Increased incidence of Cylindrospermopsis raciborskii in temperate zones--is climate change responsible?

Saturated constructed wetlands for the remediation of cylindrospermopsin and microcystin-LR: Plants, microbes, and biodegradation pathways

Harmful cyanobacterial blooms will be more intense and frequent in the future, contaminating surface waters with cyanotoxins and posing a threat to communities heavily reliant on surface water usage for crop irrigation. Constructed wetlands (CWs) are proposed to ensure safe crop irrigation, but more research is needed before implementation. The present study operated 28 mesocosms in continuous mode mimicking horizontal sub-surface flow CWs. Mesocosms were fed with synthetic lake water and spiked periodically with two cyanotoxins, microcystin-LR (MC-LR) and cylindrospermopsin (CYN), at environmentally relevant cyanotoxins concentrations (10 μg L-1). The influence of various design factors, including plant species, porous media, and seasonality, was explored. The mesocosms achieved maximum MC-LR and CYN mass removal rates of 95 % and 98 %, respectively. CYN removal is reported for the first time in CWs mimicking horizontal sub-surface flow CWs. Planted mesocosms consistently outperformed unplanted mesocosms, with Phragmites australis exhibiting superior cyanotoxin mass removal compared to Juncus effusus. Considering evapotranspiration, J. effusus yielded the least cyanotoxin-concentrated effluent due to the lower water losses in comparison with P. australis. Using the P-kC* model, different scaling-up scenarios for future piloting were calculated and discussed. Additionally, bacterial community structure was analyzed through correlation matrices and differential taxa analyses, offering valuable insights into their removal of cyanotoxins. Nevertheless, attempts to validate microcystin-LR biotransformation via the known mlrA gene degradation pathway were unfruitful, indicating alternative enzymatic degradation pathways occurring in such complex CW systems. Further investigation into the precise molecular mechanisms of removal and the identification of transformation products is needed for the comprehensive understanding of cyanotoxin mitigation in CW. This study points towards the feasibility of horizontal sub-surface flow CWs to be employed to control cyanotoxins in irrigation or recreational waters.

Assessing alternative lake management actions for climate change adaptation

Lake management actions are required to protect lake ecosystems that are being threatened by climate change. Freshwater lakes in semiarid regions are of upmost importance to their region. Simulations of the subtropical Lake Kinneret project that rising temperatures will cause change to phytoplankton species composition, including increased cyanobacteria blooms, endangering lake ecosystem services. Using lake ecosystem models, we examined several management actions under climate change, including two alternatives of desalinated water introduction into the lake, hypolimnetic water withdrawal, watershed management changes and low versus high lake water level. To account for prediction uncertainty, we utilized an ensemble of two 1D hydrodynamic-biogeochemical lake models along with 500 realizations of meteorological conditions. Results suggest that supplying desalinated water for local use, thus releasing more natural waters through the Jordan River, increasing nutrient flow, may reduce cyanobacteria blooms, mitigating climate change effects. However, these results are accompanied by considerable uncertainty.

Climate change impact on sub-tropical lakes - Lake Kinneret as a case study

Climate change is anticipated to alter lake ecosystems by affecting water quality, potentially resulting in loss of ecosystem services. Subtropical lakes have high temperatures to begin with and are expected to exhibit higher temperatures all year round which might affect the thermal structure and ecological processes in a different manner than lakes in temperate zones. In this study the ecosystem response of the sub-tropical Lake Kinneret to climate change was explored using lake ecosystem models. Projection reliability was increased by using a weather generator and ensemble modelling, confronting uncertainty of both climate projections and lake models. The study included running two 1D hydrodynamic-biogeochemical models over one thousand realizations of two gradual temperature increase scenarios that span over 49 years. Our predictions show that an increase in air temperature would have subtle effects on stratification properties but may result in considerable changes to biogeochemical processes. Water temperature rise would cause a reduction in dissolved oxygen. Both of these changes would produce elevated phosphate and lowered ammonium concentrations. In turn, these changes are predicted to modify the phytoplankton community, expressed chiefly in increased cyanobacteria blooms at the expense of green phytoplankton and dinoflagellates; these changes may culminate in overall reduction of primary production. Identification of these trends would not be possible without the use of many realizations of climate scenarios. The use of ensemble modelling increased prediction reliability and highlighted elements of uncertainty. Though we use Lake Kinneret, the patterns identified most likely indicate processes that are expected in sub-tropical lakes in general.

Microbial community structures and bacteria-Cylindrospermopsis raciborskii interactions in Yilong Lake

Cylindrospermopsis raciborskii-dominated harmful algae blooms have been reported globally in recent years. However, our understanding of the ecology of C. raciborskii in natural conditions is still poor. In this study, we collected the water samples from a C. raciborskii-blooming lake, Yilong Lake, in Yunnan province, China, and used both culture-dependent and culture-independent approaches to investigate their microbial communities and the interactions between C. raciborskii and the other bacteria. The composition and diversity of microbial communities were revealed with 16S rRNA gene high-throughput sequencing data analysis. Microbial co-occurrences analysis suggests C. raciborskii may have complex associations with other bacteria. Based on co-inoculation tests, we obtained 14 strains of bacterial strains from the water samples that exhibited either algicidal or promoting effects on a strain of C. raciborskii. Two bacterial isolates exhibited a consistent performance between co-occurrence analysis and experimental results. Effects of these bacteria-algae interspecies interactions on the bloom event are discussed. All these results may provide new insights into the C. raciborskii-dominated blooms and how its interspecies relationships with other bacteria may influence the bloom events in eutrophic waters throughout the world.

Current status and prognosis of Raphidiopsis raciborskii distribution in Bulgaria as part of the southeastern region of Europe

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.

The Raphidiopsis (= Cylindrospermopsis) raciborskii pangenome updated: Two new metagenome-assembled genomes from the South American clade

Two Raphidiopsis (=Cylindrospermopsis) raciborskii metagenome-assembled genomes (MAGs) were recovered from two freshwater metagenomic datasets sampled in 2011 and 2012 in Pampulha Lake, a hypereutrophic, artificial, shallow reservoir, located in the city of Belo Horizonte (MG), Brazil. Since the late 1970s, the lake has undergone increasing eutrophication pressure, due to wastewater input, leading to the occurrence of frequent cyanobacterial blooms. The major difference observed between PAMP2011 and PAMP2012 MAGs was the lack of the saxitoxin gene cluster in PAMP2012, which also presented a smaller genome, while PAMP2011 presented the complete sxt cluster and all essential proteins and clusters. The pangenome analysis was performed with all Raphidiopsis/Cylindrospermopsis genomes available at NCBI to date, with the addition of PAMP2011 and PAMP2012 MAGs (All33 subset), but also without the South American strains (noSA subset), and only among the South American strains (SA10 and SA8 subsets). We observed a substantial increase in the core genome size for the 'noSA' subset, in comparison to 'All33' subset, and since the core genome reflects the closeness among the pangenome members, the results strongly suggest that the conservation level of the essential gene repertoire seems to be affected by the geographic origin of the strains being analyzed, supporting the existence of a distinct SA clade. The Raphidiopsis pangenome comprised a total of 7943 orthologous protein clusters, and the two new MAGs increased the pangenome size by 11%. The pangenome based phylogenetic relationships among the 33 analyzed genomes showed that the SA genomes clustered together with 99% bootstrap support, reinforcing the metabolic particularity of the Raphidiopsis South American clade, related to its saxitoxin producing unique ability, while also indicating a different evolutionary history due to its geographic isolation.

Challenges in supplying empirical proof for predictions derived from Species Distribution Models (SDMs): the case of an invasive cyanobacterium

Species distribution models (SDMs) calibrated with bioclimatic variables revealed a high probability for range expansion of the invasive toxin producing cyanobacterium, Raphidiopsis raciborskii to Sweden, where no reports of its presence have hitherto been recorded. While predictions focused on the importance of climate variables for possible invasion, other barriers to dispersal and successful colonization need to be overcome by the species for successful invasion. In this study, we combine field-based surveys of R. raciborskii (microscopy and molecular analysis using species-specific primers) of 11 Swedish lakes and in-silico screening of environmental DNA using 153 metagenomic datasets from lakes across Europe to validate the SDMs prediction. Field-based studies in lakes with high/low predicted probability of occurrence did not detect the presence of R. raciborskii, and in-silico screening only detected hints of its presence in 5 metagenomes from lakes with probability ranging from 0.059 to 0.825. The inconsistencies between SDMs results and both field-based/in-silico monitoring could be due to either sensitivity of monitoring approaches in detecting early invasions or uncertainties in SDMs that focused solely on climate drivers. However, results highlight the necessity of proactive monitoring with high temporal and spatial frequency.

Laboratory growth capacity of an invasive cyanobacterium (Microcystis aeruginosa) on organic substrates from surface waters of permafrost peatlands

Within a global warming trend, invasive cyanobacteria, abundant in tropical and temperate regions, can migrate northward and colonize thermokarst lakes in permafrost-affected territories. For a better understanding of the cyanobacterial proliferation mechanism in those lakes, we performed laboratory growth of typical invasive cyanobacteria, Microcystis aeruginosa, onto various organic-rich solutions representative of permafrost peatlands. Aqueous leachates of lichen, moss and peat were the most favorable substrates for massive growth. The growth in the presence of all organic substrates produced an increase in solution pH by two units and a sizable (30-50%) decrease in the concentration of dissolved organic carbon. The observed increase in the dissolved organic carbon aromaticity degree likely reflected preferential cyanobacterial uptake of aliphatic, optically transparent organic substances. Cyanobacterial growth over a bloom period can create a carbon sink (uptake of 2.5 and 8.3 g C-CO₂ m^-2 d^-1) that can offset the net heterotrophic status of thermokarst lakes in permafrost peatlands, thus switching the lake status from a C source to a C sink. Therefore, predictions of future carbon exchanges with the atmosphere in surface waters of permafrost peatlands require explicit accounting for the possibility of invasive cyanobacterial growth.

Recent Advances in the Ecology of Bloom-Forming Raphidiopsis (Cylindrospermopsis) raciborskii: Expansion in China, Intraspecific Heterogeneity and Critical Factors for Invasion

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.

Near-term phytoplankton forecasts reveal the effects of model time step and forecast horizon on predictability

As climate and land use increase the variability of many ecosystems, forecasts of ecological variables are needed to inform management and use of ecosystem services. In particular, forecasts of phytoplankton would be especially useful for drinking water management, as phytoplankton populations are exhibiting greater fluctuations due to human activities. While phytoplankton forecasts are increasing in number, many questions remain regarding the optimal model time step (the temporal frequency of the forecast model output), time horizon (the length of time into the future a prediction is made) for maximizing forecast performance, as well as what factors contribute to uncertainty in forecasts and their scalability among sites. To answer these questions, we developed near-term, iterative forecasts of phytoplankton 1-14 days into the future using forecast models with three different time steps (daily, weekly, fortnightly), that included a full uncertainty partitioning analysis at two drinking water reservoirs. We found that forecast accuracy varies with model time step and forecast horizon, and that forecast models can outperform null estimates under most conditions. Weekly and fortnightly forecasts consistently outperformed daily forecasts at 7-day and 14-day horizons, a trend that increased up to the 14-day forecast horizon. Importantly, our work suggests that forecast accuracy can be increased by matching the forecast model time step to the forecast horizon for which predictions are needed. We found that model process uncertainty was the primary source of uncertainty in our phytoplankton forecasts over the forecast period, but parameter uncertainty increased during phytoplankton blooms and when scaling the forecast model to a new site. Overall, our scalability analysis shows promising results that simple models can be transferred to produce forecasts at additional sites. Altogether, our study advances our understanding of how forecast model time step and forecast horizon influence the forecastability of phytoplankton dynamics in aquatic systems and adds to the growing body of work regarding the predictability of ecological systems broadly.

Iron enrichment from hypoxic hypolimnion supports the blooming of Raphidiopsis raciborskii in a tropical reservoir

Occurring worldwide, blooms of Raphidiopsis raciborskii threaten the use of water resources especially in tropical and subtropical waterbodies. Its high flexibility in the uses of light and macronutrients (C, N, P) frustrates any bloom prediction and control based on macronutrients regulation. To identify the critical factors promoting periodic blooms of R. raciborskii, the trends of meteorological, hydrodynamic, physical, and chemical variables (including macro- and micronutrients: N, P, Fe) were analyzed in a Chinese tropical large reservoir (Dashahe reservoir) over five years. It was hypothesized that Fe availability, mediated by the mixing pattern of the reservoir, played a crucial role in the periodic blooms of the cyanobacterium. To have a more complete understanding, the effects of Fe on growth of a local R. raciborskii strain were tested in a monoculture experiment. The biomass and relative abundance of R. raciborskii in the reservoir showed a clear seasonal trend, with relative abundance > 50% in summer/autumn (July to October). Three habitat types along a dominance gradient were identified in the reservoir and 17 variables were used to compare them. Statistical analysis and habitat comparison showed that temperature and stratification, dissolved Fe and N concentrations in the epilimnion, and dissolved Fe and oxygen concentrations in the hypolimnion were the critical factors driving the dynamics of R. raciborskii in the study reservoir. The habitat dominated by R. raciborskii was characterized by a relatively low availability of macro resources (Z_eu/Z_m < 1, SRP < 0.01 mg/L, DIN < 0.3 mg/L) and by a high Fe availability supplemented from hypoxic hypolimnion. The dependence of growth on Fe concentration increase was confirmed in culture where the maximum was reached at 0.689 mg Fe /L. Our results suggest that a high Fe bioavailability, also originating from the hypoxic hypolimnion, influences the dynamics R. raciborskii and favors the blooms of the species. As a consequence, Fe concentrations in the water column as well as oxygen measurements along the water column should be routinely included in the monitoring programs aimed at predicting and controlling R. raciborskii blooms.

Harmonizing science and management options to reduce risks of cyanobacteria

Management of cyanobacteria has become an increasingly complex venture. Cyanobacteria risks have amplified as society moves forward in an era of accelerated global changes. The cyanobacteria management "pendulum" has progressively shifted from prevention to mitigation, with management considerations often put forth after bloom formation. A universal system (i.e., one-size-fits-all management) fails to provide a management path forward due to the inherent complexities of each lake. A tailored management plan is needed: the right species at the right time in the right place (i.e., the three Rs). The three Rs represent a customizable management strategy that is flexible and informed by advances in scientific understanding to lower cyanobacteria-associated risks. Identifying thresholds in risk tolerance, where thresholds are defined by community collectives, is essential to frame cyanobacteria management targets and to decide on what management interventions are warranted.

Schindler's legacy: from eutrophic lakes to the phosphorus utilization strategies of cyanobacteria

David Schindler and his colleagues pioneered studies in the 1970s on the role of phosphorus in stimulating cyanobacterial blooms in North American lakes. Our understanding of the nuances of phosphorus utilization by cyanobacteria has evolved since that time. We review the phosphorus utilization strategies used by cyanobacteria, such as use of organic forms, alternation between passive and active uptake, and luxury storage. While many aspects of physiological responses to phosphorus of cyanobacteria have been measured, our understanding of the critical processes that drive species diversity, adaptation and competition remains limited. We identify persistent critical knowledge gaps, particularly on the adaptation of cyanobacteria to low nutrient concentrations. We propose that traditional discipline-specific studies be adapted and expanded to encompass innovative new methodologies and take advantage of interdisciplinary opportunities among physiologists, molecular biologists, and modellers, to advance our understanding and prediction of toxic cyanobacteria, and ultimately to mitigate the occurrence of blooms.

Heterocyte production, gene expression and phylogeography in Raphidiopsis ( = Cylindrospermopsis) Raciborskii

Raphidiopsis ( = Cylindrospermopsis) raciborskii was described as a subtropical-tropical cyanobacterium, later reported expanding into temperate regions. Heterocyte presence used to distinguish Cylindrospermopsis from the very similar Raphidiopsis, but recently the two genera were recognized as one and unified. This study aimed to investigate how heterocyte production is related to nitrogen (N) limitation in heterocytous and non-heterocytous strains of R.raciborskii. High N-concentrations did not inhibit heterocyte development in some strains, while prolonged N-starvation periods never stimulated production in others. RT-qPCR was used to examine the genetic background, through the expression patterns of nifH, ntcA and hetR. While gene expression increased under N-restriction, N-sufficiency did not suppress nifH transcripts as previously observed in other diazotrophyc cyanobacteria, suggesting that heterocyte production in R. raciborskii is not regulated by N-availability. Heterocytous and non-heterocytous strains were genotypically characterized to assess their phylogenetic relationships,. In the phylogenetic tree, clusters were intermixed and confirmed Raphidiopsis and Cylindrospermopsis as the same genus. The tree supported previous findings of earlier splitting of American strains, while contesting the African origin hypothesis. The existence of two lines of Chinese strains, with distinct evolutionary patterns, is a significant addition that could lead to new hypotheses of the species biogeography.

Species distribution models as a tool for early detection of the invasive Raphidiopsis raciborskii in European lakes

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.

Nutrient Regulation of Relative Dominance of Cylindrospermopsin-Producing and Non-cylindrospermopsin-Producing Raphidiopsis raciborskii

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.

Precision early detection of invasive and toxic cyanobacteria: A case study of Raphidiopsis raciborskii

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.

Influence of monoterpenoids on the growth of freshwater cyanobacteria

Abstract

Cyanobacteria are characterized by a very high tolerance to environmental factors. They are found in salt water, fresh water, thermal springs, and Antarctic waters. The wide spectrum of habitats suitable for those microorganisms is related to their particularly effective metabolism; resistance to extreme environmental conditions; and the need for only limited environmental resources such as water, carbon dioxide, simple inorganic salts, and light. These metabolic characteristics have led to cyanobacterial blooms and the production of cyanotoxins, justifying research into effective ways to counteract the excessive proliferation of these microorganisms. A new and interesting idea for the immediate reduction of cyanobacterial abundance is to use natural substances with broad-spectrum biological activity to restore phytoplankton diversity. This study describes the effects of selected monoterpenoid derivatives on the development of cyanobacterial cultures. In the course of the study, some compounds ((±)-citronellal, (+)-α-pinene) showed the ability to inhibit the colonization of the tested photosynthetic bacteria, while others (eugenol, eucalyptol) stimulated the growth of these microorganisms. By analyzing the results of these experiments, information was obtained on the mutual relations of cyanobacteria and the tested monoterpenes, which are present in the aquatic environment.

Key points

• Monoterpenoids significantly inhibit the growth of single cyanobacterial strains.

• Monoterpenoids can inhibit the growth of cyanobacterial consortia.

• Natural substances can control the growth of freshwater cyanobacteria.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11260-8.

Hazardous cyanobacteria integrity response to velocity gradient and powdered activated carbon in water treatment plants

Although some studies have investigated the impact caused by chemicals used on water treatment (coagulants and oxidants) on cyanobacteria integrity, the isolated effect of shear stress during coagulation is still not fully understood. This study evaluated the impact of different velocity gradients, mixing times, and the addition of powdered activated carbon (PAC) on the integrity of Microcystis aeruginosa, Raphidiopsis raciborskii, and Dolichospermum circinale, known producers of toxin and taste and odor (T&O) compounds. No association was found between R. raciborskii cell lysis and velocity gradient, with or without PAC, demonstrating the high resilience of this taxon to shear stress. In contrast, an association was found for M. aeruginosa at the highest velocity gradient evaluated (1000 s^-1) and for D. circinale above the lowest velocity gradient studied (600 s^-1). After PAC addition, there was a reduction in the chances of finding M. aeruginosa intact cells above velocity gradient 800 s^-1 at 45 s, while D. circinale show cell lysis in all the scenarios expect at 600 s^-1 and 10 s of agitation. The additional impact of PAC on cell lysis may lead to more release of metabolites and shows the need to adjust the hydraulic conditions in the rapid mixing stage, especially when more "fragile" cyanobacteria are present. Neither cyanobacterial cell size nor morphology was shown to be relevant to shear stress sensitivity, indicating that cell wall composition might have been an important factor in controlling cell lysis.

Development of Genus-Specific PCR Primers for Molecular Monitoring of Invasive Nostocalean Cyanobacteria

The geographical range of invasive cyanobacteria with high toxigenic potential is widening because of eutrophication and global warming, thus, monitoring their appearance is necessary for safe water quality control. Most invasive cyanobacteria are nostocalean species, and their accurate identification by classical morphological methods may be problematic. In this study, we developed polymerase chain reaction (PCR) primers to selectively identify five invasive cyanobacterial genera, namely, Chrysosporum, Cuspidothrix, Cylindrospermopsis, Raphidiopsis, and Sphaerospermopsis, using genetic markers such as rbcLX, rpoB, rpoC1, and cpcBA, and determined the amplification conditions for each pair of primers. The primer performances were verified on single or mixed nostocalean cyanobacterial isolates. The five primers allowed selective identification of all the target genera. In field samples collected during summer, when cyanobacteria flourished in the Nakdong River, the respective PCR product was observed in all samples where the target genus was detected by microscopic analysis. Besides, weak bands corresponding to Sphaerospermopsis and Raphidiopsis were observed in some samples in which these genera were not detected by microscopy, suggesting that the cell densities were below the detection limit of the microscopic method used. Thus, the genus-specific primers developed in this study enable molecular monitoring to supplement the current microscopy-based monitoring.

Four decades of progress in cylindrospermopsin research: The ins and outs of a potent cyanotoxin

The cyanotoxin cylindrospermopsin (CYN), a toxic metabolite from cyanobacteria, is of particular concern due to its cosmopolitan occurrence, aquatic bioaccumulation, and multi-organ toxicity. CYN is the second most often recorded cyanotoxin worldwide, and cases of human morbidity and animal mortality are associated with ingestion of CYN contaminated water. The toxin poses a great challenge for drinking water treatment plants and public health authorities. CYN, with the major toxicity manifested in the liver, is cytotoxic, genotoxic, immunotoxic, neurotoxic and may be carcinogenic. Adverse effects are also reported for endocrine and developmental processes. We present a comprehensive review of CYN over the past four decades since its first reported poisoning event, highlighting its global occurrence, biosynthesis, toxicology, removal, and monitoring. In addition, current data gaps are identified, and future directions for CYN research are outlined. This review is beneficial for understanding the ins and outs of this environmental pollutant, and for robustly assessing health hazards posed by CYN exposure to humans and other organisms.

Occurrence of Raphidiopsis raciborskii blooms in cool waters: Synergistic effects of nitrogen availability and ecotypes with adaptation to low temperature

Raphidiopsis raciborskii is a diazotrophic and potentially toxic cyanobacterium. To date, this species has successfully invaded many regions from the tropics to sub-tropical and temperate regions, typically forming blooms at temperatures greater than 25 °C. However, there have been a few cases in which R. raciborskii blooms have occurred at low temperatures (below 15 °C), but its cause and mechanisms remain unclear. In this study, field investigations revealed that R. raciborskii blooms occurred at 10-15 °C in Lake Xihu, Yunnan, China. The biomass of R. raciborskii was found to be positively related to nitrate concentrations in this lake. Three strains of R. raciborskii, two isolated from Lake Xihu (CHAB 6611 and CHAB 6612) and one from Lushui Reservoir in central China (CHAB 3409), were used for growth experiments at 15 °C. The three strains exhibited genotypic (16S rRNA and ITS-L genes) and physiological differences in response to nitrogen concentrations at low temperature. The growth rates of strains CHAB 6611 and CHAB 6612 increased with nitrogen concentration while CHAB 3409 could not grow at 15 °C. Furthermore, the growth and phenotypic responses of CHAB 6611 and CHAB 6612 to nitrogen concentrations were different, despite the closer genetic relationship shared by these two strains. Thus, increased nitrogen concentration in water may enhance the biological availability and utilization of nitrogen by R. raciborskii, which is the external promoter, leading to improving the resistance of R. raciborskii to low temperature. The internal cause is the presence of ecotypes in R. raciborskii populations with adaptation to low temperature. With increasing global eutrophication, the distribution range of R. raciborskii as well as the scale of its blooms will increase. As such, the risk of exposure of aquatic biota and humans to cylindrospermopsin is also expected to increase.

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

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

Global scanning of cylindrospermopsin: Critical review and analysis of aquatic occurrence, bioaccumulation, toxicity and health hazards

Cylindrospermopsin (CYN), a cyanotoxin produced by harmful algal blooms, has been reported worldwide; however, there remains limited understanding of its potential risks to surface water quality. In the present study, we critically reviewed available literature regarding the global occurrence, bioaccumulation, and toxicity of CYN in aquatic systems with a particular focus on freshwater. We subsequently developed environmental exposure distributions (EEDs) for CYN in surface waters and performed probabilistic environmental hazard assessments (PEHAs) using guideline values (GVs). PEHAs were performed by geographic region, type of aquatic system, and matrix. CYN occurrence was prevalent in North America, Europe, and Asia/Pacific, with lakes being the most common system. Many global whole water EEDs exceeded guideline values (GV) previously developed for drinking water (e.g., 0.5 μg L-1) and recreational water (e.g., 1 μg L-1). GV exceedances were higher in the Asia/Pacific region, and in rivers and reservoirs. Rivers in the Asia/Pacific region exceeded the lowest drinking water GV 73.2% of the time. However, lack of standardized protocols used for analyses was alarming, which warrants improvement in future studies. In addition, bioaccumulation of CYN has been reported in mollusks, crustaceans, and fish, but such exposure information remains limited. Though several publications have reported aquatic toxicity of CYN, there is limited chronic aquatic toxicity data, especially for higher trophic level organisms. Most aquatic toxicity studies have not employed standardized experimental designs, failed to analytically verify treatment levels, and did not report purity of CYN used for experiments; therefore, existing data are insufficient to derive water quality guidelines. Considering such elevated exceedances of CYN in global surface waters and limited aquatic bioaccumulation and toxicity data, further aquatic monitoring, environmental fate and mechanistic toxicology studies are warranted to robustly assess and manage water quality risks to public health and the environment.

Phylogeographic, toxicological and ecological evidence for the global distribution of Raphidiopsis raciborskii and its northernmost presence in Lake Nero, Central Western Russia

Raphidiopsis raciborskii is a freshwater, potentially toxigenic cyanobacterium, originally described as a tropical species that is spreading to northern regions over several decades. The ability of R. raciborskii to produce cyanotoxins - in particular the alkaloid cylindrospermopsin (CYN), which is toxic to humans and animals - is of serious concern. The first appearance of R. raciborskii in Russia was noted in Lake Nero in the summer of 2010. This is the northernmost (57°N) recorded case of the simultaneous presence of R. raciborskii and detection of CYN. In this study, the data from long-term monitoring of the R. raciborskii population, temperature and light conditions in Lake Nero were explored. CYN and cyr/aoa genes present in environmental samples were examined using HPLC/MS-MS and PCR analysis. A R. raciborskii strain (R104) was isolated and its morphology, toxigenicity and phylogeography were studied. It is supposed that the trigger factor for the strong development of R. raciborskii in Lake Nero in summer 2010 may have been the relatively high water temperature, reaching 29-30 °C. Strain R. raciborskii R104 has straight trichomes and can produce akinetes, making it morphologically similar to European strains. Phylogeographic analysis based on nifH gene and 16S-23S rRNA ITS1 sequences showed that the Russian strain R104 grouped together with R. raciborskii strains isolated from Portugal, France, Germany and Hungary. The Russian strain R104 does not contain cyrA and cyrB genes, meaning that it - like all European strains - cannot produce CYN. Thus, while recent invasion of R. raciborskii into Lake Nero has occurred, morphological, genetic, and toxicological data supported the spreading of this cyanobacterium from other European lakes. Detection of CYN and cyr/aoa genes in environmental samples indicated the cyanobacterium Aphanizomenon gracile as a likely producer of CYN in Lake Nero. The article also discusses data on the global biogeography of R. raciborskii. Genetic similarity between R. raciborskii strains isolated from very remote continents might be related to the ancient origin of the cyanobacterium inhabiting the united continents of Laurasia and Gondwana, rather than comparably recent transoceanic exchange between R. raciborskii populations.

Assessing the origin of a massive cyanobacterial bloom in the Río de la Plata (2019): Towards an early warning system

The Río de la Plata estuary drains the second largest river basin of South America. The occurrence of frequent cyanobacterial blooms of the Microcystis and Dolichospermum complex in the Uruguayan coast are associated with high flows of Uruguay River due to rainy years. In summer 2019, a massive cyanobacterial bloom reached up to the Uruguayan Atlantic coast. This study seeks to unveil the origin and the environmental conditions that favored the occurrence of the last cyanobacterial bloom in the Río de la Plata, and to contribute with the development of an early warning system of cyanobacterial scum on Montevideo beaches. A complementary approach was applied with Sentinel-2 imagery, environmental data of monitoring programs of Salto Grande Reservoir and Montevideo beaches, hydro-meteorological information, and hydroelectric dam operation. Images were analyzed with the Normalized Difference Chlorophyll Index (NDCI), which allowed evaluating several water bodies within the same ranges. Positive anomalous rainfall increased river flows, particularly that of Uruguay and Negro rivers, which caused the opening of the dam spillways. NDCI maps showed that areas with high values (NDCI_>0.06) in Salto Grande reservoir kept a similar surface area before and after the prolonged overflow period (8.7-7.8 km², before and after). In the Río Negro reservoirs, however, NDCI_>0.06 coverage remarkably changed (62.5 km², Palmar reservoir), with a subsequent 56-fold reduction in the post-discharge of surface water. Twenty days after opening the spillways, Montevideo beaches were closed to swimming and the NDCI_>0.06 surface reached 51.7 km² in the Río de la Plata coast. The dynamics of NDCI areas, the downstream bloom discharge, and the predicted Río de la Plata residual currents, suggest that the cyanobacterial bloom originated in the Negro River (Palmar reservoir). This bloom event was one of the worst that occurred in the Río de la Plata in last 20 years, circulated along the Uruguayan sub-corridor to the Atlantic coast along 690 km from its origin, and lasted three months on Montevideo coast. This is the first study that addresses the impact of cyanobacterial blooms from the Negro River reservoirs on the Río de la Plata estuary. Therefore, the Negro River basin is where the main efforts should be directed to mitigate massive cyanobacterial blooms.

Species diversity and seasonal dynamics of filamentous cyanobacteria in urban reservoirs for drinking water supply in tropical China

Filamentous cyanobacteria have been observed to become the dominant species in reservoirs, especially in small reservoirs for drinking water supply in southern China. The occurrences of filamentous cyanobacteria blooms in such reservoirs add additional costs for water plants by decreasing the filtration efficiency and the potential of toxin production. To serve the purpose of drinking water supply, the effective risk assessment requires the dynamic pattern of filamentous cyanobacteria. This study seasonally collected samples from 25 reservoirs in Dongguan, one of the most important 'world factories' in China in July, December and March, and investigated the temporal dynamics of phytoplankton, particularly cyanobacteria community. Our investigation showed that filamentous cyanobacteria, Planktothrix sp, Limnothrix sp. and Cylindrospermopsis raciborskii dominated in these reservoirs and climate-related water temperature was the primary factor for the seasonal shift of filamentous cyanobacteria. High abundance of filamentous cyanobacteria occurred in the high water level period with increasing temperature but less relevant with nutrient conditions. Our study observed the seasonal dynamics of filamentous cyanobacteria in tropical urban reservoirs and highlighted the association between temperature and filamentous cyanobacteria. our data and analysis provided an evidence that increased temperature could increase the likelihood of frequency and intensity of filamentous cyanobacteria blooms. In the scenario of global warming, more frequent monitoring of filamentous cyanobacteria and the potential to produce toxin should be considered for water quality and reservoir management.

Interspecific competition reveals Raphidiopsis raciborskii as a more successful invader than Microcystis aeruginosa

As a successful invasive cyanobacterial species, Raphidiopsis raciborskii is distributed globally and shows a tendency to replace Microcystis aeruginosa in many subtropical and temperate waters, but the ecological traits that contribute to its invasiveness are still unclear. In this study, we found that R. raciborskii occurred in 149 sites in 42 lakes in eastern and central China and coexisted with M. aeruginosa at most sites. Based on field results, a combination of invasion and competition experiments using a biomass gradient to evaluate the invasiveness and competitiveness of R. raciborskii compared with M. aeruginosa was conducted. In invasive groups, both R. raciborskii and M. aeruginosa were shown to have positive specific growth rates, indicating that R. raciborskii could coexist with M. aeruginosa. Furthermore, R. raciborskii was shown to grow faster from invasion while M. aeruginosa reduced growth for invasion. In competitive groups, R. raciborskii reached a higher maximum biomass and grew longer than M. aeruginosa. The specific growth rate of R. raciborskii was not inhibited by M. aeruginosa biomass, whereas the growth of M. aeruginosa was inhibited by R. raciborskii biomass. It was shown during the whole experiment that R. raciborskii tended to replace M. aeruginosa to become dominant owing to its faster growth rate and the eventual decline in growth of M. aeruginosa. With an increase in biomass of M. aeruginosa, the vegetative cell size and filament length of R. raciborskii gradually increased. This study has demonstrated that the inherent invasive traits of R. raciborskii, size differences, niche differences, and relative fitness differences between R. raciborskii and M. aeruginosa are crucial reasons for the invasive success of R. raciborskii. Our results revealed the invasiveness and domination of R. raciborskii from a new perspective.

Phylogenetic relationships and genetic divergence of paralytic shellfish toxin- and cylindrospermopsin- producing Cylindrospermopsis and raphidiopsis

Cylindrospermopsis and Raphidiopsis (C/R group) are closely related species responsible for cyanobacterial blooms worldwide. Paralytic shellfish toxins (PSTs) and cylindrospermopsins (CYNs) have been identified in different C/R group strains. However, the evolutionary relationship between PST- and CYN-producing strains has not been systematically evaluated. In this study, C/R group strains and their toxin biosynthesis genes were evaluated by phylogenetic analysis and sequence comparison. None of the tested strains are able to produce PSTs and CYNs simultaneously. The C/R group strains were clustered into five clades, including two non-toxic, two CYN-producing and one PST-producing clades. A high degree of similarity was observed for rpoC1 (> 96%) and ITS-L (> 97%) sequences within each clade with the exception of the ITS-L (87% to 100%) region in CYN-producing R. curvata, which has been shown to contain variable sequence insertions. Genomic analysis revealed that sxtY and sxtZ could be found in both toxic and non-toxic strains. The transposase gene IS4 was only observed in strains from the PST-producing clade. The sxt and cyr gene clusters share five gene families with similar functions. The amino acid sequences of the adenylyl-sulfate kinase genes, sxtO and cyrN, are more similar (45% to 81%) than other pairs of genes (8.0% to 40%). SxtO and CyrN proteins from C/R group strains forms an independent clade on the phylogenetic tree with a high degree of sequence similarity (78% to 100%). In conclusion, PST- and CYN- producing C/R group species can be classified into different clades based on their phylogenetic profile. The sxtO and cyrN genes have probably diverged from a single ancestral adenylyl-sulfate kinase gene, and may be specifically used for toxin biosynthesis in C/R group species.

Effects of photochemical and microbiological changes in terrestrial dissolved organic matter on its chemical characteristics and phytotoxicity towards cyanobacteria

Previous studies have shown that under laboratory conditions, dissolved organic matter (DOM) leached from plants can be differentially more phytotoxic to cyanobacteria, compared to green algae. This study examined how DOM source and transformation processes (microbial and photochemical) affect its chemical composition and phytotoxicity towards a cultured species of cyanobacteria (Raphidiopsis raciborskii) using a factorial experimental design. To complement cyanobacterial bioassays, the chemical composition and associated changes in DOM were determined using spectroscopic (nuclear magnetic resonance (NMR) and absorbance) and elemental analyses. Sunlight exposed DOM from leaves of the terrestrial plants, Casuarina cunninghamiana and Eucalyptus tereticornis had the most phytotoxic effect compared to DOM not exposed to sunlight. This phytotoxic DOM was characterised by relatively low nitrogen content, containing highly coloured and relatively high molecular mass constituents. Both mixed effect model and PCA approaches to predict inhibition of photosynthetic yield indicated phytotoxicity could be predicted (P < 0.001) based upon the following parameters: C: N ratio; gilvin, and lignin-derived phenol content of DOM. Parallel proton-detected 1D and 2D NMR techniques showed that glucose anomers were the major constituents of fresh leachate. With ageing, glucose anomers disappeared and products of microbial transformation appeared, but there was no indication of the appearance of additional phytotoxic compounds. This suggests that reactive oxygen species may be responsible, at least partially, for DOM phytotoxicity. This study provides important new information highlighting the characteristics of DOM that link with phytotoxic effects.

Subtropical freshwater phytoplankton show a greater response to increased temperature than to increased pCO2

Global increases in atmospheric CO₂ and temperatures will impact aquatic systems, with freshwater habitats being affected. Some studies suggest that these conditions will promote cyanobacterial dominance. There is a need for a clearer picture of how algal species and strains within species will respond to higher temperatures and CO₂, especially in combination. This study examined two chlorophytes (Monoraphidium and Staurastrum), and two strains of the cyanobacterium Raphidiopsis raciborskii (straight S07 and coiled C03), to determine how the combination of higher temperature and CO₂ levels will affect their growth and maximum cell concentrations. Continuous cultures were used to compare the steady state cell concentrations at 28 °C and 30 °C, and CO₂ partial pressures (pCO₂), 400 and 750 ppm for all cultures, and in addition 1000 ppm at 28 °C for R. raciborskii strains. This study showed that, for all species, water temperature had a greater effect than higher pCO₂ on cell concentrations. There were clear differences in response between the chlorophyte species, with Monoraphidium preferring 28 °C and Staurastrum preferring 30 °C. There were also differences in response of the R. raciborskii strains to increasing temperature and pCO₂, with S07 having a greater increase in cell concentration. Genome analysis of R. raciborskii showed that the straight strain has five additional carbon acquisition genes (β-CA, chpY, cmpB, cmpD and NdhD4), indicative of increased carbon metabolism. These differences in the strains' response to elevated pCO₂ will lead to changes in the species population structure and distribution in the water column. This study shows that it is important to examine the effects of both pCO₂ and temperature, and to consider strain variation, to understand how species composition of natural systems may change under future climate conditions.

Cyanobacterial removal by a red soil-based flocculant and its effect on zooplankton: an experiment with deep enclosures in a tropical reservoir in China

As one kind of cheap, environmentally-friendly and efficient treatment materials for direct control of cyanobacterial blooms, modified clays have been widely concerned. The present study evaluated cyanobaterial removal by a red soil-based flocculant (RSBF) with a large enclosure experiment in a tropical mesotrophic reservoir, in which phytoplankton community was dominated by Microcystis spp. and Anabaena spp. The flocculant was composed of red soil, chitosan and FeCl₃. Twelve enclosures were used in the experiment: three replicates for each of one control and three treatments RSBF₁₅ (15 mg FeCl₃ l^-1), RSBF₂₅ (25 mg FeCl₃ l^-1), and RSBF₃₅ (35 mg FeCl₃ l^-1). The results showed that the red soil-based flocculant can significantly remove cyanobacterial biomass and reduce concentrations of nutrients including total nitrogen, nitrate, ammonia, total phosphorus, and orthophosphate. Biomass of Microcystis spp. and Anabaena spp. was reduced more efficiently (95%) than other filamentous cyanobacteria (50%). In the RSBF₁₅ treatment, phytoplankton biomass recovered to the level of the control group after 12 days and cyanobacteria quickly dominated. Phytoplankton biomass in the RSBF₂₅ treatment also recovered after 12 days, but green algae co-dominated with cyanobacteria. A much later recovery of phytoplankton until the day of 28 was observed under RSBF₃₅ treatment, and cyanobacteria did no longer dominate the phytoplankton community. The application of red soil-based flocculant greatly reduces zooplankton, especially rotifers, however, Copepods and Cladocera recovered fast. Generally, the red soil-based flocculant can be effective for urgent treatments at local scales in cyanobacteria dominating systems.

First report of anatoxin-a producing cyanobacteria in Australia illustrates need to regularly up-date monitoring strategies in a shifting global distribution

Routine monitoring of toxic cyanobacteria depends on up-to-date epidemiological information about their distribution. In Australia, anatoxin producing cyanobacteria are not regularly tested for and thought to be rare if not absent from the continent. Our study investigated the presence of anatoxin-a (ATX-a) producing cyanobacteria in surface water samples (n = 226 from 67 sampling locations) collected from 2010 to 2017 across the state of Victoria, Australia. We (1) detected the presence and distribution of anaC (anatoxin-a synthetase C) gene sequences previously associated with various cyanobacteria, including Cuspidothrix issatschenkoi, Aphanizomenon sp., D. circinale, Anabaena sp., and Oscillatoria sp., from 31 sampling locations, and (2) determined the concentration of ATX-a in samples tested using ELISA, in two instances detected at >4 µg · L-1. These data present the first confirmation of ATX-a producers in Australia. Our study indicates that ATX-a should be included in regular testing of cyanobacterial blooms in Australia and highlights the importance of regular investigation of the distributions of toxic cyanobacteria worldwide, particularly amid the known expanding distribution of many cyanobacterial taxa in a period of increased eutrophication and rising surface water temperatures.

A Comprehensive Review: Development of Electrochemical Biosensors for Detection of Cyanotoxins in Freshwater

Cyanobacteria harmful algal blooms are increasing in frequency and cyanotoxins have become an environmental and public concern in the U.S. and worldwide. In this Review, the majority of reported studies and developments of electrochemical affinity biosensors for cyanotoxins are critically reviewed and discussed. Essential background information about cyanobacterial toxins and electrochemical biosensors is combined with the rapidly moving development of electrochemical biosensors for these toxins. Current issues and future challenges for the development of useful electrochemical biosensors for cyanotoxin detection that meet the demands for applications in field freshwater samples are discussed. The major aspects of the entire review article in a prescribed sequence include (i) the state-of-the-art knowledge of the toxicity of cyanotoxins, (ii) important harmful algal bloom events, (iii) advisories, guidelines, and regulations, (iv) conventional analytical methods for determination of cyanotoxins, (v) electrochemical transduction, (vi) recognition receptors, (vii) reported electrochemical biosensors for cyanotoxins, (viii) summary of analytical performance, and (ix) recent advances and future trends. Discussion includes electrochemical techniques and devices, biomolecules with high affinity, numerous array designs, various detection approaches, and research strategies in tailoring the properties of the transducer-biomolecule interface. Scientific and engineering aspects are presented in depth. This review aims to serve as a valuable source to scientists and engineers entering the interdisciplinary field of electrochemical biosensors for detection of cyanotoxins in freshwaters.

High dispersal levels and lake warming are emergent drivers of cyanobacterial community assembly in peri-Alpine lakes

Disentangling the relative importance of deterministic and stochastic processes in shaping natural communities is central to ecology. Studies about community assembly over broad temporal and spatial scales in aquatic microorganisms are scarce. Here, we used 16S rDNA sequence data from lake sediments to test for community assembly patterns in cyanobacterial phylogenies across ten European peri-Alpine lakes and over a century of eutrophication and climate warming. We studied phylogenetic similarity in cyanobacterial assemblages over spatial and temporal distance, and over environmental gradients, comparing detected patterns with theoretical expectations from deterministic and stochastic processes. We found limited evidence for deviation of lake communities from a random assembly model and no significant effects of geographic distance on phylogenetic similarity, suggesting no dispersal limitation and high levels of stochastic assembly. We detected a weak influence of phosphorus, but no significant effect of nitrogen levels on deviation of community phylogenies from random. We found however a significant decay of phylogenetic similarity for non-random communities over a gradient of air temperature and water column stability. We show how phylogenetic data from sedimentary archives can improve our understanding of microbial community assembly processes, and support previous evidence that climate warming has been the strongest environmental driver of cyanobacterial community assembly over the past century.

Causal relationships of Raphidiopsis (formerly Cylindrospermopsis) dynamics with water temperature and N:P-ratios: A meta-analysis across lakes with different climates based on inferential modelling

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.

The occurrence of cyanobacteria blooms in freshwater ecosystems and their link with hydro-meteorological and environmental variations in Tanzania

Cyanobacteria (blue-green algae) are photosynthetic bacteria that under favorable environmental conditions produce secondary metabolites (cyanotoxins) which are harmful to the environment, including humans. The mass proliferation of harmful cyanobacteria is termed CyanoHABs. CyanoHABs can adapt to different climatic fluctuations, therefore, understanding their dynamics in freshwater systems is crucial. Variation in climatic and hydrological processes, changing land use and economic growth all influence the occurrence and distribution of CyanoHABs. There have been inadequate CyanoHAB studies at local scales, therefore their occurrence and dynamics cannot be generalized. This study reviews and synthesizes cases of CysnoHAB occurrence, magnitude, and timing and how these are linked with climatic and hydrological variations in the United Republic of Tanzania. In this study, a scoping review approach was adopted. Research articles, reports, and databases were consulted. The most common species of toxin-producing cyanobacteria were identified in different water bodies in Tanzania, as well as the record of mass fatality of birds (Lesser Flamingo) in Lake Manyara, which in almost all cases occurred during dry years. While previous studies on CyanoHAB dynamics and their links to climate, hydrological, and environmental changes have not been undertaken in Tanzania, there are studies in Lake Victoria and Tanganyika. Therefore, there should be an immediate response from water users, managers, researchers, and water authorities to address and actively engage in monitoring and managing the risks associated with CyanoHABs in Tanzania.

Environmental factors driving the occurrence of the invasive cyanobacterium Sphaerospermopsis aphanizomenoides (Nostocales) in temperate lakes

Cyanobacterial blooms are an increasing threat worldwide. Invasions of certain cyanobacterial species, mainly towards higher latitudes, add to this concern as they enrich the pool of potential bloom-formers in the invaded region. Among the numerous causes of this escalating process, climate warming is commonly considered the most crucial factor, but empirical studies of this issue are lacking. The aim of our study was to identify physical, chemical and biological factors related to the occurrence of an invasive cyanobacterium at the northern border of its putative current range, and thus enabling its expansion. This study focuses on the relatively little studied species Sphaerospermopsis aphanizomenoides (Nostocales, Cyanobacteria; synonyms: Aphanizomenon aphanizomenoides, Anabaena aphanizomenoides), which is predicted to become one of the main nuisance species of the future. Forty-nine freshwater lakes located between latitudes 51° and 55°N were examined for the presence of S. aphanizomenoides, and environmental factors that could drive its occurrence were studied simultaneously. To identify factors correlated with the presence of the species, principal component analysis (PCA) and Mann-Whitney U test were performed. Water temperature did not differentiate lakes with or without S. aphanizomenoides, however the study was conducted in a particularly hot summer. Total phosphorus concentration was identified as the primary driving factor of the occurrence of S. aphanizomenoides. The species grew in poor light conditions and high phytoplankton biomass, mainly in shallow lakes. As shown by detrended correspondence analysis (DCA), the species accompanied shade tolerant, eutrophic species of native and invasive cyanobacteria as well as eukaryotic algae. Our results indicate that eutrophication may be the primary factor enabling the increasing occurrence of S. aphanizomenoides in temperate environments, and suggest that this process may stimulate expansion of cyanobacterial species towards high latitudes.

Genetic toxicity of water contaminated by microcystins collected during a cyanobacteria bloom

Microcystin-LR (MCLR) is a toxin mainly produced by Microcystis aeruginosa, cyanobacteria most commonly found in eutrophic environments. Cyanobacteria blooms have affected Salto Grande reservoir (Americana, State of São Paulo/Brazil) for several decades, often observed during periods of drought. In this study, the genotoxic effects of MCLR (95% purity) and water samples contaminated by this toxin were evaluated during cyanobacteria bloom using assays with the test organism Allium cepa. The results showed genotoxic action for pure microcystin and cytotoxic, genotoxic and mutagenic action for water samples collected during flowering. Chromosomal aberration assays have shown that MCLR induces chromosomal breaks that persist in the daughter cells as MN. Therefore, it is possible to infer a clastogenic action for this toxin. The MCLR present in the environmental samples was shown to be more cytogenotoxic for the cells than the different concentrations tested in this study with the pure substance. This amplified toxic action can be related to a synergistic effect between the MCLR and other compounds present in the environmental samples. The genotoxicity studies with MCLR show inconsistent and inconclusive results, so this toxin needs to be better investigated in order to obtain further information about the action mode of it is on the biological system.

Cylindrospermopsis raciborskii Virus and host: genomic characterization and ecological relevance

Cylindrospermopsis (Raphidiopsis) raciborskii is an invasive, filamentous, nitrogen-fixing cyanobacterium that forms frequent blooms in freshwater habitats. While viruses play key roles in regulating the abundance, production and diversity of their hosts in aquatic ecosystems, the role(s) of viruses in the ecology of C. raciborskii is almost unexplored. Progress in this field has been hindered by the absence of a characterized virus-host system in C. raciborskii. To bridge this gap, we sequenced the genome of CrV-01T, a previously isolated cyanosiphovirus, and its host, C. raciborskii strain Cr2010. Analyses suggest that CrV-01T represents a distinct clade of siphoviruses infecting, and perhaps lysogenizing, filamentous cyanobacteria. Its genome contains unique features that include an intact CRISPR array and a 12 kb inverted duplication. Evidence suggests CrV-01T recently gained the ability to infect Cr2010 and recently lost the ability to form lysogens. The cyanobacterial host contains a CRISPR-Cas system with CRISPR spacers matching protospacers within the inverted duplication of the CrV-01T genome. Examination of metagenomes demonstrates that viruses with high genetic identity to CrV-01T, but lacking the inverted duplication, are present in C. raciborskii blooms in Australia. The unique genomic features of the CrV/Cr2010 system offers opportunities to investigate in more detail virus-host interactions in an ecologically important bloom-forming cyanobacterium.

A European Multi Lake Survey dataset of environmental variables, phytoplankton pigments and cyanotoxins

Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includes in-situ lake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment.

Environmental factors influencing the quantitative distribution of microcystin and common potentially toxigenic cyanobacteria in U.S. lakes and reservoirs

Many species of cyanobacteria are capable of producing toxins and causing nuisance blooms, however response to environmental conditions is likely taxon-specific. Environmental factors influencing cyanobacterial composition and toxin production in lakes have been examined in many studies; yet are often confined to individual water bodies, or to a small number of systems within the same region. Here, data from the 2012 USEPA National Lakes Assessment are used to examine relationships between biovolume of common potentially-toxigenic cyanobacteria (Aphanizomenon spp., Cylindrospermopsis spp., Dolichospermum spp., Microcystis spp. and Planktothrix spp.) and environmental variables across the entire conterminous United States, and results are compared across nine distinct ecoregions. Total phosphorus and water clarity were identified as the most influential environmental factors correlated with phytoplankton community composition. The Northern, Southern and Temperate Plains ecoregions displayed the highest biovolumes of potentially toxigenic taxa on average, as well as highest mean concentrations of microcystin. In those three ecoregions, samples with microcystin concentrations greater than 1 ppb were primarily dominated by Planktothrix spp. while in all other ecoregions Dolichospermum spp. was the dominant genus. Canonical Correlation Analysis revealed a strong association between high microcystin concentrations and high nutrient concentrations (total nitrogen and total phosphorus), and between high microcystin concentrations and low percentage of watershed forest cover. Results from this study indicate that the likely occurrence of potentially toxigenic taxa in lakes and reservoirs is predictable on a biogeographical basis, depending on morphological and water quality characteristics. Data from this study may be useful to regional managers attempting to prevent or mitigate nuisance cyanobacterial blooms.

Determination of the Role of Microcystis aeruginosa in Toxin Generation Based on Phosphoproteomic Profiles

Microcystis aeruginosa is the most common species responsible for toxic cyanobacterial blooms and is considered a significant contributor to the production of cyanotoxins, particularly the potent liver toxins called microcystins. Numerous studies investigating Microcystis spp. blooms have revealed their deleterious effects in freshwater environments. However, the available knowledge regarding the global phosphoproteomics of M. aeruginosa and their regulatory roles in toxin generation is limited. In this study, we conducted comparative phosphoproteomic profiling of non-toxic and toxin-producing strains of M. aeruginosa. We identified 59 phosphorylation sites in 37 proteins in a non-toxic strain and 26 phosphorylation sites in 18 proteins in a toxin-producing strain. The analysis of protein phosphorylation abundances and functions in redox homeostasis, energy metabolism, light absorption and photosynthesis showed marked differences between the non-toxic and toxin-producing strains of M. aeruginosa, indicating that these processes are strongly related to toxin generation. Moreover, the protein-protein interaction results indicated that BJ0JVG8 can directly interact with the PemK-like toxin protein B0JQN8. Thus, the phosphorylation of B0JQN8 appears to be associated with the regulatory roles of toxins in physiological activity.

Toxicity-associated changes in the invasive cyanobacterium Cylindrospermopsis raciborskii in response to nitrogen fluctuations

The cyanobacterium Cylindrospermopsis raciborskii is of particular concern due to its ability to fix nitrogen (N), sporadic bloom, potential toxicity and apparent invasiveness. However, the toxicity associated behavior and response of toxic C. raciborskii under N fluctuations in water have been poorly investigated. The present study initiated based on the field survey in which Cylindrospermopsis species was found to have a high fitness under nitrate concentrations fluctuating from 0.02 mg L^-1 to 2.90 mg L^-1 in Chinese freshwater lakes. Examination on the role of short-term N fluctuations was conducted in two C. raciborskii strains which were exposed to a range of N concentrations supplied in two patterns, namely one-time pattern and ten-time pattern in which the equal amount of N was divided into ten-time accretions. The results showed the growth of both strains were not vulnerable to the transient nutrient fluctuations. The toxic strain showed considerable toxicological flexibility with the highest yield of cylindrospermopsin (CYN) obtained in the absence of N and the lowest in full medium. Generally, larger amounts of total CYN were observed at lower N levels, indicating that N deficiency promoted the intracellular accumulation and simultaneously restrained the extracellular release of CYN. Furthermore, CYN production was significantly different in two N supply patterns. The maximum quotas of intracellular and extracellular CYN in one-time pattern were respectively 2.79-3.53 and 3.94-7.20 times higher compared to the ten-time pattern. To our knowledge, our results are the first evidence of toxicity variations of C. raciborskii to the impermanent N fluctuations, shedding new light on its toxicological plasticity.

Behavior of Cylindrospermopsis raciborskii during coagulation and sludge storage - higher potential risk of toxin release than Microcystis aeruginosa?

Owing to the global warming and its strong adaptability, Cylindrospermopsis raciborskii has spread world-wide. However, as one of toxic cyanobacteria in many drinking water sources, it has not been drawn proper consideration in drinking water treatment plants so far. The investigation aimed at unveiling the fate of C. raciborskii during polyaluminum ferric chloride (PAFC) coagulation and sludge storage, revealing its differences from Microcystis aeruginosa. Results showed that C. raciborskii cells were effectively removed intactly under optimum coagulation conditions, but PAFC at higher dosages (>10 mg/L) triggered additional cylindrospermopsins release. In sludge storage, coagulated C. raciborskii cells suffered severe oxidative damage, leading to significant cylindrospermopsins release after day 6. C. raciborskii manifested different behaviors from M. aeruginosa which cells didn't release much microcystins during coagulation and sludge storage. This was mostly due to their differences in physiology and morphology. In flocs, M. aeruginosa could be enveloped by coagulant which can protect cells against the nasty attack from outside, whereas C. raciborskii with long filaments was hard to be wrapped and prone to suffering oxidative damage. These results confirmed C. raciborskii had a higher risk of toxin release in water production process than M. aeruginosa, which should deserve more attention.

Genome variation in nine co-occurring toxic Cylindrospermopsis raciborskii strains

Cyanobacteria form harmful algal blooms and are highly adapted to a range of habitats, in part due to their phenotype plasticity. This plasticity is partially the result of co-existence of multiple strains within a single population. The toxic cyanobacterium Cylindrospermopsis raciborskii has remarkable phenotypic plasticity, strain variation and environmental adaptation resulting in an expansion of its global range. To understand the genetic basis of the high level of plasticity within a C. raciborskii population, the genomes of nine co-occurring strains were compared. The strains differed in morphology, toxin cell quotas and physiology, despite being obtained from a single water sample. Comparative genomics showed that three coiled strains were 3.9 Mbp in size, with 3544 ± 11 genes, while straight strains were 3.8 Mbp in size, with 3485 ± 20 genes. The core proteome comprised 86% of the genome and consisted of 2891 orthologous groups (OGs), whereas the variable genome comprised ∼14% (847 OGs), and the strain specific genome only ∼1% (433 OGs).There was a high proportion of variable strain-specific genes for the very closely related strains, which may underpin strain differentiation. The variable genes were associated with environmental responses and adaptation, particularly phage defence, DNA repair, membrane transport, and stress, illustrative of the adaptability of the strains in response to environmental and biological stressors. This study shows that high genomic variability exists between co-occurring strains and may be the basis of strain phenotypic differences and plasticity of populations. Therefore management and prediction of blooms of this harmful species requires different approaches to capture this strain variability.

Genomic and Genotypic Characterization of Cylindrospermopsis raciborskii: Toward an Intraspecific Phylogenetic Evaluation by Comparative Genomics

Cylindrospermopsis raciborskii is a freshwater cyanobacterial species with increasing bloom reports worldwide that are likely due to factors related to climate change. In addition to the deleterious effects of blooms on aquatic ecosystems, the majority of ecotypes can synthesize toxic secondary metabolites causing public health issues. To overcome the harmful effects of C. raciborskii blooms, it is important to advance knowledge of diversity, genetic variation, and evolutionary processes within populations. An efficient approach to exploring this diversity and understanding the evolution of C. raciborskii is to use comparative genomics. Here, we report two new draft genomes of C. raciborskii (strains CENA302 and CENA303) from Brazilian isolates of different origins and explore their molecular diversity, phylogeny, and evolutionary diversification by comparing their genomes with sequences from other strains available in public databases. The results obtained by comparing seven C. raciborskii and the Raphidiopsis brookii D9 genomes revealed a set of conserved core genes and a variable set of accessory genes, such as those involved in the biosynthesis of natural products, heterocyte glycolipid formation, and nitrogen fixation. Gene cluster arrangements related to the biosynthesis of the antifungal cyclic glycosylated lipopeptide hassallidin were identified in four C. raciborskii genomes, including the non-nitrogen fixing strain CENA303. Shifts in gene clusters involved in toxin production according to geographic origins were observed, as well as a lack of nitrogen fixation (nif) and heterocyte glycolipid (hgl) gene clusters in some strains. Single gene phylogeny (16S rRNA sequences) was congruent with phylogeny based on 31 concatenated housekeeping protein sequences, and both analyses have shown, with high support values, that the species C. raciborskii is monophyletic. This comparative genomics study allowed a species-wide view of the biological diversity of C. raciborskii and in some cases linked genome differences to phenotype.