Interactions between Scenedesmus and Microcystis may be used to clarify the role of secondary metabolites

Microcystis sp. are major players in the global intensification of toxic cyanobacterial blooms endangering the water quality of freshwater bodies. A novel green alga identified as Scenedesmus sp., designated strain huji (hereafter S. huji), was isolated from water samples containing toxic Microcystis sp. withdrawn from Lake Kinneret (Sea of Galilee), Israel, suggesting that it produces secondary metabolites that help it withstand the Microcystis toxins. Competition experiments suggested complex interaction between these two organisms and use of spent cell-free media from S. huji caused severe cell lysis in various Microcystis strains. We have isolated active metabolites from the spent S. huji medium. Application of the concentrated allelochemicals interfered with the functionality and perhaps the integrity of the Microcystis cell membrane, as indicated by the rapid effect on the photosynthetic variable fluorescence and leakage of phycobilins and ions. Although some activity was observed towards various bacteria, it did not alter growth of eukaryotic organisms such as the green alga Chlamydomonas reinhardtii.

Multiannual variations in phytoplankton populations: what distinguished the blooms of Aphanizomenon ovalisporum in Lake Kinneret in 2010 from 2009?

The reasons for large multiannual fluctuations in phytoplankton biomass and composition in freshwater lakes are complex and involve many biotic and abiotic parameters. Here we studied the 2009 and 2010 summer-autumn blooms of the toxic, cylindrospermopsin producer, Aphanizomenon ovalisporum (hereafter Aphanizomenon) in Lake Kinneret (Sea of Galilee), Israel. During the summer the total dissolved phosphate concentration in the lake is very low, close to the detection level, limiting the development of phytoplankton. Earlier we showed that Aphanizomenon blooms are associated with a large rise in alkaline phosphatase (Apase) activity in the water body and that cylindrospermopsin produced by Aphanizomenon induces the PHO regulon, including secretion of Apase, in other alga thereby improving its own phosphate supply. Aphanizomenon transcripts of PHO and AOA (involved in cylindrospermopsin biosynthesis) genes in Lake Kinneret appeared much earlier in 2010 than in 2009 suggesting that the phytoplankton became phosphate-limited already at the beginning of its summer bloom in 2010 but much later in 2009. Water inflow and lake water temperatures were significantly higher in 2010 but the incoming nutrients were consumed by the much larger phytoplankton biomass early in 2010 before the beginning of the Aphanizomenon bloom. An analysis of abiotic and biological parameters provides an explanation for the very different development of Aphanizomenon populations during 2009 and 2010.

Casting a net: fibres produced by Microcystis sp. in field and laboratory populations

The reasons for the apparent dominance of the toxic cyanobacterium Microcystis sp., reflected by its massive blooms in many fresh water bodies, are poorly understood. We show that in addition to a large array of secondary metabolites, some of which are toxic to eukaryotes, Microcystis sp. secretes large amounts of fibrous exopolysaccharides that form extremely long fibres several millimetres in length. This phenomenon was detected in field and laboratory cultures of various Microcystis strains. In addition, we have identified and characterized three of the proteins associated with the fibres and the genes encoding them in Microcystis sp. PCC 7806 but were unable to completely delete them from its genome. Phylogenetic analysis of the most abundant one, designated IPF-469, showed its presence only in cyanobacteria. Its closest relatives were detected in Synechocystis sp. PCC 6803 and in Cyanothece sp. strains; in the latter the genomic organization of the IPF-469 was highly conserved. IPF-469 and the other two proteins identified here, a haloperoxidase and a haemolysin-type calcium-binding protein, may be part of the fibres secretion pathway. The biological role of the fibres in Microcystis sp. is discussed.

A CARD-FISH protocol for the identification and enumeration of cyanobacterial akinetes in lake sediments

Akinetes are the dormant cells of Nostocales (cyanobacteria) that enable the organisms to survive harsh environmental conditions while resting in bottom sediments. The germination of akinetes assists the dispersal and persistence of the species. The assessment of the akinete pool in lake sediments is essential to predict the bloom formation of the Nostocales population. We present here the implementation of an improved catalysed reporter deposition (CARD)-fluorescence in situ hybridization (FISH) protocol to assist the identification and quantification of akinetes in sediment samples. Several 16S rRNA gene oligonucleotide probes were evaluated for labelling akinetes of various species of Anabaena, Aphanizomenon and Cylindrospermopsis. Akinetes of all the taxa studied were successfully labelled and could be easily detected by their bright fluorescence signal. The probes' specificity was tested with 32 strains of different taxa. All six Cylindrospermopsis raciborskii strains were labelled with a specific probe for its 16S rRNA gene. A more general probe labelled 73% of the Anabaena and Aphanizomenon strains. The counting data of field samples obtained with CARD-FISH and the regular light microscopy approach did not differ significantly, confirming the suitability of both methods. The CARD-FISH approach was found to be less time-consuming because of better visibility of akinetes.

The languages spoken in the water body (or the biological role of cyanobacterial toxins)

Although intensification of toxic cyanobacterial blooms over the last decade is a matter of growing concern due to bloom impact on water quality, the biological role of most of the toxins produced is not known. In this critical review we focus primarily on the biological role of two toxins, microcystins and cylindrospermopsin, in inter- and intra-species communication and in nutrient acquisition. We examine the experimental evidence supporting some of the dogmas in the field and raise several open questions to be dealt with in future research. We do not discuss the health and environmental implications of toxin presence in the water body.

Invasion of Nostocales (cyanobacteria) to Subtropical and Temperate Freshwater Lakes - Physiological, Regional, and Global Driving Forces

Similar to the increased number of studies on invasive plants and animals in terrestrial and aquatic ecosystems, many reports were recently published on the invasion of Nostocales (cyanobacteria) to freshwater environments worldwide. Invasion and proliferation of Nostocales in new habitats have the potential to significantly alter the structure of the native community and to modify ecosystem functioning. But most importantly, they influence the water quality due to a variety of toxic compounds that some species produce. Therefore a special attention was given to the invasion and persistence of toxic cyanobacteria in many aquatic ecosystems. Here we summarize the currently published records on the invasion of two Nostocales genera, Cylindrospermopsis and Aphanizomenon, to lakes and water reservoirs in subtropical and temperate zones. These invading species possess traits thought to be common to many invasive organisms: high growth rate, high resource utilization efficiency and overall superior competitive abilities over native species when local conditions vary. Assuming that dispersion routes of cyanobacteria have not been changed much in recent decades, their recent establishment and proliferation in new habitats indicate changes in the environment under which they can exploit their physiological advantage over the native phytoplankton population. In many cases, global warming was identified as the major driving force for the invasion of Nostocales. Due to this uncontrollable trend, invasive Nostocales species are expected to maintain their presence in new habitats and further expand to new environments. In other cases, regional changes in nutrient loads and in biotic conditions were attributed to the invasion events.

Massive multiplication of genome and ribosomes in dormant cells (akinetes) of Aphanizomenon ovalisporum (Cyanobacteria)

Akinetes are dormancy cells commonly found among filamentous cyanobacteria, many of which are toxic and/or nuisance, bloom-forming species. Development of akinetes from vegetative cells is a process that involves morphological and biochemical modifications. Here, we applied a single-cell approach to quantify genome and ribosome content of akinetes and vegetative cells in Aphanizomenon ovalisporum (Cyanobacteria). Vegetative cells of A. ovalisporum were naturally polyploid and contained, on average, eight genome copies per cell. However, the chromosomal content of akinetes increased up to 450 copies, with an average value of 119 genome copies per akinete, 15-fold higher than that in vegetative cells. On the basis of fluorescence in situ hybridization, with a probe targeting 16S rRNA, and detection with confocal laser scanning microscopy, we conclude that ribosomes accumulated in akinetes to a higher level than that found in vegetative cells. We further present evidence that this massive accumulation of nucleic acids in akinetes is likely supported by phosphate supplied from inorganic polyphosphate bodies that were abundantly present in vegetative cells, but notably absent from akinetes. These results are interpreted in the context of cellular investments for proliferation following a long-term dormancy, as the high nucleic acid content would provide the basis for extended survival, rapid resumption of metabolic activity and cell division upon germination.

An AbrB-like protein might be involved in the regulation of cylindrospermopsin production by Aphanizomenon ovalisporum

Certain filamentous cyanobacteria, including Aphanizomenon ovalisporum, are potentially toxic owing to the formation of the hepatotoxin cylindrospermopsin. We previously identified a gene cluster in A. ovalisporum likely to be involved in cylindrospermopsin biosynthesis, including amidinotransferase (aoaA) and polyketide-synthase (aoaC), transcribed on the reverse strands. Analysis of the genomic region between aoaA and aoaC identified two transcription start points for each of these genes, differentially expressed under nitrogen and light stress conditions. The transcript abundances of these genes and the cylindrospermopsin level were both affected by nitrogen availability and light intensity. Gel shift assays and DNA affinity columns isolated a protein that specifically binds to a 150 bp DNA fragment from the region between aoaA and aoaC, and MS/MS analyses identified similarity to AbrB in other cyanobacteria and in Bacillus sp. Comparison of the native AbrB isolated from A. ovalisporum with that obtained after cloning and overexpression of abrB in Escherichia coli identified specific post-translational modifications in the native cyanobacterial protein. These modifications, which are missing in the protein expressed in E. coli, include N-acetylation and methylation of specific residues. We discuss the possible role of these modifications in the regulation of cylindrospermopsin production in Aphanizomenon.

Towards clarification of the biological role of microcystins, a family of cyanobacterial toxins

Microcystins constitute a serious threat to the quality of drinking water worldwide. These protein phosphatase inhibitors are formed by various cyanobacterial species, including Microcystis sp. Microcystins are produced by a complex microcystin synthetase, composed of peptide synthetases and polyketide synthases, encoded by the mcyA-J gene cluster. Recent phylogenetic analysis suggested that the microcystin synthetase predated the metazoan lineage, thus dismissing the possibility that microcystins emerged as a means of defence against grazing, and their original biological role is not clear. We show that lysis of Microcystis cells, either mechanically or because of various stress conditions, induced massive accumulation of McyB and enhanced the production of microcystins in the remaining Microcystis cells. A rise in McyB content was also observed following exposure to microcystin or the protease inhibitors micropeptin and microginin, also produced by Microcystis. The extent of the stimulation by cell extract was strongly affected by the age of the treated Microcystis culture. Older cultures, or those recently diluted from stock cultures, hardly responded to the components in the cell extract. We propose that lysis of a fraction of the Microcystis population is sensed by the rest of the cells because of the release of non-ribosomal peptides. The remaining cells respond by raising their ability to produce microcystins thereby enhancing their fitness in their ecological niche, because of their toxicity.

Oral toxicity of the cyanobacterial toxin cylindrospermopsin in mice: long-term exposure to low doses

The hepatotoxin cylindrospermopsin, a sulfated-guanidinium alkaloid with substituted dioxypyrimidine (uracil) moiety, was isolated from several cyanobacteria species. The acute toxicity of cylindrospermopsin was well established based on intraperitoneal and oral exposure; however, only a few long-term subacute exposure studies were performed to permit a reliable guideline value for cylindrospermopsin in drinking water. In the study reported herein, female and male mice were exposed to cylindrospermopsin in their drinking water. Cylindrospermopsin-containing, Aphanizomenon ovalisporum (cyanobacterium)-free medium was provided as the only source of drinking water, whereas a control group was given a fresh medium for cyanobacteria as drinking water. Over a period of 42 weeks, experiment groups were exposed to cylindrospermopsin concentration, gradually increased from 100 to 550 microg L(-1) (daily exposure ranged between 10 and 55 microg kg(-1) day(-1)). Body and organ weights were recorded, and serum and hematology analyses were performed 20 and 42 weeks after the beginning of the experiment. The most pronounced effect of cylindrospermopsin was elevated hematocrit levels in both male and female mice after 16 weeks of exposure to cylindrospermopsin. The observed changes in the hematocrit level were accompanied by deformation of red blood cells, which were changed into acanthocyte. Based on these results, a daily cylindrospermopsin dose of 20 microg kg(-1) day(-1) (equivalent to 200 microg L(-1)) is proposed as the lowest-observed-adverse-effect level for both male and female mice.

Oral toxicity of the cyanobacterial toxin cylindrospermopsin in mice: long-term exposure to low doses

The hepatotoxin cylindrospermopsin, a sulfated-guanidinium alkaloid with substituted dioxypyrimidine (uracil) moiety, was isolated from several cyanobacteria species. The acute toxicity of cylindrospermopsin was well established based on intraperitoneal and oral exposure; however, only a few long-term subacute exposure studies were performed to permit a reliable guideline value for cylindrospermopsin in drinking water. In the study reported herein, female and male mice were exposed to cylindrospermopsin in their drinking water. Cylindrospermopsin-containing, Aphanizomenon ovalisporum (cyanobacterium)-free medium was provided as the only source of drinking water, whereas a control group was given a fresh medium for cyanobacteria as drinking water. Over a period of 42 weeks, experiment groups were exposed to cylindrospermopsin concentration, gradually increased from 100 to 550 microg L(-1) (daily exposure ranged between 10 and 55 microg kg(-1) day(-1)). Body and organ weights were recorded, and serum and hematology analyses were performed 20 and 42 weeks after the beginning of the experiment. The most pronounced effect of cylindrospermopsin was elevated hematocrit levels in both male and female mice after 16 weeks of exposure to cylindrospermopsin. The observed changes in the hematocrit level were accompanied by deformation of red blood cells, which were changed into acanthocyte. Based on these results, a daily cylindrospermopsin dose of 20 microg kg(-1) day(-1) (equivalent to 200 microg L(-1)) is proposed as the lowest-observed-adverse-effect level for both male and female mice.

Ecological implications of the emergence of non-toxic subcultures from toxic Microcystis strains

Two toxic, microcystin-producing, Microcystis sp. strains KLL MG-K and KLL MB-K were isolated as single colonies on agar plates from Lake Kinneret, Israel. Two non-toxic subcultures, MG-J and MB-J spontaneously succeeded the toxic ones under laboratory conditions. Southern analyses showed that MG-J and MB-J are lacking at least 34 kb of the mcy region, encoding the microcystin synthetase. Analyses of the 16S rRNA genes, the intergenic spacer region between cpcB and cpcA and the patterns of the polymerase chain reaction products of randomly amplified polymorphic DNA and highly iterated palindrome, and presence of mobile DNA elements did not allow unequivocal distinction between toxic and non-toxic subcultures. Laboratory and field experiments indicated an advantage of the toxic strain over its non-toxic successor. When grown separated by a membrane, which allowed passage of the media but not the cells, MG-K severely inhibited the growth of MG-J. Furthermore, when MG strains were placed in dialysis bags in Lake Kinneret during the season in which Microcystis is often observed, cells of MG-J lysed, whereas MG-K survived. Mechanisms whereby the non-toxic subcultures emerged and prevailed over the corresponding toxic ones under laboratory conditions, as well as a possible role of microcystin under natural conditions, are discussed.

The cyanobacterial toxin cylindrospermopsin inhibits pyrimidine nucleotide synthesis and alters cholesterol distribution in mice

The hepatotoxin Cylindrospermopsin, a sulfated-guanidinium alkaloid with substituted dioxypyrimidine (uracil) moiety, was isolated from several cyanobacteria species. Our previous studies on the toxicity of cylindrospermopsin and its derivatives suggested that the uracil moiety is crucial for the toxicity and that such toxicity could partly stem from competitive binding of the toxin to a catalytic site(s) involved in the synthesis of pyrimidine nucleotides (i.e., uridine). In the present study we demonstrated that cylindrospermopsin inhibited in a noncompetitive manner the in vitro activity of uridine monophosphate (UMP) synthase complex (responsible for the conversion of orotic acid to UMP) in a cell free liver extract from mice, with an inhibition constant, KI, of 10 microM. Exposure of mice to cylindrospermopsin at subacute concentrations, via drinking water, only slightly affected the in vitro activity of UMP synthase. The typical metabolic disorder associated with the inhibition of UMP synthase activity, known as "orotic aciduria," was not observed under these conditions, but other anomalous metabolic responses related to cholesterol metabolism were developed.

Interlaboratory comparison trial on cylindrospermopsin measurement

The hepatotoxin cylindrospermopsin (CYN) is a potent inhibitor of protein synthesis in mammalian cells. It is produced by freshwater cyanobacterial blooms in countries such as Australia, the United States, Israel, Thailand, and Brazil. An interlaboratory comparison was organized as a first step to evaluate the measurement of CYN in lyophilized cyanobacterial cells. Six laboratories from Europe, Israel, and Australia participated in the trial. All of the methods used for extraction of the toxin and the high-performance liquid chromatography (HPLC) analysis were satisfactory on the basis of statistical evaluation, according to ISO standards 5725-1 and -2. Further comparison of all the extraction methods by the organizer indicated that the most effective extraction procedure used 5% formic acid to prevent interference in chromatograms by contaminant compounds when analyzed using HPLC employing isocratic conditions of 5% (v/v) aqueous methanol plus 0.1% (v/v) trifluoroacetic acid as the mobile phase.

Effects of the marine unicellular alga Nannochloropsis sp. to reduce the plasma and liver cholesterol levels in male rats fed on diets with cholesterol

The effects of Nannochloropsis were studied on rats consuming hypercholesterolemic diets. The whole biomass and the hexane/ethanol extract increased the plasma and hepatic eicosapentaenoic and docosahexaenoic acids levels, and reduced the cholesterol levels. We also observed a higher level of propionate, and a lower ratio between acetate and propionate. These data suggest the efficacy of Nannochloropsis in reducing cholesterol levels.

Dinoflagellate-cyanobacterium communication may determine the composition of phytoplankton assemblage in a mesotrophic lake

The reasons for annual variability in the composition of phytoplankton assemblages are poorly understood but may include competition for resources and allelopathic interactions. We show that domination by the patch-forming dinoflagellate, Peridinium gatunense, or, alternatively, a bloom of a toxic cyanobacterium, Microcystis sp., in the Sea of Galilee may be accounted for by mutual density-dependent allelopathic interactions. Over the last 11 years, the abundance of these species in the lake displayed strong negative correlation. Laboratory experiments showed reciprocal, density-dependent, but nutrient-independent, inhibition of growth. Application of spent P. gatunense medium induced sedimentation and, subsequently, massive lysis of Microcystis cells within 24 hr, and sedimentation and lysis were concomitant with a large rise in the level of McyB, which is involved in toxin biosynthesis by Microcystis. P. gatunense responded to the presence of Microcystis by a species-specific pathway that involved a biphasic oxidative burst and activation of certain protein kinases. Blocking this recognition by MAP-kinase inhibitors abolished the biphasic oxidative burst and affected the fate (death or cell division) of the P. gatunense cells. We propose that patchy growth habits may confer enhanced defense capabilities, providing ecological advantages that compensate for the aggravated limitation of resources in the patch. Cross-talk via allelochemicals may explain the phytoplankton assemblage in the Sea of Galilee.

A novel gene encoding amidinotransferase in the cylindrospermopsin producing cyanobacterium Aphanizomenon ovalisporum

The hepatotoxin cylindrospermopsin is produced by several cyanobacteria species, which may flourish in tropical and sub-tropical lakes. Biosynthesis of cylindrospermopsin is poorly understood but its chemical nature, and feeding experiments with stable isotopes, suggested that guanidinoacetic acid is the starter unit and indicated involvement of a polyketide synthase. We have identified a gene encoding an amidinotransferase from the cylindrospermopsin producing cyanobacterium Aphanizomenon ovalisporum. This is the first report on an amidinotransferase gene in cyanobacteria. It is likely to be involved in the formation of guanidinoacetic acid. The aoaA is located in a genomic region bearing genes encoding a polyketide synthase and a peptide synthetase, further supporting its putative role in cylindrospermopsin biosynthesis.

Phytoplankton-mediated redox cycle of iron in the epilimnion of Lake Kinneret

The biological-mediated redox cycle of Fe was studied in the epilimnion of Lake Kinneret (Sea of Galilee), a mesotrophic lake in Israel. Multi-annual lake water sampling and incubation experiments were carried out to study Fe(III) reduction by natural phytoplankton populations and their possible role in inhibiting Fe(II) oxidation. The reduction characteristics of the dinoflagellate Peridinium gatunense, the dominant lake alga, were further examined in the laboratory. The steady-state concentration of Fe(II) calculated from the assessed reduction and oxidation rates was compared with Fe(II) measured in the lake in order to evaluate the significance of these processes to the lake Fe redox cycle. Nanomolar concentrations of Fe(II) were measured in the oxygenated, high pH, upper water layer of the lake throughout the year. Reduction rates of Fe by natural phytoplankton assemblages ranged between 0.1 and 10 nM/h. The highest reduction rates, determined in dinoflagellate-dominated lake waters, coincided with the highest concentrations of Fe(II) measured simultaneously in the lake. Iron(II) oxidation rates calculated from the measured lake Fe(II) and the obtained reduction rates were significantly slower than published abiotic Fe(II) oxidation rates. Indeed, Fe(II) oxidation rates measured in algal-enriched lake water were 30-fold slowerthan Fe(II) oxidation rates in natural water, demonstrating the potential for Fe(II) stabilization by the lake phytoplankton.

Uracil moiety is required for toxicity of the cyanobacterial hepatotoxin cylindrospermopsin

A new natural derivative of the sulfated guanidinium zwitterionic toxin cylindrospermopsin, 7-epi-cylindrospermopsin, was recently isolated from the cyanobacterium Aphanizomenon ovalisporum (Forti). The toxicity of the molecule (LD50 ip 5 d), estimated by mouse bioassay, was 200 microg/kg mouse, a value similar to that of cylindrospermopsin. Treatment of cylindrospermopsin with chlorine solution or chlorine-related oxidants produced two new derivatives. The chemical structure of these products was elucidated by nuclear magnetic resonance (NMR) and mass spectrometry (MS) techniques and toxicity was determined. In the first derivative, the vinylic proton at position 5 of the pyrimidine ring was substituted by chlorine to yield 5-chlorocylindrospermopsin. The other product is a truncated one, where C-6 of the pyrimidine ring was oxidized to a carboxylic acid. A trivial name, cylindrospermic acid, was given to this compound. Both products showed no toxic effects even at doses 50 times higher than the LD50 of cylindrospermopsin (10 mg/kg mouse ip). Based on these results, the pyrimidine ring is postulated as the molecule component essential for the toxicity of cylindrospermopsin.

CHARACTERIZATION OF A GENE ENCODING THE LIGHT-HARVESTING VIOLAXANTHIN-CHLOROPHYLL PROTEIN OF NANNOCHLOROPSIS SP. (EUSTIGMATOPHYCEAE)

In contrast to vascular plants, green algae, and diatoms, the major light-harvesting complex of the marine eustigmatophyte genus Nannochloropsis is a violaxanthin-chlorophyll a protein complex that lacks chlorophylls b and c. The isolation of a single polypeptide from the light-harvesting complex of Nannochloropsis sp. (IOLR strain) was previously reported (Sukenik et al. 1992). The NH₂ -terminal amino acid sequence of this polypeptide was significantly similar to NH₂ -terminal sequences of the light-harvesting fucoxanthin, chlorophyll a/c polypeptides from the diatom Phaeodactylum tricornutum Bohlin. Using polyclonal antibodies raised to the Nannochloropsis light-harvesting polypeptide, a gene encoding this polypeptide was isolated from a cDNA expression library. The deduced amino acid sequence of the Nannochloropsis violaxanthin-chlorophyll a polypeptide reveals a 36 amino acid presequence followed by 173 amino acids that constitute the mature polypeptide. The mature polypeptide has 30%-40% sequence identity to the diatom fucoxanthin-chlorophyll a/c polypeptides and less then 27% identity to the green algal and vascular plant light-harvesting chlorophyll polypeptides that bind both chlorophylls a and b. Its molecular mass, as deduced from the gene sequence, is 18.4 kDa with three putative transmembrane helices and several residues that may be involved in chlorophyll binding. The cDNA encoding the violaxanthin-chlorophyll a polypeptide was used to isolate and characterize a 10 kb genomic fragment containing the entire gene. The open reading frame was interrupted by five introns ranging in size from 123 to 449 bp. The intron borders have typical eukaryotic GT … AG sequences.

7-Epicylindrospermopsin, a toxic minor metabolite of the cyanobacterium Aphanizomenon ovalisporum from lake Kinneret, Israel

A toxic minor metabolite, 7-epicylindrospermopsin (1), was isolated from a culture of the cyanobacterium Aphanizomenon ovalisporum isolated from Lake Kinneret in Israel. Homonuclear and inverse-heteronuclear 2D NMR techniques, as well as HRMS and comparison of the NMR data with model compounds, enabled the structure determination of the new compound. Four polymethoxy-1-alkenes, 3-6, were isolated from the lipophilic extract of the cyanobacterium as well.