Life-history responses of Daphnia sinensis simultaneously exposed to Microcystis aeruginosa and Cylindrospermopsis raciborskii.

Many cyanobacterial species co-occur commonly in a freshwater ecosystem and can be consumed simultaneously by zooplankton. Both Microcystis aeruginosa and Cylindrospermopsis raciborskii are the dominant species and coexist in eutrophic tropical waters, and they together are assumed to have exert a stronger effect on the life history traits of cladocerans than a single cyanobacterial species. In the present study, we tested the hypothesis with life-history experiments of Daphnia sienesis, a large cladoceran species in tropics. In the experiments, M. aeruginosa and C. raciborskii were used as a mixture of food with Chlorella pyrenoidosa for the experimental animals. D. sienesis showed excellent growth and survival on sole diets of C. pyrenoidosa (CP). By contrast, Daphnia's growth decreased significantly and reproduction was completely inhibited when cyanobacteria comprised 100% of the food offered. The supplementation of C. pyrenoidosa into cyanobacterial diets significantly decreased their harmful effects on Daphnia, who improved the life history traits with the reduction of cyanobacterial percentage in mixed foods, irrespective of cyanobacterial species. Compared with the cultures of D. sienesis fed with a single cyanobacteria species, the animals in the treatments fed both M.aeruginosa-FACHB469 (F469) and C. raciborskii N8 (N8) had a lower growth rate in all the proportion of C. pyrenoidosa (25% CP, 50% CP or 75% CP). The strongest synergistic inhibition by the two cyanobacterial strains was found in the treatments with the 25% CP (i.e., 25% CP + 37.5% F469 + 37.5% N8), and no animal survived to maturity and reproduced. Thus, the simultaneous exposure to the two cyanobacterial species should be taken into account in assessing the ecological risks of cyanobacterial blooms, since multiple cyanobacterial coexistence can result in strong synergistic inhibition on growth and reproduction of zooplankton.

Exposure of Nile Tilapia (Oreochromis niloticus) Fingerlings to a Saxitoxin-Producing Strain of Raphidiopsis (Cylindrospermopsis) raciborskii (Cyanobacterium) Reduces Growth Performance and Increases Mortality Rate.

Blooms of toxin-producing cyanobacteria have been more frequent and lasting because of the eutrophication of freshwater ecosystems, including those used for aquaculture. The aim of the present study was to investigate the effects of chronic exposure to a saxitoxin-producing strain of Raphidiopsis (Cylindrospermopsis) raciborskii on the performance of Nile tilapia (Oreochromis niloticus) fingerlings over a 60-d period. The fingerlings were cultivated under the following conditions: 1) water without cyanobacterium (WATER), 2) R. raciborskii in ASM-1 culture medium (CYANO), and 3) ASM-1 culture medium without cyanobacterium (ASM). Exposure to the CYANO treatment led to a significant increase in the mortality rate (p < 0.05) and a significant reduction in growth (p < 0.05) compared to fingerlings submitted to the ASM and WATER treatments, in which similar survival and growth were found (p > 0.05). Saxitoxin toxicity was dependent on the weight of the fingerling (p < 0.05), with maximum mortality caused by the ingestion of 13.66 µg saxitoxin equivalent L-1 g-1 . The present results clearly show the harm caused by saxitoxins to the production of Nile tilapia fingerlings in the early growth phase. These findings underscore the importance of maintaining adequate water quality in aquaculture activities to minimize the risk of saxitoxin-producing cyanobacterial blooms and avoid economic losses among producers. Environ Toxicol Chem 2020;39:1409-1420. © 2020 SETAC.

Isolation and Characterization of Cylindrospermopsis raciborskii Strains from Finished Drinking Water.

In the summer of 2015, an intense cyanobacterial bloom producing geosmin/2-methylisoborneol (MIB) occurred in the Roxo freshwater reservoir in Alentejo, Portugal. The drinking water supplied from the Roxo water treatment plant (WTP) exhibited an unpleasant odor/taste and a significant cyanobacteria density was detected in the finished water at the exit of the WTP. Cyanobacteria were not evaluated downstream of the WTP, namely, at the city reservoir. The aim of this work was to isolate and characterize viable cyanobacteria present in finished water (exit of the WTP and city reservoir) that withstand conventional water treatment. Treated water samples collected at both sites were inoculated in Z8 culture medium to provide the conditions for putative cyanobacterial growth. After 30 days, filamentous cyanobacteria were observed in cultures inoculated with samples from the exit point of the WTP. Viable trichomes were isolated and identified as Cylindrospermopsis raciborskii by morphometric and molecular analysis. None of the isolates were cylindrospermopsin/microcystin producers, as confirmed by ELISA and amplification of corresponding genes (PS/PKS and mcyA-cd/mcyAB/mcyB). ELISA results were positive for saxitoxin, but saxitoxin and derivatives were not detected by liquid chromatography with fluorescence detection (LC-FLD), nor were their related genes (sxtA/sxtA4/sxtB/sxtM/sxtPer/sxtI). To our knowledge, this is the first report on the establishment of cultures of C. raciborskii that resisted water treatment processes.

Is the presence of Central European strains of Raphidiopsis (Cylindrospermopsis) raciborskii a threat to a freshwater fish? An in vitro toxicological study in common carp cells.

As yet European strains of Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii) have not been found to produce known cyanotoxins although their extracts have caused adverse effects in mammals, as shown using in vitro and in vivo experimental models. The present study investigated whether R. raciborskii isolated from Western Poland and Ukraine can affect fish cells using in vitro exposures of hepatocytes and red blood cells (RBC), and brain homogenates obtained from common carp (Cyprinus carpio) to 1.0% and 0.1% extracts of 7 strains. The studied extracts evoked different responses of catalase activity in hepatocytes with both increase and decrease observed under low and high concentrations. The cellular thiol pool was also altered with most extracts inducing a decrease in the activity of glutathione-S-transferase, and Ukrainian strains leading to an increase in glutathione level and a decrease in metallothionein content. All the studied extracts induced comparable reactive oxygen species formation, lipid peroxidation, protein carbonylation and DNA fragmentation in hepatocytes, and all but one increased the activity of caspase-3. Only one extract caused lysosomal membrane destabilization as measured by neutral red retention in RBC. In contrast to extracts of Ukrainian isolates, exposure of brain homogenates to extracts of Polish strains induced an increase in acetylcholinesterase activity suggesting the neurotoxic action of their exudates. The results indicate that both Polish and Ukrainian strains of R. raciborskii may pose a toxicological risk to freshwater fish, and further, that Polish strains may produce compound(s) evoking neurotoxic effects.

Application of N-TiO2 for visible-light photocatalytic degradation of Cylindrospermopsis raciborskii - More difficult than that for photodegradation of Microcystis aeruginosa ?

Nowadays Cylindrospermopsis raciborskii (C. raciborskii) and the metabolites produced, such as cylindrospermopsin (CYN), pose a serious threat to the ecosystem. Advanced oxidation technologies have been verified as constituting a very promising means to eliminate the risk from harmful algae. But so far little research has focused on the visible-light photocatalytic destruction of C. raciborskii cells and the degradation of their metabolites. In our study, N-doped TiO2 (N-TiO2) was used to degrade C. raciborskii and the degradation was compared with that of the Microcystis aeruginosa (M. aeruginosa). Results showed that although the photodegradation of C. raciborskii was more difficult than that of M. aeruginosa, the treatment with N-TiO2 was still satisfactory. After adding 200 mg/L N-TiO2, C. raciborskii cells (5 × 106 cells/mL) were completely destroyed within 20 h under visible light irradiation, and nearly 90% of the organic matter and CYN in the suspensions were also degraded, thereby markedly improving the water quality. The photocatalytic process starts with damage to the cell membrane resulting in the leakage of internal components. Subsequently, the leaked metabolites were oxidised by the reactive oxidizing species produced by N-TiO2. Thus, the application of N-TiO2 is a promising method for the treatment of C. raciborskii.

The effects of interspecific interactions between bloom forming cyanobacteria and Scenedesmus quadricauda (chlorophyta) on their photophysiology.

Eutrophication and enhanced external nutrient loading of lakes and seas are most clearly reflected by increased cyanobacterial blooms, which are often toxic. Freshwater cyanobacteria produce a number of bioactive secondary metabolites, some of which have allelopathic properties, significantly influencing the biological processes of other algae, thereby affecting species composition and succession of the phytoplankton. The goal of this work was to investigate the influence of bloom-forming cyanobacterial exudates on the photophysiology of the green alga Scenedesmus quadricauda by chlorophyll fluorescence analysis. We were able to prove the effect of algal cell-free filtrates on the performance of S. quadricauda and demonstrate for the first time that the freshwater picocyanobacterium Cyanobium gracile has strong negative impact on the coexisting green alga. Neither the cyanotoxin (MYC, CYN and ATX) producing, nor the non-toxic strains showed any systematic effect on the production of S. quadricauda. Various strains of the cyanobacterium Cylindrospermopsis raciborskii inhibited the performance of the green alga independently of their origin. Our results urge further studies for a better understanding of the factors affecting the release of allelopathic compounds and the mechanisms of their effects on target organisms.

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.

Distribution of invasive Cylindrospermopsis raciborskii in the East-Central Europe is driven by climatic and local environmental variables.

Mechanisms behind expansion of an invasive cyanobacterium Cylindrospermopsis raciborskii have not been fully resolved, and different hypotheses, such as global warming, are suggested. In the East-Central Europe, it is widely occurring in western part of Poland but only in single locations in the East due to some limiting factors. Therefore, broad-scale phytoplankton survey including 117 randomly selected lakes in Poland and Lithuania was conducted. The results showed that C. raciborskii occurred widely in western part of Poland but was absent from other regions and Lithuania except one lake. The regions in which C. raciborskii was present had higher annual mean air temperature, higher maximum air temperature of the warmest month and higher minimum temperature of the coldest month, demonstrating that average air temperature, and indirectly, the duration of growing season might be more important factor driving C. raciborskii distribution than measured in situ water temperature. In turn, the presence of C. raciborskii in single localities may be more related to physiological adaptations of separated ecotype. Collectively, these results provide novel evidence on the influence of temperature on C. raciborskii distribution in East-European regions but also indicate high ecological plasticity of this species.

Differences in cyanobacterial strain responses to light and temperature reflect species plasticity.

Microcystis aeruginosa and Cylindrospermopsis raciborskii are two cyanobacterial species that dominate freshwaters globally. Multiple strains of each species with different physiology occur, however, many studies have focused only on one or two strains, limiting our understanding of both strain variation and characterisation of the species. Therefore, in this study we examined the variation in growth and morphology of multiple isolates of both species, isolated from two adjacent Australian reservoirs. Four M. aeruginosa strains (=isolates) (one colony-forming, three single-celled morphology) and eight C. raciborskii isolates (five with straight trichomes, three with coiled trichomes) were cultured individually in a factorial designed experiment with four light intensities (L: 10, 30, 50 and 100µmol photons m-2s-1) and two temperatures (T: 20 and 28°C). The specific growth rate (µ), cell volume, and final cell concentration was measured. The light attenuation coefficient (kj), a measure of self-shading, was calculated. The results showed that the intraspecific variation was greater than the interspecific variation. The µ of all isolates of M. aeruginosa and C. raciborskii ranged from 0.16 to 0.55d-1 and 0.15 to 0.70d-1, respectively. However, at a specific light and temperature the mean µ of all M. aeruginosa isolates and C. raciborskii isolates were similar. At the species level, M. aeruginosa had higher growth rates at higher light intensity but lower temperature (L100T20), while straight C. raciborskii had higher growth rates at lower light intensity but higher temperature (L50T28), and coiled C. raciborskii had higher growth rates at higher light intensity and higher temperature (L100T28). The final cell concentrations of M. aeruginosa were higher than C. raciborskii. However, C. raciborskii isolates had greater variation in µ, kj and cell volume than M. aeruginosa. kj varied with light and temperature, and decreased with surface-to-volume ratio within each species. kj was lower for M. aeruginosa compared to C. raciborskii as expected based on cell size, but interestingly, C. raciborskii coiled isolates had lower kj than the straight isolates suggesting lower effect of self-shading. This study highlights the extent of strain variation to environmental conditions and to species variability.

Understanding the winning strategies used by the bloom-forming cyanobacterium Cylindrospermopsis raciborskii.

The cyanobacterium Cylindrospermopsis raciborskii is a widespread species increasingly being recorded in freshwater systems around the world. It is of particular concern because strains in some geographic areas are capable of producing toxins with implications for human and animal health. Studies of this species have increased rapidly in the last two decades, especially in the southern hemisphere where toxic strains are prevalent. A clearer picture is emerging of the strategies adopted by this species to bloom and out-compete other species. This species has a high level of flexibility with respect to light and nutrients, with higher temperatures and carbon dioxide also promoting growth. There are two types of toxins produced by C. raciborskii: cylindrospermopsins (CYNs) and saxitoxins (STXs). The toxins CYNs are constitutively produced irrespective of environmental conditions and the ecological or physiological role is unclear, while STXs appear to serve as protection against high salinity and/or water hardness. It is also apparent that strains of this species can vary substantially in their physiological responses to environmental conditions, including CYNs production, and this may explain discrepancies in findings from studies in different geographical areas. The combination of a flexible strategy with respect to environmental conditions, and variability in strain response makes it a challenging species to manage. Our ability to improve bloom prediction will rely on a more detailed understanding of the complex physiology of this species.

An overview of cyanobacterial bloom occurrences and research in Africa over the last decade.

Cyanobacterial blooms are a current cause for concern globally, with vital water sources experiencing frequent and increasingly toxic blooms in the past decade. These increases are resultant of both anthropogenic and natural factors, with climate change being the central concern. Of the more affected parts of the world, Africa has been considered particularly vulnerable due to its historical predisposition and lag in social economic development. This review collectively assesses the available information on cyanobacterial blooms in Africa as well as any visible trends associated with reported occurrences over the last decade. Of the 54 countries in Africa, only 21 have notable research information in the area of cyanobacterial blooms within the last decade, although there is substantial reason to attribute these blooms as some of the major water quality threats in Africa collectively. The collected information suggests that civil wars, disease outbreaks and inadequate infrastructure are at the core of Africa's delayed advancement. This is even more so in the area of cyanobacteria related research, with 11 out of 21 countries having recorded toxicity and physicochemical parameters related to cyanobacterial blooms. Compared to the rest of the continent, peripheral countries are at the forefront of research related to cyanobacteria, with countries such as Angola having sufficient rainfall, but poor water quality with limited information on bloom occurrences. An assessment of the reported blooms found nitrogen concentrations to be higher in the water column of more toxic blooms, validating recent global studies and indicating that phosphorous is not the only factor to be monitored in bloom mitigation. Blooms occurred at low TN: TP ratios and at temperatures above 12°C. Nitrogen was linked to toxicity and temperature also had a positive effect on bloom occurrence and toxicity. Microcystis was the most ubiquitous of the cyanobacterial strains reported in Africa and the one most frequently toxic. Cylindrospermopsis was reported more in the dry, north and western parts of the continent countries as opposed to the rest of the continent, whilst Anabaena was more frequent on the south eastern regions. In light of the entire continent, the inadequacy in reported blooms and advances in this area of research require critical intervention and action.

The role of heterocytes in the physiology and ecology of bloom-forming harmful cyanobacteria.

Dolichospermum flos-aquae and Cylindrospermopsis raciborskii are two cyanobacteria species which cause harmful blooms around the world. Both these species share the capacity to fix atmospheric nitrogen in heterocytes (cell where fixation occurs). While Dolichospermum can express heterocytes at rather regular intervals across the filament, Cylindrospermopsis can only express heterocytes at the end of the filament. The aim of this study was to experimentally assess the role of heterocyte position in the eco-physiological responses of these bloom forming cyanobacteria. Replicated monocultures of each species were grown at different eutrophication scenarios (limiting and sufficient nitrogen and phosphorus concentrations, in factorial design). Dolichospermum reached high biomass regardless of the nitrogen (and phosphorus) provided, suggesting that this species could bloom in situations with and without nitrogen limitation. In contrast, Cylindrospermopsis reached high biomass only when nitrogen supply was high; its biomass was 15-20 times lower when relying on nitrogen fixation. Hence, despite its ability to fix nitrogen, blooms of Cylindrospermopsis would be expected only under high total nitrogen availability. In Dolichospermum heterocytes occurred only in the scenarios without supplied nitrogen while in Cylindrospermopsis heterocytes occurred regardless of nitrogen availability. Yet, in both species nitrogen fixation occurred (heterocytes were functional) only when nitrogen was limiting, and nitrogen fixation increased significantly at higher phosphorus concentration. Finally, in the absence of supplied nitrogen, filament length in Dolichospermum was the longest, while filaments in Cylindrospermopsis were the shortest (up to 13 times shorter than at nitrogen sufficiency). Therefore, heterocyte expression in Dolichospermum, and filament length in Cylindrospermopsis seem good proxies of nitrogen fixation. The eco-physiological responses recorded here help understand the distribution of these species along nutrient gradients in nature.

Intercellular transfer along the trichomes of the invasive terminal heterocyst forming cyanobacterium Cylindrospermopsis raciborskii CS-505.

Cylindrospermopsis raciborskii CS-505 is an invasive freshwater filamentous cyanobacterium that when grown diazotrophically may develop trichomes of up to 100 vegetative cells while differentiating only two end heterocysts, the sole sites for their N2-fixation process. We examined the diazotrophic growth and intercellular transfer mechanisms in C. raciborskii CS-505. Subjecting cultures to a combined-nitrogen-free medium to elicit N2 fixation, the trichome length remained unaffected while growth rates decreased. The structures and proteins for intercellular communication showed that while a continuous periplasmic space was apparent along the trichomes, the putative septal junction sepJ gene is divided into two open reading frames and lacks several transmembrane domains unlike the situation in Anabaena, differentiating a 5-fold higher frequency of heterocysts. FRAP analyses also showed that the dyes calcein and 5-CFDA were taken up by heterocysts and vegetative cells, and that the transfer from heterocysts and 'terminal' vegetative cells showed considerably higher transfer rates than that from vegetative cells located in the middle of the trichomes. The data suggest that C. raciborskii CS-505 compensates its low-frequency heterocyst phenotype by a highly efficient transfer of the fixed nitrogen towards cells in distal parts of the trichomes (growing rapidly) while cells in central parts suffers (slow growth).

Phylogeny and biogeography of the invasive cyanobacterium Cylindrospermopsis raciborskii.

Cylindrospermopsis raciborskii is a toxic cyanobacterium with an invasive nature. The species is found in all the main continents but its origin and dispersal routes on a worldwide perspective remain yet mostly unknown. In this study, 27 isolates of C. raciborskii gathered worldwide have been used for an in-deep phylogenetic analyses with a concatenated system of three genetic markers (16 rRNA, 16S-23S ITS larger subunit, and RNA polymerase rpoC1) comprehending 3,188 bp. Our results provide support for an origin of C. raciborskii in the American continent. Dispersal routes included afterward a spread into the African continent and then Asia and Australia, being Europe the last continent to be colonized by this species. Our phylogenetic inferences suggest that C. raciborskii seem to have a well-defined dispersal behavior with a well-established population structure around the world.

The effect of temperature on the sensitivity of Daphnia magna to cyanobacteria is genus dependent.

In the present study, the authors investigated the effects of 6 different genera of cyanobacteria on multiple endpoints of Daphnia magna in a 21-d life table experiment conducted at 3 different temperatures (15 °C, 19 °C, and 23 °C). The specific aims were to test if the effect of temperature on Daphnia's sensitivity to cyanobacteria differed among different cyanobacteria and if the rank order from most to least harmful cyanobacteria to Daphnia reproduction changed or remained the same across the studied temperature range. Overall, the authors observed a decrease in harmful effects on reproduction with increasing temperature for Microcystis, Nodularia, and Aphanizomenon, and an increase in harmful effects with increasing temperature for Anabaena and Oscillatoria. No effect of temperature was observed on Daphnia sensitivity to Cylindrospermopsis. Harmful effects of Microcystis and Nodularia on reproduction appear to be mirrored by a decrease in length. On the other hand, harmful effects of Anabaena, Aphanizomenon, and Oscillatoria on reproduction were correlated with a decrease in intrinsic rate of natural increase, which was matched by a later onset of reproduction in exposures to Oscillatoria. In addition, the results suggest that the cyanobacteria rank order of harmfulness may change with temperature. Higher temperatures may increase the sensitivity of D. magna to the presence of some cyanobacteria (Anabaena and Oscillatoria) in their diet, whereas the harmful effects of others (Microcystis, Nodularia, and Aphanizomenon) may be reduced by higher temperatures.

Elevated CO2 causes changes in the photosynthetic apparatus of a toxic cyanobacterium, Cylindrospermopsis raciborskii.

We studied the physiological acclimation of growth, photosynthesis and CO2-concentrating mechanism (CCM) in Cylindrospermopsis raciborskii exposed to low (present day; L-CO2) and high (1300ppm; H-CO2) pCO2. Results showed that under H-CO2 the cell specific division rate (µc) was higher and the CO2- and light-saturated photosynthetic rates (Vmax and Pmax) doubled. The cells' photosynthetic affinity for CO2 (K0.5CO2) was halved compared to L-CO2 cultures. However, no significant differences were found in dark respiration rates (Rd), pigment composition and light harvesting efficiency (α). In H-CO2 cells, non-photochemical quenching (NPQ), associated with state transitions of the electron transport chain (ETC), was negligible. Simultaneously, a reorganisation of PSII features including antenna connectivity (JconPSIIα), heterogeneity (PSIIα/ß) and effective absorption cross sectional area (σPSIIα/ß) was observed. In relation to different activities of the CCM, our findings suggest that for cells grown under H-CO2: (1) there is down-regulation of CCM activity; (2) the ability of cells to use the harvested light energy is altered; (3) the occurrence of state transitions is likely to be associated with changes of electron flow (cyclic vs linear) through the ETC; (4) changes in PSII characteristics are important in regulating state transitions.

Influence of the microbial loop on trophodynamics and toxicity of cadmium complexed by cyanobacterium exudates.

Cadmium (Cd) is an important industrial and toxic metal. Its fate and toxicity in the environment may be mediated by association with dissolved organic materials excreted by phytoplankton. The aim of the present study was to investigate the trophodynamics and toxicity of Cd complexed with Cylindrospermopsis raciborskii exudates in a plankton food chain. The microbial loop involves heterotrophic bacteria as the primary consumer, which is supplemented with a Cd-exudate complex. The secondary consumer (protozoan Paramecium caudatum) was fed on the bacteria, and the tertiary consumer (copepod Mesocyclops longisetus) on the protozoa. The Cd complexing properties of the exudate were determined before the experiments, to ensure that all Cd was furnished to the organisms as the organic complex alone. The results showed that free Cd2+ ions caused the highest observed toxicity to bacteria, while Cd complexed to the cyanobacterial exudate was less toxic, but could be bioavailable and transferred through the food web. This study is a contribution to aquatic ecosystem management and to current knowledge of Cd dynamics, bioavailability and interaction with aquatic planktonic organisms.

Combined exposure to cyanobacteria and carbaryl results in antagonistic effects on the reproduction of Daphnia pulex.

In aquatic ecosystems, Daphnia are exposed to a wide variety of natural and chemical stressors that can cause interactive effects resulting in an increased impact on aquatic ecosystems. The authors therefore investigated the interactive effects of harmful cyanobacteria (cyanoHABs) with carbaryl in Daphnia pulex, because cyanobacteria have become an important concern for aquatic ecosystems. Daphnia were exposed for 21 d to 4 selected cyanobacteria (Aphanizomenon sp., Cylindrospermopsis raciborskii, Microcystis aeruginosa, and Oscillatoria sp), carbaryl, and all binary combinations of carbaryl and each individual cyanobacterium. Results were analyzed with both the independent action and the concentration addition model. The estimated median effect concentration (EC50) for carbaryl was comparable between the experiments, ranging from 2.28 µg/L to 5.94 µg/L. The EC50 for cyanobacteria ranged from 13.45% of the total diet ratio for Microcystis to 66.69% of the diet ratio for Oscillatoria. In combination with carbaryl, the response of Daphnia to 3 of the 4 cyanobacteria demonstrated antagonistic deviation patterns (p < 0.05). Exposure to combinations of carbaryl and Cylindrospermopsis did not result in statistically significant deviations from both reference models. The results may have important implications for pesticide risk assessment, underlining the impact of interactive effects on aquatic organisms. Based on these results, the authors suggest that both the independent action and the concentration addition model can serve as a protective approach in risk assessment of carbaryl in the presence of cyanobacterial blooms.

Influence of biotic and abiotic factors on the allelopathic activity of the cyanobacterium Cylindrospermopsis raciborskii strain LEGE 99043.

Allelopathy is considered to be one of the factors underlying the global expansion of the toxic cyanobacterium Cylindrospermopsis raciborskii. Although the production and release of allelopathic compounds by cyanobacteria is acknowledged to be influenced by environmental parameters, the response of C. raciborskii remains generally unrecognized. Here, the growth and allelopathic potential of C. raciborskii strain LEGE 99043 towards the ubiquitous microalga Ankistrodesmus falcatus were analyzed under different biotic and abiotic conditions. Filtrates from C. raciborskii cultures growing at different cell densities displayed broad inhibitory activity. Moreover, higher temperature, higher light intensity as well phosphate limitation further enhanced this activity. The distinct and comprehensive patterns of inhibition verified during the growth phase, and under the tested parameters, suggest the action of several, still unidentified allelopathic compounds. It is expectable that the observed increase in allelopathic activity can result in distinct ecological advantages to C. raciborskii.

What drives the distribution of the bloom-forming cyanobacteria Planktothrix agardhii and Cylindrospermopsis raciborskii?

The cyanobacteria Planktothrix agardhii and Cylindrospermopsis raciborskii are bloom-forming species common in eutrophic freshwaters. These filamentous species share certain physiological traits which imply that they might flourish under similar environmental conditions. We compared the distribution of the two species in a large database (940 samples) covering different climatic regions and the Northern and Southern hemispheres, and carried out laboratory experiments to compare their morphological and physiological responses. The environmental ranges of the two species overlapped with respect to temperature, light and total phosphorus (TP); however, they responded differently to environmental gradients; C. raciborskii biovolume changed gradually while P. agardhii shifted sharply from being highly dominated to a rare component of the phytoplankton. As expected, P. agardhii dominates the phytoplankton with high TP and low light availability conditions. Contrary to predictions, C. raciborskii succeeded in all climates and at temperatures as low as 11 °C. Cylindrospermopsis raciborskii had higher phenotypic plasticity than P. agardhii in terms of pigments, individual size and growth rates. We conclude that the phenotypic plasticity of C. raciborskii could explain its ongoing expansion to temperate latitudes and suggest its future predominance under predicted climate-change scenarios.