Microcystis Blooms Research Articles

Towards understanding the role of extracellular polymeric substances in cyanobacterial Microcystis aggregation and mucilaginous bloom formation

The development of mucilaginous cyanobacterial Microcystis blooms is a serious environmental and ecological problem, and information on the bloom-formation mechanism has been lacking until now. The aggregation of microbial cells was attributed to the matrix of extracellular polymeric substances (EPS). In this study, the quantitative role of EPS matrix in Microcystis aggregation and mucilaginous bloom formation was investigated. The results showed that when EPS matrix was extracted, the aggregation abilities decreased by 27.6% and 57.4% for the lab-cultured Microcystis suspension and the field-sampled Microcystis aggregates, respectively. The extended DLVO theory revealed that EPS extraction increased the energy barrier and the values of the second energy minimum, which accounted for the deteriorated aggregation. Further analysis showed an increasing attraction energy of EPS matrix during the Microcystis bloom development, whereas the predominant contribution originated from tightly bound EPS (TB-EPS) and loosely bound EPS (LB-EPS) for the lab-cultured and field-sampled Microcystis samples. The heterogeneous energy contribution of EPS subfractions was found to be associated with the variations in organic contents. Specifically, Microcystis aggregates exhibited a higher organic content of TB-EPS than of LB-EPS compared with the lab-cultured Microcystis suspension, whereas organic content in only the LB-EPS fraction for the bloom samples was significantly higher (p<0.01) than that of the Microcystis aggregates. Based on these results, a conceptual model of EPS function was proposed in which TB-EPS plays an important role in the formation of Microcystis aggregates, after which LB-EPS contributed to the subsequent development from Microcystis aggregates to mucilaginous bloom formation.

Effects of linear alkylbenzene sulfonate on the growth and toxin production of Microcystis aeruginosa isolated from Lake Dianchi.

The exogenous organic pollutant linear alkylbenzene sulfonate (LAS) pollution and Microcystis bloom are two common phenomena in eutrophic lakes, but the effects of LAS alone on Microcystis remain unclear. In the present study, we investigated the effects of LAS on the growth, photochemical efficiency, and microcystin production of Microcystis aeruginosa in the laboratory. Results showed that low LAS (≤10mg/L) concentrations improved the growth of M. aeruginosa (12days of exposure). High LAS (20mg/L) concentrations inhibited the growth of M. aeruginosa on the first 8days of exposure; afterward, growth progressed. After 12days of exposure, the concentrations of chlorophyll a in algal cells were not significantly affected by any of LAS concentrations (0.05 to 20mg/L) in the present study; by contrast, carotenoid and protein concentrations were significantly inhibited when LAS concentrations reached as high as 20mg/L. After 6 and 12days of exposure, low LAS (≤10mg/L) concentrations enhanced the maximal photochemical efficiency (Fv/Fm) and the maximal electron transport rate (ETRmax) of M. aeruginosa. Furthermore, LAS increased the microcystin production of M. aeruginosa. Extracellular and intracellular microcystin contents were significantly increased after M. aeruginosa was exposed to high LAS concentrations. Our results indicated that LAS in eutrophic lakes may increase the risk of Microcystis bloom and microcystin production.

Response of Daphnia's antioxidant system to spatial heterogeneity in Cyanobacteria concentrations in a lowland reservoir.

Many species and clones of Daphnia inhabit ecosystems with permanent algal blooms, and they can develop tolerance to cyanobacterial toxins. In the current study, we examined the spatial differences in the response of Daphnia longispina to the toxic Microcystis aeruginosa in a lowland eutrophic dam reservoir between June (before blooms) and September (during blooms). The reservoir showed a distinct spatial pattern in cyanobacteria abundance resulting from the wind direction: the station closest to the dam was characterised by persistently high Microcystis biomass, whereas the upstream stations had a significantly lower biomass of Microcystis. Microcystin concentrations were closely correlated with the cyanobacteria abundance (r = 0.93). The density of daphniids did not differ among the stations. The main objective of this study was to investigate how the distribution of toxic Microcystis blooms affects the antioxidant system of Daphnia. We examined catalase (CAT) activity, the level of the low molecular weight antioxidant glutathione (GSH), glutathione S-transferase (GST) activity and oxidative stress parameters, such as lipid peroxidation (LPO). We found that the higher the abundance (and toxicity) of the cyanobacteria, the lower the values of the antioxidant parameters. The CAT activity and LPO level were always significantly lower at the station with the highest M. aeruginosa biomass, which indicated the low oxidative stress of D. longispina at the site with the potentially high toxic thread. However, the low concentration of GSH and the highest activity of GST indicated the occurrence of detoxification processes at this site. These results demonstrate that daphniids that have coexisted with a high biomass of toxic cyanobacteria have effective mechanisms that protect them against the toxic effects of microcystins. We also conclude that Daphnia's resistance capacity to Microcystis toxins may differ within an ecosystem, depending on the bloom's spatial distribution.

Photosynthetic adaptation mechanism of Microcystis (Cyanophyceae) related to changes of colony size in a eutrophic lake

:Microcystis blooms occur frequently in eutrophic lakes, and they are composed of many different-sized colonies. Many studies have addressed the competitive advantage of colony size, e.g. colony buoyancy regulation mechanism, low nutrient stress tolerance, low irradiance stress; however, studies are lacking for growth strategies of different-sized Microcystis colonies responding to photosynthetic regulation. This study compared photosynthetic activities and photosynthetic pigment composition of different-sized Microcystis. The results showed that the photosynthetic activity caused a V-shaped curve as colony size increased, i.e. medium-sized colonies (125–400 μm) had the lowest photosynthetic activity. The higher photosynthetic activity of small colonies (< 125 μm) was probably caused by the higher light-harvesting efficiency of phycobilisomes (PBS) and a relatively low nonphotochemical quenching (qN). The high photosynthesis of large colonies (> 400 μm) was attributed to the high efficiency of energy transition of the electron transport chains in photosystem II. Also, large colonies tolerated high irradiance stress because of their high qN, which was related to high cell quotas of PBS. All these results suggested that both small and large colonies had their own ecological adaptation strategies when they were suspended at different water layers, providing an explanation for how Microcystis survives adverse natural conditions, grows rapidly and forms serious surface blooms.

The fate of Microcystis aeruginosa cells during the ferric chloride coagulation and flocs storage processes

Microcystis blooms could cause severe problems for drinking water supplies with their associated microcystins (MCs). As the majority of MCs are retained inside the cells, the effective removal of the intact Microcystis cells to avoid the release of additional MCs plays an important role in drinking water treatment. This study evaluated the effect of ferric chloride (FeCl3) coagulation and the flocs storage process on the integrity of Microcystis aeruginosa cells and the intracellular MCs release (and possible degradation) in both processes. Multiple analysis techniques including scanning electron microscopy and chlorophyll fluorescence were used to assess the integrity of M. aeruginosa. In the coagulation process, the coagulant dosage and mechanical actions caused no cell damage, and all the cells remained intact. Furthermore, 100 mg/L FeCl3 was effective in removing the extracellular MCs. In the flocs storage process, a number of intracellular MCs were released into the supernatant, but the cells remained viable up to 10 d.

Vertical distribution of Microcystis colony size in Lake Taihu: Its role in algal blooms

This study investigated the vertical distributions of Microcystis cell density and colony size in Lake Taihu where algal blooms occur frequently. Measurements were made from April 2011 to January 2012 to gain a seasonal outlook on the role of such distributions in the blooms. It was found that large colonies tended to accumulate on the water surface, but the cell density fluctuated widely. The cell density in the water column increased continuously from spring to summer (i.e., April to October) and decreased after late autumn, showing apparent seasonal variations. The abrupt occurrence and disappearance of Microcystis blooms over short periods of time were not caused by the rapid growth of Microcystis but by the rise and accumulation of large Microcystis colonies at the water surface, both of which are affected by colony size. The ascent velocity of large colonies was higher than that of small ones, which enables large colonies to more readily overcome the stirring effects of water flows, waves and perturbations to rise to the surface. The results of canonical correspondence analysis (CCA) of Microcystis vertical distribution in relation to environmental factors suggested that nutrient concentrations and temperature were the main influencing factors related to bloom formation by Microcystis in Lake Taihu during our investigation.

Intra-habitat Differences in the Composition of the Methanogenic Archaeal Community between the Microcystis-Dominated and the Macrophyte-Dominated Bays in Taihu Lake

A regime shift between a macrophyte-dominated clear state and a phytoplankton-dominated turbid state can have considerable impact on ecosystem structure and function of shallow lakes. However, very little is known about the response of the methanogenic archaeal community in the sediment during this regime shift. We investigated the methanogenic archaeal community at two sites in the large, shallow, eutrophic Taihu Lake over the course of one year. One site is located in Meiliang Bay and is dominated by Microcystis blooms, and the other site is located in East Taihu Bay and is dominated by aquatic macrophytes. Terminal restriction fragment length polymorphism (T-RFLP) and phylogenetic analyses of archaeal 16S rRNA genes were used to analyze the methanogenic community. Higher ratio of methanogens in Archaea was observed in East Taihu Bay than in Meiliang Bay. The methanogenic archaeal community was dominated by the Methanobacteriales and the LDS cluster in macrophytes-dominated East Taihu Bay, while it was dominated by the Methanosarcinaceae, Methanobacteriales, and the LDS cluster in Microcystis-dominated Meiliang Bay. Clustering analysis of all of the samples revealed differences in the composition of the methanogenic archaeal communities between the two sites that were independent of seasonal variations. Further statistical analysis indicated that the chlorophyll a (Chla) concentration had a profound impact on the composition of the methanogenic archaeal community in Meiliang Bay, whereas it was primarily influenced by total organic carbon (TOC) levels in East Taihu Bay. Overall, this investigation demonstrates that intra-habitat differences in the composition of methanogenic archaeal communities are likely driven by changes in the available organic materials.

Connecting the blooms: tracking and establishing the origin of the record-breaking Lake Erie Microcystis bloom of 2011 using DGGE

Connecting the blooms: tracking and establishing the origin of the record-breaking Lake Erie Microcystis bloom of 2011 using DGGE

The influence of physical and physiological processes on the spatial heterogeneity of a Microcystis bloom in a stratified reservoir

A three-dimensional coupled hydrodynamic and ecological model, ELCOM–CAEDYM, was extended to include buoyancy control dynamics for cyanobacteria, and validated in the stratified Daecheong Reservoir (Korea). Specifically, the model was used to explore the physical and biological processes that determined the temporal and spatial variability of Microcystis aeruginosa (hereafter Microcystis) biomass during an abnormally intense mono-specific bloom event. Inclusion of the buoyancy control function within the coupled model considerably improved the model predictability by capturing the biomass accumulation at the surface during the bloom, and the shift of the dominant group from green algae to cyanobacteria. Results indicated that physical processes, particularly inflow mixing, played a dominant role in determining the spatial heterogeneity of Microcystis biomass through the local control of nutrient availability. In addition, the shallow mixed layer depth (zm) relative to the euphotic depth (zp) under a stable thermal stratification provided a perfect physical habitat for the dominance of this cyanobacteria relative to other species, due to their buoyancy control capability. This work demonstrates that the coupled hydrodynamic and ecological modeling has advanced to a stage where it may be used to interpret field data and subject to a suitable level of validation, the model may be used as a management decision support tool.

Physiology and molecular biology of aquatic cyanobacteria.

Cyanobacteria thrive in every illuminated aquatic environment known, contributing at least 25% of primary productivity worldwide. Given their importance in carbon and nutrient cycles, cyanobacteria are essential geochemical agents that have shaped the composition of the Earth's crust, oceans and atmosphere for billions of years. The high diversity of cyanobacteria is reflected in the panoply of unique physiological adaptations across the phylum, including different strategies to optimize light harvesting or sustain nitrogen fixation, but also different lifestyles like psychrotrophy, and oligotrophy. Some cyanobacteria produce secondary metabolites of cryptic function, many of which are toxic to eukaryotes. Consequently, bloom-forming toxic cyanobacteria are global hazards that are of increasing concern in surface waters affected by anthropogenic nutrient loads and climate change. While focusing on cyanobacteria in aquatic environments, the collection of papers herein touches on this broad range of topics. Regarding the role of cyanobacteria in the ocean, the importance of the unicellular picocyanobacteria is paramount. Indeed, Synechococcus and Prochlorococcus are the abundant genera in the oligotrophic open sea. Given that cyanobacteria are under regular assault imposed by photooxidative stresses, and that such stresses constrain photosynthetic performance, the paper by Mella-Flores et al. (2012) describes different strategies employed by model strains of Synechococcus and Prochlorococcus to high light and UV. Nonetheless, in the ocean, Synechococcus is composed of several clades that likely define particular ecotypes. Detection and enumeration of distinct Synechococcus clades is essential in understanding how these organisms co-exist with one another. To this end, Ahlgren and Rocap (2012) developed a suite of qPCR tools enabling a thorough analysis of Synechococcus community structure and dynamics. Nitrogen fixation by the unicellular Crocosphaera watsonii is now recognized to be a significant source of new N to marine environments. Two papers describe physiological properties of Crocosphaera relevant to their ecological importance. Sohm et al. (2011) demonstrate that a strain of C. watsonii produces high amounts of extracellular polysaccharides. The high level of organic carbon production suggests that this strain may directly support significant heterotrophy in the marine C cycle. Given that arsenate is a toxic anion that can interfere with the transport and metabolism of phosphate, Dyhrman and Haley (2011) report mechanisms of arsenate resistance in Crocosphaera, whose habitat includes P-limited ocean gyres. Such regions often have high arsenate:phosphate ratios, thus detoxification mechanisms are an important strategy for N fixation in these oligotrophic regions. The structure and physiological performance of arctic cyanobacterial mats is of particular interest, given their dominant contribution to biomass at high latitudes, and the stresses imposed by global warming on such fragile environments. Lionard et al. (2012) examine the composition of an artic microbial mat, and demonstrate the robust adaptation of the cyanobacteria to osmotic stress, a likely outcome of climate change. The characterization of chlorophyll-b containing Prochlorothrix spp. by Pinevich et al. (2012) indicate that despite the scarcity of this genus in aquatic environments, molecular methods suggest it is more widely distributed. The authors compare the physiological characteristics of the two known species and discuss the evolution of their light harvesting apparatus. Four papers explore the physiology of bloom-forming and toxic cyanobacteria. Kaplan et al. (2012) address the enigmatic function of the microcystins and cylindrospermopsins, describing experimental evidence for their role in cell-cell communication among bloom-formers. Sukenik et al. (2012) explore the spread of Aphanizomenon and Cylindrospermopsis to temperate freshwater environments. The appearance of such blooms is a likely consequence of global warming along with anthropogenic nutrient load, leading to a sustained invasion of into the middle latitudes. Given that management of phosphorus inputs is a strategy for limiting algal blooms in freshwaters, Saxton et al. (2012) describe the phosphorus quotas (intracellular and total) of the toxic colonial cyanobacterium, Microcystis aeruginosa. In response to increased phosphorus, Microcystis exhibited a higher cell P quota, yet a stable growth rate compared to lower phosphorus treatments. Such work has implications in modeling Microcystis blooms from P loadings into freshwaters. Gagnon and Pick (2012) contribute a study showing that the potent neurotoxin anatoxin-a is associated with nitrogen availability in Aphanizomenon. Understanding environmental cues for toxin production is essential in ultimately mitigating bloom toxigenicity. Together, these important contributions cover many of the issues tackled in the field today: the role of cyanobacteria in the global carbon and nutrient cycles, cyanobacterial evolution, the adaptation of cyanobacteria to climate change and the consequences of toxigenicity. The studies described in this collection provide an excellent introduction to these topics, while providing important clues regarding the future directions that studies of cyanobacteria will take.

Seasonal dynamics of water bloom-forming Microcystis morphospecies and the associated extracellular microcystin concentrations in large, shallow, eutrophic Dianchi Lake

The increasing occurrence of Microcystis blooms is of great concern to public health and ecosystem due to the potential hepatotoxic microcystins (MCs) produced by these colonial cyanobacteria. In order to interpret the relationships between variations of Microcystis morphospecies and extracellular MC concentrations, the seasonal dynamics of phytoplankton community composition, MC concentrations, and environmental parameters were monitored monthly from August, 2009 to July, 2010. The results indicated that Microcystis dominated total phytoplankton abundance from May to December (96%–99% of total biovolume), with toxic Microcystis viridis and non-toxic Microcystis wesenbergii dominating after July (constituting 65%–95% of the Microcystis population), followed by M. viridis as the sole dominant species from November to January (49%–93%). Correlation analysis revealed that water temperature and nutrient were the most important variables accounting for the occurrence of M. wesenbergii, while the dominance of M. viridis was related with nitrite and nitrate. The relatively low content of MCs was explained by the association with a large proportion of M. viridis and M. wesenbergii, small colony size of Microcystis populations, and low water temperature, pH and dissolved oxygen. The extracellular MC (mean of 0.5±0.2μg/L) of water samples analyzed by enzyme-linked immunosorbent assay (ELISA) demonstrated the low concentrations of MC in Dianchi Lake which implied the low potential risk for human health in the basin. The survey provides the first whole lake study of the occurrence and seasonal variability of Microcystis population and extracellular MCs that are of particular interest for water quality monitoring and management.

Solidago canadensis L. extracts to control algal (Microcystis) blooms in ponds

Landscape ponds often suffer from the overgrowth of nuisance algae (including cyanobacteria). The use of Solidago canadensis L. as an algicide in a natural water column was evaluated in this study. The results show that when S. canadensis L. extracts were added to the outdoor enclosures, Microcystis biomass decreased by more than 70% after 5d and more than 80% after 25d. The initial negative effects of the extracts on the water quality were not long-lasting. In addition, the extracts had lower toxicity to Daphnia magna and zebrafish than to Microcystis aeruginosa. In conclusion, S. canadensis extracts effectively controlled Microcystis growth in a natural water column, and it is reasonable to expect that the extracts have fewer adverse effects on the aquatic ecosystem. This study suggests that using this plant as an algicide to control Microcystis blooms in landscape ponds is feasible.

Growth inhibition of the cyanobacterium Microcystis aeruginosa and degradation of its microcystin toxins by the fungus Trichoderma citrinoviride

Harmful cyanobacterial blooms are recognized as a rapidly expanding global problem that threatens human and ecosystem health. Many bacterial strains have been reported as possible agents for inhibiting and controlling these blooms. However, such algicidal activity is largely unexplored for fungi. In this study, a fungal strain kkuf-0955, isolated from decayed cyanobacterial bloom was tested for its capability to inhibit phytoplankton species in batch cultures. The strain was identified as Trichoderma citrinoviride Based on its morphological characteristics and DNA sequence. Microcystis aeruginosa co-cultivated with living fungal mycelia rapidly decreased after one day of incubation, and all cells completely died and lysed after 2 days. The fungal filtrate of 5-day culture also exhibited an inhibitory effect on M. aeruginosa, and this inhibition increased with the amount of filtrate and incubation time. Conversely, green algae and diatoms have not been influenced by either living fungal mycelia or culture filtrate. Interestingly, the fungus was not only able to inhibit Microcystis growth but also degraded microcystin produced by this cyanobacterium. The toxins were completely degraded within 5 days of incubation with living fungal mycelia, but not significantly changed with fungal filtrate. This fungus could be a potential bioagent to selectively control Microcystis blooms and degrade microcystin toxins.

Nutrients drive transcriptional changes that maintain metabolic homeostasis but alter genome architecture in Microcystis.

The cyanobacterium Microcystis aeruginosa is a globally distributed bloom-forming organism that degrades freshwater systems around the world. Factors that drive its dispersion, diversification and success remain, however, poorly understood. To develop insight into cellular-level responses to nutrient drivers of eutrophication, RNA sequencing was coupled to a comprehensive metabolomics survey of M. aeruginosa sp. NIES 843 grown in various nutrient-reduced conditions. Transcriptomes were generated for cultures grown in nutrient-replete (with nitrate as the nitrogen (N) source), nitrogen-reduced (with nitrate, urea or ammonium acting as the N sources) and phosphate-reduced conditions. Extensive expression differences (up to 696 genes for urea-grown cells) relative to the control treatment were observed, demonstrating that the chemical variant of nitrogen available to cells affected transcriptional activity. Of particular note, a high number of transposase genes (up to 81) were significantly and reproducibly up-regulated relative to the control when grown on urea. Conversely, phosphorus (P) reduction resulted in a significant cessation in transcription of transposase genes, indicating that variation in nutrient chemistry may influence transcription of transposases and may impact the highly mosaic genomic architecture of M. aeruginosa. Corresponding metabolomes showed comparably few differences between treatments, suggesting broad changes to gene transcription are required to maintain metabolic homeostasis under nutrient reduction. The combined observations provide novel and extensive insight into the complex cellular interactions that take place in this important bloom-forming organism during variable nutrient conditions and highlight a potential unknown molecular mechanism that may drive Microcystis blooms and evolution.

Systemic, early-season Microcystis blooms in western Lake Erie and two of its major agricultural tributaries (Maumee and Sandusky rivers)

Recurrent, massive cyanobacterial blooms composed mainly of the genus Microcystis indicate a broad-scale re-eutrophication of Lake Erie. In the past, ameliorating eutrophication relied on intentionally decreasing point-source tributary nutrient, especially phosphorus, loads to the lake. However, recent research has shown that tributaries load not only nutrients but also bloom-levels of phytoplankton, including Microcystis. We built on this previous work by sampling earlier in the year and in much smaller tributaries in both the Maumee and Sandusky systems. We found Microcystis wet biomasses in these tributaries averaged 3.16mg/L (±0.59mg/L, one standard error of the mean) in 2009 and 3.42mg/L (±0.55mg/L) in 2010. Importantly, we found Microcystis in small ditches in March, much earlier than previously observed. Microcystis biomass did not directly correspond to measured phosphorus, chlorophyll, or phycocyanin concentrations likely reflecting complexities associated with lagged physiological responses and/or non-linear growth relationships. Consequently, our findings emphasize that Microcystis blooms form a more broad-scale problem than previously documented, occurring far upstream much earlier in the year.

The responses of the taxa composition of particle-attached bacterial community to the decomposition of Microcystis blooms

The changes of taxa within the particle-attached bacterial assemblage during the decomposition of Microcystis blooms were investigated under darkness and anoxic condition in mesocosm experiments. During 14days of darkness incubation, chlorophyll-a (Ch-a) concentration decreased from 2000μg/L to 5μg/L. Samples were collected on days 0, 2 and 14 for bacterial 16S rRNA analysis, based on rapid decreases in the Chl-a concentration of water column. The total bacterial community DNA was extracted and 16S rRNA genes were amplified by polymerase chain reaction, cloned and sequenced of selected samples. The results showed that the abundance of attached bacteria increased significantly, and the composition of the particle-attached bacterial communities varied temporally during the decomposition of Microcystis blooms. The bacterial assemblage appeared to be dominated by members of Bacteroidetes, Alphaproteobacteria and Betaproteobacteria. Shift of some genera of Alphaproteobacteria and Alphaproteobacteria was also observed. Additionally, we found that the family Sphingomonas, affiliated with Alphaproteobacteria, identified as a microcystin-degrading bacterium, dominated the particle-attached bacterial communities. The results from the present study, together with previously published data highlighted the need for more studies concerning the bacterial degradation process in order to trace the environmental fate of microcystins in field conditions.

Characterization of the Microcystis Bloom and Its Nitrogen Supply in San Francisco Estuary Using Stable Isotopes

A suite of particulate and dissolved organic and inorganic stable isotopes were needed to determine the source of the nutrients and cells that initiate and sustain the toxic cyanobacteria bloom of Microcystis in San Francisco Estuary. Particulate and dissolved inorganic and organic matter in water and plankton samples were collected biweekly during Microcystis blooms in 2007 and 2008. Stable isotopes for particulate and dissolved organic matter, nitrate, and water (POM-δ 13C, POM-δ 15N, DOC-δ 13C, C/N ratio, NO3-δ 15N, NO3-δ 18O, H2O-δ 18O and H2O-δ 2H) were compared with Microcystis cell abundance, dissolved organic carbon, chlorophyll a, and toxic total microcystins concentration, as well as physical and chemical water quality variables, including streamflow. The isotopic composition of particulate organic matter, nitrate, and water differed for the Sacramento and San Joaquin Rivers and varied along the salinity gradient. The variation of particulate organic matter and water isotopes suggested Microcystis primarily entered the estuary from the San Joaquin and Old Rivers, where it was most abundant. Nitrate isotopes along with streamflow variables indicated that the San Joaquin River was a source of nitrate to the estuary. However, stable isotope comparison of the nitrogen in Microcystis cells with the dissolved inorganic nitrate in the San Joaquin River indicated that nitrate was not the primary source of nitrogen that supported the bloom. Instead, ammonium from the Sacramento River was the likely sole source of the nitrogen for most of the bloom. Selective uptake of ammonium may have further contributed to the magnitude of the Microcystis bloom which increased with the percent of ammonium within the total dissolved inorganic nitrogen pool.

Variation of Microcystis and microcystins coupling nitrogen and phosphorus nutrients in Lake Erhai, a drinking-water source in Southwest Plateau, China

Lake Erhai is the second largest lake of Southwest China and an important drinking water source. The lake is currently defined as the preliminary stage of eutrophic states, but facing a serious threat with transfer into intensive eutrophication. The present study examined the dynamics of Microcystis blooms and toxic Microcystis in Lake Erhai during 2010, based on quantitative real-time PCR method using 16S rRNA gene specific for Microcystis and microcystin systhesis gene (mcy), and chemical analysis on microcystin (MC) concentrations. Total Microcystis cell abundance at 16 sampling sites were shown as an average of 1.7 × 10(7) cells l(-1) (1.3 × 10(2)-3.8 × 10(9) cells l(-1)). Microcystin LR (MC-LR) and microcystin RR (MC-RR) were the main variants. The strong southwesterly winds, anticlockwise circular flows and geographical characteristics of lake and phytoplankton community succession impacted the distribution patterns of Chl a and MC in the lake. The concentration of Chl a and MC and abundances of total Microsytis and MC-producing Microsystis (MCM) were shown to be positively correlated with pH, DO and TP, negatively correlated with SD, NO₃-N, TN/Chl a and TN/TP, and not correlated with NH₄-N, TN, dissolved total nitrogen (DTN) and water temperatures. When TN/TP decrease, Microcystis tended to dominate and MC concentrations tended to increase, suggesting that the "TN/TP rule" can be partially applied to explain the correlation between the cyanobacterial blooms and nutrients N and P only within a certain nutrient level. It is speculated that N and P nutrients and the associated genes (e.g., mcy) may jointly drive MC concentration and toxigenicity of Microcystis in Lake Erhai.

Fate and Persistence of Particulate and Dissolved Microcystin-LA from Microcystis Blooms

ABSTRACTFew studies have estimated fate and persistence of the hepatotoxic microcystins (MCs) in situ, making ecological and human health risk assessments challenging. We determined fate and persistence of MC congeners during 2 years of Microcystis blooms in a small, shallow, closed-basin lake in Ontario, Canada. In situ half-lives were compared to estimates obtained in vitro under controlled temperature and light. The blooms produced elevated microcystin-LA (MC-LA) (maximum ∼4.2 mg L−1) with minor concentrations of MC-LR, -RR, and -YR. Dissolved MC-LA declined more slowly and persisted longer than particulate MC-LA with in situ half-lives (total 1.5–8.5 days) shorter than in vitro (total 6.8–60.0 days). Half-lives in 2010 were two to eight times shorter compared to 2009, likely due to differences in bloom phenology and species/strain composition. In vitro, higher temperature (4°C → 25°C in dark), and irradiance (dark → 45 → 260 μE m−2s−1 at 25°C) accelerated particulate and dissolved MC-LA decline, respectively. MC-RR accumulated in surface sediments while MC-LA was near detection despite elevated surface water concentrations. MC-LA appears to persist longer in surface waters than the equally toxic MC-LR, requiring almost the entire recreational season (9.5 weeks) to reach guideline concentrations (20 μg L−1).

Temporal evolution and vertical stratification of Microcystis toxic potential during a first bloom event

In order to study the setup of a Microcystis bloom and the evolution of its toxic potential, we studied the temporal and vertical variations in Microcystis aeruginosa abundance, microcystins (MC) concentrations (intracellular and extracellular), and the relative proportion of potentially microcystin-producing cells (MC-producing cells) in relation to physicochemical parameters in the recently setup Moroccan reservoir “Yaacoub Al Mansour.” The Microcystis bloom appeared relatively late in the season and was associated with a low proportion of MC-producing cells in the water surface layer, probably related to non-limiting nutrient concentrations. Interestingly, the setup of the bloom leads to a vertical gradient, showing a decrease in Microcystis cell abundance inversely coupled with an increase in the proportion of MC-producing cells. Thus, this can be the result of the growth where non-MC-producing cells remain in the lighted water layer easier than MC-producing ones. Nevertheless, parameters other than light intensity may influence the toxic potential of bloom as no vertical pattern was observed concerning microcystins cellular quotas. The high microcystins concentrations measured in the deep water layer have also proved the importance of considering the deep part of aquatic ecosystem in the management of health risks associated with cyanobacterial proliferations.