cyanobacteria Planktothrix.

Efficient RNA extraction methods are needed to study transcript regulation. Such methods must lyse the cell without degrading the genetic material. For cyanobacteria this can be particularly challenging because of the presence of the cyanobacteria cell envelope. The great breath of cyanobacterial shape and size (unicellular, colonial, or filamentous multicellular) created a variety of cell lysis methods. However, there is still a lack of reliable techniques for nucleic acid extraction for several types of cyanobacteria. Here we designed and tested 15 extraction methods using physical, thermic or chemical stress on the filamentous cyanobacteria Planktothrix agardhii. Techniques based on the use of beads, sonication, and heat shock appeared to be too soft to break the Planktothrix agardhii cell envelope, whereas techniques based on the use of detergents degraded the cell envelope but also the RNA. Two protocols allowed to successfully obtain good-quality RNA. The first protocol consisted to manually crush the frozen cell pellet with a pestle and the second was based on the use of high-intensity ultra-sonication. When comparing these two, the high-intensity ultra-sonication protocol was less laborious, faster and allowed to extract 3.5 times more RNA compared to the liquid nitrogen pestle protocol. The high-intensity ultra-sonication protocol was then tested on five Planktothrix strains, this protocol allowed to obtain >8.5 µg of RNA for approximatively 3.5 × 108 cells. The extracted RNA were characterized by 260/280 and 260/230 ratio > to 2, indicating that the samples were devoid of contaminant, and RNA Quality Number > to 7, meaning that the integrity of RNA was preserved with this extraction method. In conclusion, the method we developed based on high-intensity ultra-sonication proved its efficacy in the extraction of Planktothrix RNA and could be helpful for other types of samples.

Cloning of some heat shock proteins genes for further transcriptional study of Planktothrix agardhii exposed to abiotic stress.

Planktothrix agardhii is one of the freshwater cyanobacteria that can produce the hepatotoxin microcystins (MC)-a real threat to human and animal health. Knowledge of the biological role of MC in producing organisms is highly desired to understand the driving force of MC production. Recently, emerging evidences have suggested that MC may have protective role in cells facing environmental stress. If this is true, one should expect differences in the cellular protective mechanisms between MC-containing and MC-deficient mutant strains. To test this hypothesis, it would be essential to investigate the consequences of the loss of MC in Planktothrix in the transcriptional responses of its heat shock proteins (Hsps) to abiotic stresses-an important component of cellular stress response. However, a crucial first step is prerequisite for the isolation of hsp genes here, as the genome of Planktothrix has not been fully published. Therefore, we have successfully isolated four hsp genes including clpC (hsp100), htpG (hsp90), groEL (hsp60), and groES (hsp10) from Planktothrix agardhii PCC 7805 using ramped annealing PCR (RAN-PCR) with consensus-degenerate hybrid oligonucleotide primers (CODEHOP) and annealing control primer (ACP) system. In addition, some putative regulatory sequences found in the upstream region of groESL operon of Planktothrix agardhii were also discussed.

Heat shock transcriptional responses in an MC-Producing Cyanobacterium (Planktothrix agardhii) and its MC-deficient mutant under high light conditions.

Microcystins (MCs) are the most commonly-reported hepatotoxins produced by various cyanobacterial taxa in fresh waters to constitute a potential threat to human and animal health. The biological role of MCs in the producer organisms is not known, and it would be very useful to understand the driving force behind the toxin production. Recent studies have suggested that MCs may have a protective function in cells facing environmental stress. Following this starting premise, we speculate that under adverse conditions the expression of stress-related genes coding for Heat Shock Proteins (Hsp) might be different in an MC-producing strain and its MC-deficient mutant. We therefore used RT-qPCR to compare the expression of 13 hsp genes of an MC-producing strain of Planktothrix agardhii (CYA126/8) and its MC-deficient ΔmcyD mutant over different periods of exposure to high light stress (HL). Three reference genes (RGs) were selected from six candidates to normalize the RT-qPCR data. Of these three RGs (rsh, rpoD, and gltA), gltA is used here for the first time as an RG in prokaryotes. Under HL stress, five genes were found to be strongly up-regulated in both strains (htpG, dnaK, hspA, groES, and groEL). Unexpectedly, we found that the MC-producing wild type strain accumulated higher levels of htpG and dnaK transcripts in response to HL stress than the MC-deficient mutant. In addition, a significant increase in the mcyE transcript was detected in the mutant, suggesting that MCs are required under HL conditions. We discuss several possible roles of MCs in the response to HL stress through their possible involvement in the protective mechanisms of the cells.

Ciliate Nassula sp. grazing on a microcystin-producing cyanobacterium (Planktothrix agardhii): impact on cell growth and in the microcystin fractions.

The proliferation of microcystins (MCs)-producing cyanobacteria (MCs) can have detrimental effects on the food chain in aquatic environments. Until recently, few studies had focused on the fate of MCs in exposed organisms, such as primary consumers of cyanobacteria. In this study, we investigate the impact of an MC-producing strain of the cyanobacterium Planktothrix agardhii on the growth and physiology of a Nassula sp. ciliate isolated from a non-toxic cyanobacterial bloom. We show that this Nassula sp. strain was able to consume and grow while feeding exclusively on an MC-producing cyanobacterium over a prolonged period of time (8 months). In short-term exposure experiments (8 days), ciliates consuming an MC-producing cyanobacterial strain displayed slower growth rate and higher levels of antioxidant enzymes than ciliates feeding on two non-MC-producing strains. Three high-performance methods (LC/MS, LC/MS-MS and ELISA) were used to quantify the free and bound MCs in the culture medium and in the cells. We show that ciliate grazing led to a marked decrease in free MCs (methanol extractable) in cells, the MCs were therefore no longer found in the surrounding culture medium. These findings suggest that MCs may have undergone redistribution (free vs bound MCs) or chemical degradation within the ciliates.

Oral toxicity of extracts of the microcystin-containing cyanobacterium Planktothrix agardhii to the medaka fish (Oryzias latipes).

As previously demonstrated the medaka fish appears to offer a good model for studies of microcystins (MCs) effects. Since cyanobacterial toxins are released with other molecules in the aquatic environment when the producers are dying, in this study, we performed additional experiments in order to compare the described effects obtained with the pure toxin microcystin-LR (MC-LR), among the most toxic MCs, to those induced by complex extracts of an MCs-producer Planktothrix agardhii, strain PMC 75.02 and a natural bloom containing the MCs-producer P. agardhii. The toxicity of these extracts containing several variants of MC was determined in adult medaka treated by gavage. Extracts of an MCs-free strain of P. agardhii (PMC 87.02) were assayed for comparison. Extracts effects were analysed on two tissues, liver and intestine by means of photon and transmission electron microscopy. MC was localized in these tissues by immunocytochemistry. No effect was detectable with extracts of the MCs-free P. agardhii strain. The two MCs-P. agardhii extracts (strain and natural bloom) were able to induce harmful effects in the liver and intestine of the medaka fish in acute intoxication by gavage. In these target organs as shown by toxin immunolocalization, reactions leading to cell disjunction and lysis were observed apparently associated with an immune reaction implying MC containing macrophages. These effects are similar to those previously described with photonic microscopy in medaka treated with pure MC-LR with additional results obtained under the electron microscope. Since no significant effect was detected with the MCs-free (PMC 87.02) extract, we then conclude that MCs, even in complex association with other cyanobacterial components, should be responsible for the toxic effects observed in treated fish.

Changes in cell turgor pressure related to uptake of solutes by Microcystis sp. strain 8401.

Uptake of several naturally occurring organic solutes by the unicellular cyanobacterium Microcystis sp. caused changes in cell turgor pressure (p(t)), which was determined by measuring the mean critical pressure (p(c)) of gas vesicles in the cells. Cells had an initial p(t) of 0.34 MPa, which decreased to 0.08 MPa in 0.15 M sucrose. In solutions of polyols, p(t) gradually recovered as the solutes penetrated the cytoplasmic membranes. From measurements of the exponential rate of turgor increase, cell volume and surface area, the permeability coefficient of the cytoplasmic membrane to each solute was calculated. Permeabilities to amino acids, ammonium ions and sodium acetate indicated little passive movement of these substances across the cell surface from solutions at high concentrations. We looked for evidence of ion trapping of acetic acid: at low pH there was a rapid rise in turgor pressure indicating a rapid uptake of this weak acid. After 20 min the turgor was lost, apparently due to loss of integrity of the cell membranes. For cells in natural habitats, studies of the permeability of cells to solutes is relevant to the problem of retaining substances that are accumulated by active uptake from solutions of low concentrations in natural waters.

Relationships between the Arctic and the Antarctic cyanobacteria; three Phormidium-like strains evaluated by a polyphasic approach.

Selected strains of filamentous Phormidium-like cyanobacteria isolated from two Arctic regions (Ellesmere Island, High Canadian Arctic and Svalbard) and from Antarctica (Antarctic peninsula, South Shetland Islands and South Orkney Islands) were studied. The polyphasic approach used included phenotypic observations of morphological features and genotypic analyses (restriction fragment length polymorphism of 16S rRNA gene, internal transcribed space, 16S rRNA gene sequence analysis). Although genotypes generally correspond to observed morphotypes, the genetic analyses revealed a high degree of biodiversity that could not be unveiled using solely morphological evaluations. According to the phylogenetic analysis, the three clones were divided into two major clades, indicating that the phylogenetic distance between Arct-Ph5/Ant-Ph68 and Ant-Ph58 was so large they belonged to different genera. The polyphyletic position of strains of the genus Phormidium was confirmed by this study, attesting the need to entirely revise classification in this taxon in the future.

Mobile DNA elements in the gas vesicle gene cluster of the planktonic cyanobacteria Microcystis aeruginosa.

Insertion sequences (IS) have been characterized in Microcystis aeruginosa gas vesicle-deficient mutants. ISMae4, a homolog of the cyanobacterial IS702, belongs to the IS5 family, subgroup ISL2. ISMae2 and ISMae3 display typical IS features and express a transposase of the IS4 and IS1 family, respectively. ISMae1 exhibits a more complex genetic structure and harbours a degenerated transposase gene distantly related to IS1 elements. Hybridizations with IS-specific DNA probes suggest that transposition of ISMae2 and ISMae3 occurred by a replicative-type mechanism. To our knowledge this is the first report showing that IS1 elements can be mobile in cyanobacteria.

The gas vesicle gene cluster from Microcystis aeruginosa and DNA rearrangements that lead to loss of cell buoyancy.

Microcystis aeruginosa is a planktonic unicellular cyanobacterium often responsible for seasonal mass occurrences at the surface of freshwater environments. An abundant production of intracellular structures, the gas vesicles, provides cells with buoyancy. A 8.7-kb gene cluster that comprises twelve genes involved in gas vesicle synthesis was identified. Ten of these are organized in two operons, gvpA(I)A(II)A(III)CNJX and gvpKFG, and two, gvpV and gvpW, are individually expressed. In an attempt to elucidate the basis for the frequent occurrence of nonbuoyant mutants in laboratory cultures, four gas vesicle-deficient mutants from two strains of M. aeruginosa, PCC 7806 and PCC 9354, were isolated and characterized. Their molecular analysis unveiled DNA rearrangements due to four different insertion elements that interrupted gvpN, gvpV, or gvpW or led to the deletion of the gvpA(I)-A(III) region. While gvpA, encoding the major gas vesicle structural protein, was expressed in the gvpN, gvpV, and gvpW mutants, immunodetection revealed no corresponding GvpA protein. Moreover, the absence of a gas vesicle structure was confirmed by electron microscopy. This study brings out clues concerning the process driving loss of buoyancy in M. aeruginosa and reveals the requirement for gas vesicle synthesis of two newly described genes, gvpV and gvpW.