Search Results (31)

Toxic and non-toxic strains of Raphidiopsis raciborskii coexist widely in natural water bodies, with the dominance of toxic strains directly influencing bloom toxicity. This study investigates how temperature affects the relative dominance of toxic R. raciborskii strains and the production of cylindrospermopsin (CYN). We conducted monoculture and co-culture experiments in nutrient-rich BG11 medium at three temperatures (16 °C, 24 °C, and 32 °C) using two pairs of strains (CS506/CS510 from Australia and QDH7/N8 from China). The results revealed that the Australian strains failed to grow at 16 °C, while the Chinese strains thrived. In a co-culture experiment, the Australian toxic strain CS506 exhibited the fastest growth at 24 °C, whereas the Chinese toxic strain QDH7 reached similar maximum cell densities across all temperatures but peaked more quickly at 24 °C and 32 °C compared to 16 °C. Regardless of temperature and strain pairs, toxic strains consistently maintained a higher relative abundance than their non-toxic counterparts. Analysis using the rate of competitive displacement (RCD) model indicated that higher temperatures accelerated the displacement of non-toxic strains by toxic ones. Total CYN concentrations in co-cultures increased with temperature, although the cell quota of CYN (Q_CYN) did not vary significantly across temperatures. In co-culture, the CYN production rate during the exponential phase was positively correlated with cell growth rate, but this correlation weakened or reversed in the stationary phase, likely due to changes in nutrient availability. These findings suggest that rising temperatures under eutrophic conditions may enhance the growth and competitive advantage of toxic R. raciborskii strains, thereby exacerbating bloom toxicity. Full article

Harmful Raphidiopsis raciborskii blooms threaten aquatic ecosystems via toxin production, hypoxia induction, and biodiversity loss. To elucidate the synergistic regulatory mechanisms of Fe³⁺ and phosphorus (P) in cyanobacterial growth, we used a sterile pure culture system under laboratory conditions. We set different phosphorus sources (organic phosphorus and inorganic phosphorus) and low phosphorus concentration of R. raciborskii culture medium for culture, and set different Fe³⁺ addition amount to determine the basic growth index of cyanobacteria cells and the phosphorus content of different components. The results revealed that under conditions of sufficient inorganic phosphorus, there was a logarithmic relationship between ferric ammonium citrate (Fe³⁺) and the specific growth rate of R. raciborskii. Fe³⁺ > 2 mg/L enhanced IPS enrichment and biomass accumulation. However, in oligotrophic or mesotrophic environments with low inorganic phosphorus concentrations, the effect of Fe³⁺ on the growth of R. raciborskii contrasted with that observed in high-IP (eutrophic) environments, exhibiting a pattern of ‘low promotion and high inhibition’. Under organic phosphorus conditions, R. raciborskii converted phosphorus by increasing alkaline phosphatase activity (APA), but this metabolic compensation failed to restore physiological functions, resulting in growth suppression and enhanced cellular phosphorus reserves. Our results establish quantitative linkages between Fe³⁺-P co-limitation thresholds and algal adaptive responses, providing mechanistic insights for controlling bloom dynamics through targeted manipulation of Fe-P bioavailability. Full article

The freshwater cyanobacterium Raphidiopsis raciborskii is an invasive species that was originally described as tropical and is now widely distributed in temperate regions. The current taxonomic position was established based on a multilevel approach with the morphological description of natural populations as well as their ultrastructural, physiological and molecular characterization. The practical problem in identifying this species is that the morphology of the trichome in the natural environment can vary considerably during population growth. The aim of this study was to investigate the morphological variability of R. raciborskii during its blooming in a temperate floodplain lake on the Middle Danube (Europe). In both cases, only straight trichomes were found. Young trichomes with one or two acuminate ends and without heterocytes, referred to as primary filaments, can be exclusively monodominant at the beginning of bloom formation and remain dominant throughout the year, leading to taxonomic confusion. In mature populations, the different morphological forms of secondary filaments may differ in the size of the filaments and in the number and size of akinetes and heterocytes formed in the trichomes. The correct taxonomic identification and early detection of R. raciborskii in natural freshwaters is extremely important for the successful control of the spread of this potentially toxic species. Full article

Extreme hydrological events have become more frequent in the Danube River Basin in recent decades. In this research, we focus on the consequences of such events on the dynamic of invasive cyanobacteria Raphidiopsis raciborskii (Wołoszyńska) Aguilera et al. (formerly known as Cylindrospermopsis raciborskii (Wołoszyńska) Seenayya et Subba Raju). In the Danube floodplain lake used as a case study, the investigated period from 2003 to 2016 was characterized by the cyclical occurrence of extreme floods (2006 and 2013) and extreme droughts (2003 and 2015). As a result, the lake changed several times from a phytoplankton turbid state to a clear state characterized by a very low phytoplankton biomass. R. raciborskii was abundant in the phytoplankton and bloomed in summer 2003 (June–September) and then in summer 2007 (June–August) and in August 2013. Extreme summer drought triggered the bloom, and water temperature was the most significant environmental variable during the bloom. The observed declining trend in total cyanobacterial biomass, including the less frequent occurrence of the R. raciborskii bloom, can be linked to the ecological disturbances in the stable state of the lake caused by extreme hydrological events. This suggests that the effects of climate change may be less detrimental in preserved natural river–floodplain systems. Full article

Cyanobacterial blooms are increasingly common during winters, especially when they are mild. The goal of this study was to determine the summer and winter phytoplankton community structure, cyanotoxin presence, and toxigenicity in a eutrophic lake susceptible to cyanobacterial blooms throughout the year, using classical microscopy, an analysis of toxic cyanometabolites, and an analysis of genes involved in biosynthesis of cyanotoxins. We also assessed whether cyanobacterial diversity in the studied lake has changed compared to what was reported in previous reports conducted several years ago. Moreover, the bloom-forming cyanobacterial strains were isolated from the lake and screened for cyanotoxin presence and toxigenicity. Cyanobacteria were the main component of the phytoplankton community in both sampling times, and, in particular, Oscillatoriales were predominant in both summer (Planktothrix/Limnothrix) and winter (Limnothrix) sampling. Compared to the winter community, the summer community was denser; richer in species; and contained alien and invasive Nostocales, including Sphaerospermopsis aphanizomenoides, Raphidiopsis raciborskii, and Raphidiopsis mediterranea. In both sampling times, the blooms contained toxigenic species with genetic determinants for the production of cylindrospermopsin and microcystins. Toxicological screening revealed the presence of microcystins in the lake in summer but no cyanotoxins in the winter period of sampling. However, several cyanobacterial strains isolated from the lake during winter and summer produced anabaenopeptins and microcystins. This study indicates that summer and winter blooms of cyanobacteria in the temperate zone can differ in biomass, structure, and toxicity, and that the toxic hazards associated with cyanobacterial blooms may potentially exist during winter. Full article

Harmful cyanobacterial blooms (HCBs) are of growing global concern due to their production of toxic compounds, which threaten ecosystems and human health. Saxitoxins (STXs), commonly known as paralytic shellfish poison, are a neurotoxic alkaloid produced by some cyanobacteria. Although many field studies indicate a widespread distribution of STX, it is understudied relative to other cyanotoxins such as microcystins (MCs). In this study, we assessed eleven U.S. urban lakes using qPCR, sxtA gene-targeting sequencing, and 16S rRNA gene sequencing to understand the spatio-temporal variations in cyanobacteria and their potential role in STX production. During the blooms, qPCR analysis confirmed the presence of the STX-encoding gene sxtA at all lakes. In particular, the abundance of the sxtA gene had a strong positive correlation with STX concentrations in Big 11 Lake in Kansas City, which was also the site with the highest quantified STX concentration. Sequencing analysis revealed that potential STX producers, such as Aphanizomenon, Dolichospermum, and Raphidiopsis, were present. Further analysis targeting amplicons of the sxtA gene identified that Aphanizomenon and/or Dolichospermum are the primary STX producer, showing a significant correlation with sxtA gene abundances and STX concentrations. In addition, Aphanizomenon was associated with environmental factors, such as conductivity, sulfate, and orthophosphate, whereas Dolichospermum was correlated with temperature and pH. Overall, the results herein enhance our understanding of the STX-producing cyanobacteria and aid in developing strategies to control HCBs. Full article

Freshwater reservoirs constitute an important source of drinking water, but eutrophication and higher temperatures increase the risk of more frequent blooms of cyanobacteria, including species that produce toxins. To improve the prediction of cyanotoxin episodes, we studied the annual occurrence of potential microcystin (MC) and saxitoxin (STX)-producing cyanobacteria in the Lobo reservoir, São Paulo State, Brazil. Relationships among environmental variables, cyanobacterial biomass, numbers of the mcyE and sxtA genotypes (genes encoding production of MC and STX, respectively), and concentrations of MC and STX were determined to address variables applicable for monitoring and predicting the dynamics of cyanobacteria and cyanotoxins in the reservoir. Microscopy confirmed the presence of potentially toxin-producing cyanobacteria at all sampling times, and qPCR detection showed the occurrence of both mcyE and sxtA in most samples. Concentrations of MC and STX were generally low (MC 0–1.54 µg L⁻¹; STX 0.03–0.21 µg L⁻¹). The highest MC level exceeded the recommended limit for human intake of 1 µg L⁻¹. The abundance of the mcyE and sxtA genes, as well as the toxin concentrations, were positively correlated with the biomass of Phormidium and Raphidiopsis. Among environmental variables, the abundance of potential toxic cyanobacteria was mainly affected by P limitation (high TN:TP ratios). Our data show that detection of the mcyE and sxtA genotypes serves as a useful and reliable predictor of toxin episodes but might be combined with chemical toxin detection to form an environmental toolbox for cyanotoxin monitoring. Full article

Changes in food quality can dramatically impair zooplankton fitness, especially in eutrophic water bodies where cyanobacteria are usually predominant. Cyanobacteria are considered a food with low nutritional value, and some species can produce bioactive secondary metabolites reported as toxic to zooplankton. Considering that cyanobacteria can limit the survival, growth and reproduction of zooplankton, we hypothesized that the dietary exposure of neotropical Daphnia species (D. laevis and D. gessneri) to saxitoxin-producing cyanobacteria impairs Daphnia feeding rates and fitness regardless of a high availability of nutritious algae. Life table and grazing assays were conducted with different diets: (1) without nutritional restriction, where neonates were fed with diets at a constant green algae biomass (as a nutritious food source), and an increasing cyanobacterial concentration (toxic and poor food source), and (2) with diets consisting of different proportions of green algae (nutritious) and cyanobacteria (poor food) at a total biomass 1.0 mg C L⁻¹. In general, the presence of high proportions of cyanobacteria promoted a decrease in Daphnia somatic growth, reproduction and the intrinsic rate of population increase (r) in both diets with more pronounced effects in the nutritionally restricted diet (90% R. raciborskii). A two-way ANOVA revealed the significant effects of species/clone and treatments in both assays, with significant interaction between those factors only in the second assay. Regarding the grazing assay, only D. laevis was negatively affected by increased cyanobacterial proportions in the diet. In the life table assay with constant nutritious food, a reduction in the reproduction and the intrinsic rate of the population increase (r) of all species were observed. In conclusion, we found adverse effects of the toxic cyanobacterial strain R. raciborskii on Daphnia fitness, regardless of the constant amount of nutritious food available, proving the toxic effect of R. raciborskii and that the nutritional quality of the food has a greater influence on the fitness of these animals. Full article

Raphidiopsis raciborskii, which harms water supply and ecological security, may expand its range with climate warming. To explore the ecological management method of R. raciborskii blooms, the co-culture system of Eichhornia crassipes and R. raciborskii was established to study the inhibitory effect and inhibition mechanism of E. crassipes on R. raciborskii compared with the shaded culture of R. raciborskii alone. It was found that after co-cultured with E. crassipes (fresh weight, 5.10 ± 0.60 g·L⁻¹) for seven days under initial high nutrient concentration, the algal cell density of co-culture groups with an initial cell density of 1.57 × 10⁸ cells·L⁻¹ and 1.57 × 10⁹ cells·L⁻¹ decreased by 63.19% and 14.87%, respectively. Meanwhile, the algal cell density in the co-culture group was significantly lower than that in the control group (p < 0.01). The algal cell density change showed that the growth of R. raciborskii was inhibited by co-cultured E. crassipes. In addition, E. crassipes could significantly reduce the nitrogen and phosphorus content in the water, but the growth state and photosynthesis of E. crassipes were not inhibited by R. raciborskii. These results suggested that E. crassipes has the potential to control R. raciborskii blooms. The study also found that when co-cultured, there was no significant difference in algal cell density between the groups supplemented with a certain amount of nutrients and the group without additional nutrients. Meanwhile, the inhibition rates of the co-culture groups were over 99% on day 4. These results showed that nutrient factors had no significant effect on the inhibitory effect of E. crassipes in the range of nutrients regulated by the experiment, and other factors played a leading role. In other words, under experimental conditions, nutrient competition was not the primary inhibitory mechanism of E. crassipes. This study also confirmed that E. crassipes had inhibitory allelopathy on R. raciborskii. The contribution and significance of allelopathy and other mechanisms could be studied in the future. Full article

This study was aimed primarily at describing the planktonic assemblages with special attention to invasive and toxin-producing cyanobacterial species in the context of ecological and health threats. The second aim was to analyze the aspect of recreational pressure, which may enhance the cyanobacterial blooms, and, as a consequence, the negative changes and loss of planktonic biodiversity. This study was carried out in recreationally used Lake Sztynorckie throughout the whole growing season of 2020 and included an assessment of the abundance and biomass of phytoplankton (cyanobacteria and algae) in relation to environmental variables. The total biomass was in the range of 28–70 mg L⁻¹, which is typical for strong blooms. The dominant filamentous cyanobacteria were Pseudanabaena limnetica, Limnothrix redekei, Planktolyngbya limnetica, and Planktothrix agarhii, and three invasive nostocalean species Sphaerospermopsis aphanizomenoides, Cuspidothrix issatschenkoi, and Raphidiopsis raciborskii. They can pose a serious threat not only to the ecosystem but also to humans because of the possibility of cyanobacteria producing cyanotoxins, such as microcystins, saxitoxins, anatoxin-a, and cylindrospermopsins, having hepatotoxic, cytotoxic, neurotoxic, and dermatoxic effects. The water quality was assessed as water bodies had bad ecological status (based on phytoplankton), were highly meso-eutrophic (based on zooplankton), and had very low trophic efficiency and low biodiversity. Full article

Water blooms caused by the invasive cyanobacterium Raphidiopsis raciborskii occur in many reservoirs in the tropical and subtropical regions of China. In recent decades, this species has spread rapidly to temperate regions. Phenotypic plasticity and climate warming are thought to promote the worldwide dispersion of R. raciborskii. However, investigations into the genetic and phenotypic diversities of this species have revealed significant intraspecific heterogeneity. In particular, competition between R. raciborskii and Microcystis aeruginosa was highly strain dependent. Although the concept of an ecotype was proposed to explain the heterogeneity of R. raciborskii strains with different geographic origins, microevolution is more reasonable for understanding the coexistence of different phenotypes and genotypes in the same environment. It has been suggested that intraspecific heterogeneity derived from microevolution is a strong driving force for the expansion of R. raciborskii. Additionally, temperature, nutrient fluctuations, and grazer disturbance are critical environmental factors that affect the population establishment of R. raciborskii in new environments. The present review provides new insights into the ecological mechanisms underlying the invasion of R. raciborskii in Chinese freshwater ecosystems. Full article

Microcystins are produced by multifaceted organisms called cyanobacteria, which are integral to Africa’s freshwater environments. The excessive proliferation of cyanobacteria caused by rising temperature and eutrophication leads to the production and release of copious amounts of microcystins, requiring critical management and control approaches to prevent the adverse environmental and public health problems associated with these bioactive metabolites. Despite hypotheses reported to explain the phylogeography and mechanisms responsible for cyanobacterial blooms in aquatic water bodies, many aspects are scarcely understood in Africa due to the paucity of investigations and lack of uniformity of experimental methods. Due to a lack of information and large-scale studies, cyanobacteria occurrence and genetic diversity are seldom reported in African aquatic ecosystems. This review covers the diversity and geographical distribution of potential microcystin-producing and non-microcystin-producing cyanobacterial taxa in Africa. Molecular analyses using housekeeping genes (e.g., 16S rRNA, ITS, rpoC1, etc.) revealed significant sequence divergence across several cyanobacterial strains from East, North, West, and South Africa, but the lack of uniformity in molecular markers employed made continent-wise phylogenetic comparisons impossible. Planktothrix agardhii, Microcystis aeruginosa, and Cylindrospermopsis raciborskii (presently known as Raphidiopsis raciborskii) were the most commonly reported genera. Potential microcystin (MCs)-producing cyanobacteria were detected using mcy genes, and several microcystin congeners were recorded. Studying cyanobacteria species from the African continent is urgent to effectively safeguard public and environmental health because more than 80% of the continent has no data on these important microorganisms and their bioactive secondary metabolites. Full article

Africa’s water needs are often supported by eutrophic water bodies dominated by cyanobacteria posing health threats to riparian populations from cyanotoxins, and Lake Victoria is no exception. In two embayments of the lake (Murchison Bay and Napoleon Gulf), cyanobacterial surveys were conducted to characterize the dynamics of cyanotoxins in lake water and water treatment plants. Forty-six cyanobacterial taxa were recorded, and out of these, fourteen were considered potentially toxigenic (i.e., from the genera Dolichospermum, Microcystis, Oscillatoria, Pseudanabaena and Raphidiopsis). A higher concentration (ranging from 5 to 10 µg MC-LR equiv. L⁻¹) of microcystins (MC) was detected in Murchison Bay compared to Napoleon Gulf, with a declining gradient from the inshore (max. 15 µg MC-LR equiv. L⁻¹) to the open lake. In Murchison Bay, an increase in Microcystis sp. biovolume and MC was observed over the last two decades. Despite high cell densities of toxigenic Microcystis and high MC concentrations, the water treatment plant in Murchison Bay efficiently removed the cyanobacterial biomass, intracellular and dissolved MC to below the lifetime guideline value for exposure via drinking water (<1.0 µg MC-LR equiv. L⁻¹). Thus, the potential health threats stem from the consumption of untreated water and recreational activities along the shores of the lake embayments. MC concentrations were predicted from Microcystis cell numbers regulated by environmental factors, such as solar radiation, wind speed in the N–S direction and turbidity. Thus, an early warning through microscopical counting of Microcystis cell numbers is proposed to better manage health risks from toxigenic cyanobacteria in Lake Victoria. Full article

Cyanobacteria are a bloom-forming ancient group of photosynthetic prokaryotes. A rise in temperature is a major contributor to its massive proliferation, namely on freshwater ecosystems, with social and economic impacts. Thus, reliable and cost-effective tools can permit the fast surveillance and assessment of temperature effects on potentially toxic cyanobacteria distribution and impacts. The occurrence of three potentially toxic cyanobacteria species was assessed on seven sampling points across three sampling years. Moreover, the association between the occurrence of those cyanobacteria species with climate change events was addressed. Here, we combined molecular and statistical methods to study the impacts of temperature on the occurrence of three globally occurring cyanotoxin-producing cyanobacteria species—Microcystis aeruginosa (microcystins), Raphidiopsis raciborskii (cylindrospermopsins and saxitoxins) and Planktothrix agardhii (microcystins and saxitoxins). Samples were collected on seven European temperate freshwater systems located on the North and Centre regions of Portugal, across three distinct sampling years with distinct ranges of air temperature. Data support that M. aeruginosa is still a common inhabitant of Portuguese freshwater ecosystems and a new trend was found on R. raciborskii recent invasion and establishment on the colder north ecosystems of Portugal. Additionally, the highest frequency of detection of both cyanobacteria was associated with warmer years. P. agardhii also revealed a new trend, being reported for the first time on North and Centre Regions of Portugal, however with no statistical relation with air temperature, demonstrating a higher ecological fitness. Distinct profiles of the statistical analysis on the three tested cyanobacteria species contribute to deepen the studies on other species as well as of our analyzed species on a global level. This assessment may help to anticipate possible repercussions on water quality and public health due to most probable alterations on cyanotoxins profile given the ecological fitness established among air temperature and PCR detection of potentially toxic cyanobacteria. Full article

Findings about CO₂ dynamics in the Earth’s ancestral atmosphere have suggested much higher concentrations in past eras. Along this line, cyanobacteria are an early evolved photosynthetic group that is suggested to have experienced both high and low CO₂ availability since their Precambrian origin, and therefore, it is reasonable to assume that these microbes have the potential to cope with these scenarios by rapidly adjusting to various carbon dioxide levels. Thus, in this work, we performed a short-term (72-h) investigation of the physiological parameters (cell growth, photosynthesis and saxitoxin production) of toxic and nontoxic strains of the cyanobacterium Raphidiopsis raciborskii challenged by an extremely high pCO₂ (40,000 ppm). Additionally, the transcriptomic profile (regarding the carbon concentrating mechanism and photosynthesis) of selected toxic and nontoxic strains is also presented. We found that short-term exposure to extremely elevated CO₂ concentrations did not affect R. raciborskii physiology regardless of toxin production. However, transcripts related to bicarbonate transporters and the RuBisCO enzyme indicated the upregulation of CCM and downregulation of the Calvin cycle, respectively. According to our findings, at least at the initial growth phase, R. raciborskii was able to cope with a very high CO₂ level, which shed light on the understanding that this species might have the potential to cope with carbon dioxide in water above the predicted levels. Full article