Raciborskii Cells Research Articles

Paramecium jenningsi effectively grazes on toxic Raphidiopsis raciborskii and degrades cylindrospermopsin: Implications for control harmful cyanobacterial blooms

Harmful cyanobacterial blooms (HCB) represent a serious threat to aquatic ecosystems due to their production for potent toxins that have negative effects on plant, animal and human health. Although grazing process can be exploited to reduce harmful cyanobacteria in eutrophic waters, the feeding capacity and detoxification capability of the grazer should be considered as key traits of a safe biocontrol agent. In this study, laboratory experiments were conducted to investigate the grazing rate of the ciliate Paramecium jenningsi on toxic Raphidiopsis raciborskii and potential cylindrospermopsin (CYN) degradation during feeding process. P. jenningsi efficiently ingested R. raciborskii cells with high growth (0.1- 0.26 d−1) and grazing rates (0.1-1.4 × 104 cyanobacterial cells ciliate −1 d−1), varying with the initial ciliate density. CYN concentrations within Raphidiopsis cells displayed an elevation under grazing conditions (5.3-7.2 pg cell−1) compared to control cultures (4.4 pg cell−1). CYN concentrations within the ciliate cells showed a reduction (degradation rate =121-276 pg ciliate−1 d−1) and became undetectable after all Raphidiopsis cells were grazed out, indicating that this ciliate could not accumulate CYN within its body during feeding process. Therefore, P. jenningsi could be used as a safe bioagent for the biocontrol of HCBs in water sources. However, a further in situ study is needed for a possible application of Paramecium species for mitigation of toxic cyanobacteria and their cyanotoxins in the natural environment.

Does temperature favour the spread of Raphidiopsis raciborskii, an invasive bloom-forming cyanobacterium, by altering cellular trade-offs?

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

Bioremoval of Cylindrospermopsis raciborskii cells and cylindrospermopsin toxin in batch culture by the yeast Aureobasidium pullulans.

This study describes the ability of a yeast strain, Aureobasidium pullulans KKUY0701 isolated from eutrophic lake to eliminate Cylindrospermopsis raciborskii and cylindrospermopsin (CYN) toxin. The anti-cyanobacterial activity of this yeast strain was evaluated by growing with living cells and filtrate of C. raciborskii. CYN bioremoval was assayed using living and heat-inactivated yeast cells. Both living cells and filtrate of this yeast strain were able to suppress the growth of C. raciborskii, with total cell death occurring at day 2 and day 3, respectively. Living and inactivated yeast cells, but not yeast filtrate, reduced CYN concentrations released into cyanobacterial cultures, indicating that this toxin might be removed from the culture medium via absorption onto yeast surface rather than enzymatic biodegradation. The adsorption experiments also confirmed the elimination of CYN by living and heat-inactivated yeast. Nevertheless, inactivated yeast exhibited higher capacity (K = 3.3) and intensity (n = 1.4) than living yeast (K = 1.9, n = 1) for CYN adsorption. The study suggests that this yeast strain could be employed for bioremediation of Cylindrospermopsis blooms in freshwaters. Additionally, heat-inactivated yeast biomass could be used in slow sand filters for elimination of CYN in drinking water treatment plants.

Driving forces for the growth of MIB-producing Planktothricoides raciborskii in a low-latitude reservoir

In comparison with the middle- and high-latitude regions, the low-latitude regions are less associated with the occurrence of 2-methylisoborneol (MIB) episodes, since most of the previously identified MIB producers favor moderate/low light/temperature conditions. Here, we report a serious MIB outbreak over the period from Jul. 2018 to Jun. 2019 in a low-latitude reservoir with a mean annual water temperature of 25.6 °C. The MIB episode lasted for a long period, from Jul. 2018 to Jan. 2019, and Planktothricoides raciborskii was confirmed to be the main MIB producer. The growth characteristics of P. raciborskii were explored through both laboratory culturing and on-site verification experiments. The results indicated that this strain was not nutrient-sensitive at TN > 800 μg L−1 and TP > 10 μg L−1, but favored moderate light intensity (54 μmol photon m−2·s−1) and high temperature (30 °C). The two bloom-forming genera, Limnothrix and Aphanizomenon, favoring lower temperature and similar or relatively higher light intensity, showed much greater proliferation, about 13 folds (Limnothrix) and 58 folds (Aphanizomenon), from Dec. to Jun.; by contrast, the high water temperature (29.9 ± 2.8 °C) and light intensity (189.1 ± 87.6 μmol photon m−2·s−1) from Jul. to Nov. were not favorable to Limnothrix or Aphanizomenon, which might have created an opportunity for the growth of MIB-producing P. raciborskii. In addition, we also found that high temperature could promote the release of MIB from P. raciborskii cells, therefore exerting increased pressure on drinking water treatment processes.

Simultaneous biodegradation of harmful Cylindrospermopsis raciborskii and cylindrospermopsin toxin in batch culture by single Bacillus strain.

This study investigates the capability of a Bacillus flexus strain isolated from decayed cyanobacterial blooms for the bioremediation of Cylindrospermopsis raciborskii and cylindrospermopsin (CYN) toxin. The algicidal activity of this strain was tested by co-cultivation with C. raciborskii cultures. CYN biodegradation was investigated in the presence of living and heat-inactivated bacterial cells or bacterial filtrate. Living bacterial cells inhibited C. raciborskii growth after 2 days of incubation with complete cell death at day 5. Bacterial filtrate caused a rapid reduction in C. raciborskii growth at the first day, with complete cell lysis at day 3. Only living cells of SSZ01 caused reduction in CYN released into the medium during the bacterial decay of C. raciborskii cells. The biodegradation rate of CYN by SSZ01 relied on initial toxin concentrations. The highest rate (42 μg CYN L-1 day-1) was obtained at the higher initial concentration (300 μg L-1), and the lowest (4μg CYN L-1 day-1) was at lower concentration (50 μg L-1). These results suggest that this bacterial strain could be employed to bioremediate cyanobacterial blooms in freshwaters. Also, the application of this bacterium in slow sand filters would give possibilities for degradation and bioremediation of cyanotoxins in drinking water treatment plants.

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.

The role of hydraulic conditions of coagulation and flocculation on the damage of cyanobacteria

Limited information exists on the damage of harmful cyanobacteria cells, such as Raphidiopsis raciborskii and Dolichospermum circinale, caused by the hydraulic conditions at water treatment plants especially when it comes to the mechanical stresses imposed by coagulation and flocculation. To close this gap, this study evaluated the impacts of rapid and slow-mixing on R. raciborskii and D. circinale cells and trichomes. The hydraulic conditions used during the experiment were selected based on AWWA, which are widely applied in the absence of specific treatability tests. Cellular integrity was evaluated by the Erythrosine B staining method and logistic regression was used to study the association between organism integrity and hydraulic conditions (i.e., velocity gradient and mixing time). Wilcoxon rank-sum test was used to verify if there was a significant reduction of the trichome length and cell integrity. Rapid-mixing (velocity gradient of 750 s−1 for 60 s) reduced the odds of finding intact D. circinale to <50%, whereas the odds of finding intact R. raciborskii cells did not significantly decrease. The odds of finding intact cells of R. raciborskii were 124 times greater than D. circinale. Rapid-mixing also reduced the length of D. circinale trichomes by approximately 50% but did not significantly decrease R. raciborskii trichomes. Slow-mixing did not significantly affect organisms or trichomes of either species. The results indicate that AWWA recommendations for coagulation may cause damage to D. circinale but not to R. raciborskii, suggesting that the operation of water treatment plants could be adjusted according to the dominant cyanobacterium present in the reservoir to avoid cell rupture and metabolite release.

Interspecific competition between Cylindrospermopsis raciborskii and Microcystis aeruginosa on different phosphorus substrates.

Phosphorus (P) is responsible for algal growth and the structural changes in algal communities. Therefore, it is essential to know whether the different phosphorus availability to different algae can change the community structure. In this study, the interspecific competition was investigated at two bloom-forming cyanobacterium, Cylindrospermopsis raciborskii and Microcystis aeruginosa, when both were treated with five different phosphate compounds, including K2HPO4, β-glycerol phosphate, (2-aminoethyl)-phosphinic acid, glyphosate, and P-free. The results of mono-culture experiments showed that the two species could utilize the dissolved organic phosphorus (DOP) and K2HPO4 (DIP) as the sole P resource. Moreover, the specific growth rates and the endogenous alkaline phosphatase activity in M. aeruginosa cells were much lower than those in C. raciborskii under DOP and DIP treatments. In the co-cultured experiments, however, a significant biomass increase in C. raciborskii was observed in all experimental P treatments, except for glyphosate, regardless of its initial cell density proportion. A 31.8-63.4% increase in cell number of C. raciborskii was found after incubated into K2HPO4, while the highest biomass of mixed samples, 17.72 × 106 cell mL-1, was observed in the (2-aminoethyl)-phosphinic acid treatment (50C50M). Additionally, higher specific growth rate was also found in C. raciborskii when compared with M. aeruginosa under P-free; the increasing proportion of C. raciborskii were 29.1% (50C50M), 16.4% (75C25M), and 36.7% (25C75M), respectively. When the mixed samples were co-cultivated under glyphosate, C. raciborskii cells appeared to be depressed, whereas the cell density of M. aeruginosa increased rapidly. The findings indicated that an excellent P competition might give some advantages for C. raciborskii dominance in natural waters with DIP limitation or DOP abundance.

Factors related to aggravated Cylindrospermopsis (cyanobacteria) bloom following sediment dredging in an eutrophic shallow lake

In recent years, Cylindrospermopsis raciborskii blooms have been widely found worldwide. Topics dealing with the mitigation of C. raciborskii bloom is of great importance for toxins produced could threaten public health. The paper first investigated C. raciborskii dynamics over three years following sediment dredging in a shallow eutrophic Lake Dongqian (China). Based on rpoC1 gene copies, C. raciborskii bloom formed with average density of 1.30 × 106 cells/L on July 2009. One year later after sediment dredging, C. raciborskii cell density decreased below 1.17 × 105 cells/L or under detected limits during summer days on 2010. While two years later, the C. raciborskii bloom period was returned with markedly increased cell density reaching up to 4.15 × 107 cells/L on October 2011, and the maximum peak density was shown at 20.3 °C that was much lower than reported optimal growth temperature. Inferred from Spearman correlation analysis, linear regression showed C. raciborskii density was significant and positive with pH and SD, whereas they were significant and negative with TP and DO. Multiple regression analysis further demonstrated that TN, TP, SRP, pH and DO provided the best model and explained 53.1% of the variance in C. raciborskii dynamics. The approaches managing nutrients reduction might not control C. raciborskii bloom as extremely low TN (avg. 0.18 mg/L) and TP concentrations (avg. 0.05 mg/L) resulted in the highest C. raciborskii cell density after sediment dredging.

Development of real-time PCR for quantification of Cylindrospermopsis raciborskii cells and potential cylindrospermopsin-producing genotypes in subtropicalreservoirs of southern China

Cylindrospermopsin (CYN) is of great concern to human and animal health due to its potential toxicity. The cyanobacterium Cylindrospermopsis raciborskii is considered the most common cyanobacterial species that produces CYN. The discovery of the cyr gene cluster responsible for CYN biosynthesis allows us to develop molecular methods that detect and quantify potentially CYN-producing C. raciborskii. This paper describes the development of real-time PCR (qPCR) assays capable of quantifying the total and CYN-producing C. raciborskii in subtropical reservoirs of southern China. We designed primers and probes specifically targeting the rpoC1 and cyrJ genes of C. raciborskii, and enabling quantification of its total cell numbers and potentially toxic genotypes, respectively. The qPCR showed strong linearity between 102 and 106 copies per reaction for both genes. This molecular method was validated against microscopic counting and a high correlation was found between them for quantifying C. raciborskii cell numbers in cyanobacterial cultures and water samples. Using qPCR, we detected potentially CYN-producing C. raciborskii in 34 of the 46 subtropical reservoirs of southern China, with the cyrJ/rpoC1 proportion ranging from 0.3 to 34.7%. CYN concentrations significantly correlated with both the copy numbers of cyrJ gene and the proportion of toxic C. raciborskii. Thus, the present real-time PCR method provides a reliable and faster method for estimating the potential toxicity of C. raciborskii blooms in this region. The wide distribution of potential CYN-producing C. raciborskii in the investigated subtropical reservoirs highlights the need for further monitoring.

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.

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.

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

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

Constitutive cylindrospermopsin pool size in Cylindrospermopsis raciborskii under different light and CO2 partial pressure conditions.

Cylindrospermopsin (CYN) and 7-deoxy-cylindrospermopsin (dCYN) are potent hepatotoxic alkaloids produced by numerous species of cyanobacteria, including the freshwater Cylindrospermopsis raciborskii. C. raciborskii is an invasive cyanobacterium, and the study of how environmental parameters drive CYN production has received significant interest from water managers and health authorities. Light and CO2 affect cell growth and physiology in photoautotrophs, and these are potential regulators of cyanotoxin biosynthesis. In this study, we investigated how light and CO2 affect CYN and dCYN pool size as well as the expression of the key genes, cyrA and cyrK, involved in CYN biosynthesis in a toxic C. raciborskii strain. For cells growing at different light intensities (10 and 100 μmol photons m(-2) s(-1)), we observed that the rate of CYN pool size production (μCYN) was coupled to the cell division rate (μc) during batch culture. This indicated that CYN pool size under our experimental conditions is constant and cell quotas of CYN (QCYN) and dCYN (QdCYN) are fixed. Moreover, a lack of correlation between expression of cyrA and total CYN cell quotas (QCYNs) suggests that the CYN biosynthesis is regulated posttranscriptionally. Under elevated CO2 (1,300 ppm), we observed minor effects on QCYN and no effects on expression of cyrA and cyrK. We conclude that the CYN pool size is constitutive and not affected by light and CO2 conditions. Thus, C. raciborskii bloom toxicity is determined by the absolute abundance of C. raciborskii cells within the water column and the relative abundance of toxic and nontoxic strains.

Parameter Optimization Algorithms for Evolving Rule Models Applied to Freshwater Ecosystems

Predictive rule models for early warning of cyanobacterial blooms in freshwater ecosystems were developed using a hybrid evolutionary algorithm (HEA). The HEA has been designed to evolve IF-THEN-ELSE model structures using genetic programming and to optimize the stochastical constants contained in the model using population-based algorithms. This paper intensively investigates the performances of the following six alternative population-based algorithms for parameter optimization (PO) of rule models within this hybrid methodology: 1) hill climbing (HC); 2) simulated annealing (SA); 3) genetic algorithm (GA); 4) differential evolution (DE); 5) covariance matrix adaptation evolution strategy (CMA-ES); and 6) estimation of distribution algorithm (EDA). The comparative study was carried out by predictive modeling of chlorophyll- ${a}$ concentrations and the potentially toxic cyanobacterium Cylindrospermopsis raciborskii cell concentrations based on water quality time-series data in Lake Wivenhoe, Queensland, Australia, from 1998 to 2009. The experimental results demonstrate that with these PO methods, the rule models discovered by the HEA proved to be both predictive and explanatory whose IF condition indicates threshold values for some crucial water quality parameters. When comparing different PO algorithms, HC always performed best followed by DE, GA, and EDA, while CMA-ES performed worst and the performance of SA varied with different data sets.

Dissolved organic phosphorus use by the invasive freshwater diazotroph cyanobacterium, Cylindrospermopsis raciborskii

This study examines the physiological responses of the diazotrophic cyanobacteria, Cylindrospermopsis raciborskii, to different dissolved organic phosphorus (DOP) compounds to explore mechanisms of environmental acclimation in this invasive species. Our results show that the specific growth rates of C. raciborskii cells in media treated with β-glycerol phosphate, d-glucose-6-phosphate, and (2-aminoethyl)-phosphinic acid were significantly higher than those of cells grown in phosphorus free media. We observed that maximal net photosynthesis was highest when cells were cultured with d-glucose-6-phosphate and lowest when cells were cultured with glyphosate. Similarly, rates of photosynthetic activity (maximum quantum yield, maximum electron transport rate, and photosynthetic efficiency) were observed to be highest in media treated with d-glucose-6-phosphate. We report that rates of alkaline phosphatase activity to the different organophosphates tested changed markedly in response to the concentration of dissolved inorganic phosphorus (DIP); a result supported by the amount of green fluorescent products revealed by ELF®97 phosphate dye (ELFP) and gene up-regulation for alkaline phosphatase (phoA). Our results indicate that C. raciborskii is able to use different organic phosphorus to support its growth when phosphorus is limited. In addition, we show that C. raciborskii has a higher availability to phosphate (COP) than phosphonate (CP). The results suggest that the strategic flexibility to environmental phosphorus might play an important role in the domination of C. raciborskii.

A role for aeration and intake depth in managing toxic Cylindrospermopsis: A comparison between off‐stream and riverine environments in the Fitzroy Basin, Australia

AbstractCylindrospermopsis is a cyanobacterial species of global concern, due to its ability to produce potent toxins and its near‐cosmopolitan distribution. To date, both management and research efforts for Cylindrospermopsis have focused on riverine settings. By contrast, off‐stream storages are inadequately studied, particularly in terms of population dynamics and toxin profiles, how these are linked with the riverine systems that seed them, and what management options are best for optimizing water quality. This is a critical gap, given that storages are key off‐take points for many water users, including for livestock watering and industrial supply. This study examined the relationship between a riverine and an off‐stream environment, with respect to the development of toxic Cylindrospermopsis blooms in central Queensland. It discusses the role of an aeration system in preventing or delaying bloom development and explores the implications of intake depth with respect to population (cell density) and toxicity. Along the impounded river, the study sites featured warm, stratified conditions and an anoxic hypolimnion. By contrast, the off‐stream storage dam lacked stratification, due to the constant aeration provided by a hypolimnetic bubbler. Cylindrospermopsis raciborskii blooms appeared in both environments, with peak cellular concentrations detected in the subsurface waters. The dynamics of the toxin profiles differed, however, with the riverine concentrations being mostly epilimnetic, whereas the dam featured toxin distributed throughout the water column. Concentrations of both cylindrospermopsin (CYN) and deoxy‐cylindrospermopsin (deoxy‐CYN) were lower in the dam, compared with the riverine sites. A one‐month lag period was evident before C. raciborskii cells appeared in the dam following their observation in the river samples, and a similar period elapsed before the dam populations reached their peak concentrations. These data are invaluable in considering the best management options for off‐stream sites that experience seeding from riverine populations, particularly for toxin producers such as Cylindrospermopsis.

Effects of different photoperiods and concentrations of phosphate on the growth of the cyanobacterium Cylindrospermopsis raciborskii (Woloszynska)

The freshwater cyanobacterium, Cylindrospermopsis raciborskii is potentially toxic and widely distributed in tropical and sub-tropical regions. This species is highly abundant throughout the Anzali Lagoon during the months of July and August. The effects of four different photoperiods (L/D 12:12, 10:14, 14:10, 8:16) as well as three different concentrations of phosphate (2 fold, 3 fold and half of the main culture) on growth rates of C. raciborskii were studied. C. raciborskii , isolated from water samples collected from the Anzali Lagoon, was cultured in Zehnder 8 media (using Z 8 without a nitrogen source) under light intensity of 3000 Lux at 25 + 2°C. A control group cultured with the main culture medium was also used. This experiment was carried out for 25 days. Inoculants and C. raciborskii cells were counted every other day throughout the experiment period using a Neubauer chamber and specific growth rates (SGR) and cell division rates (G) were estimated. The maximum growth rate (8x10 6 ± 5 cells ml -1 ) was observed at 12 L:12 D photoperiod followed by 14 L:10 D, 10 L:14 D and 8 L:16 D. The results obtained in the present study especially with 14 L:10 D and 12 L:12 D were in complete agreement with general growth pattern of C. raciborskii observed in warmer months with rather longer daytime in tropical and subtropical areas. The maximum growth rates were observed in the group treated with 2 fold PO 4 concentration. The strategy of growth in this species may depend on nitrogen fixation, when other algae were nitrogen limited and fast PO 4 uptake even if very low concentration is available. Key words: Cyanobacteria, Cylindrospermopsis raciborskii, Anzali Lagoon, photoperiod, phosphate, growthrate.

Physical and chemical processes promoting dominance of the toxic cyanobacterium Cylindrospermopsis raciborskii

The freshwater cyanobacterium, Cylindrospermopsis raciborskii (Wo’oszyńska) Seenayya and Subba Raju is a common species in lakes and reservoirs globally. In some areas of the world it can produce cyto- and hepatotoxins (cylindrospermopsins, saxitoxins), making blooms of this species a serious health concern for humans. In the last 10–15 years, there has been a considerable body of research conducted on the ecology, physiology and toxin production of this species and this paper reviews these studies with a focus on the cylindrospermopsin (CYN)-producing strains. C. raciborskii has low light requirements, close to neutral buoyancy, and a wide temperature tolerance, giving it the capacity to grow in many lentic waterbodies. It also has a flexible strategy with respect to nitrogen (N) utilisation; being able to switch between utilising fixed and atmospheric N as sources of N fluctuate. Additionally this species has a high phosphate (DIP) affinity and storage capacity. Like many cyanobacteria, it also has the capacity to use dissolved organic phosphorus (DOP). Changes in nutrient concentrations, light levels and temperature have also been found to affect production of the toxin CYN by this species. However, optimal toxin production does not necessarily occur when growth rates are optimal. Additionally, different strains of C. raciborskii vary in their cell quota of CYN, making it difficult to predict toxin concentrations, based on C. raciborskii cell densities. In summary, the ecological flexibility of this organism means that controlling blooms of C. raciborskii is a difficult undertaking. However, improved understanding of factors promoting the species and toxin production by genetically capable strains will lead to improved predictive models of blooms.

Extreme differences in akinete, heterocyte and cylindrospermopsin concentrations with depth in a successive bloom involving Aphanizomenon ovalisporum (Forti) and Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Subba Raju

This paper describes the species-specific responses recorded from two toxin-producing cyanobacteria Aphanizomenon ovalisporum (Forti) and Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Subba Raju as they approached an overwintering phase in Cobaki Lake (New South Wales, Australia). Each species was examined from samples collected at every metre from a time series of depth profiles and analyzed with respect to growth of specialized cells (heterocytes and akinetes), presence of cylindrospermopsins and the accompanying seasonal dynamics of the water body. Growth and dominance of each species was linked to differing seasonal environmental conditions. Both A. ovalisporum and C. raciborskii produced specialized cells during the bloom. As the water chemistry changed, A. ovalisporum produced akinetes before experiencing a rapid decline in cell numbers. In contrast, C. raciborskii continued to bloom without producing detectable akinetes. Peak C. raciborskii cell concentrations (83,160 cells mL −1) occurred in the late autumn, when surface water temperatures were 19.1 °C, and were accompanied by concentrations of total cylindrospermopsins in the hypolimnion exceeding 100 μg L −1. These toxin concentrations were highly positively correlated with conductivity, soluble iron, bioavailable nutrient species and heterocyte densities. This is the first field study to provide evidence that C. raciborskii, despite being traditionally considered a tropical species, can be highly toxic in cooler waters especially when accompanied by strong stratification involving an anoxic semi-saline hypolimnion. This has serious implications for both water quality management and human health risks in those subtropical climates where C. raciborskii is present.