Interactions between intracellular Na+ levels and saxitoxin production in Cylindrospermopsis raciborskii T3

Review of the occurrence, treatment technologies, and detection methods for saxitoxins in freshwaters

The increasing occurrence of saxitoxins in freshwaters is becoming a concern for water treatment facilities owing to its structural properties which make it resistant to oxidation at pH < 8. Hence, it is crucial to be able to monitor these toxins in surface and drinking water to protect public health. This review aims to outline the current state of knowledge related to the occurrence of saxitoxins in freshwaters and its removal strategies and provide a critical assessment of the detection methods to provide a basis for further development. Temperature and nutrient content are some of the factors that influence the production of saxitoxins in surface waters. A high dose of sodium hypochlorite with sufficient contact time or activated carbon has been shown to efficiently remove extracellular saxitoxins to meet the drinking water guidelines. While HILIC-MS has proven to be a powerful technology for more sensitive and reliable detection of saxitoxin and variants after solid phase extraction, ELISA is cost-effective and easy to use and is used by Ohio EPA for surveillance with a limit of detection of 0.015 μg/L. However, there is a need for the development of cost-effective and sensitive techniques that can quantify the variants of saxitoxin.

Unveiling the link between Raphidiopsis raciborskii blooms and saxitoxin levels: Evaluating water quality in tropical reservoirs, Brazil

The proliferation of Raphidiopsis raciborskii blooms has sparked concerns regarding potential human exposure to heightened saxitoxins (STXs) levels. Thus, comprehending how environmental elements drive the proliferation of this STXs-producing species can aid in predicting human exposure risks. This study aimed to explore the link between cyanobacteria R. raciborskii, STXs cyanotoxins, and environmental factors in 37 public supply reservoirs in the tropical region and assess potential health hazards these toxins pose in the reservoir waters. A Structural Equation Model was used to assess the impact of environmental factors (water volume and physical and chemical variables) on R. raciborskii biomass and STXs levels. Furthermore, the potential risk of STXs exposure from consuming untreated reservoir water was evaluated. Lastly, the cumulative distribution function (CDF) of STXs across the reservoirs was computed. Our findings revealed a correlation between R. raciborskii biomass and STXs concentrations. Total phosphorus emerged as a critical environmental factor positively influencing species biomass and indirectly affecting STXs levels. pH significantly influenced STXs concentrations, indicating different factors influencing R. raciborskii biomass and STXs. Significantly, for the first time, the risk of STXs exposure was gauged using the risk quotient (HQ) for untreated water consumption from public supply reservoirs in Brazil's semi-arid region. Although the exposure risks were generally low to moderate, the CDF underscored the risk of chronic exposure due to low toxin concentrations in over 90% of samples. These outcomes emphasize the potential expansion of R. raciborskii in tropical settings due to increased phosphorus, amplifying waterborne STXs levels and associated intoxication risks. Thus, this study reinforces the importance of nutrient control, particularly phosphorus regulation, as a mitigation strategy against R. raciborskii blooms and reducing STXs intoxication hazards.

Proteomic and toxicological analysis of the response of dinoflagellate Alexandrium catenella to changes in NaNO3 concentration

Dinoflagellates of the genus Alexandrium cause Harmful Algal Blooms (HABs) in coastal waters worldwide, damaging marine environments, aquaculture, and human health. They synthesize potent neurotoxic alkaloids known as PSTs (i.e., Paralytic Shellfish Toxins), the etiological agents of PSP (i.e., Paralytic Shellfish Poisoning). In recent decades, the eutrophication of coastal waters with inorganic nitrogen (e.g., nitrate, nitrite, and ammonia) has increased the frequency and scale of HABs. PSTs concentrations within Alexandrium cells can increase by up to 76% after a nitrogen enrichment event; however, the mechanisms that underlie their biosynthesis in dinoflagellates remains unclear. This study combines mass spectrometry, bioinformatics, and toxicology and investigates the expression profiles of PSTs in Alexandrium catenella grown in 0.4, 0.9 and 1.3 mM NaNO₃. Pathway analysis of protein expression revealed that tRNA amino acylation, glycolysis, TCA cycle and pigment biosynthesis were upregulated in 0.4 mM and downregulated in 1.3 mM NaNO₃ compared to those grown in 0.9 mM NaNO₃. Conversely, ATP synthesis, photosynthesis and arginine biosynthesis were downregulated in 0.4 mM and upregulated in 1.3 mM NaNO₃. Additionally, the expression of proteins involved in PST biosynthesis (sxtA, sxtG, sxtV, sxtW and sxtZ) and overall PST production like STX, NEO, C1, C2, GTX1-6 and dcGTX2 was higher at lower nitrate concentrations. Therefore, increased nitrogen concentrations increase protein synthesis, photosynthesis, and energy metabolism and decrease enzyme expression in PST biosynthesis and production. This research provides new clues about how the changes in the nitrate concentration can modulate different metabolic pathways and the expression of PST biosynthesis in toxigenic dinoflagellates.

Influence of Environmental Factors on Occurrence of Cyanobacteria and Abundance of Saxitoxin-Producing Cyanobacteria in a Subtropical Drinking Water Reservoir in Brazil

Blooms of cyanobacteria are frequent in Brazilian water reservoirs used for drinking water. The warning for the presence of potential toxin-producing cyanobacteria is typically based on time-consuming microscopy, rather than specific molecular detection of toxic genes in cyanobacteria. In this study, we developed a quantitative PCR assay for the detection of cyanobacteria producing the neurotoxin saxitoxin (STX). The assay targets the sxtA gene in the sxt gene cluster. Potential and dominant STX-producers in the Itupararanga reservoir were the genera Raphidiopsis, Aphanizomenon and Geitlerinema. Numbers of the sxtA gene varied from 6.76 × 103 to 7.33 × 105 cells mL−1 and correlated positively with SXT concentrations in the water. Concentrations of STX and the sxtA gene also correlated positively with TN:TP ratio and pH, but correlated negatively with inorganic nutrients and turbidity, confirming that regulation of the SXT production was impacted by environmental variables. In contrast, the occurrence of another cyanotoxin, microcystin, did not correlate with any environmental variables. The developed qPCR assay was found to be a rapid and robust approach for the specific quantification of potential STX-producing cyanobacteria and should be considered in future investigations on toxic cyanobacteria to provide an early warning of potential toxin episodes.

Competitive interactions as a mechanism for chemical diversity maintenance in Nodularia spumigena

Nodularia spumigena is a bloom-forming diazotrophic cyanobacterium inhabiting brackish waters worldwide. This species produces non-ribosomal peptides (NRPs), including the hepatotoxin nodularin, often referred to as cyanotoxin. Several known classes of NRPs have various biological activities, although their modes of action are poorly understood. In the Baltic N. spumigena, there is a high NRP chemodiversity among strains, allowing their grouping in specific chemotypes and subgroups. Therefore, it is relevant to ask whether the NRP production is affected by intraspecific interactions between the co-existing strains. Using a novel approach that combines culture technique and liquid chromatography-tandem mass spectrometry for the NRP analysis, we examined N. spumigena strains under mono- and co-culture conditions. The test strains were selected to represent N. spumigena belonging to the same or different chemotype subgroups. In this setup, we observed physiological and metabolic responses in the test strains grown without cell contact. The changes in NRP levels to co-culture conditions were conserved within a chemotype subgroup but different between the subgroups. Our results suggest that intraspecific interactions may promote a chemical diversity in N. spumigena population, with higher NRP production compared to a single-strain population. Studying allelochemical signalling in this cyanobacterium is crucial for understanding toxicity mechanisms and plankton community interactions in the Baltic Sea and other aquatic systems experiencing regular blooms.

The combined effect of clethodim (herbicide) and nitrogen variation on allelopathic interactions between Microcystis aeruginosa and Raphidiopsis raciborskii

The large-scale use of herbicides deteriorates water quality and threatens aquatic biodiversity. Unfortunately, there are few studies on the ecological effects of herbicides on toxin-producing strains of cyanobacteria under changing nutrient conditions. The objective of the present study was to investigate the effects of the herbicide clethodim and nitrogen variation on the allelopathic interactions and toxin production of Microcystis aeruginosa BCCUSP232 and Raphidiopsis raciborskii (formerly known as Cylindrospermopsis raciborskii) ITEPA1. M. aeruginosa had increased cell density when exposed to the clethodim (H +) (23.55 mg/L), whereas the highest cell density of R. raciborskii was observed in the treatment with clethodim plus limited nitrogen. Also, the cell-free exudate of R. raciborskii significantly stimulated the growth of M. aeruginosa on day 3 of the experiment. The concentration of chlorophyll-a in M. aeruginosa cultures generally increased in all the treatments, while in R. raciborskii cultures, the opposite occurred. Total microcystins (MCs) content of M. aeruginosa in the mixed cultures was 68% higher in nitrogen-enriched conditions than the control. A similar increase in MC content occurred in M. aeruginosa unialgal culture treated with R. raciborskii exudate. Total saxitoxin concentration was 81% higher in mixed cultures of R. raciborskii simultaneously exposed to high nitrogen and clethodim. Similarly, unialgal cultures of R. raciborskii exposed to either high nitrogen or clethodim had higher saxitoxins concentrations than the control. The intracellular H₂O₂ content of M. aeruginosa cultures decreased, whereas, in R. raciborskii cultures, it increased during exposure to high nitrogen and clethodim. Only R. raciborskii had a significant variation in peroxidase activity. The activities of glutathione S-transferase of both strains were higher in the presence of clethodim. These results revealed that nitrogen enrichment and the presence of clethodim might lead to the excessive proliferation of M. aeruginosa and R. raciborskii and increased production of cyanotoxins in aquatic environments.

Saxitoxins from the freshwater cyanobacterium Raphidiopsis raciborskii can contaminate marine mussels

Raphidiopsis raciborskii (formerly Cylindrospermopsis raciborskii) is a freshwater cyanobacterium potentially producing saxitoxins (STX) and cylindrospermopsin. Its ecophysiological versatility enables it to form blooms in the most diverse types of environments, from tropical to temperate, and from relatively pristine to polluted. In Peri Lake, located in the subtropical south of Brazil, growing populations of STX-producing R. raciborskii have been detected since 1994, posing risks to the use of its waters that supply a population of about 100,000 inhabitants. Despite the existence of a monitoring system for the presence and toxicity of cyanobacteria in Peri Lake water, no assessment has been made in the coastal region, downstream of outflowing lake water, thereby potentially making available a toxic biomass to natural and cultivated shellfish populations in the salt water ecosystem. To address this problem, the present study evaluated environmental variables and STX concentration by profiling the outflowing waters between Peri Lake and the adjacent coastal zone. Laboratory experiments were carried out with three strains of R. raciborskii in order to confirm the effect of salinity on STX production and verify if Perna Perna mussels fed with R. raciborskii cultures would absorb and accumulate STX. Results showed that environmental concentrations of STX reach high levels (up to 6.31 µg L^-1 STX eq.), especially in the warmer months, reaching the coastal zone. In laboratory tests, it was found that the strains tolerate salinities between 4 and 6 and that salinity influences the production of STX. In addition, mussels fed with R. raciborskii effectively absorb and accumulate STX, even in typically marine salinities (22 to 30), suggesting that R. raciborskii biomass remains available and toxic despite salinity shock. These results draw attention to the ecological and health risk associated with R. raciborskii blooms, both in the lake environment and in the adjacent marine environment, calling attention to the need to improve the monitoring and management systems for water and shellfish toxicity in the region of interest, as well as other places where toxic cyanobacteria of limnic origin can reach the coastal zone.

Physiological changes induced by sodium chloride stress in Aphanizomenon gracile, Cylindrospermopsis raciborskii and Dolichospermum sp

Due to anthropogenic activities, associated with climate change, many freshwater ecosystems are expected to experience an increase in salinity. This phenomenon is predicted to favor the development and expansion of freshwater cyanobacteria towards brackish waters due to their transfer along the estuarine freshwater-marine continuum. Since freshwater cyanobacteria are known to produce toxins, this represents a serious threat for animal and human health. Saxitoxins (STXs) are classified among the most powerful cyanotoxins. It becomes thus critical to evaluate the capacity of cyanobacteria producing STXs to face variations in salinity and to better understand the physiological consequences of sodium chloride (NaCl) exposure, in particular on their toxicity. Laboratory experiments were conducted on three filamentous cyanobacteria species isolated from brackish (Dolichospermum sp.) and fresh waters (Aphanizomenon gracile and Cylindrospermopsis raciborskii) to determine how salinity variations affect their growth, photosynthetic activity, pigment composition, production of reactive oxygen species (ROS), synthesis of compatible solutes and STXs intracellular quotas. Salinity tolerance was found to be species-specific. Dolichospermum sp. was more resistant to salinity variations than A. gracile and C. raciborskii. NaCl variations reduced growth in all species. In A. gracile, carotenoids content was dose-dependently reduced by NaCl. By contrast, in C. raciborskii and Dolichospermum sp., variations in carotenoids content did not show obvious relationships with NaCl concentration. While in Dolichospermum sp. phycocyanin and phycoerythrin increased within the first 24 h exposure to NaCl, in both A. gracile and C. raciborskii, these pigments decreased proportionally to NaCl concentration. Low changes in salinity did not impact STXs production in A. gracile and C. raciborskii while higher increase in salinity could modify the toxin profile and content of C. raciborskii (intracellular STX decreased while dc-GTX2 increased). In estuaries, A. gracile and C. raciborskii would not be able to survive beyond the oligohaline area (i.e. salinity > 5). Conversely, in part due to its ability to accumulate compatible solutes, Dolichospermum sp. has the potential to face consequent salinity variations and to survive in the polyhaline area (at least up to salinity = 24).

Monitoring of saxitoxin production in lakes in Denmark by molecular, chromatographic and microscopic approaches

Diversity of phytoplankton in three eutrophic and two mesotrophic lakes in Denmark was determined by microscopic and HPLC pigment analyses to identify and quantify potential saxitoxin (STX) producing cyanobacteria. Potential dominant STX-producers were identified to the filamentous genera Dolichospermum, Cuspidothrix, Phormidium and Planktolyngbya. Presence of STX production was documented by extraction of five intracellular STXs that included (in declining concentration in the cyanobacteria) dc-neo-STX, neo-STX, dc-STX, STX and GTX. Total concentrations of the five STXs varied from 9 to 142 fg per potential STX producer, corresponding to 87 to 985 ng L^-1 in the lakes. For molecular detection of the STX-producers, a quantitative PCR method was developed by design of a new robust primer set with broad coverage to target the sxtA gene that is common to all STX-producing cyanobacteria. After validation, copy numbers of the sxtA gene were determined to vary from about 10⁴ (mesotrophic lakes) to 10⁸ per mL (the most eutrophic lake). A moderate but significant correlation was observed between abundance of the sxtA copies and concentrations of the five intracellular STXs. The qPCR assay was found to be a rapid and robust procedure for quantification of STX producers. Saxitoxin and its analogs appeared not to cause health concerns in the lakes, but commercial fishing for pike perch in the most eutrophic lake should be monitored to test for food web accumulation of STXs.

Freshwater neurotoxins and concerns for human, animal, and ecosystem health: A review of anatoxin-a and saxitoxin

Toxic cyanobacteria are a concern worldwide because they can adversely affect humans, animals, and ecosystems. However, neurotoxins produced by freshwater cyanobacteria are understudied relative to microcystin. Thus, the objective of this critical review was to provide a comprehensive examination of the modes of action, production, fate, and occurrence of the freshwater neurotoxins anatoxin-a and saxitoxin as they relate to human, animal, and ecosystem health. Literature on freshwater anatoxin-a and saxitoxin was obtained and reviewed for both laboratory and field studies. Current (2020) research identifies as many as 41 anatoxin-a producing species and 15 saxitoxin-producing species of freshwater cyanobacteria. Field studies indicate that anatoxin-a and saxitoxin have widespread distribution, and examples are given from every continent except Antarctica. Human and animal health concerns can range from acute to chronic. However, few researchers studied chronic or sublethal effects of freshwater exposures to anatoxin-a or saxitoxin. Ecosystem health also is a concern, as the effects of toxicity may be far reaching and include consequences throughout the food web. Several gaps in knowledge were identified for anatoxin-a and saxitoxin, including triggers of production and release, environmental fate and degradation, primary and secondary exposure routes, diel variation, food web effects, effects of cyanotoxin mixtures, and sublethal health effects on individual organisms and populations. Despite the gaps, this critical review facilitates our current understanding of freshwater neurotoxins and thus can serve to `` guide future research on anatoxin-a, saxitoxin, and other cyanotoxins.

Changes in pH and dissolved inorganic carbon in water affect the growth, saxitoxins production and toxicity of the cyanobacterium Raphidiopsis raciborskii ITEP-A1

Raphidiopsis raciborskii is a widely distributed, potentially toxic cyanobacterium described as a tropical-subtropical species. However, its occurrence in temperate regions has been expanding. Understanding the environmental factors underlying the expansion and colonization success of Raphidiopsis has been the object of numerous studies. However, less is known regarding its responses to pH and inorganic carbon in water. Thus, the aim of the present study was to investigate the effects of changes in pH and dissolved inorganic carbon on growth and saxitoxins production in the strain R. raciborskii ITEP-A1. We incubated batch cultures with different unbuffered and buffered pH (neutral-acid and alkaline) and inorganic carbon availability (CO₂-rich air bubbling and the addition of NaHCO₃) to assess the effect of these factors on the growth, toxin production as well as saxitoxins composition of the cyanobacterium. The carbon concentrating mechanism (CCM) system of ITEP-A1 was also characterized by an in silico analysis of its previously sequenced genome. The growth and saxitoxins production of R. raciborskii were affected. The addition of sodium bicarbonate and air bubbling enhanced the growth of the cyanobacterium in alkaline pH. In contrast, saxitoxins production and relative toxicity were decreased. Moreover, significant changes in the cellular composition of saxitoxins were strongly related to pH changes. ITEP-A1 potentially expresses the low-flux bicarbonate transporter BicA, an efficient CCM which uptakes most of its carbon from HCO₃^-. Hence, increasing the diffusion of CO₂ in alkaline eutrophic lakes is likely to increase R. raciborskii dominance, but produce less toxic blooms.

Clethodim (herbicide) alters the growth and toxins content of Microcystis aeruginosa and Raphidiopsis raciborskii

Increased agricultural intensification goes with the widespread use of herbicides that adversely affect aquatic biodiversity. The effects of herbicides on toxin-producing cyanobacteria have been poorly studied. The present study aimed to investigate the toxicological and physiological effects of the herbicide clethodim on Raphidiopsis raciborskii (a.k.a. Cylindrospermopsis raciborskii) ITEPA1 and Microcystis aeruginosa BCCUSP232. On day four of the experiment, the exposure to 25 mg/L clethodim resulted in the highest cell density of R. raciborskii. Similarly, exposure to the 1, 5, 20, and 50 mg/L clethodim treatments resulted in the highest cell densities of M. aeruginosa on day 4 of the experiment. Medium effect concentrations (EC₅₀) after 96 h of exposure of both strains to clethodim were 192.98 mg/L and 168.73 mg/L for R. raciborskii and M. aeruginosa, respectively. The presence of clethodim significantly increased the total microcystin content of M. aeruginosa compared to the control cultures. At 400 mg/L, total saxitoxins content of R. raciborskii was 27% higher than that of the control cultures on day 4. In contrast, cultures exposed to 100 mg/L clethodim had the lowest saxitoxins levels per cell quota. There was an increase in the levels of intracellular hydrogen peroxide in both species during exposure to clethodim, which was followed by significant changes (p < 0.05) in the activity of antioxidant enzymes such as peroxidase and superoxide dismutase. These results revealed that the presence of low levels of clethodim in the aquatic environment might lead to the excessive proliferation of cyanobacteria and alteration of their cyanotoxins content.

Sensitivity of freshwater species under single and multigenerational exposure to seawater intrusion

Salinization of coastal freshwater ecosystems is already occurring in some regions of the world. This phenomenon raises serious concerns on the protection of coastal freshwater ecosystems, since many of them support and shelter a large number of species and are considered hotspots of biodiversity. This work intended to assess the adverse effects that salinization, caused by the intrusion of seawater (SW), may pose to freshwater organisms. In this study, three specific goals were addressed: (i) to assess if sodium chloride (NaCl) may be used as a surrogate of natural SW at early-stages of risk assessment; (ii) to identify the most sensitive freshwater species to salinity NaCl; and (iii) to determine if increased tolerance to salinity may be acquired after multigenerational exposure to low levels of salinization (induced with NaCl). A total of 12 standard monospecific bioassays were carried out by exposing organisms from different taxonomic groups (Cyanobacteria: one species, Tracheophyta: two species, Rotifera: one species, Arthropoda: two species and Mollusca: one species) to a series of concentrations of NaCl (ranging from 0.95 to 22.8 mS cm-1) or dilutions of SW (ranging from 1.70 to 52.3 mS cm-1). In general, NaCl exerted similar or higher toxicity than SW, both at lethal and sublethal levels, suggesting that it may be proposed as a protective surrogate of SW for first tiers of salinization risk assessment. Among all tested species, the cyanobacterium Cylindrospermopsis raciborskii, the daphnid Daphnia longispina and the rotifer Brachionus plicatilis were the most sensitive taxa to salinization (EC50 ≤ 4.38 mS cm-1). Given their position at the basis of the food web, it is suggested that small increments of salinity may be enough to induce structural changes in freshwater communities or induce changes in trophic relations. No clear evidences of increased tolerance after multigenerational exposure to low levels of salinity were found.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Guanidinium Toxins and Their Interactions with Voltage-Gated Sodium Ion Channels

Guanidinium toxins, such as saxitoxin (STX), tetrodotoxin (TTX) and their analogs, are naturally occurring alkaloids with divergent evolutionary origins and biogeographical distribution, but which share the common chemical feature of guanidinium moieties. These guanidinium groups confer high biological activity with high affinity and ion flux blockage capacity for voltage-gated sodium channels (NaV). Members of the STX group, known collectively as paralytic shellfish toxins (PSTs), are produced among three genera of marine dinoflagellates and about a dozen genera of primarily freshwater or brackish water cyanobacteria. In contrast, toxins of the TTX group occur mainly in macrozoa, particularly among puffer fish, several species of marine invertebrates and a few terrestrial amphibians. In the case of TTX and analogs, most evidence suggests that symbiotic bacteria are the origin of the toxins, although endogenous biosynthesis independent from bacteria has not been excluded. The evolutionary origin of the biosynthetic genes for STX and analogs in dinoflagellates and cyanobacteria remains elusive. These highly potent molecules have been the subject of intensive research since the latter half of the past century; first to study the mode of action of their toxigenicity, and later as tools to characterize the role and structure of NaV channels, and finally as therapeutics. Their pharmacological activities have provided encouragement for their use as therapeutants for ion channel-related pathologies, such as pain control. The functional role in aquatic and terrestrial ecosystems for both groups of toxins is unproven, although plausible mechanisms of ion channel regulation and chemical defense are often invoked. Molecular approaches and the development of improved detection methods will yield deeper understanding of their physiological and ecological roles. This knowledge will facilitate their further biotechnological exploitation and point the way towards development of pharmaceuticals and therapeutic applications.

Influence of environmental variables on saxitoxin yields by Cylindrospermopsis raciborskii in a mesotrophic subtropical reservoir

Saxitoxins are a class of toxins produced by at least two groups of evolutionarily distant organisms (cyanobacteria and dinoflagellates). While the toxicity of these toxins is relatively well characterized, to date little is known about their drivers and ecological functions, especially in lower latitude tropical and subtropical freshwater ecosystems. In the present study, we aimed to obtain a better understanding of the main drivers of saxitoxin concentrations in aquatic environments. We investigated the relationships among saxitoxin concentrations in a mesotrophic subtropical reservoir dominated by the cyanobacteria Cylindrospermopsis raciborskii with physical, chemical and biological water variables. The highest saxitoxin concentrations were 0.20 μg·L^-1, which occurred in the samples with the highest densities of C. raciborskii (maximum of 4.3 × 10⁴ org·mL^-1) and the highest concentration of dissolved nutrients (nitrate from 0.2 to 0.8 μg·L^-1, ortophosphate from 0.3 to 8.5 μg·L^-1). These correlations were confirmed by statistical analyses. However, the highest saxitoxin relative concentrations (per trichome) were associated with lower C. raciborskii densities, suggesting that saxitoxin production or the selection of saxitoxin-producing strains was associated with the adaptation of this species to conditions of stress. Our results indicate that C. raciborskii toxin yields vary depending on the enrichment conditions having potential implications for reservoir management.

Low dose extended exposure to saxitoxin and its potential neurodevelopmental effects: A review

Saxitoxin (STX) and its analogs, the paralytic shellfish toxins (PSTs), are a group of potent neurotoxins well known for their role in acute paralytic poisoning by preventing the generation of action potentials in neuronal cells. They are found in both marine and freshwater environments globally and although acute exposure from the former has previously received more attention, low dose extended exposure from both sources is possible and to date has not been investigated. Given the known role of cellular electrical activity in neurodevelopment this pattern of exposure may be a significant public health concern. Additionally, the presence of PSTs is likely to be an ongoing and possibly increasing problem in the future. This review examines the neurodevelopmental toxicity of STX, the risk of extended or repeated exposure to doses with neurodevelopmental effects, the potential implications of this exposure and briefly, the steps taken and difficulties faced in preventing exposure.

Moving towards adaptive management of cyanotoxin-impaired water bodies

The cyanobacteria are a phylum of bacteria that have played a key role in shaping the Earth's biosphere due to their pioneering ability to perform oxygenic photosynthesis. Throughout their history, cyanobacteria have experienced major biogeochemical changes accompanying Earth's geochemical evolution over the past 2.5+ billion years, including periods of extreme climatic change, hydrologic, nutrient and radiation stress. Today, they remain remarkably successful, exploiting human nutrient over‐enrichment as nuisance “blooms.” Cyanobacteria produce an array of unique metabolites, the functions and biotic ramifications of which are the subject of diverse ecophysiological studies. These metabolites are relevant from organismal and ecosystem function perspectives because some can be toxic and fatal to diverse biota, including zooplankton and fish consumers of algal biomass, and high‐level consumers of aquatic food sources and drinking water, including humans. Given the long history of environmental extremes and selection pressures that cyanobacteria have experienced, it is likely that that these toxins serve ecophysiological functions aimed at optimizing growth and fitness during periods of environmental stress. Here, we explore the molecular and ecophysiological mechanisms underlying cyanotoxin production, with emphasis on key environmental conditions potentially controlling toxin production. Based on this information, we offer potential management strategies for reducing cyanotoxin potentials in natural waters; for cyanotoxins with no clear drivers yet elucidated, we highlight the data gaps and research questions that are still lacking. We focus on the four major classes of toxins (anatoxins, cylindrospermopsins, microcystins and saxitoxins) that have thus far been identified as relevant from environmental health perspectives, but caution there may be other harmful metabolites waiting to be elucidated.

Abiotic variables affect STX concentration in a meso-oligotrophic subtropical coastal lake dominated by Cylindrospermopsis raciborskii (Cyanophyceae)

The cyanobacterium Cylindrospermopsis raciborskii is capable of producing toxins including saxitoxin (STX). Few studies have verified the influence of environmental variables on the production of STX and most have only been studied in the laboratory. The goal of this work was to identify the abiotic variables related to STX concentration in situ. The relationship among STX concentration and the physical variables, nutrients and chlorophyll-a (chl-a) concentration was examined in a meso-oligotrophic subtropical coastal lake dominated by C. raciborskii. A generalized linear model was developed, incorporating all variables measured monthly over a 45-month monitoring period. Conductivity and dissolved inorganic nitrogen (DIN) concentration provided the greatest explanatory power for STX concentration in situ. Previous studies suggested that C. raciborskii cells exposed to stress associated with higher ionic concentrations appear to activate the biosynthesis of STX suggesting that STX can elicit changes cell permeability and may contribute to the homeostasis of this organism. An increase of DIN concentration results in a higher concentration of STX which may be related to a reduced metabolic demand, since the uptake of inorganic nitrogen requires less energy than N₂-fixation. Thus, increased DIN can favor the growth of C. raciborskii population or improve cellular homeostasis, both potentially increasing STX concentration in the aquatic system, which was observed through a delayed response pattern. The developed model, while providing only a moderate predictive power, can assist in the understanding of the environmental variables associated with increases in STX concentration, and in monitoring and minimizing the risks of toxic blooms.

Influence of nitrogen availability on the expression of genes involved in the biosynthesis of saxitoxin and analogs in Cylindrospermopsis raciborskii

The development of cyanobacterial blooms in inland aquatic ecosystems is greatly promoted by nutrient availability, especially nitrogen and phosphorous. When blooms are dominated by toxigenic species the harmful effects of nutrient loading becomes particularly relevant. Among toxic species, Cylindrospermopsis raciborskii found in South American ecosystems is characterized by the production of saxitoxin and analogs (Paralytic Shellfish Poisoning, PSP), for which the factors that trigger their production have not been elucidated. In this study, the effect of nitrate availability on the relative transcript abundance of two genes (sxtU and sxtI), both involved in different steps of PSP biosynthetic pathway, was addressed in C. raciborskii MVCC19 by qPCR. The relative transcript abundance of both genes significantly increased from the beginning to the end of growth, independent of nitrate availability in the culture medium. Differences between the genes in terms of the levels of relative expression were also found, implying that during growth in nitrate-rich or nitrate-deprived conditions C. raciborskii MVCC19 has the ability to produce different kind of PSP molecules. The presence of nifH transcripts in the nitrogen-depleted treatment confirmed that in the absence of nitrate C. raciborskii fixed atmospheric N₂. Moreover, after transferring filaments to nitrate-rich conditions the synthesis of nifH mRNA continued for few hours, suggesting that cell adjustments enabling the utilization of soluble nitrogen sources are not immediate. Our results show that biosynthesis of saxitoxin and analogs in C. raciborskii is not related to nitrate availability, but rather is linked to cyanobacteria growth rate.

The genetics, biosynthesis and regulation of toxic specialized metabolites of cyanobacteria

The production of toxic metabolites by cyanobacterial blooms represents a significant threat to the health of humans and ecosystems worldwide. Here we summarize the current state of the knowledge regarding the genetics, biosynthesis and regulation of well-characterized cyanotoxins, including the microcystins, nodularin, cylindrospermopsin, saxitoxins and anatoxins, as well as the lesser-known marine toxins (e.g. lyngbyatoxin, aplysiatoxin, jamaicamides, barbamide, curacin, hectochlorin and apratoxins).

Microbial communities reflect temporal changes in cyanobacterial composition in a shallow ephemeral freshwater lake

The frequency of freshwater cyanobacterial blooms is at risk of increasing as a consequence of climate change and eutrophication of waterways. It is increasingly apparent that abiotic data are insufficient to explain variability within the cyanobacterial community, with biotic factors such as heterotrophic bacterioplankton, viruses and protists emerging as critical drivers. During the Australian summer of 2012-2013, a bloom that occurred in a shallow ephemeral lake over a 6-month period was comprised of 22 distinct cyanobacteria, including Microcystis, Dolichospermum, Oscillatoria and Sphaerospermopsis. Cyanobacterial cell densities, bacterial community composition and abiotic parameters were assessed over this period. Alpha-diversity indices and multivariate analysis were successful at differentiating three distinct bloom phases and the contribution of abiotic parameters to each. Network analysis, assessing correlations between biotic and abiotic variables, reproduced these phases and assessed the relative importance of both abiotic and biotic factors. Variables possessing elevated betweeness centrality included temperature, sodium and operational taxonomic units belonging to the phyla Verrucomicrobia, Planctomyces, Bacteroidetes and Actinobacteria. Species-specific associations between cyanobacteria and bacterioplankton, including the free-living Actinobacteria acI, Bacteroidetes, Betaproteobacteria and Verrucomicrobia, were also identified. We concluded that changes in the abundance and nature of freshwater cyanobacteria are associated with changes in the diversity and composition of lake bacterioplankton. Given this, an increase in the frequency of cyanobacteria blooms has the potential to alter nutrient cycling and contribute to long-term functional perturbation of freshwater systems.

Elevated Na(+) and pH influence the production and transport of saxitoxin in the cyanobacteria Anabaena circinalis AWQC131C and Cylindrospermopsis raciborskii T3

Saxitoxins (STX), neurotoxic alkaloids, fall under the umbrella of paralytic shellfish toxins produced by marine dinoflagellates and freshwater cyanobacteria. The genes responsible for the production of STX have been proposed, but factors that influence their expression and induce toxin efflux remain unclear. Here we characterize the putative STX NorM-like MATE transporters SxtF and SxtM. Complementation of the antibiotic-sensitive strain Escherichia coli KAM32 with these transporters decreased fluoroquinolone sensitivity, indicating that while becoming evolutionary specialized for STX transport these transporters retain relaxed specificity typical of this class. The transcriptional response of STX biosynthesis (sxtA) along with that of the STX transporters (sxtM and sxtF from Cylindrospermopsis raciborskii T3, and sxtM from Anabaena circinalis AWQC131C) were assessed in response to ionic stress. These data, coupled with a measure of toxin intracellular to extracellular ratios, provide an insight into the physiology of STX export. Cylindrospermopsis raciborskii and Anabaena circinalis exhibited opposing responses under conditions of ionic stress. High Na(+) (10 mM) induced moderate alterations of transcription and STX localization, whereas high pH (pH 9) stimulated the greatest physiological response. Saxitoxin production and cellular localization are responsive to ionic strength, indicating a role of this molecule in the maintenance of cellular homeostasis.

Impact of environmental factors on the regulation of cyanotoxin production

Cyanobacteria are capable of thriving in almost all environments. Recent changes in climatic conditions due to increased human activities favor the occurrence and severity of harmful cyanobacterial bloom all over the world. Knowledge of the regulation of cyanotoxins by the various environmental factors is essential for effective management of toxic cyanobacterial bloom. In recent years, progress in the field of molecular mechanisms involved in cyanotoxin production has paved the way for assessing the role of various factors on the cyanotoxin production. In this review, we present an overview of the influence of various environmental factors on the production of major group of cyanotoxins, including microcystins, nodularin, cylindrospermopsin, anatoxins and saxitoxins.

Growth and saxitoxin production by Cylindrospermopsis raciborskii (cyanobacteria) correlate with water hardness

The cosmopolitan and increasing distribution of Cylindrospermopsis raciborskii can be attributed to its ecophysiological plasticity and tolerance to changing environmental factors in water bodies. In reservoirs in the semi-arid region of Brazil, the presence and common dominance of C. raciborskii have been described in waters that are considered hard. We investigated the response of a Brazilian C. raciborskii strain to water hardness by evaluating its growth and saxitoxin production. Based on environmental data, a concentration of 5 mM of different carbonate salts was tested. These conditions affected growth either positively (MgCO₃) or negatively (CaCO₃ and Na₂CO₃). As a control for the addition of cations, MgCl₂, CaCl₂ and NaCl were tested at 5 or 10 mM, and MgCl₂ stimulated growth, NaCl slowed but sustained growth, and CaCl₂ inhibited growth. Most of the tested treatments increased the saxitoxin (STX) cell quota after six days of exposure. After 12 days, STX production returned to concentrations similar to that of the control, indicating an adaptation to the altered water conditions. In the short term, cell exposure to most of the tested conditions favored STX production over neoSTX production. These results support the noted plasticity of C. raciborskii and highlight its potential to thrive in hard waters. Additionally, the observed relationship between saxitoxin production and water ion concentrations characteristic of the natural environments can be important for understanding toxin content variation in other harmful algae that produce STX.

Interpreting the possible ecological role(s) of cyanotoxins: compounds for competitive advantage and/or physiological aide?

To date, most research on freshwater cyanotoxin(s) has focused on understanding the dynamics of toxin production and decomposition, as well as evaluating the environmental conditions that trigger toxin production, all with the objective of informing management strategies and options for risk reduction. Comparatively few research studies have considered how this information can be used to understand the broader ecological role of cyanotoxin(s), and the possible applications of this knowledge to the management of toxic blooms. This paper explores the ecological, toxicological, and genetic evidence for cyanotoxin production in natural environments. The possible evolutionary advantages of toxin production are grouped into two main themes: That of "competitive advantage" or "physiological aide". The first grouping illustrates how compounds produced by cyanobacteria may have originated from the need for a cellular defence mechanism, in response to grazing pressure and/or resource competition. The second grouping considers the contribution that secondary metabolites make to improved cellular physiology, through benefits to homeostasis, photosynthetic efficiencies, and accelerated growth rates. The discussion also includes other factors in the debate about possible evolutionary roles for toxins, such as different modes of exposures and effects on non-target (i.e., non-competitive) species. The paper demonstrates that complex and multiple factors are at play in driving evolutionary processes in aquatic environments. This information may provide a fresh perspective on managing toxic blooms, including the need to use a "systems approach" to understand how physico-chemical conditions, as well biological stressors, interact to trigger toxin production.

WITHDRAWN: Cyanobacterial bioactive molecules - Biosynthesis and genetic regulation

This article has been withdrawn at the request of the editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

PSP toxin release from the cyanobacterium Raphidiopsis brookii D9 (Nostocales) can be induced by sodium and potassium ions

Paralytic shellfish poisoning (PSP) toxins are a group of naturally occurring neurotoxic alkaloids produced among several genera of primarily freshwater cyanobacteria and marine dinoflagellates. Although saxitoxin (STX) and analogs are all potent Na(+) channel blockers in vertebrate cells, the functional role of these compounds for the toxigenic microorganisms is unknown. Based upon the known importance of monovalent cations (such as sodium) in the maintenance of cellular homeostasis and ion channel function, we examined the effect of high extracellular concentrations of these ions on growth, cellular integrity, toxin production and release to the external medium in the filamentous freshwater cyanobacterium, Raphidiopsis brookii D9; a gonyautoxins (GTX2/3) and STX producing toxigenic strain. We observed a toxin export in response to high (17 mM) NaCl and KCl concentrations in the growth medium that was not primarily related to osmotic stress effects, compared to the osmolyte mannitol. Addition of exogenous PSP toxins with the same compositional profile as the one produced by R. brookii D9 was able to partially mitigate this effect of high Na⁺ (17 mM). The PSP toxin biosynthetic gene cluster (sxt) in D9 has two genes (sxtF and sxtM) that encode for a MATE (multidrug and toxic compound extrusion) transporter. This protein family, represented by NorM in the bacterium Vibrio parahaemolyticus, confers resistance to multiple cationic toxic agents through Na⁺/drug antiporters. Conserved domains for Na⁺ and drug recognition have been described in NorM. For the D9 sxt cluster, the Na⁺ recognition domain is conserved in both SxtF and SxtM, but the drug recognition domain differs between them. These results suggest that PSP toxins are exported directly in response to the presence of monovalent cations (Na⁺, K⁺) at least at elevated concentrations. Thus, the presence of both genes in the sxt cluster from strain D9 can be explained as a selective recognition mechanism by the SxtF/M transporters for GTX2/3 and STX. We propose that these toxins in cyanobacteria could act extracellularly as a protective mechanism to ensure homeostasis against extreme salt variation in the environment.

Environmental conditions that influence toxin biosynthesis in cyanobacteria

Over the past 15 years, the genetic basis for production of many cyanobacterial bioactive compounds has been described. This knowledge has enabled investigations into the environmental factors that regulate the production of these toxins at the molecular level. Such molecular or systems level studies are also likely to reveal the physiological role of the toxin and contribute to effective water resource management. This review focuses on the environmental regulation of some of the most relevant cyanotoxins, namely the microcystins, nodularin, cylindrospermopsin, saxitoxins, anatoxins and jamaicamides.

Inhibition of copper uptake in yeast reveals the copper transporter Ctr1p as a potential molecular target of saxitoxin

Saxitoxin is a secondary metabolite produced by several species of dinoflagellates and cyanobacteria which targets voltage-gated sodium and potassium channels in higher vertebrates. However, its molecular target in planktonic aquatic community members that co-occur with the toxin producers remains unknown. Previous microarray analysis with yeast identified copper and iron-homeostasis genes as being differentially regulated in response to saxitoxin. This study sought to identify the molecular target in microbial cells by comparing the transcriptional profiles of key copper and iron homeostasis genes (CTR1, FRE1, FET3, CUP1, CRS5) in cells exposed to saxitoxin, excess copper, excess iron, an extracellular Cu(I) chelator, or an intracellular Cu(I) chelator. Protein expression and localization of Ctr1p (copper transporter), Fet3p (multicopper oxidase involved in high-affinity iron uptake), and Aft1p (iron regulator) were also compared among treatments. Combined transcript and protein profiles suggested saxitoxin inhibited copper uptake. This hypothesis was confirmed by intracellular Cu(I) imaging with a selective fluorescent probe for labile copper. On the basis of the combined molecular and physiological results, a model is presented in which the copper transporter Ctr1p serves as a molecular target of saxitoxin and these observations are couched in the context of the eco-evolutionary role this toxin may serve for species that produce it.

The chemical ecology of cyanobacteria

This review covers the literature on the chemically mediated ecology of cyanobacteria, including ultraviolet radiation protection, feeding-deterrence, allelopathy, resource competition, and signalling. To highlight the chemical and biological diversity of this group of organisms, evolutionary and chemotaxonomical studies are presented. Several technologically relevant aspects of cyanobacterial chemical ecology are also discussed.

The inhibitory effect of calcium on Cylindrospermopsis raciborskii (cyanobacteria) metabolism

Cylindrospermopsis raciborskii (Woloszynska) Seenaya & Subba Raju is a freshwater cyanobacterium of worldwide distribution. In the North-eastern region of Brazil many eutrophic water reservoirs are characterized by the dominance of C. raciborskii, with recurrent occurrence of blooms. These water bodies have high conductivity due to a high ionic concentration, and are defined as hard (with high values of CaCO3). In this study, we investigated the long-term effect (12 days) of high calcium concentration (8 mM Ca(2+)) on C. raciborskii (T3 strain) growth, morphology, toxin content, and metabolism. Changes in protein expression profiles were investigated by proteomic analysis using 2D gel electrophoresis and mass spectrometry. A continued exposure to calcium had a pronounced effect on C. raciborskii (T3): it limited growth, decreased thricome length, increased chlorophyll-a content, altered toxin profile (although did not affect PST content, saxitoxin + neosaxitoxin), and inhibited the expression of proteins related to primary metabolism.

A putative gene cluster from a Lyngbya wollei bloom that encodes paralytic shellfish toxin biosynthesis

Saxitoxin and its analogs cause the paralytic shellfish-poisoning syndrome, adversely affecting human health and coastal shellfish industries worldwide. Here we report the isolation, sequencing, annotation, and predicted pathway of the saxitoxin biosynthetic gene cluster in the cyanobacterium Lyngbya wollei. The gene cluster spans 36 kb and encodes enzymes for the biosynthesis and export of the toxins. The Lyngbya wollei saxitoxin gene cluster differs from previously identified saxitoxin clusters as it contains genes that are unique to this cluster, whereby the carbamoyltransferase is truncated and replaced by an acyltransferase, explaining the unique toxin profile presented by Lyngbya wollei. These findings will enable the creation of toxin probes, for water monitoring purposes, as well as proof-of-concept for the combinatorial biosynthesis of these natural occurring alkaloids for the production of novel, biologically active compounds.

On the chemistry, toxicology and genetics of the cyanobacterial toxins, microcystin, nodularin, saxitoxin and cylindrospermopsin

The cyanobacteria or "blue-green algae", as they are commonly termed, comprise a diverse group of oxygenic photosynthetic bacteria that inhabit a wide range of aquatic and terrestrial environments, and display incredible morphological diversity. Many aquatic, bloom-forming species of cyanobacteria are capable of producing biologically active secondary metabolites, which are highly toxic to humans and other animals. From a toxicological viewpoint, the cyanotoxins span four major classes: the neurotoxins, hepatotoxins, cytotoxins, and dermatoxins (irritant toxins). However, structurally they are quite diverse. Over the past decade, the biosynthesis pathways of the four major cyanotoxins: microcystin, nodularin, saxitoxin and cylindrospermopsin, have been genetically and biochemically elucidated. This review provides an overview of these biosynthesis pathways and additionally summarizes the chemistry and toxicology of these remarkable secondary metabolites.

Biosynthetic intermediate analysis and functional homology reveal a saxitoxin gene cluster in cyanobacteria

Saxitoxin (STX) and its analogues cause the paralytic shellfish poisoning (PSP) syndrome, which afflicts human health and impacts coastal shellfish economies worldwide. PSP toxins are unique alkaloids, being produced by both prokaryotes and eukaryotes. Here we describe a candidate PSP toxin biosynthesis gene cluster (sxt) from Cylindrospermopsis raciborskii T3. The saxitoxin biosynthetic pathway is encoded by more than 35 kb, and comparative sequence analysis assigns 30 catalytic functions to 26 proteins. STX biosynthesis is initiated with arginine, S-adenosylmethionine, and acetate by a new type of polyketide synthase, which can putatively perform a methylation of acetate, and a Claisen condensation reaction between propionate and arginine. Further steps involve enzymes catalyzing three heterocyclizations and various tailoring reactions that result in the numerous isoforms of saxitoxin. In the absence of a gene transfer system in these microorganisms, we have revised the description of the known STX biosynthetic pathway, with in silico functional inferences based on sxt open reading frames combined with liquid chromatography-tandem mass spectrometry analysis of the biosynthetic intermediates. Our results indicate the evolutionary origin for the production of PSP toxins in an ancestral cyanobacterium with genetic contributions from diverse phylogenetic lineages of bacteria and provide a quantum addition to the catalytic collective available for future combinatorial biosyntheses. The distribution of these genes also supports the idea of the involvement of this gene cluster in STX production in various cyanobacteria.

Comparative gene expression of PSP-toxin producing and non-toxic Anabaena circinalis strains

Blooms of the freshwater cyanobacterium Anabaena circinalis are recognised as an important health risk worldwide due to the production of a range of toxins such as saxitoxin (STX) and its derivatives, also known as paralytic shellfish poisoning (PSP) toxins. In this study the transcriptional profile of PSP toxin-producing and non-toxic strains of A. circinalis was investigated by means of a DNA microarray approach. Additionally, gene expression was studied after exposure of toxic A. circinalis cultures to lidocaine hydrochloride at 1 microM for 2 h. Under standard growth conditions, a limited number of putative toxic-strain distinctive DNA fragments, identified in previous studies, were preferentially expressed in toxic versus non-toxic strains. The same genes did not significantly change their expression after exposure to 1 microM lidocaine, conditions previously shown to induce STX production in the cyanobacterium Cylindrospermopsis raciborskii T3. Lidocaine supplementation, however, enhanced the transcription of genes involved in physiological adaptive responses and bloom formation in cyanobacteria, such as the gas vesicle structural protein A and phycocyanin. The heat shock protein HSP-70 and the chlorophyll-a binding protein isiA were significantly repressed by lidocaine exposure. Stress response proteins and genes implicated in secondary metabolism were repressed, including phosphopantetheinyl transferases. The BGGM1 DNA microarray, used in this study, was shown to be suitable for gene expression studies in cultured toxic cyanobacteria and allowed the analysis of gene transcripts associated with surface scum formation by toxic A. circinalis.

Saxitoxin, a toxic marine natural product that targets a multitude of receptors

Saxitoxin (STX) was discovered early last century and can contaminate seafood and drinking water, and over time has become an invaluable research tool and an internationally regulated chemical weapon. Among natural products, toxins obtain a unique reputation from their high affinity and selectivity for their target pharmacological receptor, which for STX has long been considered to only be the voltage gated sodium channel. In recent times however, STX has been discovered to also bind to calcium and potassium channels, neuronal nitric oxide synthase, STX metabolizing enzymes and two circulatory fluid proteins, namely a transferrin-like family of proteins and a unique protein found in the blood of pufferfish.

Evidence for differences in the metabolism of saxitoxin and C1+2 toxins in the freshwater cyanobacterium Cylindrospermopsis raciborskii T3

The activity of paralytic shellfish poisoning (PSP) toxins biosynthetic enzymes was assayed in the cyanobacterium Cylindrospermopsis raciborskii T3 after inhibiting protein synthesis with chloramphenicol (CAM). The production of C1+2 and saxitoxin (STX) was sensitive to CAM with STX levels decreasing by 70% after 24-h exposure to the antibiotic. PSP toxin production was strongly promoted by arginine supplementation, with a maximum 476% increase in intracellular STX concentrations after 24-h exposure to 10 mM of the amino acid. However, arginine had no stimulating effect on PSP toxin levels if supplemented in combination with CAM at 10 microg l(-1). Addition of agmatine and proline to C. raciborskii T3 cultures in the presence of 10 microg l(-1) CAM increased C1+2 toxins levels, while having a negative or no effect on STX accumulation. In vitro, PSP toxin levels increased naturally in cyanobacterial extracts, with CAM and arginine having no influence on either C1+2 or STX synthesis. The evidence presented in this study suggests a possible difference between the metabolism of STX and the C1+2 toxins and indicated a high turnover rate of STX biosynthetic enzymes in C. raciborskii T3.

The effect of temperature on growth and production of paralytic shellfish poisoning toxins by the cyanobacterium Cylindrospermopsis raciborskii C10

Cylindrospermopsis raciborskii is a cyanobacterium which produces either cylindrospermopsine or paralytic shellfish poisoning (PSP) toxins. We studied the effect of temperature on growth and production of PSP toxins by C. raciborskii C10, isolated from a freshwater reservoir in Brazil. We analyzed the extracellular and intracellular content of PSP toxins at two different temperatures: 19 and 25 degrees C. C. raciborskii C10 produces STX, GTX2, and GTX3 at both temperatures. dcSTX was also detected at 25 degrees C in the intracellular extracts obtained at the end of the stationary phase. The growth achieved at 25 degrees C and estimated by optical density at 700 nm was three times greater than at 19 degrees C. However, no significant differences were observed in the content of PSP toxins in either the cells or the extracellular media. The kinetics of accumulation of PSP toxins within the cells rather than in the media suggests an active PSP toxins-export process that is not related to cell lysis. The extracellular accumulation of PSP toxins at 19 degrees C suggested a biotransformation of STX to the epimers GTX2 and GTX3. The stability of the PSP toxins produced by C. raciborskii C10 was high enough for them to remain active in the media after 30 days (at 25 degrees C) or after 50 days (at 19 degrees C).

Identification of an Na(+)-dependent transporter associated with saxitoxin-producing strains of the cyanobacterium Anabaena circinalis

Blooms of the freshwater cyanobacterium Anabaena circinalis are recognized as an important health risk worldwide due to the production of a range of toxins such as saxitoxin (STX) and its derivatives. In this study we used HIP1 octameric-palindrome repeated-sequence PCR to compare the genomic structure of phylogenetically similar Australian isolates of A. circinalis. STX-producing and nontoxic cyanobacterial strains showed different HIP1 (highly iterated octameric palindrome 1) DNA patterns, and characteristic interrepeat amplicons for each group were identified. Suppression subtractive hybridization (SSH) was performed using HIP1 PCR-generated libraries to further identify toxic-strain-specific genes. An STX-producing strain and a nontoxic strain of A. circinalis were chosen as testers in two distinct experiments. The two categories of SSH putative tester-specific sequences were characterized by different families of encoded proteins that may be representative of the differences in metabolism between STX-producing and nontoxic A. circinalis strains. DNA-microarray hybridization and genomic screening revealed a toxic-strain-specific HIP1 fragment coding for a putative Na(+)-dependent transporter. Analysis of this gene demonstrated analogy to the mrpF gene of Bacillus subtilis, whose encoded protein is involved in Na(+)-specific pH homeostasis. The application of this gene as a molecular probe in laboratory and environmental screening for STX-producing A. circinalis strains was demonstrated. The possible role of this putative Na(+)-dependent transporter in the toxic cyanobacterial phenotype is also discussed, in light of recent physiological studies of STX-producing cyanobacteria.