An advanced technique for immuno-labelling of microcystins in cryosectioned cells of Microcystis aeruginosa PCC 7806 (cyanobacteria): Implementations of an experiment with varying light scenarios and culture densities

Changes in CO2 concentration drive a succession of toxic and non-toxic strains of Microcystis blooms

The dynamic changes between toxic and non-toxic strains of Microcystis blooms have always been a hot topic. Previous studies have found that low CO2 favors toxic strains, but how changing dissolved CO2 (CO2 [aq]) in water body influences the succession of toxic and non-toxic strains in Microcystis blooms remains uncertain. Here, we combined laboratory competition experiments, field observations, and a machine learning model to reveal the links between CO2 changes and the succession. Laboratory experiments showed that under low CO2 conditions (100-150 ppm), the toxic strains could make better use of CO2 (aq) and be dominant. The non-toxic strains demonstrated a growth advantage as CO2 concentration increased (400-1000 ppm). Field observations from June to November in Lake Taihu showed that the percentage of toxic strains increased as CO2 (aq) decreased. Machine learning highlighted links between the inorganic carbon concentration and the proportion of advantageous strains. Our findings provide new insights for cyanoHABs prediction and prevention.

Synchrotron-based X-ray fluorescence microscopy mapping the ionome of a toxic freshwater cyanobacterium

Harmful algal blooms (HABs) pose a major environmental concern across the globe. In abundance, cyanobacteria, or so-called green-blue algae can produce extremely dangerous cyanotoxins that harm humans and animals. This study focused on the mapping and distribution of intracellular macro-and micronutrients of the wide-spread freshwater cyanobacteria Microcystis aeruginosa (M. aeruginosa). Towards a better understanding of trace metal uptake and homeostasis throughout the cell cycle, we quantitatively mapped the spatial distribution of the elements P, K, Fe, Ca, Zn, Mn, and Cu across the ultrastructure of frozen-hydrated single cells using state-of-the-art X-ray nanofluorescence imaging at the Advanced Photon Source (APS) at Argonne National Laboratory. Bulk cellular nutrient and trace metal content correlated well with the total intracellular elemental content in individual cells obtained by quantitative synchrotron X-ray fluorescence measurements. Multi-dimensional mappings showed P and K atoms colocalized as discrete semicircular hotspots that were analyzed with respect to their stoichiometry. Elevated Cu and Ca concentrations were detected along division plane of cells. P and K were found to have similar spatial elemental distribution with about 65% and 69% of the total cellular P and K, respectively, located at the hotspots. The P and K colocalization were refined further using nanotomography, showing a K envelope surrounding the P core. Inorganic P and organic P compounds were specified using solution-state 31P nuclear magnetic resonance (NMR) spectroscopy from M. aeruginosa. Of the total extracted P determined by 31P NMR spectroscopy, 47% were found to be nucleotides while only 11% were polyphosphates. Multimodal X-ray imaging provides a better understanding of intracellular biochemical processes in cyanobacteria, helping us monitor and combat an emerging environmental threat.

Differential Labeling of Chemically Modified Peptides and Lipids among Cyanobacteria Planktothrix and Microcystis

The cyanoHAB forming cyanobacteria Microcystis and Planktothrix frequently produce high intracellular amounts of microcystins (MCs) or anabaenopeptins (APs). In this study, chemically modified MCs and APs have been localized on a subcellular level in Microcystis and Planktothrix applying copper-catalyzed alkyne-azide cycloaddition (CuACC). For this purpose, three different non-natural amino acids carrying alkyne or azide moieties were fed to individual P. agardhii strains No371/1 and CYA126/8 as well as to M. aeruginosa strain Hofbauer showing promiscuous incorporation of various amino acid substrates during non-ribosomal peptide synthesis (NRPS). Moreover, CYA126/8 peptide knock-out mutants and non-toxic strain Synechocystis PCC6803 were processed under identical conditions. Simultaneous labeling of modified peptides with ALEXA405 and ALEXA488 and lipid staining with BODIPY 505/515 were performed to investigate the intracellular location of the modified peptides. Pearson correlation coefficients (PCC) obtained from confocal images were calculated between the different fluorophores and the natural autofluorescence (AF), and between labeled modified peptides and dyed lipids to investigate the spatial overlap between peptides and the photosynthetic complex, and between peptides and lipids. Overall, labeling of modified MCs (M. aeruginosa) and APs (P. agardhii) using both fluorophores revealed increased intensity in MC/AP producing strains. For Synechocystis lacking NRPS, no labeling using either ALEXA405 or ALEXA488 was observed. Lipid staining in M. aeruginosa and Synechocystis was intense while in Planktothrix it was more variable. When compared with AF, both modified peptides and lipids showed a heterologous distribution. In comparison, the correlation between stained lipids and labeled peptides was not increased suggesting a reduced spatial overlap.

Cyanobacterial blooms in wastewater treatment facilities: Significance and emerging monitoring strategies

Municipal wastewater treatment facilities (WWTFs) are prone to the proliferation of cyanobacterial species which thrive in stable, nutrient-rich environments. Dense cyanobacterial blooms frequently disrupt treatment processes and the supply of recycled water due to their production of extracellular polymeric substances, which hinder microfiltration, and toxins, which pose a health risk to end-users. A variety of methods are employed by water utilities for the identification and monitoring of cyanobacteria and their toxins in WWTFs, including microscopy, flow cytometry, ELISA, chemoanalytical methods, and more recently, molecular methods. Here we review the literature on the occurrence and significance of cyanobacterial blooms in WWTFs and discuss the pros and cons of the various strategies for monitoring these potentially hazardous events. Particular focus is directed towards next-generation metagenomic sequencing technologies for the development of site-specific cyanobacterial bloom management strategies. Long-term multi-omic observations will enable the identification of indicator species and the development of site-specific bloom dynamics models for the mitigation and management of cyanobacterial blooms in WWTFs. While emerging metagenomic tools could potentially provide deep insight into the diversity and flux of problematic cyanobacterial species in these systems, they should be considered a complement to, rather than a replacement of, quantitative chemoanalytical approaches.

Chemically labeled toxins or bioactive peptides show a heterogeneous intracellular distribution and low spatial overlap with autofluorescence in bloom-forming cyanobacteria

Harmful algal blooms formed by colony-forming cyanobacteria deteriorate water resources by producing cyanotoxins, which frequently occur at high intracellular concentrations. We aimed to localize toxic microcystins (MCs) and bioactive anabaenopeptins (APs) at the subcellular level under noninvasive conditions. Since both metabolites are synthesized nonribosomally, the relaxed specificity of key enzymes catalyzing substrate activation allowed chemical labeling through a standard copper-catalyzed click chemistry reaction. The genera Planktothrix and Microcystis specifically incorporated unnatural amino acids such as N-propargyloxy-carbonyl-L-lysine or O-propargyl-L-tyrosine, resulting in modified AP or MC peptides carrying the incorporated alkyne moiety. The labeled cells were quantitatively differentiated from the unlabeled control cells. MCs and APs occurred intracellularly as distinct entities showing a cell-wide distribution but a lowered spatial overlap with natural autofluorescence. Using the immunofluorescence technique, colocalization with markers of individual organelles was utilized to relate the distribution of labeled MCs to cellular compartments, e.g., using RbcL and FtsZ (cytosol) and PsbA (thylakoids). The colocalization correlation coefficients calculated pairwise between organelles and autofluorescence were highly positive as opposed to the relatively low positive indices derived from labeled MCs. The lower correlation coefficients imply that only a portion of the labeled MC molecules were related spatially to the organelles in the cell.

Relationship between Photosynthetic Capacity and Microcystin Production in Toxic Microcystis Aeruginosa under Different Iron Regimes

Blooms of harmful cyanobacteria have been observed in various water bodies across the world and some of them can produce intracellular toxins, such as microcystins (MCs), which negatively impact aquatic organisms and human health. Iron participates significantly in cyanobacterial photosynthesis and is proposed to be linked to MC production. Here, the cyanobacteria Microcystis aeruginosa was cultivated under different iron regimes to investigate the relationship between photosynthetic capacity and MC production. The results showed that iron addition increased cell density, cellular protein concentration and the Chl-a (chlorophyll-a) content. Similarly, it can also up⁻regulate photosynthetic capacity and promote MC⁻leucine⁻arginine (MC⁻LR) production, but not in a dose⁻dependent manner. Moreover, a significant positive correlation between photosynthetic capacity and MC production was observed, and electron transport parameters were the most important parameters contributing to the variation of intracellular MC⁻LR concentration revealed by Generalized Additive Model analysis. As the electron transport chain was affected by iron variation, adenosine triphosphate production was inhibited, leading to the alteration of MC synthetase gene expression. Therefore, it is demonstrated that MC production greatly relies on redox status and energy metabolism of photosynthesis in M. aeruginosa. In consequence, more attention should be paid to the involvement of photosynthesis in the regulation of MC production by iron variation in the future.

Microcystin-LR Binds Iron, and Iron Promotes Self-Assembly

The microcystin-producing Microcystis aeruginosa PCC 7806 and its close strain, the nonproducing Microcystis aeruginosa PCC 7005, grow similarly in the presence of 17 μM iron. Under severe iron deficient conditions (0.05 μM), the toxigenic strain grows slightly less than in iron-replete conditions, while the nonproducing microcystin strain is not able to grow. Isothermal titration calorimetry performed at cyanobacterial cytosol or meaningful environmental pHs values shows a microcystin-LR dissociaton constant for Fe²⁺ and Fe³⁺ of 2.4 μM. Using atomic force microscopy, 40% of microcystin-LR dimers were observed, and the presence of iron promoted its oligomerization up to six units. Microcystin-LR binds also Mo⁶⁺, Cu²⁺, and Mn²⁺. Polymeric microcystin binding iron may be related with a toxic cell colony advantage, providing enhanced iron bioavailability and perhaps affecting the structure of the gelatinous sheath. Inside cells, with microcystin implicated in the fitness of the photosynthetic machinery under stress conditions, the toxin would be involved in avoiding metal-dependent Fenton reactions when photooxidation causes disassembly of the iron-rich photosystems. Additionally, it could be hypothesized that polymerization-depolymerization dynamics may be an additional signal that could trigger changes (for example, in the binding of microcystin to proteins).

Effects of elevated CO2 on dynamics of microcystin-producing and non-microcystin-producing strains during Microcystis blooms

In an attempt to elucidate the effects of different CO2 concentrations (270, 380, and 750 μL/L) on the competition of microcystin-producing (MC-producing) and non-MC-producing Microcystis strains during dense cyanobacteria blooms, an in situ simulation experiment was conducted in the Meiliang Bay of Lake Taihu in the summer of 2012. The abundance of total Microcystis and MC-producing Microcystis genotypes was quantified based on the 16S rDNA and mcyD gene using real-time PCR. The results showed that atmospheric CO2 elevation would significantly decrease the pH value and increase the dissolved inorganic carbon (DIC) concentration. Changes in CO2 concentration did not show significant influence on the abundance of total Microcystis population. However, CO2 concentrations may be an important factor in determining the subpopulation structure of Microcystis. The enhancement of CO2 concentrations could largely increase the competitive ability of non-MC-producing over MC-producing Microcystis, resulting in a higher proportion of non-MC-producing subpopulation in treatments using high CO2 concentrations. Concurrently, MC concentration in water declined when CO2 concentrations were elevated. Therefore, we concluded that the increase of CO2 concentrations might decrease potential health risks of MC for human and animals in the future.

Functions, compositions, and evolution of the two types of carboxysomes: polyhedral microcompartments that facilitate CO2 fixation in cyanobacteria and some proteobacteria

Cyanobacteria are the globally dominant photoautotrophic lineage. Their success is dependent on a set of adaptations collectively termed the CO2-concentrating mechanism (CCM). The purpose of the CCM is to support effective CO2 fixation by enhancing the chemical conditions in the vicinity of the primary CO2-fixing enzyme, D-ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), to promote the carboxylase reaction and suppress the oxygenase reaction. In cyanobacteria and some proteobacteria, this is achieved by encapsulation of RubisCO within carboxysomes, which are examples of a group of proteinaceous bodies called bacterial microcompartments. Carboxysomes encapsulate the CO2-fixing enzyme within the selectively permeable protein shell and simultaneously encapsulate a carbonic anhydrase enzyme for CO2 supply from a cytoplasmic bicarbonate pool. These bodies appear to have arisen twice and undergone a process of convergent evolution. While the gross structures of all known carboxysomes are ostensibly very similar, with shared gross features such as a selectively permeable shell layer, each type of carboxysome encapsulates a phyletically distinct form of RubisCO enzyme. Furthermore, the specific proteins forming structures such as the protein shell or the inner RubisCO matrix are not identical between carboxysome types. Each type has evolutionarily distinct forms of the same proteins, as well as proteins that are entirely unrelated to one another. In light of recent developments in the study of carboxysome structure and function, we present this review to summarize the knowledge of the structure and function of both types of carboxysome. We also endeavor to cast light on differing evolutionary trajectories which may have led to the differences observed in extant carboxysomes.

Comparative proteomics between natural Microcystis isolates with a focus on microcystin synthesis

BACKGROUND

Microcystis aeruginosa is a species of cyanobacteria commonly found in a number of countries and frequently related to animal poisoning episodes due to its capacity to produce the cyanotoxin known as microcystin. Despite vast literature on microcystin structures and their deleterious effects, little is known about its synthesis by cyanobacteria. Therefore, this study used proteomic tools to compare two M. aeruginosa strains, contrasting them for microcystin production.

RESULTS

2-DE gels were performed and 30 differential protein spots were chosen. Among them, 11 protein spots were unique in the toxin producing strain and 8 in the non-toxin producing strain, and 14 protein spots were shown on both 2-DE gels but expressed differently in intensity. Around 57% of the tandem mass spectrometry identified proteins were related to energy metabolism, with these proteins being up-regulated in the toxin producing strain.

CONCLUSIONS

These data suggest that the presence of higher quantities of metabolic enzymes could be related to microcystin metabolism in comparison to the non-toxin producing strain. Moreover, it was suggested that the production of microcystin could also be related to other proteins than those directly involved in its production, such as the enzymes involved in the Calvin cycle and glycolysis.

The languages spoken in the water body (or the biological role of cyanobacterial toxins)

Although intensification of toxic cyanobacterial blooms over the last decade is a matter of growing concern due to bloom impact on water quality, the biological role of most of the toxins produced is not known. In this critical review we focus primarily on the biological role of two toxins, microcystins and cylindrospermopsin, in inter- and intra-species communication and in nutrient acquisition. We examine the experimental evidence supporting some of the dogmas in the field and raise several open questions to be dealt with in future research. We do not discuss the health and environmental implications of toxin presence in the water body.

Reversal in competitive dominance of a toxic versus non-toxic cyanobacterium in response to rising CO2

Climate change scenarios predict a doubling of the atmospheric CO(2) concentration by the end of this century. Yet, how rising CO(2) will affect the species composition of aquatic microbial communities is still largely an open question. In this study, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and non-toxic strain of the freshwater cyanobacterium Microcystis aeruginosa. The toxic strain was able to reduce dissolved CO(2) to lower concentrations than the non-toxic strain, and became dominant in competition at low CO(2) levels. Conversely, the non-toxic strain could grow at lower light levels, and became dominant in competition at high CO(2) levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether microcystins might have a role in this reversal of competitive dominance, we performed further competition experiments with the wild-type strain M. aeruginosa PCC 7806 and its mcyB mutant impaired in microcystin production. The microcystin-producing wild type had a strong selective advantage at low CO(2) levels but not at high CO(2) levels. Our results thus demonstrate both in theory and experiment that rising CO(2) levels can alter the community composition and toxicity of harmful algal blooms.

Comparative protein expression in different strains of the bloom-forming cyanobacterium Microcystis aeruginosa

Toxin production in algal blooms presents a significant problem for the water industry. Of particular concern is microcystin, a potent hepatotoxin produced by the unicellular freshwater species Microcystis aeruginosa. In this study, the proteomes of six toxic and nontoxic strains of M. aeruginosa were analyzed to gain further knowledge in elucidating the role of microcystin production in this microorganism. This represents the first comparative proteomic study in a cyanobacterial species. A large diversity in the protein expression profiles of each strain was observed, with a significant proportion of the identified proteins appearing to be strain-specific. In total, 475 proteins were identified reproducibly and of these, 82 comprised the core proteome of M. aeruginosa. The expression of several hypothetical and unknown proteins, including four possible operons was confirmed. Surprisingly, no proteins were found to be produced only by toxic or nontoxic strains. Quantitative proteome analysis using the label-free normalized spectrum abundance factor approach revealed nine proteins that were differentially expressed between toxic and nontoxic strains. These proteins participate in carbon-nitrogen metabolism and redox balance maintenance and point to an involvement of the global nitrogen regulator NtcA in toxicity. In addition, the switching of a previously inactive toxin-producing strain to microcystin synthesis is reported.

Effects of arsenate on microcystin content and leakage of Microcystis strain PCC7806 under various phosphate regimes

Both arsenic pollution and eutrophication are prominent environmental issues when considering the problem of global water pollution. It is important to reveal the effects of arsenic species on cyanobacterial growth and toxin yields to assess ecological risk of arsenic pollution or at least understand naturally occurring blooms. The sensitivity of cyanobacteria to arsenate has often been linked to the structural similarities of arsenate and phosphate. Thus, we approached the effect of arsenate with concentrations from 10(-8) to 10(-4) M on Microcystis strain PCC7806 under various phosphate regimes. The present study showed that Microcystis strain PCC7806 was arsenate tolerant up to 10(-4) M. And such tolerance was without reference to both content of intra- and extra-cellular phosphate. It seems that arsenate involved the regulation of microcystin synthesis and cellular polyphosphate contributed to microcystin production of Microcystis responding to arsenate, since there was a positive linear correlation of the cellular microcystin quota with the exposure concentration of arsenate when the cells were not preconditioned to phosphate starvation. It is presumed that arsenate could help to actively export microcystins from living Microcystis cells when preconditioned to phosphate starvation and incubated with the medium containing 1 microM phosphate. This study firstly provided evidence that microcystin content and/or release of Microcystis might be impacted by arsenate if it exists in harmful algal blooms.

The molecular genetics and regulation of cyanobacterial peptide hepatotoxin biosynthesis

Over the last 10 years, we have witnessed major advances in our understanding of natural product biosynthesis, including the genetic basis for toxin production by numerous groups of cyanobacteria. Cyanobacteria produce an unparalleled array of bioactive secondary metabolites, including alkaloids, polyketides and non-ribosomal peptides, some of which are potent toxins. This review addresses the molecular genetics underlying the production of hepatotoxins, microcystin and nodularin in fresh and brackish water. These toxins pose a serious threat to human health and their occurrence in water supplies is increasing, because of the prevalence of toxic algal blooms worldwide. Toxin biosynthesis gene-cluster-associated transposition and the natural transformability of certain species suggest a broader distribution of toxic cyanobacterial taxa. The information gained from the discovery of these toxin biosynthetic pathways has enabled the genetic screening of various environments for drinking-water quality management. Understanding the role of cyanotoxins in the producing microorganisms and the environmental regulation of their biosynthesis genes may also suggest the means of controlling toxic-bloom events.

Topology and enhanced toxicity of bound microcystins in Microcystis PCC 7806

The cyanobacterium Microcystis is a potent producer of microcystins and cyanopeptolins and causes most of the toxicity outbreaks in freshwaters worldwide. Microcystins are mainly stored in the cells and little is found in the water. The intracellular concentration of microcystins in Microcystis PCC 7806 was at least 0.9 mM, although the solubility of microcystins in water was only about 10 microM. This low solubility does not allow the solubilisation of such high amounts of microcystins in the cytosol of Microcystis. Differential fractionation of cell constituents showed that microcystins and cyanopeptolins were bound to a protein fraction primarily composed of phycobilins. The percentage of microcystins and cyanopeptolins found in the thylakoid membranes was very low. Phycobilins may be the major proteins that have binding sites for these oligopeptides. A molar ratio near to 6 was observed for microcystins to the phycobilin (alphabeta) monomer. The binding of the microcystins to the protein was rather weak and allowed rapid dissociation of microcystins from the protein-matrix. Toxicity assays with Thamnocephalus platyurus showed that native microcystin when still bound to cyanobacterial protein was more toxic than an equivalent amount that has been desorbed from the protein by treatment with methanol. It is suggested that phycobilins serve in the gut of grazers as carrier molecules for the rapid transport of microcystin from lysed cells of Microcystis to the epithelium where the uptake of microcystins occurs. Because protein-bound microcystin does not bind to C18 cartridges, this behaviour can be the cause of many analytical discrepancies observed. The blue-coloured water observed upon the collapse of Microcystis blooms may be extremely toxic because the released phycobilins may carry the major fraction of microcystins.

Impact of inorganic carbon availability on microcystin production by Microcystis aeruginosa PCC 7806

Batch culture experiments with the cyanobacterium Microcystis aeruginosa PCC 7806 were performed in order to test the hypothesis that microcystins (MCYSTs) are produced in response to a relative deficiency of intracellular inorganic carbon (C(i,i)). In the first experiment, MCYST production was studied under increased C(i,i) deficiency conditions, achieved by restricting sodium-dependent bicarbonate uptake through replacement of sodium bicarbonate in the medium with its potassium analog. The same experimental approach was used in a second experiment to compare the response of the wild-type strain M. aeruginosa PCC 7806 with its mcyB mutant, which lacks the ability to produce MCYSTs. In a third experiment, the impact of varying the C(i,i) status on MCYST production was examined without suppressing the sodium-dependent bicarbonate transporter; instead, a detailed investigation of a dark-light cycle was performed. In all experiments, a relative C(i,i) deficiency was indicated by an elevated variable fluorescence signal and led to enhanced phycocyanin cell quotas. Higher MCYST cell quotas (in the first and third experiments) and increased total (intracellular plus extracellular) MCYST production (in the first experiment) were detected with increased C(i,i) deficiency. Furthermore, the MCYST-producing wild-type strain and its mcyB mutant showed basically the same response to restrained inorganic carbon uptake, with elevated variable fluorescence and phycocyanin cell quotas with increased C(i,i) deficiency. The response of the wild type, however, was distinctly stronger and also included elevated chlorophyll a cell quotas. These differences indicate the limited ability of the mutant to adapt to low-C(i,i) conditions. We concluded that MCYSTs may be involved in enhancing the efficiency of the adaptation of the photosynthetic apparatus to fluctuating inorganic carbon conditions in cyanobacterial cells.