Search

This review summarizes the present state of knowledge regarding the toxic, bloom-forming cyanobacterium, Microcystis, with a specific focus on its geographic distribution, toxins, genomics, phylogeny, and ecology. A global analysis found documentation suggesting geographic expansion of Microcystis, with recorded blooms in at least 108 countries, 79 of which have also reported the hepatatoxin microcystin. The production of microcystins (originally "Fast-Death Factor") by Microcystis and factors that control synthesis of this toxin are reviewed, as well as the putative ecophysiological roles of this metabolite. Molecular biological analyses have provided significant insight into the ecology and physiology of Microcystis, as well as revealed the highly dynamic, and potentially unstable, nature of its genome. A genetic sequence analysis of 27 Microcystis species, including 15 complete/draft genomes are presented. Using the strictest biological definition of what constitutes a bacterial species, these analyses indicate that all Microcystis species warrant placement into the same species complex since the average nucleotide identity values were above 95%, 16S rRNA nucleotide identity scores exceeded 99%, and DNA-DNA hybridization was consistently greater than 70%. The review further provides evidence from around the globe for the key role that both nitrogen and phosphorus play in controlling Microcystis bloom dynamics, and the effect of elevated temperature on bloom intensification. Finally, highlighted is the ability of Microcystis assemblages to minimize their mortality losses by resisting grazing by zooplankton and bivalves, as well as viral lysis, and discuss factors facilitating assemblage resilience.

Although toxic cyanobacterial blooms in western Lake Erie threaten drinking water supplies and are promoted by nutrient loading, the precise nutrient regime that selects specific cyanobacteria populations is poorly understood. Here, we assess shifts in cyanobacterial abundances and global gene-expression patterns in response to natural and manipulated gradients in nitrogen and phosphorus to identify gene pathways that facilitate dominance by different cyanobacteria. Gradients in soluble reactive phosphorus shaped cyanobacterial communities and elicited the largest transcriptomic responses. Under high-P conditions (closest to the mouth of the Maumee River), Anabaena and Planktothrix were the dominant cyanobacterial populations, and experimental P and ammonium enrichment promoted nitrogen fixation gene (nifH) expression in Anabaena. For Microcystis, experimental additions of P up-regulated genes involved in phage defense, genomic rearrangement, and nitrogen acquisition but led to lower abundances. Within offshore, low-P regions of the western basin of Lake Erie, Microcystis up-regulated genes associated with P scavenging (pstSCAB, phoX) and dominated cyanobacterial communities. Experimental additions of ammonium and urea did not alter Microcystis abundances but did up-regulate protease inhibitors (aer and mcn gene sets) and microcystin synthetase genes (mcy), with urea enrichment yielding significant increases in microcystin concentrations. Our findings suggest that management plans that reduce P loads alone may not significantly reduce the risk of cyanobacterial blooms in western Lake Erie but rather may promote a shift among cyanobacterial populations (Microcystis, Anabaena, and Planktothrix) toward a greater dominance by toxic strains of Microcystis.

Freshwater cyanobacterial harmful algal blooms (CyanoHABs) caused by algae in the genus Microcystis have been increasing in frequency and severity in recent decades. Microcystis blooms threaten aquatic organisms through effects associated with the rapid increase of biomass and the production of the hepatotoxin microcystin (MC) by toxic strains. Among fish, effects of blooms are likely to be more severe for early life stages, and physiological impacts on this life stage could significantly impact recruitment and fish populations. This study explores the effects of Microcystis blooms on the development of fish using the model organism, the Japanese medaka (Oryzias latipes), under realistic exposure conditions. Medaka embryos were exposed to natural blooms collected from New York City (USA) lakes, lab cultures of Microcystis, and MC-LR solutions. Field collected samples were more toxic than lab cultures (even when compared at the same algal density or MC concentration), causing decreased survival, premature time to hatch, reduced body length, yolk sac edema, and decreased heart rate, while lab culture exposures only resulted in bradycardia. Heart rate was the most sensitive endpoint measured, being depressed in embryos exposed to both lab cultures and field collected blooms. Generalized linear model analysis indicated bradycardia was statistically associated with both cell densities of blooms and MC concentrations, while single factor analysis indicated that MC concentrations had a stronger correlation compared to cell densities. However, MC exposure could not fully explain the effects observed, as exposures to MC-LR solutions alone were not able to reduce heart rate as severely as algal exposures. Collectively, these experiments indicate that factors beyond exposure to MC or even isolated Microcystis strains influence heart rate of fish exposed to Microcystis blooms. Enhanced mortality, depressed heart rate, and abnormal development observed in response to environmentally realistic exposures of Microcystis blooms could affect success of fish at both individual or population levels.

The bloom-forming, toxic cyanobacterium Microcystis synthesizes multiple secondary metabolites and has been shown to deter zooplankton grazing. However, the biochemical and/or molecular basis by which Microcystis deters zooplankton remains unclear. This global transcriptomic study explored the response of Microcystis to direct and indirect exposures to multiple densities of two cladoceran grazers, Daphnia pulex and D. magna . Higher densities of both daphnids significantly reduced Microcystis cell densities and elicited a stronger transcriptional response in Microcystis . While many putative grazer deterrence genes (encoding microcystin, aeruginosin, cyanopeptolin, and microviridin) were largely unaffected by zooplankton, transcripts for heat shock proteins ( hsp ) increased in abundance. Beyond metabolites and hsp , large increases in the abundances of transcripts from photosynthetic processes were observed, evidencing energy acquisition pathways were stimulated by grazing. In addition, transcripts of genes associated with the production of extracellular polysaccharides and gas vesicles significantly increased in abundance. These genes have been associated with colony formation and may have been invoked to deter grazers. Collectively, this study demonstrates that daphnid grazers induce a significant transcriptomic response in Microcystis , suggesting this cyanobacterium upregulates specific biochemical pathways to adapt to predation. IMPORTANCE This work explores the transcriptomic responses of Microcystis aeruginosa following exposure to grazing by two cladocerans, Daphnia magna and D. pulex . Contrary to previous hypotheses, Microcystis did not employ putative grazing deterrent secondary metabolites in response to the cladocerans, suggesting they may have other roles within the cell, such as oxidative stress protection. The transcriptional metabolic signature during intense grazing was largely reflective of a growth and stress response, although increasing abundances of transcripts encoding extracellular polysaccharides and gas vesicles were potentially related to predator avoidance.

Since the mid-twentieth century, both nutrient delivery rates and the frequency of harmful algal blooms (HABs) in coastal aquatic ecosystems have intensified. Recent studies have shown that nitrogen (N) or phosphorus (P) can limit primary production in freshwater systems, and Microcystis is able to utilize both inorganic and organic forms of N. The present study quantified the microcystin synthetase gene (mcyD) and the ribosomal RNA gene (16S) to assess how various nutri- ent sources affected the growth of toxic and non-toxic strains of Microcystis during natural blooms. During the present study, dense Microcystis blooms (>10 6 cell equivalents l -1 ) were observed within 2 contrasting ecosystems in the eastern USA: a tidal tributary and a eutrophic lake. In both systems, all Microcystis populations were stimulated by N more frequently than P during nutrient amendment experiments. The abundance of toxic strains of Microcystis was enhanced by nutrient enrichment more frequently (83% of experiments) than non-toxic strains (58% of experiments), suggesting that toxic strains may have a greater demand for both nutrients. Furthermore, abundances of toxic strains of Microcystis were enhanced by inorganic N more frequently (67% of experiments) than organic N (8% of experiments), while non-toxic strains were stimulated by organic N (50% of experiments) more frequently than inorganic N (25% of experiments). Inorganic P increased abundances of toxic strains of Microcystis more frequently than non-toxic strains (42 and 33% of experiments, respec- tively). Therefore, the dominance of toxic Microcystis may be influenced by both the concentration and species of nutrients, with higher concentrations of inorganic N and/or P likely promoting blooms dominated by toxic strains and potentially yielding higher microcystin concentrations.

Background While transcriptomics have become a valuable tool for linking physiology and ecology in aquatic microbes, the temporal dynamics of global transcriptomic patterns in Microcystis have rarely been assessed. Furthermore, while many microbial studies have explored expression of nutrient transporter genes, few studies have concurrently measured nutrient assimilation rates. Here, we considered how the global transcriptomic patterns and physiology of the cyanobacterium, Microcystis aeruginosa, changed daily as cells were grown from replete to deficient nitrogen (N) conditions and then back to replete conditions. Results During N deprivation, Microcystis downregulated genes involved in photosynthesis and respiration, carbon acquisition, lipid metabolism, and amino acid biosynthesis while upregulating genes involved in N acquisition and transport. With increasing N stress, both the strength of expression and number of genes being differentially expressed increased, until N was restored at which point these patterns reversed. Uptake of 15N-labeled nitrate, ammonium and urea reflected differential expression of genes encoding transporters for these nutrients, with Microcystis appearing to preferentially increase transcription of ammonium and urea transporters and uptake of these compounds during N deprivation. Nitrate uptake and nitrate transporter expression were correlated for one set of transporters but not another, indicating these were high and low affinity nitrate transporters, respectively. Concentrations of microcystin per cell decreased during N deprivation and increased upon N restoration. However, the transcript abundance of genes involved in the synthesis of this compound was complex, as microcystin synthetase genes involved in peptide synthesis were downregulated under N deprivation while genes involved in tailoring and transport were upregulated, suggesting modification of the microcystin molecule under N stress as well as potential alternative functions for these genes and/or this toxin. Conclusions Collectively, this study highlights the complex choreography of gene expression, cell physiology, and toxin synthesis that dynamic N levels can elicit in this ecologically important cyanobacterium. Differing expression patterns of genes within the microcystin synthetase operon in response to changing N levels revealed the potential limitations drawing conclusions based on only one gene in this operon. Electronic supplementary material The online version of this article (doi:10.1186/s12864-015-2275-9) contains supplementary material, which is available to authorized users.

Historically, phosphorus (P) has been considered the primary limiting nutrient for phytoplankton assemblages in freshwater ecosystems. This review, supported by new findings from Lake Erie, highlights recent molecular, laboratory, and field evidence that the growth and toxicity of some non-diazotrophic blooms of cyanobacteria can be controlled by nitrogen (N). Cyanobacteria such as Microcystis possess physiological adaptations that allow them to dominate low-P surface waters, and in temperate lakes, cyanobacterial densities can be controlled by N availability. Beyond total cyanobacterial biomass, N loading has been shown to selectively promote the abundance of Microcystis and Planktothrix strains capable of synthesizing microcystins over strains that do not possess this ability. Among strains of cyanobacteria capable of synthesizing the N-rich microcystins, cellular toxin quotas have been found to depend upon exogenous N supplies. Herein, multi-year observations from western Lake Erie are presented demonstrating that microcystin concentrations peak in parallel with inorganic N, but not orthophosphate, concentrations and are significantly lower (p < 0.01) during years of reduced inorganic nitrogen loading and concentrations. Collectively, this information underscores the importance of N as well as P in controlling toxic cyanobacteria blooms. Furthermore, it supports the premise that management actions to reduce P in the absence of concurrent restrictions on N loading may not effectively control the growth and/or toxicity of non-diazotrophic toxic cyanobacteria such as the cosmopolitan, toxin-producing genus, Microcystis.

Cyanobacteria blooms caused by species such as Microcystis have become commonplace in many freshwater ecosystems. Although phosphorus (P) typically limits the growth of freshwater phytoplankton populations, little is known regarding the molecular response of Microcystis to variation in P concentrations and sources. For this study, we examined genes involved in P acquisition in Microcystis including two high-affinity phosphate-binding proteins (pstS and sphX) and a putative alkaline phosphatase (phoX). Sequence analyses among ten clones of Microcystis aeruginosa and one clone of Microcystis wesenbergii indicates that these genes are present and conserved within the species, but perhaps not the genus, as phoX was not identified in M. wesenbergii. Experiments with clones of M. aeruginosa indicated that expression of these three genes was strongly upregulated (50- to 400-fold) under low inorganic P conditions and that the expression of phoX was correlated with alkaline phosphatase activity (p 0.005). In contrast, cultures grown exclusively on high levels of organic phosphorus sources (adenosine 5'-monophosphate, β-glycerol phosphate, and D: -glucose-6-phosphate) or under nitrogen-limited conditions displayed neither high levels of gene expression nor alkaline phosphatase activity. Since Microcystis dominates phytoplankton assemblages in summer when levels of inorganic P (P(i)) are often low and/or dominate lakes with low P(i) and high organic P, our findings suggest this cyanobacterium may rely on pstS, sphX, and phoX to efficiently transport P(i) and exploit organic sources of P to form blooms.

Whole transcriptome shotgun sequencing (RNA-seq) was used to assess the transcriptomic response of the toxic cyanobacterium Microcystis aeruginosa during growth with low levels of dissolved inorganic nitrogen (low N), low levels of dissolved inorganic phosphorus (low P), and in the presence of high levels of high molecular weight dissolved organic matter (HMWDOM). Under low N, one third of the genome was differentially expressed, with significant increases in transcripts observed among genes within the nir operon, urea transport genes (urtBCDE), and amino acid transporters while significant decreases in transcripts were observed in genes related to photosynthesis. There was also a significant decrease in the transcription of the microcystin synthetase gene set under low N and a significant decrease in microcystin content per Microcystis cell demonstrating that N supply influences cellular toxicity. Under low P, 27% of the genome was differentially expressed. The Pho regulon was induced leading to large increases in transcript levels of the alkaline phosphatase phoX, the Pst transport system (pstABC), and the sphX gene, and transcripts of multiple sulfate transporter were also significantly more abundant. While the transcriptional response to growth on HMWDOM was smaller (5-22% of genes differentially expressed), transcripts of multiple genes specifically associated with the transport and degradation of organic compounds were significantly more abundant within HMWDOM treatments and thus may be recruited by Microcystis to utilize these substrates. Collectively, these findings provide a comprehensive understanding of the nutritional physiology of this toxic, bloom-forming cyanobacterium and the role of N in controlling microcystin synthesis.