Search

Feeding effects are crucial for evaluating the capacity of zooplankton to regulate phytoplankton populations within freshwater ecosystems. To examine the impact of the bloom-forming cyanobacteria Raphidiopsis raciborskii, which occurs in tropical and subtropical freshwaters, on the growth of zooplankton Daphnia in relation to toxins, filament length and fatty acid content, we fed D. magna with R. raciborskii only (cylindrospermopsin (CYN)-producing and non-CYN-producing, as the negative controls), Chlorella pyrenoidosa only (as the positive control) and a mixed diet containing R. raciborskii (CYN-producing and non-CYN-producing) and C. pyrenoidosa. Consequently, our findings revealed that the toxic effect of CYN-producing R. raciborskii strains on Daphnia was mitigated by the coexistence of C. pyrenoidosa containing stearidonic acid (SDA, C18:4 ω3) in mixed diets. This was evident in the elevated survival rate compared that from diets containing only R. raciborskii and a significantly higher reproduction and population intrinsic increase rate compared to diets consisting of only R. raciborskii or C. pyrenoidos. Additionally, a strong positive correlation was observed between arachidonic acid (ARA, 20:4ω6) and the population intrinsic increase rate of Daphnia; notably, R. raciborskii strains were found to be rich in the ω6 polyunsaturated fatty acid ARA. These outcomes reinforce the crucial role of polyunsaturated fatty acids in predicting the population increase of crustacean zooplankton, which has long been neglected. Furthermore, our results underscore the potential effectiveness of zooplankton, particularly in temperate lakes, in controlling CYN-producing R. raciborskii populations. [ABSTRACT FROM AUTHOR]

Eutrophication events resulting from increased primary production are becoming common under changing environmental conditions that include habitat degradation and disturbance of food webs. They cause several detrimental ecological consequences including the intoxication of animals that ingest toxic Cyanobacteria. We tested whether the tadpole of an anthropophilic frog (Scinax longilineus) would avoid feeding on a potentially toxic Cyanobacteria using Raphidiopsis raciborskii as a model. We maintained tadpoles in water collected from a natural monospecific growth of R. raciborskii at the original concentration (100%) and concentrations of 0, 25, 50 and 75% of the original sample diluted in rested tap water. We evaluated the amount of R. raciborskii ingested by the tadpoles and conducted behavioral tests by exposing tadpoles to an aversive stimulus and quantifying their escape response. Tadpoles ate the least R. raciborskii the highest the concentration of this Cyanobacteria was in the water. Tadpoles exposed to 50 and 75% of the original concentration of the R. raciborskii sample had a reduction of time spent escaping, what is likely to compromise their ability to escape a real predator attack. Tadpoles at the highest concentration (100%) did not show significant changes in their escape performance, however they practically stopped feeding, what would not be a solution in the long term (i.e., duration of a real bloom). We conclude that tadpoles of S. longilineus can recognize the presence of R. raciborskii and avoid its ingestion, with potential sub-lethal effects represented by a reduced ability to escape predators. [ABSTRACT FROM AUTHOR]

Feeding effects are crucial for evaluating the capacity of zooplankton to regulate phytoplankton populations within freshwater ecosystems. To examine the impact of the bloom-forming cyanobacteria Raphidiopsis raciborskii, which occurs in tropical and subtropical freshwaters, on the growth of zooplankton Daphnia in relation to toxins, filament length and fatty acid content, we fed D. magna with R. raciborskii only (cylindrospermopsin (CYN)-producing and non-CYN-producing, as the negative controls), Chlorella pyrenoidosa only (as the positive control) and a mixed diet containing R. raciborskii (CYN-producing and non-CYN-producing) and C. pyrenoidosa. Consequently, our findings revealed that the toxic effect of CYN-producing R. raciborskii strains on Daphnia was mitigated by the coexistence of C. pyrenoidosa containing stearidonic acid (SDA, C18:4 ω3) in mixed diets. This was evident in the elevated survival rate compared that from diets containing only R. raciborskii and a significantly higher reproduction and population intrinsic increase rate compared to diets consisting of only R. raciborskii or C. pyrenoidos. Additionally, a strong positive correlation was observed between arachidonic acid (ARA, 20:4ω6) and the population intrinsic increase rate of Daphnia; notably, R. raciborskii strains were found to be rich in the ω6 polyunsaturated fatty acid ARA. These outcomes reinforce the crucial role of polyunsaturated fatty acids in predicting the population increase of crustacean zooplankton, which has long been neglected. Furthermore, our results underscore the potential effectiveness of zooplankton, particularly in temperate lakes, in controlling CYN-producing R. raciborskii populations.

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. [ABSTRACT FROM AUTHOR]

The increase in frequency and prevalence of cylindrospermopsin (CYN)-producing cyanobacteria blooms have become a growing problem worldwide. Therefore, the long-term effects of CYN on the physiological processes of the macrophyte, Lemna trisulca, were examined. During a 5-day exposure to a range of CYN concentrations (0.5, 1 and 5 μg ml−1), the low levels of stress experienced by L. trisulca stimulated its growth rate and photosynthesis and led to an increase in the synthesis of photosynthetic pigments. In turn, it was shown that a 3-week exposure to the highest tested CYN concentration (5 μg ml−1) resulted in the inhibition of biomass accumulation (by 7.6%) compared with that of the control plants, but necrosis and chlorosis were not observed. A negative correlation with the concentration of carotenoids was also observed, which may suggest that the antioxidant mechanisms of L. trisulca are effective only up to a certain CYN concentration. An increase in protein content in the tissues of macrophytes treated with concentrations of CYN ranging from 0.5 to 5 μg ml−1 indicates that the toxin did not inhibit protein synthesis but increased the metabolism of L. trisulca, potentially to provide energy for defence. The toxin also affects the homeostasis of mineral ions in cells. This is the first description of the physiological response of L. trisulca to the long-term effects of a wide range of CYN concentrations. [ABSTRACT FROM AUTHOR]

Raphidiopsis raciborskii is a toxic, invasive bacteria with a defined biogeographic pattern attributed to the generation of ecotypes subjected to local environmental filters and to phenotypic plasticity. The interactions taking place between the cyanobacterium and the other bacteria inhabiting the external polysaccharide-rich matrix surrounding the cells, or phycosphere, may be ecotype-specific and would have different influence on the carbon and nutrient cycling in the ecosystem. Here, we describe the bacterial community or microbiome (assessed by 16S rRNA metagenomics) associated to two R. raciborskii strains that have been described as different ecotypes: the saxitoxin-producer MVCC19 and the non-toxic LB2897. Our results showed that both ecotypes share 50% of their microbiomes and differ in their dominant taxa. The taxon having the highest abundance in the microbiome of MVCC19 was Neorhizobium (22.5% relative abundance), while the dominant taxon in LB2897 was the Planctomycetes SM1A02 (26.2% relative abundance). These groups exhibit different metabolic capabilities regarding nitrogen acquisition (symbiotic nitrogen-fixing in Neorhizobium vs. anammox in SM1A02), suggesting the existence of ecotype-specific microbiomes that play a relevant role in cyanobacterial niche-adaptation. In addition, as saxitoxin and analogues are nitrogen-rich (7 atoms per molecule), we hypothesise that saxitoxin-producing R. raciborskii benefits from external sources of nitrogen provided by the microbiome bacteria. Based on these findings, we propose that the mechanisms involved in the assembly of the cyanobacterial microbiome community are ecotype-dependent.

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. [ABSTRACT FROM AUTHOR]

Raphidiopsis raciborskii is a notorious bloom-forming and filamentous cyanobacterium that has been extensively investigated into its toxicity, phylogeny, and spreading potential. Studies have demonstrated that this species has spanned different climates from tropical zones to temperate regions, suggesting that R. raciborskii is becoming a cosmopolitan species in freshwater systems around the world. In fact, it has been proposed that several characteristics of R. raciborskii may explain its spread and dominance. In particular, R. raciborskii is known to display a high extent of physiological plasticity regarding nutrients, light regimes, and temperatures. Moreover, this species illustrates different ecotypes with distinct environmental requirements. Here, we present an overview of R. raciborskii's global distribution and adaptation strategy based on the recent findings from genome variance, toxicity, and ecophysiology. The expansion of its geographical distribution can be linked to its genome, toxicity, and ecophysiology. The variable genes are mainly associated with the stress response, phage defense, DNA repair, cell cycle control, and membrane transport, illustrating the species' adaptability in response to changing environments. In fact, the species shows rapid adaptability to low and/or variable nutrient availability, especially changing phosphorus availability. Moreover, the variabilities of strains within the population extend their flexibility to adapt and acclimate to ambient environment. In addition, cylindrospermopsins (CYN) appear to have a potential biological role in facilitating theirs dominance or bloom. These strategies of R. raciborskii make it a challenge to manage in a freshwater system, reflecting the management of its bloom from further evidence of the complex ecophysiology, toxicity, and genome of this species. [ABSTRACT FROM AUTHOR]

Microcystis aeruginosa has always been regarded as the main culprit of cyanobacterial blooms in freshwater. However, in recent years, Raphidiopsis raciborskii has gradually replaced M. aeruginosa as the culprit of cyanobacterial blooms in some tropical and subtropical shallow lakes. To reveal which one plays a more dominant role, interactions between cylindrospermospin (CYN)-producing R. raciborskii and microcystins (MCs)-producing or non-MCs-producing M. aeruginosa strains were studied using bialgal cultures at different initial ratios of biomasses of the two species at 25 °C. During the co-cultivation, the M. aeruginosa strains inhibited the growth and heterocyst formation of R. raciborskii filaments, and thus occupied a dominant position during the co-cultivation regardless of the initial biomass ratios in the cultures. In addition, the MCs-producing M. aeruginosa strain contributed to a higher portion of the total biomass and exerted a stronger inhibitory effect on R. raciborskii compared with the non-MCs-producing strain. However, the growth of both MCs-producing and non-MCs-producing M. aeruginosa strains was stimulated by R. raciborskii in the co-cultures compared with M. aeruginosa monoculture, indicating that M. aeruginosa could outcompete R. raciborskii if given enough time, enabling it to develop into the dominant species even in very low initial concentration. To our best knowledge, this is the first report on the loss of heterocyst formation by a species of cyanobacteria that resulted from interactions between two different species of cyanobacteria. These findings indicate that it is difficult for R. raciborskii to replace the dominant position of M. aeruginosa under the experimental environmental condition, and the allelopathic effects of M. aeruginosa on R. raciborskii could significantly contribute to the success of M. aeruginosa. [ABSTRACT FROM AUTHOR]

Raphidiopsis raciborskii can cause harmful cyanobacterial blooms when concentrations of environmental phosphorus (P) are very low, thus the physiological and molecular mechanisms involved in the acclimation to P need to be characterized better. The growth, chlorophyll fluorescence, alkaline phosphatase, and expression of genes directly involved in P assimilation were compared in the R. raciborskii FACHB 1496 strain grown with and without inorganic P. The specific growth rate (μ), Chl a, and six fluorescence parameters (minimal fluorescence (F0), maximal fluorescence (Fm), maximal variable fluorescence (Fv), electron transport flux (further than QA) per RC (ET0/RC), quantum yield of the electron transport in PSII (ØE0), and the probability that an electron from a trapped exciton is moved into the electron transport chain beyond QA− (ψ0)) markedly decreased in R. raciborskii in response to experimental P-deficiency. In contrast, the relative variable fluorescence at the J-step (VJ), trapped energy flux (leading to QA reduction) per RC (TR0/RC), and alkaline phosphatase activity significantly increased. In addition, gene expressions involved in the alkaline phosphatase (phoA1 and phoA2), high-affinity inorganic P transporter (pstS1), phosphonate transporter and metabolism (phnD and phnM), and nucleotidase (nucH) were significantly upregulated under P deficiency. However, physiological and molecular responses were resumed rapidly after P re-supplementation following P-deficient conditions. Our results highlight that R. raciborskii can perform coordinated and complex cellular and physiological responses to cope with P deficiency, reflecting R. raciborskii's multi-faceted machinery to respond to environmental P fluctuations. [ABSTRACT FROM AUTHOR]

Cyanobacterial dominance or blooms can influence ecosystem structure in reservoirs, yet there are only few studies of its effect on the resource use and trophic structure of zooplankton. We hypothesised that zooplankton traits would exhibit a strong response to the increase of invasive and toxic cyanobacteria.We investigated the effect of an invasive bloom‐forming cyanobacterium Raphidiopsis raciborskii (formerly Cylindrospermopsis raciborskii) on zooplankton functional traits (e.g., growth, reproduction, allochthonous resource use and realised niche) through laboratory experiments and 3‐year field investigations in two subtropical reservoirs. The realised niche (i.e., resource and habitat use) was quantified using stable isotopes (termed isotopic niches).We fed the cladoceran zooplankton Moina sp. with Scenedesmus obliquus (chlorophyte), R. raciborskii, and a mixture of S. obliquus and R. raciborskii in laboratory experiments. A diet of R. raciborskii alone depressed the growth and reproduction of Moina sp. compared to a diet of S. obliquus alone, but a mixture of S. obliquus and R. raciborskii greatly alleviated the inhibition effects caused by cyanobacteria.Under natural field conditions, we did not find a strong inhibition effect of R. raciborskii dominance on the zooplankton, but a high biomass of R. raciborskii was associated with an increase (17%) in the relative allochthonous resource (terrestrially derived organic matter) use by cladocerans (but not for copepods), even though the chlorophyll‐a concentration increased from 17 μg/L in the non‐bloom period to 28 μg/L during periods of R. raciborskii dominance or blooms. Furthermore, a high biomass of R. raciborskii increased the contribution of inter‐taxa variation (taxa sorting) to the expanded isotopic niches of the zooplankton community.Our study helps to clarify cyanobacteria‐zooplankton interactions and highlights the importance of cyanobacterial dominance or blooms in regulating cross‐ecosystem resource use and biogeochemical cycling mediated by zooplankton in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Raphidiopsis raciborskii is a globally distributed bloom-forming cyanobacterium and produces toxin cylindrospermopsin (CYN), which results in public and environmental health consequences. piece of evidence To understand their blooming dynamics and to identify the environmental factors that determine their success in the tropical reservoir, a monthly monitoring program was conducted from May 2019 to April 2020. Variations in phytoplankton communities, environmental parameters (temperature, pH, dissolved oxygen, turbidity, dissolved inorganic nutrients) and CYN were simultaneously determined from a total of 36 samples in the Ea Nhai reservoir in Vietnam. The phytoplankton is composed of a dominant cyanobacteria community formed mainly by filamentous R. raciborskii with the biovolume of up to 66.8 mm3 L-1. PCA and Pearson correlation results showed the biovolume and variation of R. raciborskii in the Ea Nhai reservoir were mainly related to temperature and nutrients (N-NH4 , P-PO4 , TP, TN). Analysis of the toxins by ELISA demonstrated the presence of CYN in all water samples with an average value of 1.24 µg L-1 and it was significantly correlated with the biovolume of the invasive species R. raciborskii. The results of the HPLC analysis confirmed the production of CYN in nine isolated Raphidiopsis strains with concentrations ranging from 0.054 to 0.584 μg g-1DW. The relatively high biovolume of R. raciborskii that is associated with the level of CYN in the water and the capacity to produce CYN of the isolated R. raciborskii strains represent a potential threat to the public health and the life of domestic and wild animals in the reservoir catchment. [ABSTRACT FROM AUTHOR]

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.

Copyright of Asian Journals of Ecotoxicology is the property of Gai Kan Bian Wei Hui and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii) can produce cylindrospermopsin (CYN) which is of great concern due to its considerable toxicity to human and animals. Its CYN-producing (toxic) and non-CYN-producing (non-toxic) strains co-exist commonly in natural water bodies, while how their relative dominance is regulated has not been addressed. In this study, we combined field investigation with laboratory experiments to assessed the relationship between toxic and non-toxic R. raciborskii abundances under different nutrient levels. The rpoC1 - and cyrJ -based qPCR was applied for quantifying total and toxic R. raciborskii abundances, respectively. The field survey showed that toxic R. raciborskii was detected in 97 of 115 reservoirs where its proportion ranged from 0.3% to 39.7% within the R. raciborskii population. Both total and toxic R. raciborskii abundances increased significantly with trophic level of these reservoirs, consistent with our monoculture and co-culture experiments showing in an increase in R. raciborskii growth with increasing nitrogen (N) or phosphorus (P) concentrations. In the monoculture experiments, growth rates of non-toxic and toxic strains from Australia or China were not significantly different under the same culture conditions. On the other hand, in the co-culture experiments, the toxic strains displayed a significantly faster growth than non-toxic strains under nutrient-replete conditions, resulting in an obvious shift toward the dominance by toxic strains from day 3 to the end of the experiments, regardless of the strain originating from Australia or China. The reverse was found under N- or P-limited conditions. Our results indicated that the toxic strains of R. raciborskii have a competitive advantage relative to the non-toxic strains in a more eutrophic world. In parallel to an increase in dominance, both toxic strains grown in the mixed population significantly increased CYN production under nutrient-replete conditions as compared to nutrient-limited conditions, suggesting that CYN may be of significance for ecological advantage of toxic R. raciborskii. These results highlight the importance of nutrient availability in regulating abundances and strain dominance of two genotypes of R. raciborskii. Our findings demonstrated that elevated nutrients would favor the growth of CYN-producing R. raciborskii and CYN production, leading to more blooms with higher toxicity at global scale. [ABSTRACT FROM AUTHOR]

Raphidiopsis raciborskii is a toxic, invasive bacteria with a defined biogeographic pattern attributed to the generation of ecotypes subjected to local environmental filters and to phenotypic plasticity. The interactions taking place between the cyanobacterium and the other bacteria inhabiting the external polysaccharide-rich matrix surrounding the cells, or phycosphere, may be ecotype-specific and would have different influence on the carbon and nutrient cycling in the ecosystem. Here, we describe the bacterial community or microbiome (assessed by 16S rRNA metagenomics) associated to two R. raciborskii strains that have been described as different ecotypes: the saxitoxin-producer MVCC19 and the non-toxic LB2897. Our results showed that both ecotypes share 50% of their microbiomes and differ in their dominant taxa. The taxon having the highest abundance in the microbiome of MVCC19 was Neorhizobium (22.5% relative abundance), while the dominant taxon in LB2897 was the Planctomycetes SM1A02 (26.2% relative abundance). These groups exhibit different metabolic capabilities regarding nitrogen acquisition (symbiotic nitrogen-fixing in Neorhizobium vs. anammox in SM1A02), suggesting the existence of ecotype-specific microbiomes that play a relevant role in cyanobacterial nicheadaptation. In addition, as saxitoxin and analogues are nitrogen-rich (7 atoms per molecule), we hypothesise that saxitoxin-producing R. raciborskii benefits from external sources of nitrogen provided by the microbiome bacteria. Based on these findings, we propose that the mechanisms involved in the assembly of the cyanobacterial microbiome community are ecotype-dependent. [ABSTRACT FROM AUTHOR]

Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii) can produce cylindrospermopsin (CYN) which is of great concern due to its considerable toxicity to human and animals. Its CYN-producing (toxic) and non-CYN-producing (non-toxic) strains co-exist commonly in natural water bodies, while how their relative dominance is regulated has not been addressed. In this study, we combined field investigation with laboratory experiments to assessed the relationship between toxic and non-toxic R. raciborskii abundances under different nutrient levels. The rpoC1- and cyrJ-based qPCR was applied for quantifying total and toxic R. raciborskii abundances, respectively. The field survey showed that toxic R. raciborskii was detected in 97 of 115 reservoirs where its proportion ranged from 0.3% to 39.7% within the R. raciborskii population. Both total and toxic R. raciborskii abundances increased significantly with trophic level of these reservoirs, consistent with our monoculture and co-culture experiments showing in an increase in R. raciborskii growth with increasing nitrogen (N) or phosphorus (P) concentrations. In the monoculture experiments, growth rates of non-toxic and toxic strains from Australia or China were not significantly different under the same culture conditions. On the other hand, in the co-culture experiments, the toxic strains displayed a significantly faster growth than non-toxic strains under nutrient-replete conditions, resulting in an obvious shift toward the dominance by toxic strains from day 3 to the end of the experiments, regardless of the strain originating from Australia or China. The reverse was found under N- or P-limited conditions. Our results indicated that the toxic strains of R. raciborskii have a competitive advantage relative to the non-toxic strains in a more eutrophic world. In parallel to an increase in dominance, both toxic strains grown in the mixed population significantly increased CYN production under nutrient-replete conditions as compared to nutrient-limited conditions, suggesting that CYN may be of significance for ecological advantage of toxic R. raciborskii. These results highlight the importance of nutrient availability in regulating abundances and strain dominance of two genotypes of R. raciborskii. Our findings demonstrated that elevated nutrients would favor the growth of CYN-producing R. raciborskii and CYN production, leading to more blooms with higher toxicity at global scale.

Changes in water level and flushing rate directly affect to a large extent the biomass of harmful cyanobacteria, and drive the shift of phytoplankton composition between cyanobacteria dominance/non-dominance in eutrophic waters. Here, we gave a theoretical formula describing the combinational effect of water level and flushing rate on cyanobacterial biomass in eutrophic and well-mixed waters. We also formulated an equation predicting the water level and flushing rate at which cyanobacteria become non-dominating in such water columns. The formulae were confronted with field observations of a low-light adapted cyanobacterium in a large coastal reservoir of southern China. Our formulae demonstrate that water level and flushing rate have an interactive effect on the equilibrium biomass of low-light adapted cyanobacteria in mixed and turbid waters. The formulae were well fitted to the field observation of Raphidiopsis raciborskii population in the reservoir during four dry seasons. In agreement with the theoretical analysis, multiple regression analysis also showed that the interaction between water level and flushing rate is able to interpret the variation of R. raciborskii biomass in the water column. The two formulae are applicable for predicting the response of low-light adapted cyanobacteria to local climate change. Our findings have practical significance in designing measures against the dominance of low light-adapted cyanobacteria in reservoirs. [ABSTRACT FROM AUTHOR]

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.

Impact of macrophyte Lemna trisulca on the growth rate and synthesis of cylindrospermopsin (CYN) by cyanobacterium Raphidiopsis raciborskii was determined. The presence of L. trisulca inhibited the biomass accumulation of the cyanobacterium by 25% compared to the control during co-cultivation. The simultaneous cultivation of these organisms slightly affected the inhibition of macrophyte growth rate by 5.5% compared to the control. However, no morphological changes of L. trisulca after incubation with cyanobacteria were observed. It was also shown that the long-term (35 days) co-cultivation of R. raciborskii and L. trisulca led to a decrease in CYN concentration in media and cyanobacterial cells by 32 and 38%, respectively, compared to the values obtained for independent cultivation of cyanobacterium. Excessive absorption of phosphate ions by L. trisulca from the medium compared to nitrate ions led to a significant increase in the nitrate:phosphate ratio in the media, which inhibits the development of cyanobacterium. The obtained results indicate that L. trisulca in the natural environment may affect the physiology of cyanobacteria. The presented study is the first assessment of the allelopathic interaction of macrophyte and R. raciborskii. [ABSTRACT FROM AUTHOR]

• During the period 2009–2022 R. raciborskii was registered in 27 % of the lakes. • R. raciborskii has colonised 14 new localities during the same period. • Its contribution to the total phytoplankton biomass has increased significantly. • The main factor for the successful expansion is high summer temperatures. • Models predict future risk of colonisation even at altitudes above 500 m asl. The cyanobacterial species Raphidiopsis raciborskii (Wołoszyńska) Aguilera et al. has a high invasiveness potential, which in less than a century leads to its cosmopolitan spread. In the temperate climate of Europe , R. raciborskii has been reported in many countries, but there is still a lack of detailed information about the current status of its distribution in lakes of Bulgaria, as a part of the southeastern range of its spread in Europe. We investigated the distribution of the species using data on the phytoplankton of 122 lakes surveyed during 13-years period (2009–2022). The species was found in 33 lakes (up to 324 m asl), and 14 new localities were registered during the studied period. The results reveal that the number of lakes with the presence of R. raciborskii (27 % of all research lakes) and its contribution to the total phytoplankton biomass, has increased significantly over the last decade. The species has successfully adapted and dominated the phytoplankton in 9 lakes, forming a bloom in 8 of them. The dominant position of R. raciborskii causes loss of species and functional diversity of phytoplankton and displaces the native bloom-forming cyanobacteria. Lakes with and without the species were compared based on the available data on bioclimatic and local environmental variables. Statistically significant differences were established with respect to water transparency, conductivity, maximum depth and maximum air temperature in the warmest month. Species distribution models (SDMs) were used to identify lakes in high risk of future invasion by R. raciborskii. The results of the SDMs implementation confirmed the high maximum air temperature and low water transparency to be important predictors of the occurrence of R. raciborskii in freshwater lakes in Bulgaria. In the areas with high summer temperatures the most suitable for R. raciborskii development were found to be shallow polymictic or medium deep lakes with small surface area and low water transparency. In areas with a suitable climate, the large, deep reservoirs with high transparency as well as macrophyte dominated lakes have a low probability of occurrence of R. raciborskii. Future colonization of lakes above 500 m asl (but most likely below 700 m asl) is also possible, especially in the conditions of global warming. SDMs account for climatic and biogeographic differences of lakes and could help in elucidating the underlying factors that control the occurrence and adaptation of R. raciborskii in a given area. [ABSTRACT FROM AUTHOR]

While warming and eutrophication have increased the frequency and magnitude of harmful cyanobacterial blooms globally, the scenario for many eutrophic tropical freshwaters is a perennial year-round bloom. Yet, the drivers of persistent blooms are less understood when conditions such as light, temperature, and nutrients favor cyanobacteria growth year-round, and especially in regions facing recurrent periods of drought. In order to understand the drivers of cyanobacteria dominance, we assessed the abiotic conditions related to the abundance and dominance of the two dominant bloom-forming genera Raphidiopsis and Microcystis, in six shallow, man-made lakes located in the semiarid Northeastern region of Brazil during a prolonged regional drought. Lower water level corresponded to increased phosphorous and nitrogen concentration and, consequently, phytoplankton biomass. Cyanobacterial biomass was also proportional to phosphorus concentrations during year-round blooms. Yet, the two dominant cyanobacterial genera, Raphidiopsis and Microcystis, seldom co-occurred temporally and the switch between them was driven by water transparency. Our results illustrate the effects of drought induced water level reductions on the biomass and composition of cyanobacterial blooms in tropical shallow man-made lakes. Given the ideal year-round conditions (i.e., high light and temperature), droughts may be expected to intensify the risk and multitude of problems associated with eutrophication. [ABSTRACT FROM AUTHOR]

R. raciborskii is known for growing under wide ranges of temperature and light. In temperate regions, however, low temperature and high light may serve as a stressful condition for invading tropical populations. The genetic basis of R. raciborskii's adaptation to this combination of stresses are unknown. In this study, the growth rate and the expression of genes that may be crucial in the response to the chill/light stress of two R. raciborskii strains (differing in their climatic origin and toxicity) exposed to low temperature and high light were examined. Results showed that AMU-DH-30, a non CYN (cylindrospermopsin) producing strain isolated from the temperate region, exhibited under stress the upregulation of genes involved in the protein translation (rbp1, nusG, hflX), membrane fluidity (desA), photosynthetic activity (ccr2 and ftsH), and the accumulation of compatible solutes (asd). In contrast, a CYN producing Australian strain CS-505 was not able to adapt quickly and to continue growth during stress conditions. Intriguingly, CS-505 and AMU-DH-30 had a similar ability to resume growth when the stress conditions subsided. Moreover, in strain CS-505 the cyrB gene was significantly upregulated under the stress conditions. The presented results shed new light on the possible mechanisms involved in the response of R.raciborskii to chill/light stress. [ABSTRACT FROM AUTHOR]

Aim: Free‐living bacteria have long been assumed to have little biogeographic signature because of their high potential for passive dispersal. Raphidiopsis raciborskii and Microcystis aeruginosa are free‐living cosmopolitan cyanobacteria with a probable tropical origin, that frequently form toxic blooms and are considered invasive species in middle latitudes. Despite these similarities, their phylogeographic patterns have seldom been directly compared. Our aim was to reconstruct the phylogeographic histories of R. raciborskii and M. aeruginosa, in order to explore whether their current distributions could be attributed to the same biogeographic events. Location: Global. Taxa: Raphidiopsis raciborskii (Nostocales) and Microcystis aeruginosa (Chroococcales). Methods: The phylogeography of these species was studied based on global genetic diversity patterns from a dataset of worldwide 16S rRNA gene sequences. Genetic diversity indices were measured globally and by latitude. Analyses based on distance matrices were performed to evaluate the correlation between genetic divergence, geographic distance and climatic conditions. Demographic history was investigated through neutrality tests and Bayesian Skyline Plot analysis. Results: For both species, the genetic diversity is highest in tropical latitudes, and the data provided evidence of recent population expansions (in the last 2,500 years). R. raciborskii showed much lower genetic diversity than M. aeruginosa. A significant phylogeographic structure was found for R. raciborskii but not for M. aeruginosa. Climatic conditions had a significant influence on the genetic structure of both species, but this influence was stronger and varied according to latitude only for R. raciborskii. Main Conclusions: Supporting the hypothesis of a tropical origin and recent dispersal to temperate habitats, both species had higher genetic diversity in tropical latitudes and showed evidence of recent population expansions. R. raciborskii populations showed a significant decline in genetic similarity with increasing geographic distance, indicating an interaction between drift and some dispersal limitation on its phylogeography. In contrast, M. aeruginosa seemed to have a high frequency of intercontinental dispersal. Finally, the particularities of each species, such as the ability to form akinetes in R. raciborskii and high genome plasticity and niche specialization in M. aeruginosa, may be associated with the lower genetic diversity of R. raciborskii when compared to M. aeruginosa. [ABSTRACT FROM AUTHOR]

Copyright of Knowledge & Management of Aquatic Ecosystems is the property of EDP Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Raphidiopsis (previously Cylindrospermopsis) raciborskii is considered as a cylindrospermopsin (CYN) producer in Thailand. There are no reports including molecular information on the genes associated with CYN production of cyanobacteria from Thailand. In the present study, 27 new R. raciborskii strains from Thailand were isolated. Their morphological features and phylogenetic positions based on the partial sequences of 16S rDNA and rbcLX were analyzed for identification. Morphological characteristics of all the strains tested, and the natural samples, were found to be similar to the original description of R. raciborskii. The rbcLX phylogenetic trees revealed that the clade of R. raciborskii is independent from those of R. mediterranea and R. curvata, whereas the 16S rDNA phylogenetic trees failed to discriminate these species. Thus, all the strains tested in the present study were identified as R. raciborskii. Furthermore, the LC/MS/MS analysis confirmed that some of the strains tested produced CYN and 7-deoxy-CYN. The presence of genes associated with CYN biosynthesis (cyrA, cyrB, cyrC, and cyrJ) in Thai Raphidiopsis strains was investigated by PCR and phylogenetic analyses. Results of PCR showed that the PCR amplicons of cyrB and cyrJ were obtained from all toxic strains, while the amplicons of cyrA and cyrC were not obtained in some toxic Thai strains. Our findings supported the observation that cyrB and cyrJ, especially the latter, may be useful for the detection of CYNproducing cyanobacteria in water resources for water supplies and fisheries. [ABSTRACT FROM AUTHOR]

Bacterioplankton community composition has become the center of research attention in recent years. Bacteria associated with toxic cyanobacteria blooms have attracted considerable interest. However, little is known about the environmental factors driving the bacteria community, including the impact of invasive cyanobacteria. Therefore, our aim has been to determine the relationships between heterotrophic bacteria and phytoplankton community composition across 24 Polish lakes with different contributions of cyanobacteria including the invasive species Raphidiopsis raciborskii. This analysis revealed that cyanobacteria were present in 16 lakes, while R. raciborskii occurred in 14 lakes. Our results show that bacteria communities differed between lakes dominated by cyanobacteria and lakes with minor contributions of cyanobacteria but did not differ between lakes with R. raciborskii and other lakes. Physical factors, including water and Secchi depth, were the major drivers of bacteria and phytoplankton community composition. However, in lakes dominated by cyanobacteria, bacterial community composition was also influenced by biotic factors such as the amount of R. raciborskii, chlorophyll-a and total phytoplankton biomass. Thus, our study provides novel evidence on the influence of environmental factors and R. raciborskii on lake bacteria communities.

Feeding effects are crucial for evaluating the capacity of zooplankton to regulate phytoplankton populations within freshwater ecosystems. To examine the impact of the bloom-forming cyanobacteria Raphidiopsis raciborskii , which occurs in tropical and subtropical freshwaters, on the growth of zooplankton Daphnia in relation to toxins, filament length and fatty acid content, we fed D. magna with R. raciborskii only (cylindrospermopsin (CYN)-producing and non-CYN-producing, as the negative controls), Chlorella pyrenoidosa only (as the positive control) and a mixed diet containing R. raciborskii (CYN-producing and non-CYN-producing) and C. pyrenoidosa . Consequently, our findings revealed that the toxic effect of CYN-producing R. raciborskii strains on Daphnia was mitigated by the coexistence of C. pyrenoidosa containing stearidonic acid (SDA, C18:4 ω3) in mixed diets. This was evident in the elevated survival rate compared that from diets containing only R. raciborskii and a significantly higher reproduction and population intrinsic increase rate compared to diets consisting of only R. raciborskii or C. pyrenoidos . Additionally, a strong positive correlation was observed between arachidonic acid (ARA, 20:4ω6) and the population intrinsic increase rate of Daphnia ; notably, R. raciborskii strains were found to be rich in the ω6 polyunsaturated fatty acid ARA. These outcomes reinforce the crucial role of polyunsaturated fatty acids in predicting the population increase of crustacean zooplankton, which has long been neglected. Furthermore, our results underscore the potential effectiveness of zooplankton, particularly in temperate lakes, in controlling CYN-producing R. raciborskii populations., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Lei, Lai, Liu, Li, Zhang and Tang.)

• Population dynamics of R. raciborskii were strongly correlated with water temperature in the temperate and Mediterranean lakes but weakly correlated in the tropical lake. • Relationships with N:P-ratios confirmed that fastest growth of R. raciborskii coincides with episodes of N-limited waters regardless of trophic states. • Distinct phenology of R. raciborskii appeared to become compromised by growing importance biotic regulating factors along warming climate. • HEA served as powerful tool for meta-analysis resulting in inferential models with predictive capacity just driven by water temperature or N:P-ratios. Raphidiopsis raciborskii is a tropical toxic cyanobacterium that is rapidly expanding to diverse lake habitats in different climate zones by sophisticated adaptation mechanisms. This meta-analysis investigated correlations of R. raciborskii with water temperature and N:P (nitrogen to phosphorus)-ratios across four lakes with different climates and trophic states by means of long-term time series and the hybrid evolutionary algorithm HEA. The results have shown that in the lakes with temperate and Mediterranean climate, R. raciborskii is strongly correlated with water temperature since germination and growth rely on rising water temperatures in spring. In contrast, there was a weaker correlation with water temperature in subtropical and tropical lakes where pelagic populations of R. raciborskii are overwintering, and are present all year round. However, the highest abundances of R. raciborskii coincided with highest water temperature for the Mediterranean, subtropical and tropical lakes, whilst in the temperate Langer See the highest abundances of R. raciborskii occurred at 24.1 °C, even though temperatures of up to 27 °C were recorded in 2013 and 2014. The correlation of R. raciborskii with N:P-ratios proved to be strongest for the meso- to eutrophic Lake Kinneret (r2 = 0.8) and lowest for the eutrophic Lake Paranoa (r2 = 0.16). However, the assumption has been confirmed that R. raciborskii is growing fastest when waters are N-limited regardless of trophic states. In terms of phenology, the temperate and Mediterranean lakes displayed "fastest growth" in spring and early summer. In contrast, the growing season in subtropical and tropical lakes lasted from spring to autumn most likely because of overwintering populations, and growing importance of direct and indirect biotic regulating factors such as competition, grazing, remineralisation of nutrients along warming climate. In order to carry out a meta-analysis of time series across four different lakes, HEA served as powerful tool resulting in inferential models with predictive capacity for population dynamics of R. raciborskii just driven by water temperature or N:P-ratios, whilst coefficients of determination r2 served as criteria for hypotheses testing. [ABSTRACT FROM AUTHOR]

R. raciborskii is known for growing under wide ranges of temperature and light. In temperate regions, however, low temperature and high light may serve as a stressful condition for invading tropical populations. The genetic basis of R. raciborskii’s adaptation to this combination of stresses are unknown. In this study, the growth rate and the expression of genes that may be crucial in the response to the chill/light stress of two R. raciborskii strains (differing in their climatic origin and toxicity) exposed to low temperature and high light were examined. Results showed that AMU-DH-30, a non CYN (cylindrospermopsin) producing strain isolated from the temperate region, exhibited under stress the upregulation of genes involved in the protein translation (rbp1, nusG, hflX), membrane fluidity (desA), photosynthetic activity (ccr2 and ftsH), and the accumulation of compatible solutes (asd). In contrast, a CYN producing Australian strain CS-505 was not able to adapt quickly and to continue growth during stress conditions. Intriguingly, CS-505 and AMU-DH-30 had a similar ability to resume growth when the stress conditions subsided. Moreover, in strain CS-505 the cyrB gene was significantly upregulated under the stress conditions. The presented results shed new light on the possible mechanisms involved in the response of R.raciborskii to chill/light stress.

• R. raciborskii. has a wide range of temperature adaptations, with the ability to spread and bloom-forming in temperate regions. • Saccharides and reduce glutathione increase the cold acclimation capacity of R. raciborskii. • The energy metabolism strategy of R. raciborskii. is temperature-dependent. • R. raciborskii. can regulate cell morphology, physiology, and photosynthetic strategies through resource trade-offs. As a tropical filamentous cyanobacterium, Raphidiopsis raciborskii has attracted much attention due to its expansion and toxin production. However, the mechanisms of its expansion to temperate regions have not been studied in detail. To address the potential strategies, the physiological and metabolomic profiles of R. raciborskii FACHB 1096 isolated from a temperate lake in China were determined and measured at different temperatures (10 °C, 15 °C, 20 °C, 25 °C, and 32 °C). The results demonstrated that temperature significantly changed cell viability, chlorophyll a content, specific growth rate, Chl a fluorescence, and filamentous shape of R. raciborskii. Low temperature decreased cell viability, specific growth rate, and photosynthetic efficiency, while the proportion of akinete and carbon fixation per unit cell were significantly increased compared with high temperature (32 °C). A constructed unimodal model indicated that filament length, cell volume, and cell length/width of R. raciborskii were significantly reduced in both high and low temperature environments. Under low-temperature conditions, R. raciborskii suffered different degrees of oxidative damage and produced corresponding antioxidant substances to resist oxidative stress, suggesting that low temperature changes the metabolic level of the cells, causing the cells to gradually switch from development to defense. Metabolomic data further confirmed that temperature change induced shifts in metabolic pathways in R. raciborskii , including starch and sucrose metabolic pathways, glutathione metabolic pathways, and the pentose phosphate pathways (PPP), as well as metabolic pathways related to the tricarboxylic acid (TCA) cycle. Our results indicated that the trade-offs of R. raciborskii cells among the growth, cell size, and metabolites can be significantly regulated by temperature, with broad implications for its global expansion in temperate waterbodies. [Display omitted] [ABSTRACT FROM AUTHOR]

The increasing occurrence of cyanobacteria blooms is of global concern, and is often associated with environmental and socio-economic problems, such as degenerated ecosystems and aquaculture impairment. The diazotrophic cyanobacterium Raphidiopsis raciborskii (R. raciborskii) grows rapidly in the tropics, and produces the toxin, cylindrospermopsin (CYN), which has harmful effects on aquatic organisms. Thus, to protect water quality and ecosystem, it is essential to have rapid and reliable methods for cyanobacteria and R. raciborskii detection and prediction so that early warning can be provided for management. Molecular assays, such as PCR, real-time quantitative PCR (qPCR), two-step PCR assays are accurate and widely used, but still require several hours from sample preparation to data analysis. In this study, insulated isothermal PCR (iiPCR) assays in conjunction with fast DNA extraction method, were developed and verified as a rapid detection assay in detecting cyanobacteria and R. raciborskii within 50 min, and also with high detection accuracy (98.8%) and the overall high agreement level (98.8%, k = 97.5%)) comparing to conventional qPCR assay. However, the limitation of the iiPCR assay is that it only generates qualitative results. Therefore, the quantified iiPCR assay, named as A-iiPCR, by coupling iiPCR device with fluorescence signal catching and interpretation instrument (Andor spectrometer with Solis spectroscopy software) was developed and verified with in situ environmental samples. The fluorescence intensity decreased accordingly with the drop of DNA concentration until reaching 1.32 ng/μL. Also, Delft 3D modelling was established to simulate R. raciborskii change in predicting spatial and temporal variabilities for reservoir management, as the simulated R. raciborskii concentration was the highest at sampling site 1, as well as temporally highest in April and October, posing as the most high-risk location and time periods for R. raciborskii bloom-forming requiring corresponding governance measures. [Display omitted] • Challenges in cyanobacteria rapid detection by Insulated Isothermal Polymerase Chain Reaction are discussed. • High agreement level comparing to conventional Quantitative Real-time PCR assay are presented. • Quantification of the Insulated Isothermal Polymerase Chain Reaction with in situ samples is discussed. • Prediction of Raphidiopsis raciborskii bloom-forming in tropical reservoir by Delft 3D modelling is presented. [ABSTRACT FROM AUTHOR]

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.

To elucidate the interspecies connectivity between cyanobacteria and other bacteria (noncyanobacteria), microbial diversity and composition were investigated through high-throughput sequencing (HTS) in a drinking water reservoir in Chongqing city, Southwest China, during Raphidiopsis raciborskii blooms. Significant temporal changes were observed in microbial community composition during the sampling period, primarily reflected by variations in relative bacterial abundance. The modularity analysis of the network demonstrated that the bacterial community forms co-occurrence/exclusion patterns in response to variations in environmental factors. Moreover, five modules involved in the dynamic phases of the R. raciborskii bloom were categorized into the Pre-Bloom, Bloom, Post-Bloom, and Non-Bloom Groups. The reservoir was eutrophic (i.e., the average concentrations of total nitrogen (TN) and total phosphorus (TP) were 2.32 and 0.07 mg L−1, respectively) during the investigation; however, Pearson's correlation coefficient showed that R. raciborskii was not significantly correlated with nitrogen and phosphorus. However, other environmental factors, such as water temperature, pH, and the permanganate index, were positively correlated with R. raciborskii. Importantly, Proteobacteria (α -, γ -Proteobacteria), Acidobacteria, Chloroflexi, and Firmicutes were preferentially associated with increased R. raciborskii blooms. These results suggested that the transition of R. raciborskii bloom-related microbial modules and their keystone species could be crucial in the development and collapse of R. raciborskii blooms and could provide a fundamental basis for understanding the linkage between the structure and function of the microbial community during bloom dynamics. [ABSTRACT FROM AUTHOR]

• Species Distribution Models predictions uncovered areas of high occurrence probability of the cyanobacteria Raphidiopsis raciborskii in locations in Europe where it has not been recorded yet. • Temperature seasonality and altitude were the main factor determining the high probability of occurrence of R. raciborskii in Europe. • In Sweden, areas of suitable climate for R. raciborskii corresponded with lakes of high concentrations of total phosphorus increasing the risk of high biomass of this cyanobacteria. In freshwater habitats, invasive species and the increase of cyanobacterial blooms have been identified as a major cause of biodiversity loss. The invasive cyanobacteria Raphidiopsis raciborskii a toxin-producing and bloom-forming species affecting local biodiversity and ecosystem services is currently expanding its range across Europe. We used species distribution models (SDMs) and regional bioclimatic environmental variables, such as temperature and precipitation, to identify suitable areas for the colonization and survival of R. raciborskii , with special focus on the geographic extent of potential habitats in Northern Europe. SDMs predictions uncovered areas of high occurrence probability of R. raciborskii in locations where it has not been recorded yet, e.g. some areas in Central and Northern Europe. In the southeastern part of Sweden, areas of suitable climate for R. raciborskii corresponded with lakes of high concentrations of total phosphorus, increasing the risk of the species to thrive. To our knowledge, this is the first attempt to predict areas at high risk of R. raciborskii colonization in Europe. The results from this study suggest several areas across Europe that would need monitoring programs to determine if the species is present or not, to be able to prevent its potential colonization and population growth. Regarding an undesirable microorganism like R. raciborskii , authorities may need to start information campaigns to avoid or minimize the spread. [ABSTRACT FROM AUTHOR]

• The electron transport of PSII was blocked in the P-deficient and AEP group. • Significant regulation of 701 genes was observed in P-deficient condition. • Carbon-phosphorus lyase plays an important role in AEP group. • Nitrogen, amino acids, and urea can partly mitigate the effects of P-depletion. Phosphorus (P) is a vital macronutrient associated with the growth and proliferation of Raphidiopsis raciborskii, an invasive and notorious bloom-forming cyanobacterium. However, the molecular mechanisms involved in P acclimation remain largely unexplored for Raphidiopsis raciborskii. Here, transcriptome sequencing of Raphidiopsis raciborskii was conducted to reveal multifaceted mechanisms involved in mimicking dipotassium phosphate (DIP), β-glycerol phosphate (Gly), 2-aminoethylphosphonic acid (AEP), and P-free conditions (NP). Chlorophyll a fluorescence parameters showed significant differences in the NP and AEP groups compared with the DIP and Gly-groups. Expression levels of genes related to phosphate transportation and uptake, organic P utilization, nitrogen metabolism, urea cycling, carbon fixation, amino acid metabolism, environmental information, the ATP-synthesis process in glycolysis, the tricarboxylic acid (TCA) cycle, and the pentose phosphate pathway were remarkably upregulated, while those related to photosynthesis, phycobiliproteins, respiration, oxidative phosphorylation, sulfur metabolism, and genetic information were markedly downregulated in the NP group relative to the DIP group. However, the expression of genes involved in organic P utilization, the urea cycle, and genetic information in the Gly-group, and carbon-phosphorus lyase, genetic information and environmental information in the AEP group were significantly increased compared to the DIP group. Together, these results indicate that Raphidiopsis raciborskii exhibits the evolution of coordination of multiple metabolic pathways and certain key genes to adapt to ambient P changes, which implies that if P is reduced to control Raphidiopsis raciborskii bloom, there is a risk that external nutrients (such as nitrogen, amino acids, and urea) will stimulate the growth or metabolism of Raphidiopsis. [Display omitted] [ABSTRACT FROM AUTHOR]

• dPCR results were highly correlated with qPCR and microscopy. • dPCR can quantify cyanobacterial abundance and potential toxicity simultaneously. • dPCR exhibited higher sensitive and precision of low density cyanobacteria. • dPCR assay provides a fast, reliable, and simple monitoring method. • The dPCR offers an early detection of invasive bloom-forming cyanobacteria. Blooms of the toxic cyanobacterium, Raphidiopsis raciborskii (basionym Cylindrospermopsis raciborskii), are becoming a major environmental issue in freshwater ecosystems globally. Our precision prevention and early detection of R. raciborskii blooms rely upon the accuracy and speed of the monitoring method. A duplex digital PCR (dPCR) monitoring approach was developed and validated to detect the abundance and toxin-producing potential of R. raciborskii simultaneously in both laboratory spiked and environmental samples. Results of dPCR were strongly correlated with traditional real time quantitative PCR (qPCR) and microscopy for both laboratory and environmental samples. However, discrepancies between methods were observed when measuring R. raciborskii at low abundance (1 − 105 cells L −1), with dPCR showing a higher precision compared to qPCR at low cell concentration. Furthermore, the dPCR assay had the highest detection rate for over two hundred environmental samples especially under low abundance conditions, followed by microscopy and qPCR. dPCR assay had the advantages of simple operation, time-saving, high sensitivity and excellent reproducibility. Therefore, dPCR would be a fast and precise monitoring method for the early warning of toxic bloom-forming cyanobacterial species and assessment of water quality risks, which can improve prediction and prevention of the impacts of harmful cyanobacterial bloom events in inland waters. [Display omitted] [ABSTRACT FROM AUTHOR]

Potentially toxic Cylindrospermopsis raciborskii blooms are of emerging concerns, as its scale is spreading from tropical regions to high latitudes, increasing the risk of aquatic biota being exposed to cylindrospermopsin (CYN). So far, CYN-producing C. raciborskii strains have only been reported in tropical waters which are commonly phosphorus (P)-deficient, where they can dominate phytoplankton communities. However, the influence of CYN on phytoplankton communities under different P status remains unclear. In this study, we first analyzed the summer observations of 120 tropical reservoirs in Guangdong Province. The proportion of potential CYN-producers was significantly higher in P-deficient and CYN-present reservoirs than that in P-sufficient or CYN-absent ones. This suggested that in P-deficient condition, the potential CYN producers might gain more advantages by the help of CYN. Then, in laboratory experiments we found that upon P deprivation, CYN did not inhibit the cell growth of other algal cells, but significantly stimulates them to secret more alkaline phosphatase (ALP) than in P-sufficient condition. Through transcriptomics, we further revealed that under such P-deficient condition, CYN remarkably induced intracellular nitrogen allocation and protein export system by activating the PIK3/Akt-cGMP/PKG signaling pathways in Scenedesmus bijugatus , thus enhancing its ALP secretion. Our study implies that CYN-induced ALP secretion is facilitated upon P deficiency, thus supporting the dominance of its producers C. raciborskii. [Display omitted] • Potential CYN-producer gain more advantage in P-deficient CYN-present reservoirs. • P deficiency enhanced the CYN-induced ALP secretion. • CYN did not inhibit the growth of three algal cells irrespective of P status. • CYN could trigger the growth of P-sufficient chlorophytes. • CYN activated N allocation and protein export to induce ALP secretion. [ABSTRACT FROM AUTHOR]

Bacterioplankton community composition has become the center of research attention in recent years. Bacteria associated with toxic cyanobacteria blooms have attracted considerable interest. However, little is known about the environmental factors driving the bacteria community, including the impact of invasive cyanobacteria. Therefore, our aim has been to determine the relationships between heterotrophic bacteria and phytoplankton community composition across 24 Polish lakes with different contributions of cyanobacteria including the invasive species Raphidiopsis raciborskii. This analysis revealed that cyanobacteria were present in 16 lakes, while R. raciborskii occurred in 14 lakes. Our results show that bacteria communities differed between lakes dominated by cyanobacteria and lakes with minor contributions of cyanobacteria but did not differ between lakes with R. raciborskii and other lakes. Physical factors, including water and Secchi depth, were the major drivers of bacteria and phytoplankton community composition. However, in lakes dominated by cyanobacteria, bacterial community composition was also influenced by biotic factors such as the amount of R. raciborskii, chlorophyll-a and total phytoplankton biomass. Thus, our study provides novel evidence on the influence of environmental factors and R. raciborskii on lake bacteria communities. [ABSTRACT FROM AUTHOR]