Dark accelerates dissolved inorganic phosphorus release of high-density cyanobacteria.

Bloom-forming cyanobacteria dramatically influence nutrient cycling in eutrophic freshwater lakes. The phosphorus (P) assimilation and release of bloom-forming cyanobacteria significantly may also affect the phosphorus source and amounts in water. To understand the phosphorus release process of bloom-forming cyanobacteria below the accumulated surface and sedimentary bloom-forming cyanobacteria, the degradation of bloom-forming cyanobacteria dominated by Microcystis spp. at different cell density in the dark was investigated over a 25-day microcosm experiment. The dissolved inorganic phosphorus (DIP) and dissolved total phosphorus (DTP) contents increased with the increment of cyanobacterial density, and the dark status markedly increased the proportion of DIP in water during the decline period of bloom-forming cyanobacteria. Meanwhile, the process of cyanobacterial apoptosis accompanied by the changes of malondialdehyde (MDA) and phosphatase (AKP) contents, and the increases of superoxide dismutase (SOD) and catalase (CAT) activities of cyanobacteria in the dark, especially in low-density groups (5.23×108 cells L-1), which further affect the physicochemical water parameters. Moreover, the DIP release from high-density cyanobacteria (7.86×107 cells L-1~5.23×108 cells L-1) resulted from the relative abundance of organophosphorus degrading bacteria in the dark. Therefore, the fast decay of cyanobacteria in the dark could accelerate DIP release, the high DIP release amount from accumulated bloom-cyanobacteria provide adequate P quickly for the sustained growth of cyanobacteria.

Microcystis aeruginosa inactivation and microcystin-LR degradation by the photo-Fenton process at the initial near-neutral pH.

Of all cyanobacteria, Microcystis aeruginosa is the most commonly found species in bloom episodes all over the world. This species is known to produce cyanopeptides with hepatotoxic effects, namely microcystins (MCs). In this regard, Advanced Oxidation Processes (AOPs) have been widely studied for cyanotoxin degradation, but very few studies focused on cyanobacteria inactivation combined with toxin removal. To our knowledge, this is the first report of the photo-Fenton process application focusing on M. aeruginosa inactivation and microcystin-LR (MC-LR) degradation. This research work aimed to evaluate the photo-Fenton process under three different conditions with regard to Fe2+/H2O2 ratios (0.6/10, 5/50, and 20/100 mg L-1) at the initial near-neutral pH. Process efficiency was measured by immediate cell density reduction, growth inhibition, effect on MC-LR concentrations, and scanning electron microscopy (SEM) to analyze any alterations in cell morphology. Growth inhibition test (GIT) results pointed to cell inactivation under all conditions tested, and MC-LR concentrations were reduced below WHO's maximum limit at medium and higher concentrations of reagents. The possible mechanisms of cell inactivation by oxidative species are discussed.

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.

Antibiotics induced alterations in cell density, photosynthesis, microcystin synthesis and proteomic expression of Microcystis aeruginosa during CuSO4 treatment.

Antibiotic contaminants have the potential to interfere with the control of cyanobacterial bloom through generating hormesis in cyanobacteria at current contamination level of ng L-1. This study investigated the influence of a mixture of four frequently detected antibiotics, amoxicillin, ciprofloxacin, sulfamethoxazole and tetracycline, during the treatment of Microcystis aeruginosa by copper sulfate (CuSO4) algaecide. CuSO4 significantly (p <  0.05) inhibited cell density, growth rate, Fv/Fm value, chlorophyll a content and microcystin production ability of M. aeruginosa in a dose-dependent manner at application doses of 0.01-0.05 mg L-1. Besides, CuSO4 inhibited oxidation-reduction process, photosynthesis and biosynthesis in M. aeruginosa at the proteomic level. Preventative application of CuSO4 to a low density (4 × 105 cells mL-1) of M. aeruginosa effectively prevented the formation of bloom at low CuSO4 doses, which is a possible route for eliminating the negative effects of CuSO4 algaecide in aquatic environments. The presence of mixed antibiotics alleviated the toxicity of CuSO4 in M. aeruginosa, through the downregulation of cation transport proteins and the upregulation of proteins related with chlorophyll a synthesis, photosynthesis, gene expression and oxidation-reduction. Mixed antibiotics also promoted microcystin synthesis in CuSO4 treated cells through the upregulation of microcystin synthetases. Mixed antibiotics significantly (p <  0.05) increased cell density, growth rate, Fv/Fm value, chlorophyll a content and microcystin production ability in CuSO4 treated cells at test concentrations of 80 and 200 ng L-1. A no-impact threshold of 20 ng L-1 for mixed antibiotics (5 ng L-1 for each antibiotic) was suggested for eliminating the interference of antibiotic contaminants on cyanobacterial bloom control.

Characterization of Cyanophages in Lake Erie: Interaction Mechanisms and Structural Damage of Toxic Cyanobacteria.

Cyanophages are abundant in aquatic environments and play a critical role in bloom dynamics, including regulation of cyanobacteria growth and photosynthesis. In this study, cyanophages from western Lake Erie water samples were screened for lytic activities against the host cell (Microcystis aeruginosa), which was also originated from Lake Erie and identified with real-time sequencing (Nanopore sequencing). M. aeruginosa was mixed with the cyanophages and their dynamic interactions were examined over two weeks using atomic force microscopy (AFM) as well as transmission electron microscopy (TEM), qPCR, phycocyanin and chlorophyll-a production, and optical absorbance measurements. The TEM images revealed a short-tailed virus (Podoviridae) in 300 nm size with unique capsid, knob-like proteins. The psbA gene and one knob-like protein gene, gp58, were identified by PCR. The AFM showed a reduction of mechanical stiffness in the host cell membranes over time after infection, before structural damage became visible. Significant inhibition of the host growth and photosynthesis was observed from the measurements of phycocyanin and chlorophyll-a concentrations. The results provide an insight into cyanobacteria-cyanophage interactions in bloom dynamics and a potential application of cyanophages for bloom control in specific situations.

Effect of light on the transformation of BDE-47 by living and autoclaved cultures of Microcystis flos-aquae and Chlorella vulgaris.

Polybrominated diphenyl ethers (PBDEs) are ubiquitous and toxic contaminants found in high concentrations in watercourses, and are not well removed by conventional wastewater treatment facilities. This study aimed to evaluate the removal and transformation of BDE-47, one of the environmentally predominant PBDE congener, by a green alga (Chlorella vulgaris) and a cyanobacterium (Microcystis flos-aquae) under different light conditions. Living and autoclaved cultures were exposed to BDE-47 at a concentration of 10 µg L-1 for 7 days. Both species removed >90% of BDE-47 very shortly after spiking. Light intensity affected the transformation of BDE-47 in living cultures of both species, since 5 to 11 times more debromination products were measured at a light intensity of 100 µmol photons m-2 s-1 than at 20 µmol photons m-2 s-1. Living cultures of M. flos-aquae transformed BDE-47 at a rate of 0.22 day-1 while no transformation was observed in the respective autoclaved cultures. On the contrary, both living and autoclaved cultures of C. vulgaris had similar BDE-47 transformation rates of 0.05-0.06 day-1. Debromination of BDE-47 was a predominant transformation pathway in cultures of C. vulgaris, with two times higher BDE-28 concentrations measured than in M. flos-aquae, while hydroxylation was more dominant with the cyanobacterium. Most BDE-47 and its debromination product BDE-28 were found on the cell surface of both species. These results reveal that different transformation mechanisms were involved in C. vulgaris and M. flos-aquae cultures and confirm the importance of species selection for the removal of PBDEs from contaminated environments.

Comparison and optimization of different methods for Microcystis aeruginosa's harvesting and the role of zeta potential on its efficiency.

This study has compared the harvesting efficiency of four flocculation methods, namely, induced by pH, FeCl3, AlCl3 and chitosan. No changes were observed on M. aeruginosa cells. Flocculation assays performed at pH 3 and 4 have shown the best harvesting efficiency among the pH-induced tests, reaching values above 90% after 8 h. The adjustment of zeta potential (ZP) to values comprised between - 6.7 and - 20.7 mV enhanced significantly the settling rates using flocculant agents, being FeCl3 the best example where increments up to 88% of harvesting efficiency were obtained. Although all the four methods tested have presented harvesting efficiencies above 91% within the first 8 h after the optimization process, the highest performance was obtained using 3.75 mg L-1 of FeCl3, which allowed reaching 92% in 4 h.

Effects of Phosphorus on Interspecific Competition between two cell-size Cyanobacteria: Synechococcus sp. and Microcystis aeruginosa.

Pico-cyanobacteria and micro-cyanobacteria coexist ubiquitously in many lakes. Differences in cell size and abilities to utilize nutrients may influence their distribution patterns. In this study, Synechococcus sp. and Microcystis aeruginosa were chosen as pico- and micro-cyanobacteria, respectively. Gradient phosphorus treatments (0.002, 0.01, 0.05, and 0.25 mg P L-1) were designed in mono- and co-cultures. Growth curves were recorded and fitted by the Monod equation. Moreover, the interspecific competition was analyzed by the Lotka-Volterra model. When mono-cultured in lower P conditions (≤ 0.01 mg P L-1), Synechococcus sp. obtained much higher biomass than M. aeruginosa. But, M. aeruginosa grew faster than Synechococcus sp. in higher P groups (≥ 0.05 mg P L-1) (p < 0.05). Synechococcus sp. has abilities to thrive in low-phosphorus environments, whereas M. aeruginosa favored high-phosphorus conditions. In co-cultures, Synechococcus sp. strongly inhibited M. aeruginosa at each P treatment.

A comparison of trichloromethane formation from two algae species during two pre-oxidation-coagulation-chlorination processes.

Microcystis aeruginosa (M. aeruginosa) commonly blooms in summer while Cyclotella meneghiniana (C. meneghiniana) outbreaks in fall in water reservoirs of Southeast China. Pre-oxidation has been demonstrated to enhance the algae removal from chemical coagulation processes. However, excessive dosage of pre-oxidant can increase the disinfection by-products formation potential (DBPsFP). Additionally, the DBPs formation mechanisms from algae during the pre-oxidation-coagulation-chlorination processes have not well elucidated. In this study, the objectives were to investigate the trichloromethane (TCM) formation, the changes of water quality indexes, and the morphology changes of algal cells from M. aeruginosa or C. meneghiniana contaminated water during potassium permanganate (KMnO4) or chlorine (Cl2) pre-oxidation-coagulation-chlorination disinfection. The results showed that the TCM yield for two algal species decreased with the dosage increase of KMnO4 pre-oxidation, but increased with the dosage increase of pre-chlorination. Therefore, the 2.0 mg/L KMnO4 or 0.5 mg/L Cl2 was proposed as the optimal dosage for preventing both M. aeruginosa blooms in summer and for C. meneghiniana outbreaks in fall. M. aeruginosa exhibited a slightly higher TCM yield than C. meneghiniana in these treatment processes. Based on the release of potassium (K) ion and SEM analysis, KMnO4 had less damage on cell integrity than Cl2 at the dosage ≤2.0 mg/L. In addition, C. meneghiniana was easier to be disrupted by both pre-oxidants than M. aeruginosa, combining with subsequent coagulation led to different value of dissolved organic carbon (DOC), UV-visible absorbance (UV254) and turbidity.

Sono-Fenton hybrid process on the inactivation of Microcystis aeruginosa: Extracellular and intracellular oxidation.

For the first time, the inactivation of Microcystis aeruginosa using sono-Fenton process at low frequency high intensity (20 kHz, 0.42 W/mL) and high frequency low intensity (800 kHz, 0.07 W/mL) was investigated, respectively. 20 kHz sono-Fenton treatment successfully reduced cyanobacterial cell number from 4.19 × 106 cells/mL to 0.45 × 106 cells/mL within 5 min treatment. Alternatively, efficient performance of 800 kHz sono-Fenton process was observed to decrease Microcystis cell number to 2.33 × 106 cells/mL after 5 min inactivation, with lower energy cost. It was found that powerful 20 kHz sonication induced pore formation on the cell wall, leading to extracellular damage, while 800 kHz irradiation with low intensity triggered intracellular uptake of chemicals, suggesting endocytosis effects. Furthermore, sono-Fenton Processes were found to be affected by the concentrations of Fenton's reagent, and pre-sonication time. Although solo Fenton treatment released microcystins in water, the degradation of microcystin-LR were achieved using 20 and 800 kHz sono-Fenton processes, respectively. The results of this work showed that severe extracellular oxidation is the vital inactivation mechanism of 20 kHz sono-Fenton process, while the internal oxidation caused by intracellularly delivered Fenton reagents is suggested to be the main cause of 800 kHz sono-Fenton inactivation, leading to much lower energy cost. This work provides alternative methods to control harmful cyanobacteria in water towards effective treatment.

Mesohaline conditions represent the threshold for oxidative stress, cell death and toxin release in the cyanobacterium Microcystis aeruginosa.

As aquatic ecosystems become increasingly affected by hydrologic alterations, drought and sea level rise a need exists to better understand the biological effects of elevated salinity on toxigenic cyanobacteria such as Microcystis aeruginosa. This study investigated the impacts of oligohaline/low mesohaline conditions and exposure time on selected physiological and biochemical responses in M. aeruginosa including cell viability, oxidative stress, antioxidant responses, in addition to microcystin synthesis and release into the surrounding environment. M. aeruginosa was able to grow in most test salinity treatments (1.4-10 ppt), as supported by cell abundance data and chlorophyll-a (chl-a) concentrations. Physiological data showed that after certain salinity thresholds (∼7ppt) were surpassed, salt stress had cascading effects, such as increased ROS production and lipid peroxidation, potentiating the decline in cellular viability. Furthermore, elevated salinity induced oxidative stress which was concomitant with a decrease in cell abundance, chl-a concentration and photochemical efficiency in the 7-10 ppt treatments. M. aeruginosa did not synthesize microcystins (MCs) in response to increased saline conditions, and mcy-D expression was not correlated with either salinity treatment or extracellular MC concentrations, indicating that salinity stress could inhibit toxin production and that released toxins were likely synthesized prior to exposure. Additionally, extracellular MC concentrations were not correlated with decreased cellular integrity, as evidenced by SYTOX analyses, suggesting that toxins may be released through mechanisms other than cellular lysis. Results from this study support that M. aeruginosa can survive with limited negative impacts to cellular structure and function up to a certain threshold between 7-10 ppt. However, after these thresholds are surpassed, there is radical decline in cell health and viability leading to toxin release. This work underscores the importance of understanding the balance between ROS production and antioxidant capacities when assessing the fate of M. aeruginosa under mesohaline conditions.

Preferential binding between intracellular organic matters and Al13 polymer to enhance coagulation performance.

Coagulation is the best available method for removing intracellular organic matter (IOM), which is released from algae cells and is an important precursor to disinfection by-products in drinking water treatment. To gain insight into the best strategy to optimize IOM removal, the coagulation performance of two Al salts, i.e., aluminum chloride (AlCl3) and polyaluminum chloride (PACl, containing 81.2% Al13), was investigated to illuminate the effect of Al species distribution on IOM removal. PACl showed better removal efficiency than AlCl3 with regard to the removal of turbidity and dissolved organic carbon (DOC), owing to the higher charge neutralization effect and greater stability of pre-formed Al13 species. High pressure size exclusion chromatography analysis indicated that the superiority of PACl in DOC removal could be ascribed to the higher binding affinity between Al13 polymer and the low and medium molecular weight (MW) fractions of IOM. The results of differential log-transformed absorbance at 254 and 350 nm indicated more significant formation of complexes between AlCl3 and IOM, which benefits the removal of tryptophan-like proteins thereafter. Additionally, PACl showed more significant superiority compared to AlCl3 in the removal of <5 kDa and hydrophilic fractions, which are widely viewed as the most difficult to remove by coagulation. This study provides insight into the interactions between Al species and IOM, and advances the optimization of coagulation for the removal of IOM in eutrophic water.

Growth suppression and apoptosis-like cell death in Microcystis aeruginosa by H2O2: A new insight into extracellular and intracellular damage pathways.

H2O2 has been suggested and applied as effective algaecide for harmful cyanobacterial bloom control, however, the transport of exogenous H2O2 into microalgal cells, the subsequent intracellular damage pathway and dose-response variations were little studied. We addressed these questions in a bloom-forming cyanobacterium Microcystis aeruginosa with H2O2 at 0.1-1.5 mM. The results showed that H2O2 at 0.4 mM and above significantly suppressed M. aeruginosa growth for over two weeks, and induced apoptosis-like death in terms of membrane potential dissipation, caspase-3 activation, chromatin condensation, and lysis induction. However, the dose-response effects were not monotonic. H2O2 at 0.7 mM resulted in the severest growth suppression among 0.1-1.5 mM treatments, including the lowest biomass for 74% loss, the highest cell lysis ratio for 79%, and the highest utilization rate of H2O2 for 0.101 mM d-1. Moreover, several evidence point to severer apoptosis-like cell death in 0.7 mM treatments, involving fastest and severest cell lysis, smallest cell size and wrinkled surface and lowest membrane potential. Therefore, the apoptosis-like cell death induced by H2O2 at moderate dosages should be a crucial cause for the non-monotonic dose-response effects on growth suppression. Additionally, intracellular H2O2 level increased rapidly within 20 min after exposure at 0.4 mM and above, directly confirming the transport of exogenous H2O2 into M. aeruginosa cells and the intracellular damages due to subsequent elevation in intracellular oxidative stress. The study demonstrates that H2O2 at moderate dosages could be a promising method for the biomass control, in a fast and efficient way, on M. aeruginosa blooms.

Quantification of cyanobacterial cells via a novel imaging-driven technique with an integrated fluorescence signature.

A novel imaging-driven technique with an integrated fluorescence signature to enable automated enumeration of two species of cyanobacteria and an alga of somewhat similar morphology to one of the cyanobacteria is presented to demonstrate proof-of-concept that high accuracy, imaging-based, rapid water quality analysis can be with conventional equipment available in typical water quality laboratories-this is not currently available. The results presented herein demonstrate that the developed method identifies and enumerates cyanobacterial cells at a level equivalent to or better than that achieved using standard manual microscopic enumeration techniques, but in less time, and requiring significantly fewer resources. When compared with indirect measurement methods, the proposed method provides better accuracy at both low and high cell concentrations. It extends the detection range for cell enumeration while maintaining accuracy and increasing enumeration speed. The developed method not only accurately estimates cell concentrations, but it also reliably distinguishes between cells of Anabaena flos-aquae, Microcystis aeruginosa, and Ankistrodesmus in mixed cultures by taking advantage of additional contrast between the target cell and complex background gained under fluorescent light. Thus, the proposed image-driven approach offers promise as a robust and cost-effective tool for identifying and enumerating microscopic cells based on their unique morphological features.

Adsorptive removal of harmful algal species Microcystis aeruginosa directly from aqueous solution using polyethylenimine coated polysulfone-biomass composite fiber.

In recent times, the treatment of harmful algal blooms (HABs) became an important environmental issue to preserve and remediate water resources globally. In the present study, the adsorptive removal of harmful algal species Microcystis aeruginosa directly from an aqueous medium was attempted. Waste biomass (Escherichia coli) was immobilized using polysulfone and coated using the cationic polymer polyethylenimine (PEI) to generate PEI-coated polysulfone-biomass composite fiber (PEI-PSBF). The density of M. aeruginosa in an aqueous medium (BG11) was significantly decreased by treatment with PEI-PSBF. additionally, analysis using FE-SEM, confirmed that the removal of M. aeruginosa algal cells by PEI-PSBF was caused by the adsorption mechanism. According to the profiles of phosphorus for the algal cell growth in M. aeruginosa cultivating samples, we found that the adsorbed M. aeruginosa onto the PEI-PSBF lost their biological activity compared to the non-treated M. aeruginosa cells.

Improved biovolume estimation of Microcystis aeruginosa colonies: A statistical approach.

The Microcystis aeruginosa complex (MAC) clusters many of the most common freshwater and brackish bloom-forming cyanobacteria. In monitoring protocols, biovolume estimation is a common approach to determine MAC colonies biomass and useful for prediction purposes. Biovolume (µm3 mL-1) is calculated multiplying organism abundance (orgL-1) by colonial volume (µm3org-1). Colonial volume is estimated based on geometric shapes and requires accurate measurements of dimensions using optical microscopy. A trade-off between easy-to-measure but low-accuracy simple shapes (e.g. sphere) and time costly but high-accuracy complex shapes (e.g. ellipsoid) volume estimation is posed. Overestimations effects in ecological studies and management decisions associated to harmful blooms are significant due to the large sizes of MAC colonies. In this work, we aimed to increase the precision of MAC biovolume estimations by developing a statistical model based on two easy-to-measure dimensions. We analyzed field data from a wide environmental gradient (800 km) spanning freshwater to estuarine and seawater. We measured length, width and depth from ca. 5700 colonies under an inverted microscope and estimated colonial volume using three different recommended geometrical shapes (sphere, prolate spheroid and ellipsoid). Because of the non-spherical shape of MAC the ellipsoid resulted in the most accurate approximation, whereas the sphere overestimated colonial volume (3-80) especially for large colonies (MLD higher than 300 µm). Ellipsoid requires measuring three dimensions and is time-consuming. Therefore, we constructed different statistical models to predict organisms depth based on length and width. Splitting the data into training (2/3) and test (1/3) sets, all models resulted in low training (1.41-1.44%) and testing average error (1.3-2.0%). The models were also evaluated using three other independent datasets. The multiple linear model was finally selected to calculate MAC volume as an ellipsoid based on length and width. This work contributes to achieve a better estimation of MAC volume applicable to monitoring programs as well as to ecological research.

Sensor manufacturer, temperature, and cyanobacteria morphology affect phycocyanin fluorescence measurements.

Sensors to measure phycocyanin fluorescence in situ are becoming widely used as they may provide useful proxies for cyanobacterial biomass. In this study, we assessed five phycocyanin sensors from three different manufacturers. A combination of culture-based experiments and a 30-sample field study was used to examine the effect of temperature and cyanobacteria morphology on phycocyanin fluorescence. Phycocyanin fluorescence increased with decrease in temperature, although this varied with manufacturer and cyanobacterial density. Phycocyanin fluorescence and cyanobacterial biovolume were strongly correlated (R 2 > 0.83, P < 0.05) for single-celled and filamentous species. The relationship was generally weak for a colonial strain of Microcystis aeruginosa. The colonial culture was divided into different colony size classes and phycocyanin measured before and after manual disaggregation. No differences were measured, and the observation that fluorescence spiked when large colonial aggregates drifted past the light source suggests that sample heterogeneity, rather than lack of light penetration into the colonies, was the main cause of the poor relationship. Analysis of field samples showed a strong relationship between in situ phycocyanin fluorescence and spectrophotometrically measured phycocyanin (R 2 > 0.7, P < 0.001). However, there was only a weak relationship between phycocyanin fluorescence and cyanobacterial biovolume for two sensors (R 2 = 0.22-0.29, P < 0.001) and a non-significant relationship for the third sensor (R 2 = 0.29, P > 0.4). The five sensors tested in our study differed in their output of phycocyanin fluorescence, upper working limits (1200 to > 12,000 µg/L), and responses to temperature, highlighting the need for comprehensive sensor calibration and knowledge on the limitations of specific sensors prior to deployment.

Review: a meta-analysis comparing cell-division and cell-adhesion in Microcystis colony formation.

The freshwater cyanobacterium Microcystis is a nuisance species. It forms large blooms on the water surface and overwhelmingly dominates the ecosystem through the formation of colonies from single cells surrounded by mucilage; however, the mechanism of colony formation is poorly understood. Two mechanisms of Microcystis colony formation have been proposed: cell-division, where cells remain attached after binary fission; and cell-adhesion, where single cells stick together. This paper examined the published literature on Microcystis colony formation to clarify the mechanism of colony formation and its relationship to environmental drivers. This meta-analysis showed that in laboratory experiments, colony formation by cell-division was mainly induced by zooplankton filtrate, high Pb2+ concentrations, the presence of the cyanobacterium Cylindrospermopsis raciborskii, heterotrophic bacteria, and low temperature and low light intensities. Alternatively, colony formation by cell-adhesion was mainly induced by zooplankton grazing, high Ca2+ concentrations, and microcystins. Therefore, colony formation by cell-division appears to be a slower process and to occur under an environmental stress factor, while cell-adhesion occurs more quickly to an environmental threat. Applying the criteria to the different morphospecies of Microcystis, it was found that under natural conditions M. ichthyoblabe colonies formed predominantly through cell-division, whereas M. wesenbergii colonies formed predominantly through cell-adhesion. This study provides new insights into the mechanisms and environmental drivers of colony formation by Microcystis.

iTRAQ-based quantitative proteomic analysis of Microcystis aeruginosa exposed to spiramycin at different nutrient levels.

Research on the combined effects of antibiotic contaminants and environmental factors in cyanobacteria is still limited. This study focused on the action and its mechanism of spiramycin combined with changes in nitrogen and phosphorus level in Microcystis aeruginosa at environmentally relevant concentrations. Though photosynthetic activity was stimulated by spiramycin at a high nutrient level, no significant correlation (p>0.05) was found between photosynthesis-related proteins and growth-related proteins, and the growth rate was inhibited by 200ngL-1 of spiramycin. At low nitrogen and low phosphorus levels, up-regulated photosynthesis-related proteins were closely correlated with (p<0.05) stress response-related, transcription-related and cell division-related proteins, which consequently led to stimulated growth of M. aeruginosa under spiramycin exposure. Spiramycin exposure also regulated the production of microcystins (MCs) and the expression of two microcystin synthetases (mcyB and mcyC). The spiramycin-induced protein secretion process and the up-regulation of ATP binding cassette transporters might contribute to the increased MC release. Enolase, superoxide dismutase, protein GrpE, DNA-directed RNA polymerase subunit alpha and serine protease were candidate target proteins of spiramycin in M. aeruginosa under different nutrient conditions. Coexisting spiramycin mitigated the threat of cyanobacteria to aquatic environments at a high nutrient level but aggravated cyanobacterial bloom at a low nitrogen level.

Physiological effects and toxin release in Microcystis aeruginosa and Microcystis viridis exposed to herbicide fenoxaprop-p-ethyl.

Fenoxaprop-p-ethyl (FPE) was studied for possible ecotoxicity on two representative toxigenic cyanobacteria including Microcystis aeruginosa and Microcystis viridis. Growth curves, chlorophyll a content, protein content, microcystin levels, oxidative stress, and apoptosis rates were measured for the two cyanobacteria after exposure to different concentrations of FPE. Results showed that the changes in chlorophyll a content and protein content were consistent with cell density, and M. viridis was more sensitive than M. aeruginosa to FPE. The results of oxidative stress indicated that FPE induced the generation of malondialdehyde (MDA) and enhanced the activities of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in these two cyanobacteria. To further explore the toxicity of FPE, apoptosis rates and toxin levels were measured for the two cyanobacteria. Different degrees of apoptosis rates were observed in the two cyanobacteria, and the apoptosis rates increased with the increase concentration of FPE. The intracellular and extracellular MC-LR were both affect by FPE. The presence of FPE in aquatic ecosystem may stimulate the synthesis and release of MC-LR, which may cause serious water pollution and pose threats to human health. These results may be useful for the ecotoxicity assessment of FPE and guiding the rational use of pesticides in agriculture.