Non-toxic Microcystis Research Articles

Spatio-temporal variation of toxin-producing gene abundance in Microcystis aeruginosa from Poyang Lake.

Microcystis aeruginosa (M. aeruginosa) causes massive blooms in eutrophic freshwater and releases microcystin. Poyang Lake is the largest freshwater lake in China and has kept a mid-nutrient level in recent years. However, there is little research on microcystin production in Poyang Lake. In this study, water and sediment samples from ten sampling sites in Poyang Lake were collected from May to December in 2020, and from January to April in 2021 respectively. Microcystis genes (mcyA, mcyB, 16s rDNA) were quantified by real-time fluorescence quantitative PCR analysis, and then the spatial and temporal variation of mcy genes, physicochemical factors, and bacterial population structure in the lake was analyzed. The relationship between the abundance of mcy genes and physicochemical factors in water column was also revealed. Results indicated that the microcystin-producing genes mcyA and mcyB showed significant differences in spatial and temporal levels as well, which is closely related to the physicochemical factors especially the water temperature (p < 0.05) and the nitrogen content (p < 0.05). The abundance of mcy genes in the sediment in December affected the abundance of mcy genes in the water column in the next year, while the toxic Microcystis would accumulate in the sediment. In addition to the toxic Microcystis, we also found a large number of non-toxic Microcystis in the water column and sediment, and the ratio of toxic to non-toxic species can also affect the toxicity production of M. aeruginosa. Overall, the results showed that M. aeruginosa toxin-producing genes in Poyang Lake distributed spatially and temporally which related to the physicochemical factors of Poyang Lake.

Changes in microcystin concentration in Lake Taihu, 13 years (2007–2020) after the 2007 drinking water crisis

Since the 2007 water crisis occurred in Lake Taihu, substantial measures have been taken to restore the lake. This study evaluates the effectiveness of these restoration activities. We examined the physicochemical parameters and the distribution of microcystin and Microcystis in both the water column and sediment during the bloom period of May 2020 to October 2020. The mean value of extracellular and intracellular microcystin content was 0.12 μg L−1 and 16.26 μg L−1, respectively. The mean value of microcystin in sediment was 172.02 ng g−1 and peaked in August. The concentration in the water and sediment was significantly lower than the historical average concentration. The abundance of toxigenic Microcystis and total Microcystis in the water column ranged from 2.61 × 102 to 2.25 × 109 copies·L−1 and 8.28 × 105 to 2.76 × 109 copies·L−1, respectively. The proportion of toxic Microcystis in the sediment ranging from 31.2% to 19.12%. The highest and lowest region was Meiliang Bay and Grass-algae type zone, respectively. The copy number of the 16S rRNA gene was 1–4 orders of magnitude higher than that of mcyA gene in populations of Microcystis, indicating that non-toxic Microcystis was the dominant form in the majority of the lake. The abundance of toxic Microcystis in the water column was positively correlated with total phosphorus, PO43--P and pH, while the water temperature played distinct role to the distribution of toxic Microcystis in sediment. Our research indicated phosphorus remains a key factor influencing the toxic Microcystis and microcystins in the water column. pH played distinct roles in the distribution of microcystins in sediment and water column. The increasing water temperature is a threat. Explicit management actions and policies, which take into account nutrient concentrations, pH, and increasing temperatures, are necessary to understand and control the distribution of microcystin and Microcystis in Lake Taihu.

Novel ecological implications of non-toxic Microcystis towards toxic ecotype in population-promoting toxic ecotype dominance at various N levels and cooperative defense against luteolin-stress.

Microcystin (MC)-producing (MC+) and MC-free (MC-) Microcystis always co-exist and interact during Microcystis-dominated cyanobacterial blooms (MCBs), where MC+Microcystis abundance and extracellular MC-content (EMC) determine the hazard extent of MCBs. The current study elucidated intraspecific interaction between MC+ and MC-Microcystis at various nitrogen (N) levels (0.5-50mg/L) and how such N-mediated interaction impacted algicidal and EMC-inhibiting effect of luteolin, a natural bioalgicide. Conclusively, MC+ and MC-Microcystis were inhibited mutually at N-limitation (0.5mg/L), which enhanced the algicidal and EMC-inhibiting effects of luteolin. However, at N-sufficiency (5-50mg/L), MC-Microcystis promoted MC+ ecotype growth and dominance, and such intraspecific interaction induced the cooperative defense of two ecotypes, weakening luteolin's algicidal and EMC-inhibiting effects. Mechanism analyses further revealed that MC+Microcystis in luteolin-stress co-culture secreted exopolymeric substances (EPSs) for self-protection against luteolin-stress and also released more EMC to induce EPS-production by MC-Microcystis as protectants, thus enhancing their luteolin-resistance and promoting their growth. This study provided novel ecological implications of MC-Microcystis toward MC+ ecotype in terms of assisting the dominant establishment of MC+Microcystis and cooperative defense with MC+ ecotype against luteolin, which guided the application of bioalgicide (i.e. luteolin) for MCBs and MCs pollution mitigation in different eutrophication-degree waters.

Environmental concentrations of anionic surfactants in lake surface microlayers enhance the toxicity of Microcystis blooms: Insight from photosynthesis, interspecies competition, and MC production

Anionic surfactants represented by linear alkylbenzene sulfonate (LAS) exhibit vertical heterogeneity of concentrations in aquatic environments owing to their amphiphilic structure. Field investigations showed that the concentration of anionic surfactants (mainly LAS) in the water surface microlayer (SML) of Lake Taihu reached 580 μg/L, higher than that in the lower layer. Floating Microcystis blooms overlap in space with the high concentration of anionic surfactants in SML. However, few studies have focused on the effects of anionic surfactants (e.g., LAS) on the interspecies competition between toxic and nontoxic Microcystis. In this study, coculture and monoculture experiments were conducted with both toxic and nontoxic Microcystis species to explore how the environmental concentration of LAS regulates the dominance of toxic Microcystis and toxicity from the perspective of photosynthesis, species dominance, and MC production. The results showed that LAS concentrations above 0.267 or 0.431 mg/L (depending on light conditions) selectively promoted the photosynthetic competitive advantage of toxic Microcystis, leading to its higher population proportion in the community. Additionally, LAS concentrations above 0.5 mg/L induced the synthesis and release of microcystins (MCs). The results of chlorophyll fluorescence analysis, electron microscopy and transcriptome sequencing suggested that compared with nontoxic Microcystis, toxic Microcystis can better resist LAS stress by dissipating excess light, maintaining an intact membrane structure and maintaining cellular homeostasis. Transcriptome sequencing revealed that the photosynthetic damage of nontoxic Microcystis might be attributed to the impacts of LAS on the absorption and assimilation of nitrogen, which finally resulted in the degradation of phycobilisomes. This study can provide novel insight for establishing standards and safety management of wastewater discharge.

Temperature-Dependent Growth Characteristics and Competition of Pseudanabaena and Microcystis

Global warming has been considered to accelerate the expansion of cyanobacterial blooms, which are frequently composed of the bloom-forming genera, Microcystis and Pseudanabaena, in freshwater ecosystems worldwide. Nonetheless, the impact of changes due to toxin production or lack thereof on the growth of co-existing strains, both arising from increasing temperature, has remained unknown to date. We conducted competition experiments involving toxic Microcystis PCC7806, a non-toxic mcyB− mutant, and two strains of Pseudanabaena (TH-1 and DC-1) identified as P. mucicola. In mono-culture, the specific growth ratio of Pseudanabaena increased; nevertheless, the maximum cell density declined with increasing temperature. The maximum growth ratios of Pseudanabaena TH-1 and Pseudanabaena DC-1 were 0.512 day−1 in the 30 °C group and 0.440 day−1 in the 35 °C group, respectively. The optimum temperature for the growth of Pseudanabaena was 25 °C. Remarkably, there was no significant disparity in the maximum cell density of Microcystis PCC7806 and the mcyB− mutant across varied temperature groups, even though their maximum growth rates differed marginally, reaching 0.280 day−1 and 0.306 day−1 in the 30 °C group, respectively. In co-cultures, the growth of Pseudanabaena TH-1 was uniformly inhibited, whereas that of DC-1 was somewhat influenced by co-culturing with toxic and non-toxic Microcystis, except for the 35 °C group, where inhibition was absent amongst DC-1 and Microcystis. Moreover, the growth of Microcystis was promoted with a co-culture of TH-1 in the 20 °C groups. Conversely, the co-culture of Microcystis with Pseudanabaena DC-1 led to the inhibition of the former in the 30 °C and 35 °C groups. With a Lotka–Volterra competition model, the results showed that Microcystis dominated or co-existed with Pseudanabaena, conforming to expectations from the wild.

Differential Comprehensive Effects of Food Quality and ZnO Nanoparticles on the Key Traits of Early Life History of Daphnia magna

In aquatic ecosystems, cladocerans, an important part of zooplankton, are often exposed to new pollutant nanoparticles and poor quality food simultaneously. To evaluate the combined effects of poor quality food and nanoparticles on the development and early reproductive performance of cladocerans, we exposed Daphnia magna, a representative cladoceran, to different concentrations of ZnO nanoparticles under the food conditions containing different proportions of non-toxic Microcystis, recorded some key indicators of the early life history, and analyzed the possible differential dose effects. The results showed that non-toxic Microcystis and ZnO nanoparticles significantly delayed the times to maturation and reproduction and also significantly reduced the survival time, the body length at maturation, the number of offspring in the first brood, and the total offspring per female of D. magna. When the food contained non-toxic Microcystis, D. magna did not reach sexual maturity at 0.25 mg L−1 ZnO nanoparticles and did not develop eggs and reproduce offspring at 0.20 mg L−1 ZnO nanoparticles, especially non-toxic Microcystis and ZnO nanoparticles had a significant or nearly significant synergistic effect on the time to maturation, time to first brood, and the total offspring per female of D. magna, whereas for other life history indicators, non-toxic Microcystis and ZnO nanoparticles showed an additive effect, based on comparing the slopes of linear fitting of the relevant indicators with nanoparticle concentration under different food quality conditions. Such findings are helpful to realistically evaluate the comprehensive stress effect on zooplankton under the complex conditions of poor food quality and coexistence of pollutants.

Environmental factors associated with cyanobacterial assemblages in a mesotrophic subtropical plateau lake: A focus on bloom toxicity.

Harmful algal blooms caused by cyanobacteria have been increasing in frequency worldwide. However, the main environmental drivers of this change are often difficult to identify because of the effects of the interaction between eutrophication and climate change. Recently, filamentous N2-fixing cyanobacteria and non-diazotrophic Microcystis have been observed to be co-existing and undergoing succession in some eutrophic lakes. However, the succession patterns of dominant cyanobacteria and the factors driving this in mesotrophic lakes are not well understood. We hypothesized that the changes in cyanobacterial assemblages in mesotrophic lakes could result in a relatively high risks of toxic blooms, and that these changes are associated with the global climatic changes. We tested these hypotheses using data from the subtropical mesotrophic Lake Erhai. We found that the high spatiotemporal variability in the cyanobacterial community, and the increase in biomass were driven primarily by the growth of bloom-forming cyanobacterial taxa. Species-specific biomasses were related to a different environmental stressor; increases in dissolved organic carbon (DOC) concentrations were statistically associated with an increase of Microcystis biomass, whereas increases in surface water temperature favored higher biomass of Pseudanabaena at low transparency and high concentration of phosphorus. In addition, low nitrogen- to- phosphorus ratios were identified as potential determinants of the abundance of N2-fixing Dolichospermum. Furthermore, changes in the concentration of DOC, total nitrogen, pH and water transparency levels were found to affect the composition of Microcystis morphotypes and genotypes mostly. This study highlights that the toxic to non-toxic Microcystis ratio might increase with the water darkening and browning (which occurs in many subtropical plateau lakes). Lake management strategies, therefore, need to consider the toxicity of cyanobacterial assemblages in mesotrophic lakes over the intensity of the cyanobacterial blooms.

Cyanobacterial Classification with the Toxicity Using MALDI Biotyper.

An abnormal growth of cyanobacteria in eutrophicated freshwaters can cause various environmental problems. In particular, Microcystis producing hepatotoxic cyclic heptapeptides microcystins (MCs) has been globally observed. Recent studies have demonstrated that matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) offers a rapid classification of cyanobacteria; however, they have not fully considered the toxicity yet. In this study, we have performed MALDI-TOF MS for intact cyanobacterial cells using Biotyper software and optimized their conditions to achieve cyanobacterial classification with the toxicity. The detection mass range used for Biotyper was extended to cover small molecules, but their intense ions were suppressed as a function of the used instrument Autoflex Speed, which enabled simultaneous observations of large molecular fingerprints and small MCs with comparable ion intensity. Hierarchical clustering of mass spectra obtained under the optimized conditions differentiated toxic and non-toxic clusters of Microcystis strains and furthermore formed a tight cluster of non-toxic strains possessing the MC biosynthesis gene mcyG. Spectral libraries were expanded to >30 genera (>80 strains) under the default and optimized conditions to improve the confidence of cyanobacterial classification. Consequently, spectral library searching allowed for characterization of cyanobacteria from a field sample as mixed toxic and non-toxic Microcystis cells, without isolating those cells.

Non-toxic and toxic Microcystis aeruginosa reduce the tolerance of Daphnia pulex to low calcium in different degrees: Based on the changes in the key life-history traits

Calcium decline and cyanobacterial blooms pose a serious threat to the crustacean zooplankton Daphnia, which has a high demand for calcium. In the present study, we exposed two different clones of Daphnia pulex to different combinations of calcium concentrations (0.1, 0.5, 1.0, 5.0, 10.0 mg L−1) and food types (100% Chlorella; 80% Chlorella and 20% non-toxic Microcystis; 80% Chlorella and 20% toxic Microcystis) for 16 days, recorded the key life-history traits, and then used an exponential rise function to fit the traits. Results showed toxic Microcystis and low calcium together negatively affected the survival, development, and reproduction of Daphnia. The negative effect of non-toxic Microcystis and low calcium only affected the development and reproduction. The survival time and reproductive performance increased exponentially with increasing calcium concentration and then approached an asymptotic maximum. Both non-toxic and toxic Microcystis reduced the asymptotic maximum of the reproductive performance. The rising rate at which they reached the asymptotes differed significantly among the three food types; i.e., the reproductive performance of Daphnia was affected in a wider range of calcium concentrations under bad food quality. The findings indicated that Microcystis impaired the tolerance of Daphnia to low calcium, which may cause serious consequences in freshwater ecosystems.

Predator-prey interactions between the ciliate Blepharisma americanum and toxic (Microcystis spp.) and non-toxic (Chlorella vulgaris, Microcystis sp.) photosynthetic microbes

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 83:211-224 (2019) - DOI: https://doi.org/10.3354/ame01913 Predator-prey interactions between the ciliate Blepharisma americanum and toxic (Microcystis spp.) and non-toxic (Chlorella vulgaris, Microcystis sp.) photosynthetic microbes Ian J. Chapman1,2, Daniel J. Franklin1, Andrew D. Turner3, Eddie J. A. McCarthy1, Genoveva F. Esteban1,* 1Bournemouth University, Department of Life and Environmental Sciences, Faculty of Science and Technology, Dorset, BH12 5BB, UK 2NSW Shellfish Program, NSW Food Authority, Taree, NSW 2430, Australia 3Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Weymouth, Dorset, DT4 8UB, UK *Corresponding author: gesteban@bournemouth.ac.uk ABSTRACT: Despite free-living protozoa being a major factor in modifying aquatic autotrophic biomass, ciliate-cyanobacteria interactions and their functional ecological roles have been poorly described, especially with toxic cyanobacteria. Trophic relationships have been neglected and grazing experiments give contradictory evidence when toxic taxa such as Microcystis are involved. Here, 2 toxic Microcystis strains (containing microcystins), 1 non-toxic Microcystis strain and a non-toxic green alga, Chlorella vulgaris, were used to investigate predator-prey interactions with a phagotrophic ciliate, Blepharisma americanum. Flow cytometric analysis for microalgal measurements and a rapid ultra high performance liquid chromatography-tandem mass spectrometry protocol to quantify microcystins showed that non-toxic photosynthetic microbes were significantly grazed by B. americanum, which sustained ciliate populations. In contrast, despite constant ingestion of toxic Microcystis, rapid egestion of cells occurred. The lack of digestion resulted in no significant control of toxic cyanobacteria densities, a complete reduction in ciliate numbers, and no observable encystment or cannibalistic behaviour (gigantism). Individual B. americanum morphological responses (biovolume and cell width) showed a significant decrease over time when sustained on non-toxic Microcystis compared to grazed C. vulgaris populations, supporting previous studies that cyanobacteria may be a relatively poor source of nutrition. Results here provide insight into the ecological interactions of ciliates and cyanobacteria, and for the first time B. americanum is shown to have the capacity to suppress potentially bloom-forming cyanobacteria. However, grazing can be significantly altered by the presence of microcystins, which could have an impact on bloom dynamics and overall community structure. KEY WORDS: Ciliate-cyanobacteria interactions · Toxic cyanobacteria · Ciliate · Grazing · Microcystis · Microcystins Full text in pdf format PreviousNextCite this article as: Chapman IJ, Franklin DJ, Turner AD, McCarthy EJA, Esteban GF (2019) Predator-prey interactions between the ciliate Blepharisma americanum and toxic (Microcystis spp.) and non-toxic (Chlorella vulgaris, Microcystis sp.) photosynthetic microbes. Aquat Microb Ecol 83:211-224. https://doi.org/10.3354/ame01913 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 83, No. 3. Online publication date: September 19, 2019 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2019 Inter-Research.

Nitrogen limitation significantly reduces the competitive advantage of toxic Microcystis at high light conditions

Microcystis is a notorious cyanobacterial genus due to its rapid growth rate, huge biomass, and producing toxins in some eutrophic freshwater environments. To reveal the regulatory factors of interspecific competition between toxic and non-toxic Microcystis, three dominant Microcystis strains were selected, and their photosynthesis, population dynamics and microcystins (MCYST) production were measured. The results suggested that nitrogen-limitation (N-limitation) had a greater restriction for the growth of toxic Microcystis than that of non-toxic Microcystis, especially when cultured at high light or high temperature based on the weight analysis of key factors. Comparison of photosynthesis showed that low light or N-rich would favor the competitive advantage of toxic Microcystis while high light combined with N-limitation would promote the competitive advantage of non-toxic Microcystis, and these two competitive advantages could be further amplified by temperature increase. Mixed competitive experiments of toxic and non-toxic Microcystis were conducted, and the results of absorption spectrum (A485/A665) and qPCR (real-time quantitative PCR) suggested that the proportion of toxic Microcystis and the half-time of succession process were significantly reduced by 69.4% and 28.4% (p < 0.01) respectively by combining N-limitation with high light intensity than that measured under N-limitation condition. N-limitation led to a significant decrease of MCYST cellular quota in Microcystis biomass, which would be further decreased to a lower level by the high light. Based on above mentioned analysis, to decrease the MCYST production of Microcystis blooms, we should control nutrient, especial nitrogen through pollutant intercepting and increase the light intensity through improving water transparency.

Effects of high and low C:P foods on the feeding of Daphnia pulex

The effects of food quality and food quantity on crustacean zooplankton’s feeding rate have been analyzed in the present study. In two experiments, we studied the clearance rates (CRs) of Daphnia pulex on different algal diets (Scenedesmus obliquus, nontoxic Microcystis sp. PCC7806 and Anabeana flos-aquae) varying in carbon to phosphorus (C:P) ratios and quantities (concentrations). In order to investigate the effects of different algal diets on the feeding of cladocerans, the mixture of S. obliquus, Microcystis sp. PCC7806 and A. flos-aquae was also fed to D. pulex during a 72-h period. Our results showed: (1) D. pulex had different clearance rates from the three mono-specific P-rich algal diets, while no significant difference among clearance rates was observed among the three mono-specific P-poor algal diets; (2) D. pulex increased its clearance rates when it fed on mono-specific P-poor S. obliquus and A. flos-aquae, while a P-poor mixture of M. aeruginosa and A. flos-aquae reduced the clearance rate; (3) A U-shaped quadratic function provided a good fit to the clearance rate and C:P ratios because the clearance rate of D. pulex in both low and high C:P categories were significantly higher than in the moderate C:P category (C:P value = 219). Our results demonstrated that filaments and C:P ratios of algae strongly influence the feeding of crustacean zooplankton. ‘Compensatory feeding’ occurred in the mono-specific, P-poor algal diets but not in the mixed diets. Moreover, the clearance rate of zooplankton was the lowest in the moderate P stoichiometric category. Our experiments suggest that the morphology and C:P ratios of algal diets have important effects on the feeding of Daphnia, and the P stoichiometric characteristics in mixed diets may play an important role in the feeding of large-sized cladocerans.

Microcystin interferes with defense against high oxidative stress in harmful cyanobacteria

Harmful cyanobacteria producing toxic microcystins are a major concern in water quality management. In recent years, hydrogen peroxide (H2O2) has been successfully applied to suppress cyanobacterial blooms in lakes. Physiological studies, however, indicate that microcystin protects cyanobacteria against oxidative stress, suggesting that H2O2 addition might provide a selective advantage for microcystin-producing (toxic) strains. This study compares the response of a toxic Microcystis strain, its non-toxic mutant, and a naturally non-toxic Microcystis strain to H2O2 addition representative of lake treatments. All three strains initially ceased growth upon H2O2 addition. Contrary to expectation, the non-toxic strain and non-toxic mutant rapidly degraded the added H2O2 and subsequently recovered, whereas the toxic strain did not degrade H2O2 and did not recover. Experimental catalase addition enabled recovery of the toxic strain, demonstrating that rapid H2O2 degradation is indeed essential for cyanobacterial survival. Interestingly, prior to H2O2 addition, gene expression of a thioredoxin and peroxiredoxin was much lower in the toxic strain than in its non-toxic mutant. Thioredoxin and peroxiredoxin are both involved in H2O2 degradation, and microcystin may potentially suppress their activity. These results show that microcystin-producing strains are less prepared for high levels of oxidative stress, and are therefore hit harder by H2O2 addition than non-toxic strains.

Assessment of different mcy genes for detecting the toxic to non-toxic Microcystis ratio in the field by multiplex qPCR

Harmful cyanobacterial blooms, especially Microcystis blooms, occur worldwide and draw widespread attention. The dynamics of microcystin-producing Microcystis and competition between microcystin-producing Microcystis and non-microcystin-producing Microcystis are key to predicting and treating Microcystis blooms. Multiplex qPCR is a useful tool to assess such issues. In this study, we developed multiplex qPCR methods with newly-designed probes and primers for the microcystin-synthesis related genes mcyA and mcyE. We used seven toxic Microcystis strains and four non-toxic Microcystis strains to compare the differences in the ratios of toxic and non-toxic Microcystis in mixed cultures, which were calculated using abundances of the genes mcyA, mcyB, mcyD, mcyE and phycocyanin (PC). We also compared traditional cell counting and multiplex qPCR. Hierarchical clustering and principal component analysis indicated that mcyD was the most suitable mcy gene for quantification in laboratory experiments. mcyB abundances were always higher; we suggest that the amount of toxic Microcystis measured using mcyB might overestimate the actual percentages.

Biometric and physiological responses of Egeria densa Planch. cultivated with toxic and non-toxic strains of Microcystis

Cyanobacterial blooms are becoming increasingly common in aquatic environments around the world, mainly due to eutrophication and climate change. Cyanotoxin-producing strains (e.g., microcystins (MC) producers) may be present in these blooms, affecting the growth of other aquatic organisms, such as aquatic macrophytes. In this study, we evaluated the morphometric and physiological responses of the aquatic macrophyte Egeria densa to the exposure to a toxic strain of Microcystis aeruginosa (MCs producer) and a non-toxic Microcystis panniformis (non-MC producer). The effects of Microcystis strains on E. densa growth and biomass were verified for five weeks (Experiment 1) and physiological responses were evaluated for 14days (Experiment 2). Prolonged exposure of E. densa to the MC producing strain reduced growth, accompanied by the inhibition of shoot and root emission. Both Microcystis strains caused a decrease in the content of photosynthetic pigments, like total chlorophyll and chlorophyll a and b, accompanied by an increase of carotenoids. At the beginning of the MC-producing strain exposure, E. densa showed an increase in the activity of the anti-oxidative enzymes superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), accompanied by an increase in the levels of malondialdehyde, indicating lipid peroxidation. During the 14th day of exposure, the activity of antioxidant enzymes remained similar to the control, suggesting that E. densa has an efficient anti-oxidative system to control the reactive oxygen species produced in response to the stress caused by microcystins. However, when prolonged exposure occurred, possible damage to proteins may have affected the growth and development of E. densa. No changes were observed in the enzymatic activity of the plants exposed to the non-MC producing strain, suggesting that this cyanobacterial strain do not cause significant damage to the development of E. densa. These results are important for understanding the anti-oxidative defense mechanisms of aquatic macrophytes when coexisting with an MC producing strain.

Distinct Bloom Dynamics of Toxic and Non-toxic Microcystis (Cyanobacteria) Subpopulations in Hoedong Reservoir (Korea)

Despite the importance of understanding the bloom mechanisms that influence cyanobacterial toxin production, the dynamics of toxic Microcystis subpopulations are largely unknown. Here, we quantified both toxic and entire (i.e., toxic and non-toxic) Microcystis populations based on the microcystin synthetase E (mcyE) and 16S ribosomal RNA genes. Samples were collected from pelagic water and sediments twice per week from October to December 2011, and we investigated the effects of physicochemical factors (pH, water temperature, dissolved oxygen, nutrients, etc.) and biological factors (ciliates and zooplankton) on the abundance of toxic and non-toxic Microcystis. During the study period, Microcystis blooms were composed of toxic and non-toxic subpopulations. Resting stage Microcystis in sediment may be closely linked to Microcystis populations in pelagic water and may contribute to the toxic subpopulation composition in surface Microcystis blooms. In pelagic water, the toxic and entire Microcystis population had a significant positive correlation with the pH and water temperature (p<0.05). However, their responses to changes in environmental factors were thought to be distinct. The ratio of the toxic to non-toxic Microcystis subpopulations was significantly (p<0.05) enhanced by a lower pH and water temperature and an increase in protozoan grazers, reflecting environmental stresses. These results suggest that the toxic and non-toxic subpopulations of Microcystis have distinct tolerance levels against these stressors. The intracellular microcystin (MC) concentration was positively associated with the abundance of the mcyE-positive Microcystis. By comparison, the MC concentration in pelagic water body (extracellular) increased when Microcystis was lysed due to environmental stresses.

Selective Inhibition of Rice Straw Extract on Growth of Cyanobacteria and Chlorophyta

Rice straw is supposed to be an environment-friendly biomaterial for inhibiting the growth of harmful blooms of the cyanobacterium Microcystis aeruginosa. The effects of rice straw extract(RSE) on algal growth, morphologic parameters(cell size), and physiological parameters(in vivo Chl-a fluorescence) were investigated using flow cytometry. We examined the selective inhibitory potential of rice straw on four cyanobacterial strains(toxic and non-toxic Microcystis aeruginosa, toxic Anabaena flos-aquae, and Microcystis ichthyoblabe), in comparison with inhibitory effects on three common freshwater green algae(Selenastrum capricornutum, Chlorella pyrenoidosa, and Scenedesmus obliqnus). Concentrations from 2.0 to 10.0 g·L-1 of RSE were found to efficiently inhibit the growth of cyanobacteria in a dose-dependent manner, simultaneously modifying the in vivo Chl-a fluorescence and cell size. The 50% growth-inhibition concentration(7 d) of A. flos-aquae, M. ichthyoblabe, M. aeruginosa(toxic strain), M. aeruginosa(non-toxic strain) was 1.72, 2.21, 2.92 and 5.72 g·L-1, respectively. Interestingly, the growth and cell size of C. pyrenoidosa and S. obliqnus increased with the addition of RSE and colony formation was observed. In the case of S. capricornutum, the inhibitory effect of RSE on growth and in vivo Chl-a fluorescence occurred at 1.0-4.0 g·L-1, while RSE induced a stimulatory effect on algal growth at 8.0-10.0 g·L-1. Taken together, the sensitivity of cyanobacteria to RSE was significantly higher than that of S. capricornutum, C. pyrenoidosa and S. obliqnus. The higher sensitivity of PSⅡ reaction center of cyanobacteria and the ability to form colonies of green algae may have important implications for the species-specific allelopathic antialgal activity of rice straw.

Competition between toxic and nontoxic Microcystis (Cyanophyceae) under different light and temperature conditions

Competition between toxic and nontoxic Microcystis (Cyanophyceae) under different light and temperature conditions

Distribution of Toxic and Non-toxic Microcystis in Korean Water Supply

Distribution of Toxic and Non-toxic Microcystis in Korean Water Supply

Competition between a toxic and a non-toxic Microcystis strain under constant and pulsed nitrogen and phosphorus supply

The toxicity of a harmful algal bloom is strongly determined by the relative abundance of non-toxic and toxic genotypes and might therefore be regulated by competition for growth-limiting resources. Here, we studied how the toxic Microcystis aeruginosa strain PCC 7806 and a non-toxic mutant compete for nitrogen and phosphorus under constant and pulsed nutrient supply. Our monoculture and competition experiments show that these closely related genotypes have distinct nutrient physiologies and that they differ in their ability to compete for nitrogen and phosphorus. The toxic wild type won the competition under nitrogen limitation, while the non-toxic mutant dominated under phosphorus limitation. Pulses of both nitrogen and phosphorus increased the dominance of the toxic genotype, which lead to an even faster competitive exclusion of the non-toxic genotype under nitrogen pulses and to coexistence of both genotypes under phosphorus pulses. Our findings indicate that the genotype level dynamics driven by resource competition can be an important factor in determining cyanobacterial bloom toxicity.