Metabolomic analysis reveals the toxicity mechanisms of bisphenol A on the Microcystis aeruginosa under different phosphorus levels.

Harmful cyanobacterial blooms have been a global environmental problem. Discharge of anthropogenic pollutants and excess nutrient import into the freshwater bodies may be the biggest drivers of bloom. Bisphenol A (BPA), a typical endocrine-disrupting compound, is frequently detected in different natural waters, which was a threat to the balance of aquatic ecosystem. Yet mechanistic understanding of the bloom and microcystin generation under combined pollution conditions is still a mystery. Herein, the cellular and metabolomic responses to BPA exposure and phosphorus (P) levels in Microcystis aeruginosa were investigated throughout its growth period. The results showed that the stress response of M. aeruginosa to BPA was characterized by a decrease in growth density, an increase in P utilization, an increase in ATPase activity, a disruption of the photosynthetic system, and an increase in the production and release of microcystins (MCs). However, these effects are highly dependent on the growth stage of the cyanobacterial cell and the magnitude of the added P concentration. In addition, exposure to a high concentration (10 µM) of BPA significantly stimulated the production of 20.7% more and the release of 29.2% more MCs from M. aeruginosa cells at a low P level. The responses of reactive oxygen species (ROS), superoxide dismutase (SOD) and malondialdehyde (MDA) suggested that exposure to BPA exposure at a low P level can lead to oxidative stress in M. aeruginosa. In addition, the differentially expressed 63 metabolites showed that cell growth, energy generation and photosynthesis were mainly regulated by the metabolic network of 3-phosphoglyceric acid (3-PGA), D-glucose 6-phosphate, UDP-α-D-galactose and UDP-N-acetyl-D-galactosamine (UDP-GalNAc) metabolism. Amino acids and lipid metabolism collectively mediated MCs production and release. These findings will provide important references for the control of harmful cyanobacterial blooms under combined pollution.

Microcystis blooms caused the decreasing richness of and interactions between free-living microbial functional genes in Lake Taihu, China.

Microcystis blooms have a marked effect on microbial taxonomical diversity in eutrophic lakes, but their influence on the composition of microbial functional genes is still unclear. In this study, the free-living microbial functional genes (FMFG) composition was investigated in the period before Microcystis blooms (March) and during Microcystis blooms (July) using a comprehensive functional gene array (GeoChip 5.0). The composition and richness of FMFG in the water column was significantly different between these two periods. The FMFG in March was enriched in the functional categories of nitrogen, sulfur, and phosphorus cycling, whereas the FMFG in July was enriched in carbon cycling, organic remediation, and metal homeostasis. Molecular ecological network analysis further demonstrated fewer functional gene interactions and reduced complexity in July than in March. Module hubs of the March network were mediated by functional genes associated with carbon, nitrogen, sulfur, and phosphorus, whereas those in July by a metal homeostasis functional gene. We also observed stronger deterministic processes in the FMFG assembly in July than in March. Collectively, this study demonstrated that Microcystis blooms induced significant changes in FMFG composition and metabolic potential, and abundance-information, which can support the understanding and management of biogeochemical cycling in eutrophic lake ecosystems.

The facilitating role of phycospheric heterotrophic bacteria in cyanobacterial phosphonate availability and Microcystis bloom maintenance.

BACKGROUND: Phosphonates are the main components in the global phosphorus redox cycle. Little is known about phosphonate metabolism in freshwater ecosystems, although rapid consumption of phosphonates has been observed frequently. Cyanobacteria are often the dominant primary producers in freshwaters; yet, only a few strains of cyanobacteria encode phosphonate-degrading (C-P lyase) gene clusters. The phycosphere is defined as the microenvironment in which extensive phytoplankton and heterotrophic bacteria interactions occur. It has been demonstrated that phytoplankton may recruit phycospheric bacteria based on their own needs. Therefore, the establishment of a phycospheric community rich in phosphonate-degrading-bacteria likely facilitates cyanobacterial proliferation, especially in waters with scarce phosphorus. We characterized the distribution of heterotrophic phosphonate-degrading bacteria in field Microcystis bloom samples and in laboratory cyanobacteria "phycospheres" by qPCR and metagenomic analyses. The role of phosphonate-degrading phycospheric bacteria in cyanobacterial proliferation was determined through coculturing of heterotrophic bacteria with an axenic Microcystis aeruginosa strain and by metatranscriptomic analysis using field Microcystis aggregate samples. RESULTS: Abundant bacteria that carry C-P lyase clusters were identified in plankton samples from freshwater Lakes Dianchi and Taihu during Microcystis bloom periods. Metagenomic analysis of 162 non-axenic laboratory strains of cyanobacteria (consortia cultures containing heterotrophic bacteria) showed that 20% (128/647) of high-quality bins from eighty of these consortia encode intact C-P lyase clusters, with an abundance ranging up to nearly 13%. Phycospheric bacterial phosphonate catabolism genes were expressed continually across bloom seasons, as demonstrated through metatranscriptomic analysis using sixteen field Microcystis aggregate samples. Coculturing experiments revealed that although Microcystis cultures did not catabolize methylphosphonate when axenic, they demonstrated sustained growth when cocultured with phosphonate-utilizing phycospheric bacteria in medium containing methylphosphonate as the sole source of phosphorus. CONCLUSIONS: The recruitment of heterotrophic phosphonate-degrading phycospheric bacteria by cyanobacteria is a hedge against phosphorus scarcity by facilitating phosphonate availability. Cyanobacterial consortia are likely primary contributors to aquatic phosphonate mineralization, thereby facilitating sustained cyanobacterial growth, and even bloom maintenance, in phosphate-deficient waters. Video Abstract.

Effects of 4-tert-butylpyrocatechol and tea polyphenol on growth, physiology and antioxidant responses in Microcystis aeruginosa.

Global warming has increased the frequency of Microcystis aeruginosa blooms, leading to the deterioration of water quality and loss of biodiversity. Therefore, developing effective strategies for controlling M. aeruginosa blooms has become an important research topic. Plant extracts, 4­tert-butylpyrocatechol (TBC) and tea polyphenol (TP) are commonly used for water purification and to increase fish immunity, which have great potential to inhibit cyanobacterial blooms. The inhibitory effects of TBC and TP on M. aeruginosa were investigated in terms of growth characteristics, cell membrane morphology, physiological, photosynthetic activities, and antioxidant enzymes activities. The results showed that TBC and TP inhibited the growth of M. aeruginosa by decreasing the chlorophyll fluorescence transients or increasing the antioxidant enzymes activities of M. aeruginosa. TBC damaged the cell morphology of M. aeruginosa, reduced extracellular polysaccharides and protein contents, and up-regulated the antioxidant activity-related gene (sod and gsh) expressions of M. aeruginosa. TP significantly decreased the photosynthetic pigment content, influenced the phycobiliprotein content, and strongly down-regulated the photosynthesis-related gene (psbA, psaB, and rbcL) relative expressions of M. aeruginosa. TBC caused significant oxidative stress, dysfunction of physiological metabolic processes, and damaged crucial biomacromolecules (e.g., lipids, proteins and polysaccharides), prompted the loss of cell integrity, ultimately leading to the death of M. aeruginosa. However, TP depressed photosynthetic activities and consequently inhibited the transfer of electrons, affected the electron transfer chain, decreased the photosynthetic efficiency, and eventually caused the death of M. aeruginosa cells. Our study showed the inhibitory effects and algicidal mechanisms of TBC and TP on M. aeruginosa, and provide a theoretical basis for restrain the overgrowth of M. aeruginosa.

Enhancing arsenate metabolism in Microcystis aeruginosa and relieving risks of arsenite and microcystins by nano-Fe2O3 under dissolved organic phosphorus conditions.

Little information is available on how nano-Fe2O3 substituted iron ions as a possible iron source impacting on algal growth and arsenate (As(V)) metabolism under dissolved organic phosphorus (DOP) (D-glucose-6-phosphate (GP)) conditions. We investigated the growth of Microcystis aeruginosa and As(V) metabolism together with their metabolites in As(V) aquatic environments with nano-Fe2O3 and GP as the sole iron and P sources, respectively. Results showed that nano-Fe2O3 showed inhibitory effects on M. aeruginosa growth and microcystin (MCs) release under GP conditions in As(V) polluted water. There was little influence on As species changes in GP media under different nano-Fe2O3 concentrations except for obvious total As (TAs) removal in 100.0 mg L-1 nano-Fe2O3 levels. As(V) metabolism dominated with As(V) biotransformation in algal cells was facilitated and arsenite (As(III)) releasing risk was relieved clearly by nano-Fe2O3 under GP conditions. The dissolved organic matter (DOM) in media exhibited more fatty acid analogs containing -CO, -CH2 =CH2, and -CH functional groups with increasing nano-Fe2O3 concentrations, but the fluorescent analogs were relatively reduced especially for the fluorescent DOM dominated by aromatic protein-like tryptophan which was significantly inhibited by nano-Fe2O3. Thus, As methylation that was facilitated in M. aeruginosa by nano-Fe2O3 in GP environments also caused more organic substances to release that absorb infrared spectra while reducing the release risks of As(III) and MCs as well as protein-containing tryptophan fractions. From 1H-NMR analysis, this might be caused by the increased metabolites of aromatic compounds, organic acid/amino acid, and carbohydrates/glucose in algal cells. The findings are vital for a better understanding of nano-Fe2O3 role-playing in As bioremediation by microalgae and the subsequent potential aquatic ecological risks.

Necrosis of Microcystis aeruginosa causing tannin derivatives in Rhus chinensis stem.

BACKGROUND: Algal infestation in Korean lakes, rivers, and in agroecosystems is a catastrophic problem resulting in contaminated drinking and agricultural irrigation water. Developing allelochemical-based algicides has previously faced difficulties, including dosage requirements and chemical instability. Despite these challenges, these algicides have enormous potential for eco-friendly use. This study presents the efficient use of tannin derivatives as antialgal chemicals modeled on a tannin-rich stem extract of Rhus chinensis in a thermal processing application. RESULTS: Tannic acids are the key component of algal necrosis in R. chinensis stem extract, and although heat extraction from the stem increased the crude extraction yield 1.8-fold, the procedure induced the conversion of tannic acids to gallic acid, resulting in lower antialgal activity. Gallotannin showed stronger antialgal activity (The 50% lethal dosage (LD50 )= 44.6 mg L-1 ) than gallic acid (LD50  = 99.2 mg L-1 ), and the nonheated extract exhibited 3.7-fold lower LD50 (0.66 g L-1 ) than the heated extract (LD50  = 2.45 g L-1 ), resulting in 2.6-fold higher content of gallotannin. CONCLUSION: These results demonstrate that heat treatment of R. chinensis stems during the extraction process is not beneficial to algal control because of the acceleration of thermal tannin degradation, despite it showing higher crude extract yields. Therefore, it is suggested extraction processes minimizing the loss of tannic acids should be the preferred methods used to develop tannin-based natural algicides for controlling algal infestation. Tannic acids showed higher toxicity into necrosis of M. aeruginosa than gallic acid where heat-processed extraction of R. chinensis stems produces more gallic acid content resulting in thermal degradation of tannic complexes than the extraction of nonthermal treatment. © 2022 Society of Chemical Industry.

Insights into the conversion of dissolved organic phosphorus favors algal bloom, arsenate biotransformation and microcystins release of Microcystis aeruginosa.

Little information is available on influences of the conversion of dissolved organic phosphorus (DOP) to inorganic phosphorus (IP) on algal growth and subsequent behaviors of arsenate (As(V)) in Microcystis aeruginosa (M. aeruginosa). In this study, the influences factors on the conversion of three typical DOP types including adenosine-5-triphosphate disodium salt (ATP), ß-glycerophosphate sodium (ßP) and D-glucose-6-phosphate disodium salt (GP) were investigated under different extracellular polymeric secretions (EPS) ratios from M. aeruginosa, and As(V) levels. Thus, algal growth, As(V) biotransformation and microcystins (MCs) release of M. aeruginosa were explored in the different converted DOP conditions compared with IP. Results showed that the three DOP to IP without EPS addition became in favor of algal growth during their conversion. Compared with IP, M. aeruginosa growth was thus facilitated in the three converted DOP conditions, subsequently resulting in potential algal bloom particularly at arsenic (As) contaminated water environment. Additionally, DOP after conversion could inhibit As accumulation in M. aeruginosa, thus intracellular As accumulation was lower in the converted DOP conditions than that in IP condition. As(V) biotransformation and MCs release in M. aeruginosa was impacted by different converted DOP with their different types. Specifically, DMA concentrations in media and As(III) ratios in algal cells were promoted in converted ßP condition, indicating that the observed dissolved organic compositions from ßP conversion could enhance As(V) reduction in M. aeruginosa and then accelerate DMA release. The obtained findings can provide better understanding of cyanobacteria blooms and As biotransformation in different DOP as the main phosphorus source.

Comment on "Models predict planned phosphorus load reduction will make Lake Erie more toxic".

Hellweger et al. (Reports, 27 May 2022, pp. 1001) predict that phosphorus limitation will increase concentrations of cyanobacterial toxins in lakes. However, several molecular, physiological, and ecological mechanisms assumed in their models are poorly supported or contradicted by other studies. We conclude that their take-home message that phosphorus load reduction will make Lake Erie more toxic is seriously flawed.

Response to Comment on "Models predict planned phosphorus load reduction will make Lake Erie more toxic".

Huisman et al. claim that our model is poorly supported or contradicted by other studies and the predictions are "seriously flawed." We show their criticism is based on an incomplete selection of evidence, misinterpretation of data, or does not actually refute the model. Like all ecosystem models, our model has simplifications and uncertainties, but it is better than existing approaches hat ignore biology and do not predict toxin concentration.

Models predict planned phosphorus load reduction will make Lake Erie more toxic.

Harmful cyanobacteria are a global environmental problem, yet we lack actionable understanding of toxigenic versus nontoxigenic strain ecology and toxin production. We performed a large-scale meta-analysis including 103 papers and used it to develop a mechanistic, agent-based model of Microcystis growth and microcystin production. Simulations for Lake Erie suggest that the observed toxigenic-to-nontoxigenic strain succession during the 2014 Toledo drinking water crisis was controlled by different cellular oxidative stress mitigation strategies (protection by microcystin versus degradation by enzymes) and the different susceptibility of those mechanisms to nitrogen limitation. This model, as well as a simpler empirical one, predicts that the planned phosphorus load reduction will lower biomass but make nitrogen and light more available, which will increase toxin production, favor toxigenic cells, and increase toxin concentrations.

[Elucidation of Phenomena Involving Cyanobacteria in Freshwater Ecosystem by Chemically Ecological Approach].

Lakes Sagami and Tsukui are reservoirs constructed by connecting to the Sagami River. Because of eutrophication of the lakes, cyanobacteria have appeared every year. This review deals with phenomena related to occurrence of cyanobacteria that have been observed for 40 years since 1974 at the lakes. These 40 years of observations raised three interesting issues including the retention of cyanobacteria on their surfaces. These phenomena have been attributed to the usual factors, such as illuminance, nutrition and water temperature, but our research results suggested that they cannot be resolved without the introduction of another factor. We have attempted to elucidate various phenomena involving cyanobacteria in lake ecosystems by chemical ecological methods using volatile organic compounds (VOCs) produced by the cyanobacteria as indicators. One of the VOCs, ß-cyclocitral, was significantly involved in the above phenomena, which was considered to be produced by the carotenoid cleavage dioxygenase (CCD) of the cyanobacteria. ß-Cyclocitral was not produced in the two known CCDs, but two additional CCDs to Microcystis aeruginosa participated to produce the ß-cyclocitral. These CCDs did not directly produce ß-cyclocitral, but it was accumulated in cells as their precursors. The released ß-cyclocitral underwent a Baeyer-Villiger-like oxidation. It was speculated that Microcystis activated the CCD genes through density stress and produced ß-cyclocitral, which acted as an allelopathic substance. As a result, the number of cells of cyanobacteria decreased, and the resulting nitrogen and phosphorus were fed to the living cyanobacteria. It is postulated that this "quorum sensing" was functioning in the above-mentioned issues.

Dark adaptation and ability of pulse-amplitude modulated (PAM) fluorometry to identify nutrient limitation in the bloom-forming cyanobacterium, Microcystis aeruginosa (Kützing).

Harmful algal blooms in inland waters are widely linked to excess phosphorus (P) loading, but increasing evidence shows that their growth and formation can also be influenced by nitrogen (N) and iron (Fe). Deficiency in N, P, and Fe differentially affects cellular photosystems and is manifested as changes in photosynthetic yield (Fv/Fm). While Fv/Fm has been increasingly used as a rapid and convenient in situ gauge of nutrient deficiency, there are few rigorous comparisons of instrument sensitivity and ability to resolve specific nutrient stresses. This study evaluated the application of Fv/Fm to cyanobacteria using controlled experiments on a single isolate and tested three hypotheses: i) single Fv/Fm measurements taken with different PAM fluorometers can distinguish among limitation by different nutrients, ii) measurements of Fv/Fm made by the addition of DCMU are comparable to PAM fluorometers, and iii) dark adaptation is not necessary for reliable Fv/Fm measurements. We compared Fv/Fm taken from the bloom-forming Microcystis aeruginosa (UTEX LB 3037) grown in nutrient-replete treatment (R) and N-, P-, and Fe-limited treatments (LN, LP, LFe, respectively), using three pulse-amplitude modulated (PAM) fluorometers and the chemical photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and evaluated the effects of dark adaptation prior to PAM measurement. There were significant differences in Fv/Fm estimates among PAM fluorometers for light- versus dark-adapted cell suspensions over the whole experiment (21 days), which were all significantly higher than the DCMU-based measurements. However, dark adaptation had no effect on Fv/Fm when comparing PAM-based values across a single nutrient treatment. All Fv/Fm methods could distinguish LN and LP from R and LFe treatments but none were able to resolve LFe from R, or LN from LP cultures. These results indicated that for most PAM applications, dark adaptation is not necessary, and furthermore that single measurements of Fv/Fm do not provide a robust measurement of nutrient limitation in Microcystis aeruginosa UTEX LB 3037, and potentially other, common freshwater cyanobacteria.

Challenges of using blooms of Microcystis spp. in animal feeds: A comprehensive review of nutritional, toxicological and microbial health evaluation.

Microcystis spp., are Gram-negative, oxygenic, photosynthetic prokaryotes which use solar energy to convert carbon dioxide (CO2) and minerals into organic compounds and biomass. Eutrophication, rising CO2 concentrations and global warming are increasing Microcystis blooms globally. Due to its high availability and protein content, Microcystis biomass has been suggested as a protein source for animal feeds. This would reduce dependency on soybean and other agricultural crops and could make use of "waste" biomass when Microcystis scums and blooms are harvested. Besides proteins, Microcystis contain further nutrients including lipids, carbohydrates, vitamins and minerals. However, Microcystis produce cyanobacterial toxins, including microcystins (MCs) and other bioactive metabolites, which present health hazards. In this review, challenges of using Microcystis blooms in feeds are identified. First, nutritional and toxicological (nutri-toxicogical) data, including toxicity of Microcystis to mollusks, crustaceans, fish, amphibians, mammals and birds, is reviewed. Inclusion of Microcystis in diets caused greater mortality, lesser growth, cachexia, histopathological changes and oxidative stress in liver, kidney, gill, intestine and spleen of several fish species. Estimated daily intake (EDI) of MCs in muscle of fish fed Microcystis might exceed the provisional tolerable daily intake (TDI) for humans, 0.04 µg/kg body mass (bm)/day, as established by the World Health Organization (WHO), and is thus not safe. Muscle of fish fed M. aeruginosa is of low nutritional value and exhibits poor palatability/taste. Microcystis also causes hepatotoxicity, reproductive toxicity, cardiotoxicity, neurotoxicity and immunotoxicity to mollusks, crustaceans, amphibians, mammals and birds. Microbial pathogens can also occur in blooms of Microcystis. Thus, cyanotoxins/xenobiotics/pathogens in Microcystis biomass should be removed/degraded/inactivated sufficiently to assure safety for use of the biomass as a primary/main/supplemental ingredient in animal feed. As an ameliorative measure, antidotes/detoxicants can be used to avoid/reduce the toxic effects. Before using Microcystis in feed ingredients/supplements, further screening for health protection and cost control is required.

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.

Precursor-Directed Biosynthesis and Fluorescence Labeling of Clickable Microcystins.

Microcystins, cyclic nonribosomal heptapeptides, are the most well-known cyanobacterial toxins. They are exceptionally well studied, but open questions remain concerning their physiological role for the producing microorganism or their suitability as lead compounds for anticancer drug development. One means to study specialized metabolites in more detail is the introduction of functional groups that make a compound amenable for bioorthogonal, so-called click reactions. Although it was reported that microcystins cannot be derivatized by precursor-directed biosynthesis, we successfully used this approach to prepare clickable microcystins. Supplementing different azide- or terminal alkyne containing amino acid analogues into the cultivation medium of microcystin-producing cyanobacteria strains, we found that these strains differ strongly in their substrate acceptance. Exploiting this flexibility, we generated more than 40 different clickable microcystins. We conjugated one of these derivatives with a fluorogenic dye and showed that neither incorporation of the unnatural amino acid analogue nor attachment of the fluorescent label significantly affects the cytotoxicity against cell lines expressing the human organic anion transporting polypeptides 1B1 or 1B3. Using time-lapse microscopy, we observed that the fluorescent microcystin is rapidly taken up into eukaryotic cells expressing these transporters.

Proteomic mechanisms for the stimulatory effects of antibiotics on Microcystis aeruginosa during hydrogen peroxide treatment.

Application of low dosage of H2O2 at early stage of cyanobacterial life cycle is a promising route for cyanobacterial bloom mitigation, which could minimize adverse effects on non-target organisms. Besides, influence of co-existing contaminants on cyanobacterial bloom mitigation under combined pollution conditions remains unclear. This study assessed the influence of a mixture of four frequently detected antibiotics (tetracycline, sulfamethoxazole, ciprofloxacin and amoxicillin) during H2O2 treatment of Microcystis aeruginosa at early growth stage. H2O2 significantly (p < 0.05) inhibited growth rate, chlorophyll a content, Fv/Fm and rETRmax in a dose-dependent manner at low doses of 0.25-1 mg L-1, through downregulating proteins involved in cell division, cellular component organization, gene expression and photosynthesis. Although H2O2 increased microcystin content in each cyanobacterial cell through the upregulation of microcystin synthetases (mcyC and mcyF), total microcystin concentration in H2O2 treated groups was significantly (p < 0.05) reduced due to the decrease of cell density. Existence of 80 and 200 ng L-1 mixed antibiotics during H2O2 treatment facilitated the scavenging of ROS by antioxidant enzymes and significantly (p < 0.05) stimulated growth, photosynthesis, microcystin synthesis and microcystin release in H2O2 treated cells, through the upregulation of proteins involved in photosynthesis, oxidation-reduction process, biosynthesis, gene expression and transport. Mixed antibiotics increased the hazard of M. aeruginosa during H2O2 treatment, through the stimulation of microcystin synthesis and release at the proteomic level. Each target antibiotic should be controlled below 5 ng L-1 before the application of H2O2 for eliminating the interference of antibiotics on cyanobacterial bloom mitigation.

Biological Stoichiometry Regulates Toxin Production in Microcystis aeruginosa (UTEX 2385).

Harmful algal blooms (HABs) are increasing in magnitude, frequency, and duration globally. Even though a limited number of phytoplankton species can be toxic, they are becoming one of the greatest water quality threats to public health and ecosystems due to their intrinsic toxicity to humans and the numerous interacting factors that undermine HAB forecasting. Here, we show that the carbon:nitrogen:phosphorus (C:N:P) stoichiometry of a common toxic phytoplankton species, Microcystis, regulates toxin quotas during blooms through a tradeoff between primary and secondary metabolism. Populations with optimal C:N (< 8) and C:P (< 200) cellular stoichiometry consistently produced more toxins than populations exhibiting stoichiometric plasticity. Phosphorus availability in water exerted a strong control on population biomass and C:P stoichiometry, but N availability exerted a stronger control on toxin quotas by regulating population biomass and C:N:P stoichiometry. Microcystin-LR, like many phytoplankton toxins, is an N-rich secondary metabolite with a C:N stoichiometry that is similar to the optimal growth stoichiometry of Microcystis. Thus, N availability relative to P and light provides a dual regulatory mechanism that controls both biomass production and cellular toxin synthesis. Overall, our results provide a quantitative framework for improving forecasting of toxin production during HABs and compelling support for water quality management that limit both N and P inputs from anthropogenic sources.

Role of divalent metals Cu2+ and Zn2+ in Microcystis aeruginosa proliferation and production of toxic microcystins.

Cu2+ and Zn2+, two ubiquitous metals in water environments, can widely trigger algae blooms at favourable environmental conditions. This paper elucidates the roles of Cu2+ and Zn2+ in the proliferation of Microcystis aeruginosa (M. aeruginosa) and synthesis of Microcystins (MCs). Findings indicate significant influences of increasing Cu2+ and Zn2+ concentrations on cell proliferation at limited available phosphorus concentrations of less than 0.1 mg/L. By contrast, Cu2+ and Zn2+ notably affected MCs production at all the inoculated phosphorus concentrations. The critical concentrations of 1 µg/L and 5 µg/L for Cu2+ and Zn2+, respectively, are determined to trigger rapid cell proliferation and MCs production. Furthermore, the impacts of Cu2+ and Zn2+ on nitrogen absorption and, subsequently, on amino acids (AAs) formation in cells, is likely key in MCs synthesis. The two AAs Alanine (Ala) and glutamic acid (Glu) demonstrate the most notable variations with the concentrations of Cu2+ & Zn2+.

Formation mechanism of the Microcystis aeruginosa bloom in the water with low dissolved phosphorus.

The utilization of phosphorus by algae in the low-phosphorus state has drawn wide concerns due to the high risk of forming algal blooms. The cyanobacteria Microcystis aeruginosa (M. aeruginosa) grew well under low-phosphorus condition by hydrolyzing dissolved organic phosphorus (DOP) to dissolved inorganic phosphorus (DIP) through alkaline phosphatase (AP). There was a negative correlation between DIP concentration and AP activity of algae. AP activity significantly increased at 0-3 d (p < 0.05), and reached the peak values of 43.06 and 49.11 King unit/gprot on day 5 for DIP (0.1 mg/L) and DOP (4.0 mg/L), respectively. The relative expression of phosphate transporter gene increased with decreasing phosphorus concentrations. The catalase activity under low-phosphorus condition increased significantly (p < 0.05) after one week, and was generally higher than 0.15 U/mgprot on day 14. Understanding the utilization efficiency and mechanism of DIP and DOP in the low-phosphorus state would help to inhibit the formation of algal blooms.

Feedback Regulation between Aquatic Microorganisms and the Bloom-Forming Cyanobacterium Microcystis aeruginosa.

The frequency and intensity of cyanobacterial blooms are increasing worldwide. Interactions between toxic cyanobacteria and aquatic microorganisms need to be critically evaluated to understand microbial drivers and modulators of the blooms. In this study, we applied 16S/18S rRNA gene sequencing and metabolomics analyses to measure the microbial community composition and metabolic responses of the cyanobacterium Microcystis aeruginosa in a coculture system receiving dissolved inorganic nitrogen and phosphorus (DIP) close to representative concentrations in Lake Taihu, China. M. aeruginosa secreted alkaline phosphatase using a DIP source produced by moribund and decaying microorganisms when the P source was insufficient. During this process, M. aeruginosa accumulated several intermediates in energy metabolism pathways to provide energy for sustained high growth rates and increased intracellular sugars to enhance its competitive capacity and ability to defend itself against microbial attack. It also produced a variety of toxic substances, including microcystins, to inhibit metabolite formation via energy metabolism pathways of aquatic microorganisms, leading to a negative effect on bacterial and eukaryotic microbial richness and diversity. Overall, compared with the monoculture system, the growth of M. aeruginosa was accelerated in coculture, while the growth of some cooccurring microorganisms was inhibited, with the diversity and richness of eukaryotic microorganisms being more negatively impacted than those of prokaryotic microorganisms. These findings provide valuable information for clarifying how M. aeruginosa can potentially modulate its associations with other microorganisms, with ramifications for its dominance in aquatic ecosystems.IMPORTANCE We measured the microbial community composition and metabolic responses of Microcystis aeruginosa in a microcosm coculture system receiving dissolved inorganic nitrogen and phosphorus (DIP) close to the average concentrations in Lake Taihu. In the coculture system, DIP is depleted and the growth and production of aquatic microorganisms can be stressed by a lack of DIP availability. M. aeruginosa could accelerate its growth via interactions with specific cooccurring microorganisms and the accumulation of several intermediates in energy metabolism-related pathways. Furthermore, M. aeruginosa can decrease the carbohydrate metabolism of cooccurring aquatic microorganisms and thus disrupt microbial activities in the coculture. This also had a negative effect on bacterial and eukaryotic microbial richness and diversity. Microcystin was capable of decreasing the biomass of total phytoplankton in aquatic microcosms. Overall, compared to the monoculture, the growth of total aquatic microorganisms is inhibited, with the diversity and richness of eukaryotic microorganisms being more negatively impacted than those of prokaryotic microorganisms. The only exception is M. aeruginosa in the coculture system, whose growth was accelerated.