Transcriptomic analysis dissects the regulatory strategy of toxic cyanobacterium Microcystis aeruginosa under differential nitrogen forms

Sulfate assimilation regulates antioxidant defense response of the cyanobacterium Synechococcus elongatus PCC 7942 to high concentrations of carbon dioxide

The adaptive evolution of cyanobacteria over a prolonged period has allowed them to utilize carbon dioxide (CO2) at the low concentrations found in the atmosphere (0.04% CO2) for growth. However, whether the exposure of cyanobacteria to high concentrations of CO2 results in oxidative stress and the activation of antioxidant defense response remains unknown, albeit fluctuations in other culture conditions have been reported to exert these effects. The current study reveals the physiological regulation of the model cyanobacterium Synechococcus elongatus PCC 7942 upon exposure to 1% CO2 and the underlying mechanism. Exposure to 1% CO2 was demonstrated to induce oxidative stress and activate antioxidant defense responses in S. elongatus. Further analysis of variations in metabolism between S. elongatus cells grown at 0.04% CO2 and exposed to 1% CO2 revealed that sulfate assimilation was enhanced after the exposure to 1% CO2. A strain of S. elongatus lacking the gene cysR, encoding a global transcriptional regulator for genes involved in sulfate assimilation, was generated by deleting the gene from the genomic DNA. A comparative analysis of the wild-type and cysR-null strains indicated the regulation of the antioxidant response by sulfate assimilation. In addition, lines of evidence were presented that suggest a role of degradation of phycobilisome in the antioxidant response of S. elongatus under conditions of 1% CO2 and sulfate limitation. This study sheds light on the in situ effects of high CO2-induced stress on the ecophysiology of cyanobacteria upon exposure to diverse scenarios from a biotechnological and ecological perspective.IMPORTANCECyanobacteria that grow autotrophically with CO2 as the sole carbon source can be subject to high-CO2 stress in a variety of biotechnological and ecological scenarios. However, physiological regulation of cyanobacteria in response to high-CO2 stress remains elusive. Here, we employed microbial physiological, biochemical, and genetic techniques to reveal the regulatory strategies of cyanobacteria in response to high-CO2 stress. This study, albeit physiological, provides a biotechnological enterprise for manipulating cyanobacteria as the chassis for CO2 conversion and sheds light on the in situ ecological effects of high CO2 on cyanobacteria.

Nitrogen-removed organic matters from cyanobacterial decomposition promote the release of nitrogen from lake sediment

Cyanobacterial blooms, which carry a lot of nitrogen (N) and phosphorus (P), have emerged as one of the most severe environmental issues in freshwater ecosystems. However, there are few studies on the effect of organic matters released during cyanobacterial decomposition in promoting N release from lake sediments that remain underexplored. An essential step is to eliminate the impact of the N contributions from cyanobacteria when evaluating sedimentary N release. The response surface methodology (RSM) was developed to optimize the struvite precipitation model, and the results indicated that 1.3 of Mg/N, 1.0 of P/N, and pH 9.5 were the optimum conditions for N removal from cyanobacterial pyrolysis liquid. Following this, calcium phosphate crystallization (at pH 10 and Ca/P = 4.98) removed residual P, and zeolite adsorption (at pH 8 and 10 g/L zeolite dosage) eliminated the remaining N. Ultimately, 99.3% of N was removed with the two methods in cyanobacterial pyrolysis liquid. The cyanobacterial pyrolysis liquid, stripped of N, was found to significantly enhance the release of N from lake sediment under anaerobic conditions, which can then be reutilized by cyanobacteria. These findings reveal that organic matter derived from cyanobacterial decomposition promotes sedimentary N release, creating a feedback loop that sustains cyanobacterial blooms in freshwater ecosystems.

From winter dormancy to spring bloom: Regulatory mechanisms in Microcystis aeruginosa post-overwintering recovery

Cyanobacterial blooms pose a significant environmental threat in freshwater ecosystems. These cyanobacteria exhibit resilience to cold and dark conditions during winter and flourish as temperature rise in warmer seasons. However, there is a limited understanding of the dynamic growth recovery process and regulatory signaling mechanisms in cyanobacteria after overwintering. In this study, we employed Microcystis aeruginosa (M. aeruginosa) as a model to simulate its growth recovery when subjected to increasing temperature after overwintering under low temperature (4 °C) and dark conditions. We investigated changes in cell growth, microcystin levels, and signaling pathways throughout this recovery phase. Our results indicated that compared to the non-overwintering treatment (T1), the overwintered treatment (T2) experienced a 55.6 % decrease in algae density and a significant reduction in microcystin-LR (MC-LR) levels within the 15-20 °C temperature range (p < 0.05). Overwintering suppressed photosynthetic efficiency during the recovery phase of M. aeruginosa, activated the antioxidant system, and impaired cellular ultrastructure, making algal cells more vulnerable to death. At the transcriptional level, overwintering down-regulated pathways such as photosynthesis, ribosome, the Calvin cycle, and oxidative phosphorylation, hindering the growth and metabolic capacity of M. aeruginosa. In conclusion, this study highlights the inhibitory impacts of overwintering on growth and metabolism of cyanobacteria during the recovery process. It provides insights into the mechanistic foundations of seasonal cyanobacterial blooms and the crucial role of signaling regulation in these processes.

Enhancing lead tolerance in spirulina using glycine betaine under high CO2 and salinity for sustainable carbon capture

Microalgae-based carbon capture and utilization (CCU) offers a promising negative emission technology that absorbs CO2 and generates valuable biomass. However, heavy metal pollutants in industrial flue gases can compromise carbon sequestration efficiency and bioproduct quality. Spirulina was investigated as a model organism for CO2 sequestration, using a modified Zarrouk's medium to integrate salt stress and an osmoprotectant to boost biomass yield under 15 % CO2. This approach enhances both biomass yield （380.83 mg L-1 d-1）and resistance to lead toxicity. Analytical assessments revealed that glycine betaine (GB) supplementation drastically reduced lead accumulation, decreasing the extracellular and intracellular contents by 39.7 % and 60.7 %, respectively. A notable decrease in extracellular dissolved organic matter was also observed. Furthermore, transcriptomic analyses confirmed that GB treatment strengthened osmotic stress responses and suppressed metal ion transport. These findings enhanced the feasibility of microalgae-based CCU technologies, marking significant progress in sustainable algal biotechnology.

Molecular response and adaptation mechanism of Microcystis aeruginosa under metalimnetic oxygen minimum conditions

Reservoirs are important drinking water sources. The metalimnetic oxygen minimum (MOM) usually occurs periodically in summer and autumn in deep-water reservoirs due to algae blooms and thermal stratification. This study aimed to explore the physiological and molecular responses of Microcystis aeruginosa (M. aeruginosa) under MOM conditions (darkness coupled with low dissolved oxygen (DO) concentration, hydrostatic pressure, and nutrient starvation). The comprehensive response of M. aeruginosa suggested that MOM conditions led to an immediate collapse of gas vesicles. This was followed by a gradual inhibition of photosynthesis by disturbing the electron transport chain and a significant downregulation of energy metabolism and carbohydrate metabolism. The active cells were approximately 5 % and > 45 % under MOM aerobic (3.0-7.0 mg/L DO) and anaerobic conditions (< 0.5 mg/L DO), respectively, for 20 days. In addition, a single exposure to darkness or pressure accelerated the decay of M. aeruginosa cells; however, MOM conditions with a low DO concentration had the opposite effect. The survival of M. aeruginosa cells under MOM conditions could be attributed to stringent response and the activation of HIF-1 signal when DO concentration decreased to < 2.0 mg/L by promoting the formation of cellular quiescence and resource redistribution. This study sheds light on the molecular response and adaptation mechanism of M. aeruginosa under MOM conditions.

Genomic synergistic efficient carbon fixation and nitrogen removal induced by excessive inorganic carbon in the anammox-centered coupling system

Given the significance of HCO3- for autotrophic anammox bacteria (AnAOB), excessive HCO3- was always provided in anammox-related systems and engineering applications. However, its impact mechanism on anammox process at genome-level remains unknown. This study firstly established an anammox-centered coupling system that entails heterotrophic partial denitrification (PD) and hydrolytic acidification (A-PDHA) fed mainly with inorganic carbon (high HCO3- concentration and low C/N ratio). Metagenomic binning and metatranscriptomics analyses indicated that high HCO3- concentration enhanced expression of natural most efficient phosphoenolpyruvate (PEP) carboxylase within AnAOB, by up to 30.59 folds. This further induced AnAOB to achieve high-speed carbon-fixing reaction through cross-feeding of phosphate and PEP precursors with heterotrophs. Additionally, the enhanced activity of transporters and catalytic enzymes (up to 4949-fold) induced by low C/N ratio enabled heterotrophs to eliminate extracellular accumulated energy precursors mainly derived from carbon fixation products of AnAOB. This maintained high-speed carbon-fixing reaction within AnAOB and supplemented heterotrophs with organics. Moreover, assimilated energy precursors stimulated nitrogen metabolism enzymes, especially NO2- reductase (968.14 times), in heterotrophs. This established an energy-saving PD-A process mediated by interspecies NO shuttle. These variation resulted in efficient nitrogen removal (>95 %) and reduced external organic carbon demand (67 %) in A-PDHA system. This study unveils the great potential of an anammox-centered autotrophic-heterotrophic coupling system for achieving cost-effective nitrogen removal and enhancing carbon fixation under excessive HCO3- doses.

Mechanistic study on the increase of Microcystin-LR synthesis and release in Microcystis aeruginosa by amino-modified nano-plastics

Ecological risk of micro/nano-plastics (MPs/NPs) has become an important environmental issue. Microcystin-leucine-arginine (MC-LR) produced by Microcystis aeruginosa (M. aeruginosa) is the most common and toxic secondary metabolites (SM). However, the influencing mechanism of MPs and NPs exposure on MC-LR synthesis and release have still not been clearly evaluated. In this work, under both acute (4d) and long-term exposure (10d), only high-concentration (10 mg/L) exposure of amino-modified polystyrene NPs (PS-NH2-NPs) promoted MC-LR synthesis (32.94 % and 42.42 %) and release (27.35 % and 31.52 %), respectively. Mechanistically, PS-NH2-NPs inhibited algae cell density, interrupted pigment synthesis, weakened photosynthesis efficiency, and induced oxidative stress, with subsequent enhancing the MC-LR synthesis. Additionally, PS-NH2-NPs exposure up-regulated MC-LR synthesis pathway genes (mcyA, mcyB, mcyD, and mcyG) combined with significantly increased metabolomics (Leucine and Arginine), thereby enhancing MC-LR synthesis. PS-NH2-NPs exposure enhanced the MC-LR release from M. aeruginosa via up-regulated MC-LR transport pathway genes (mcyH) and the shrinkage of plasma membrane. Our results provide new insights into the long-time coexistence of NPs with algae in freshwater systems might pose a potential threat to aquatic environments and human health.

Insights into cyanobacterial blooms through the lens of omics

Cyanobacteria are oxygen-producing photosynthetic bacteria that convert carbon dioxide into biomass upon exposure to sunlight. However, favorable conditions cause harmful cyanobacterial blooms (HCBs), which are the dense accumulation of biomass at the water surface or subsurface, posing threats to freshwater ecosystems and human health. Understanding the mechanisms underlying cyanobacterial bloom formation is crucial for effective management. In this regard, recent advancements in omics technologies have provided valuable insights into HCBs, which have raised expectations to develop more effective control methods in the near future. This literature review aims to present the genomic architecture, adaptive mechanisms, microbial interactions, and ecological impacts of HCBs through the lens of omics. Genomic analysis indicates that the genome plasticity of cyanobacteria has enabled their resilience and effective adaptation to environmental changes. Transcriptomic investigations have revealed that cyanobacteria use various strategies for adapting to environmental stress. Additionally, metagenomic and metatranscriptomic analyses have emphasized the significant role of the microbial community in regulating HCBs. Finally, we offer perspectives on potential opportunities for further research in this field.

Toxic mechanisms of the antiviral drug arbidol on microalgae in algal bloom water at transcriptomic level

Increasing antivirals in surface water caused by their excessive consumption pose serious threats to aquatic organisms. Our recent research found that the input of antiviral drug arbidol to algal bloom water can induce acute toxicity to the growth and metabolism of Microcystis aeruginosa, resulting in growth inhibition, as well as decrease in chlorophyll and ATP contents. However, the toxic mechanisms involved remained obscure, which were further investigated through transcriptomic analysis in this study. The results indicated that 885-1248 genes in algae were differentially expressed after exposure to 0.01-10.0 mg/L of arbidol, with the majority being down-regulated. Analysis of commonly down-regulated genes found that the cellular response to oxidative stress and damaged DNA bonding were affected, implying that the stress defense system and DNA repair function of algae might be damaged. The down-regulation of genes in porphyrin metabolism, photosynthesis, carbon fixation, glycolysis, tricarboxylic acid cycle, and oxidative phosphorylation might inhibit chlorophyll synthesis, photosynthesis, and ATP supply, thereby hindering the growth and metabolism of algae. Moreover, the down-regulation of genes related to nucleotide metabolism and DNA replication might influence the reproduction of algae. These findings provided effective strategies to elucidate toxic mechanisms of contaminants on algae in algal bloom water.

Integrated physio-biochemical and transcriptomic analysis reveals the joint toxicity mechanisms of two typical antidepressants fluoxetine and sertraline on Microcystis aeruginosa

Selective serotonin reuptake inhibitor (SSRI) antidepressants are of increasing concern worldwide due to their ubiquitous occurrence and detrimental effects on aquatic organisms. However, little is known regarding their effects on the dominant bloom-forming cyanobacterium, Microcystis aeruginosa. Here, we investigated the individual and joint effects of two typical SSRIs fluoxetine (FLX) and sertraline (SER) on M. aeruginosa at physio-biochemical and molecular levels. Results showed that FLX and SER had strong growth inhibitory effects on M. aeruginosa with the 96-h median effect concentrations (EC50s) of 362 and 225 μg/L, respectively. Besides, the mixtures showed an additive effect on microalgal growth. Meanwhile, both individual SSRIs and their mixtures can inhibit photosynthetic pigment synthesis, cause oxidative damage, destroy cell membrane, and promote microcystin-leucine-arginine (MC-LR) synthesis and release. Moreover, the mixtures enhanced the damage to photosynthesis, antioxidant system, and cell membrane and facilitated MC-LR synthesis and release compared to individuals. Furthermore, transcriptomic analysis revealed that the dysregulation of the key genes related to transport, photosystem, protein synthesis, and non-ribosomal peptide structures was the fundamental molecular mechanism underlying the physio-biochemical responses of M. aeruginosa. These findings provide a better understanding of the toxicity mechanisms of SSRIs to microalgae and their risks to aquatic ecosystems.

Algicidal activity synchronized with nitrogen removal by actinomycetes: Algicidal mechanism, stress response of algal cells, denitrification performance, and indigenous bacterial community co-occurrence

The harmful algal blooms (HABs) can damage the ecological equilibrium of aquatic ecosystems and threaten human health. The bio-degradation of algal by algicidal bacteria is an environmentally friendly and economical approach to control HABs. This study applied an aerobic denitrification synchronization algicidal strain Streptomyces sp. LJH-12-1 (L1) to control HABs. The cell-free filtrate of the strain L1 showed a great algolytic effect on bloom-forming cyanobacterium, Microcystis aeruginosa (M. aeruginosa). The optimal algicidal property of strain L1 was indirect light-dependent algicidal with an algicidal rate of 85.0%. The functional metabolism, light-trapping, light-transfer efficiency, the content of pigments, and inhibition of photosynthesis of M. aeruginosa decreased after the addition of the supernatant of the strain L1 due to oxidative stress. Moreover, 96.05% nitrate removal rate synchronized with algicidal activity was achieved with the strain L1. The relative abundance of N cycling functional genes significantly increased during the strain L1 effect on M. aeruginosa. The algicidal efficiency of the strain L1 in the raw water was 76.70% with nitrate removal efficiency of 81.4%. Overall, this study provides a novel route to apply bacterial strain with the property of denitrification coupled with algicidal activity in treating micro-polluted water bodies.

Simulating short-term light responses of photosynthesis and water use efficiency in sweet sorghum under varying temperature and CO2 conditions

Climate change, characterized by rising atmospheric CO2 levels and temperatures, poses significant challenges to global crop production. Sweet sorghum, a prominent C4 cereal extensively grown in arid areas, emerges as a promising candidate for sustainable bioenergy production. This study investigated the responses of photosynthesis and leaf-scale water use efficiency (WUE) to varying light intensity (I) in sweet sorghum under different temperature and CO2 conditions. Comparative analyses were conducted between the A n-I, g s-I, T r-I, WUEi-I, and WUEinst-I models proposed by Ye et al. and the widely utilized the non-rectangular hyperbolic (NRH) model for fitting light response curves. The Ye's models effectively replicated the light response curves of sweet sorghum, accurately capturing the diminishing intrinsic WUE (WUEi) and instantaneous WUE (WUEinst) trends with increasing I. The fitted maximum values of A n, g s, T r, WUEi, and WUEinst and their saturation light intensities closely matched observations, unlike the NRH model. Despite the NRH model demonstrating high R 2 values for A n-I, g s-I, and T r-I modelling, it returned the maximum values significantly deviating from observed values and failed to generate saturation light intensities. It also inadequately represented WUE responses to I, overestimating WUE. Across different leaf temperatures, A n, g s, and T r of sweet sorghum displayed comparable light response patterns. Elevated temperatures increased maximum A n, g s, and T r but consistently declined maximum WUEi and WUEinst. However, WUEinst declined more sharply due to the disproportionate transpiration increase over carbon assimilation. Critically, sweet sorghum A n saturated at current atmospheric CO2 levels, with no significant gains under 550 μmol mol-1. Instead, stomatal closure enhanced WUE under elevated CO2 by coordinated g s and T r reductions rather than improved carbon assimilation. Nonetheless, this response diminished under simultaneously high temperature, suggesting intricate interplay between CO2 and temperature in modulating plant responses. These findings provide valuable insights into photosynthetic dynamics of sweet sorghum, aiding predictions of yield and optimization of cultivation practices. Moreover, our methodology serves as a valuable reference for evaluating leaf photosynthesis and WUE dynamics in diverse plant species.

The Microcystis-microbiome interactions: origins of the colonial lifestyle

Species of the Microcystis genus are the most common bloom-forming toxic cyanobacteria worldwide. They belong to a clade of unicellular cyanobacteria whose ability to reach high biomasses during blooms is linked to the formation of colonies. Colonial lifestyle provides several advantages under stressing conditions of light intensity, ultraviolet light, toxic substances and grazing. The progression from a single-celled organism to multicellularity in Microcystis has usually been interpreted as individual phenotypic responses of the cyanobacterial cells to the environment. Here, we synthesize current knowledge about Microcystis colonial lifestyle and its role in the organism ecology. We then briefly review the available information on Microcystis microbiome and propose that changes leading from single cells to colonies are the consequence of specific and tightly regulated signals between the cyanobacterium and its microbiome through a biofilm-like mechanism. The resulting colony is a multi-specific community of interdependent microorganisms.

Physiology, microcystin production, and transcriptomic responses of Microcystis aeruginosa exposed to calcium and magnesium

Calcium ions (Ca2+) and magnesium ions (Mg2+) are pivotal in the community composition and stability of harmful cyanobacteria, yet the physiological and molecular responses remains poorly understood. This study aims to explore these responses in the high microcystin producer Microcystis aeruginosa (M. aeruginosa). Results indicate that the growth of M. aeruginosa is inhibited by Ca2+/Mg2+ exposure (0.5-10 mM), while Fv/Fm photosynthetic parameters and extracellular microcystin-leucine-arginine (MC-LR) concentrations increase. Additionally, MC-LR release is significantly elevated under exposure to Ca2+/Mg2+, posing potential aquatic environmental risks. Transcriptomic analysis reveals downregulation of genes related to cell architecture, membrane transport, and metabolism, while the genes linked to photosynthesis electron transmission and heavy metal-responsive transcriptional regulators are upregulated to adapt to environmental changes. Further analysis reveals that Ca2+ and Mg2+ primarily impact sulfur metabolism and transport of amino acids and mineral within cells. These findings provide insights into M. aeruginosa cells responses to Ca2+ and Mg2+ exposure.

Investigation of dissolved organic matter's influence on the toxicity of cadmium to the cyanobacterium Microcystis aeruginosa by biochemical and molecular assays

Rapid economic development has increased the accumulation of dissolved organic matter (DOM) and heavy metals in aquatic environments. In addition, Microcystis aeruginosa can cause the outbreak of cyanobacteria bloom and can produce microcystin, which poses a threat to human water safety. Therefore, this study analyzed the biochemical and molecular assays of DOM (0, 1, 3, 5, 8, 10 mg C L-1) extracted from four different sources on the toxicity of cadmium (Cd) to M. aeruginosa. The results showed that the addition of different concentrations of DOM from sediment, biochar, and humic acid alleviated the toxicity of Cd to M. aeruginosa. But the addition of rice hulls DOM at high concentrations (8 and 10 mg L-1) significantly reduced the normal growth and metabolic activities of M. aeruginosa. DOM from four different sources promoted the expression level of microcystin-related gene mcyA and the production of microcystin-leucine-arginine (MC-LR), and mcyA was positively correlated with MC-LR. DOM from biochar, sediment, and humic acid were able to bind Cd through complexation. The results will help to understand the toxic effects of heavy metals on toxic-producing cyanobacteria in the presence of DOM, and provide certain reference for the evaluation of water environmental health.

Differential effects of nitrate and ammonium on the growth of algae and microcystin production by nitrogen-fixing Nostoc sp. and non-nitrogen-fixing Microcystis aeruginosa

Cyanotoxins produced by cyanobacteria are a significant threat to human health. However, their responses to nitrogen (N) supplies could differ between N-fixing and non-N-fixing species, which has been poorly understood. This study aimed to compare the responses of the non-N-fixing Microcystis aeruginosa and N-fixing Nostoc sp. to varying concentrations of nitrate and ammonium. This comparison had been conducted by analyzing chlorophyll-a contents, maximum quantum efficiencies of photosystem II, microcystin production, and related gene expressions. Our findings revealed that nitrate substantially stimulated the growth of both M. aeruginosa and Nostoc sp. with biomass increase by 366.2 ± 56.5 and 93.0 ± 14.0%, respectively, at 16 mg-N/L. In contrast, high ammonium concentrations suppressed their growth. Furthermore, the intracellular concentration of microcystins produced by M. aeruginosa was higher under high nitrate. Extracellular microcystins showed an opposite trend to increases in nitrate and ammonium. Ammonium increases the production and releases microcystin from Nostoc sp. N metabolism genes showed a similar trend with toxin formation genes, which were up-regulated under the high N treatments. This study provides valuable insights into the impacts of N supplies on growths of N- and non-N-fixing cyanobacteria, as well as microcystin production, which helps to develop effective strategies for managing cyanobacterial blooms.

Effect of butachlor on Microcystis aeruginosa: Cellular and molecular mechanisms of toxicity

The rapid development of agriculture increases the release of butachlor into aquatic environments. As a dominant species causing cyanobacterial blooms, Microcystis aeruginosa (M. aeruginosa) can produce microcystin and poses threats to aquatic ecosystems and human health. However, the impact of butachlor on M. aeruginosa remains unclarified. Therefore, the physiochemical responses of M. aeruginosa to butachlor were investigated, and the relevant underlying molecular mechanism was highlighted. There were no significant changes (P > 0.05) in the growth and physiology of M. aeruginosa at the low concentrations of butachlor (0-0.1 mg/L), which evidenced a high level of butachlor tolerance in Microcystis aeruginosa. For the high concentrations of butachlor (4-30 mg/L), the inhibition of photosynthetic activity, disruption of cell ultrastructure, and oxidative stress were dominant toxic effects on M. aeruginosa. Additionally, the impaired cellular integrity and lipid peroxidation may be attributed to the substantial elevations of extracellular microcystin-LR concentration. Downregulation of genes associated with photosynthesis, energy metabolism, and oxidative stress was inferred to be responsible for the growth suppression of M. aeruginosa in 30 mg/L butachlor treatment. The upregulation of gene sets involved in nitrogen metabolism may illustrate the specific effort to sustain the steady concentration of intracellular microcystin-LR. These findings dissect the response mechanism of M. aeruginosa to butachlor toxicity and provide valuable reference for the evaluation of potential risk caused by butachlor in aquatic environments.

The effects of exogenous amino acids on production of microcystin variants in Microcystis aeruginosa

Dissolved free amino acids are a significant component of dissolved organic nitrogen (DON) in natural waterbodies. The effects of four amino acids (glutamic acid, phenylalanine, leucine, and arginine) on the growth and microcystins (MCs) production of Microcystis aeruginosa were studied in batch culture. The profiles of five MCs variants and the expression levels of target genes involved in MCs biosynthesis and nitrogen metabolism were measured. When amino acids were used as the sole nitrogen source instead of nitrate at different levels (0.5, 2.0 and 8.0 mg/L based on N) in BG-11 medium, algal cell growth and intracellular MCs quotas were inhibited slightly by the treatments with glutamic acid and arginine. The treatments with phenylalanine and leucine, on the other hand, had a strong inhibitory effect on algal cell growth and MCs production. Moreover, the concentrations of Chlorophyll a, phycocyanin and allophycocyanin in cells cultured in glutamic acid, leucine and phenylalanine were lower than those in the control group with nitrate as nitrogen source. The existence of leucine or phenylalanine can lead to a significant increase in the relative abundance of MCs variants structured with the corresponding amino acids. The expression of microcystin-producing gene mcyD was downregulated while the gene pipX associated with nitrogen metabolism was upregulated during the cultivation of M. aeruginosa with amino acids as sole nitrogen source. M. aeruginosa undergoes significant alterations due to exogenous amino acids and exhibits advanced strategies for MCs production.

Synthesis and characterization of a novel magnetic resin (m-MAR resin) and its removal performance for alkaline amino acids

Alkaline amino acids as dissolved organic nitrogen (DON) have raised much concern in drinking water treatment due to poor removal in conventional treatment process and high potential for nitrogenous disinfection by-products (N-DBPs). This work was intended to devise a new magnetic adsorption resin (noted as m-MAR resin) for the efficient reduction of alkaline amino acids and explore the application potential of combined MIEX and m-MAR resins. The distribution and composition of DON and amino acids was clarified for different water sources in Lake Taihu basin, in which alkaline amino acids accounted for a higher proportion. The removal of different nitrogenous organics by MIEX resin was also examined, where the resin was effective in removing phycocyanin (65.6%) and glutamic acid (74.2%), reducing the generation of disinfection by-products (DBPs). The m-MAR resin was manufactured and characterized to cope with alkaline amino acids, and batch experiments were undertaken to investigate its adsorption behaviors on histidine and arginine under different operating conditions. The maximal adsorption capacities of arginine and histidine onto m-MAR resin were 2.84 mg/g and 1.62 mg/g, respectively, which was better than MIEX resin. The removal mechanism of the two basic amino acids by m-MAR resin was mainly due to the hydrogen bonding and the acid-base reaction. Moreover, the reusability of the m-MAR resin was elucidated after six successive adsorption-desorption cycles. Finally, the effectiveness of combined MIEX and m-MAR resin in treating DON derived from Microcystis aeruginosa reached 35.2% and the DON concentration in Lake Taihu could be reduced from 0.56 to 0.16 mg/L, which simultaneously decreased the generation potential of N-DBPs. The enhancement of coagulation by the combined process of m-MIER and m-MAR as pretreatment was estimated.