Extracellular microcystin prediction based on toxigenic Microcystis detection in a eutrophic lake

Study on the inhibition mechanism of eucalyptus tannins against Microcystis aeruginosa

Microcystis aeruginosa is the competitively dominant algal species in eutrophic waters and poses a serious threat to the aquatic ecological environment. To investigate the effects of eucalyptus tannins (TFL) and black water in eucalyptus plantations on M. aeruginosa, this study exposed M. aeruginosa to different concentrations (0 (control), 20, 50, 80, 110, and 140 mg L-1) of tannic acid (TA; hydrolyzed tannins, HT; reagent tannin), epigallocatechin gallate (EGCG; condensed tannins, CT; reagent tannin), eucalyptus tannins (TFL, complex tannin) and mixed TFL + Fe3+ solution (tannin: Fe3+ molar ratio = 1:10). The cell density, chlorophyll-a (Chl-a) content, superoxide dismutase (SOD) activity, malondialdehyde (MDA) and soluble protein (SP) contents of algae under tannin stress were determined, and the algal cell density treated with under the combination of TFL and Fe3+ was determined. The results showed a reduction in the Chl-a content of algal cells, which inhibited photosynthesis; leading to membrane lipid peroxidation; and the complexation of soluble proteins resulting in blocked protein synthesis were the main mechanisms by which tannins inhibited the growth of M. aeruginosa. TFL achieved the same inhibition of algal cells as the tannin reagent at the same concentration. At 4 d, TFL at 80 mg L-1 and above could achieve more than 54.87 % algal density inhibition. The inhibition rate of 80 mg L-1 and above TFL + Fe3+ on algal density was more than 75 %, indicating that TFL + Fe3+ had a stronger inhibitory effect on algal density. The results may facilitate the resource utilization of eucalyptus harvesting residues, explorations of the potential application of eucalyptus tannins in the control of M. aeruginosa, and provide new ideas for ecological algal inhibition in eucalyptus plantations.

Revealing Physiochemical Factors and Zooplankton Influencing Microcystis Bloom Toxicity in a Large-Shallow Lake Using Bayesian Machine Learning

Toxic cyanobacterial blooms have become a severe global hazard to human and environmental health. Most studies have focused on the relationships between cyanobacterial composition and cyanotoxins production. Yet, little is known about the environmental conditions influencing the hazard of cyanotoxins. Here, we analysed a unique 22 sites dataset comprising monthly observations of water quality, cyanobacterial genera, zooplankton assemblages, and microcystins (MCs) quota and concentrations in a large-shallow lake. Missing values of MCs were imputed using a non-negative latent factor (NLF) analysis, and the results achieved a promising accuracy. Furthermore, we used the Bayesian additive regression tree (BART) to quantify how Microcystis bloom toxicity responds to relevant physicochemical characteristics and zooplankton assemblages. As expected, the BART model achieved better performance in Microcystis biomass and MCs concentration predictions than some comparative models, including random forest and multiple linear regression. The importance analysis via BART illustrated that the shade index was overall the best predictor of MCs concentrations, implying the predominant effects of light limitations on the MCs content of Microcystis. Variables of greatest significance to the toxicity of Microcystis also included pH and dissolved inorganic nitrogen. However, total phosphorus was found to be a strong predictor of the biomass of total Microcystis and toxic M. aeruginosa. Together with the partial dependence plot, results revealed the positive correlations between protozoa and Microcystis biomass. In contrast, copepods biomass may regulate the MC quota and concentrations. Overall, our observations arouse universal demands for machine-learning strategies to represent nonlinear relationships between harmful algal blooms and environmental covariates.

Seasonal changes in the bacterial community structure of three eutrophicated urban lakes in Mexico city, with emphasis on Microcystis spp

Artificial urban lakes commonly have physicochemical conditions that contribute to rapid anthropogenic eutrophication and development of cyanobacterial blooms. Microcystis is the dominat genus in most freshwater bodies and is one of the main producter of microcystins. Using 454-pyrosequencing we characterized the bacterial community, with special emphasis on Microcystis, in three recreational urban lakes from Mexico City in both wet and dry seasons. We also evaluated some physicochemical parameters that might influence the presence of Microcystis blooms, and we associated the relative abundance of heterotrophic and autotrophic bacterial communities with their possible metabolic capacities. A total of 14 phyla, 18 classes, 39 orders, 53 families and 48 bacterial genera were identified in both seasons in the three urban lakes. Cyanobacteria had the highest relative abundance followed by Proteobacteria and Actinobacteria. Microcystis was the dominant taxon followed by Arthrospira, Planktothrix and Synechococcus. We also found heterotrophic bacteria associated with the blooms, such as Rhodobacter, Pseudomonas, Sphingomonas and, Porphyrobacter. The highest richness, diversity and dominance were registered in the bacterial community of the Virgilio Uribe Olympic Rowing-Canoeing Track in both seasons, and the lowest values were found in the Chapultepec Lake. The canonical correspondence analysis showed that dissolved oxygen and NO₃-N concentrations might explain the presence of Microcystis blooms. The metabolic prediction indicated that these communities are involved in photosynthesis, oxidative phosphorylation, methane metabolism, carbon fixation, and nitrogen and sulfur metabolism. The lakes studied had a high prevalence of Microcystis, but average values of microcystins did not exceed the maximum permissible level established by the United States Environmental Protection Agency for recreational and cultural activities. The presence of cyanobacteria and microcystins at low to moderate concentrations in the three lakes could result in ecosystem disruption and increase animal and human health risks.

Cloning and Expression of Genes for Biodegrading Nodularin by Sphingopyxis sp. USTB-05

Biodegradation is efficient for removing cyanobacterial toxins, such as microcystins (MCs) and nodularin (NOD). However, not all the microbial strains with the microcystin-biodegrading enzymes MlrA and MlrC could biodegrade NOD. Studies on genes and enzymes for biodegrading NOD can reveal the function and the biodegradation pathway of NOD. Based on successful cloning and expression of the USTB-05-A and USTB-05-C genes from Sphingopyxis sp. USTB-05, which are responsible for the biodegradation of MCs, the pathway for biodegrading NOD by these two enzymes was investigated in this study. The findings showed that the enzyme USTB-05-A converted cyclic NOD (m/z 825.4516) into its linear type as the first product by hydrolyzing the arginine and Adda peptide bond, and that USTB-05-C cut off the Adda and glutamic acid peptide bond of linearized NOD (m/z 843.4616) and produced dimeric Adda (m/z 663.4377) as the second product. Further, based on the homology modeling of enzyme USTB-05-A, site-directed mutants of USTB-05-A were constructed and seven crucial sites for enzyme USTB-05-A activity were found. A complete enzymatic mechanism for NOD biodegradation by USTB-05-A in the first step was proposed: glutamic acid 172 and histidine 205 activate a water molecule facilitating a nucleophilic attack on the arginine and Adda peptide bond of NOD; tryptophan 176 and tryptophan 201 contact the carboxylate side chain of glutamic acid 172 and accelerate the reaction rates; and histidine 260 and asparagine 264 function as an oxyanion hole to stabilize the transition states.

Allelopathically inhibitory effects of eucalyptus extracts on the growth of Microcystis aeruginosa

Microcystis aeruginosa (M. aeruginosa), as the dominant algae in eutrophic water bodies, has caused a serious harm to the local eco-environment. A biological tool, employing allelopathic inhibitory of eucalyptus to control M. aeruginosa, has been receiving tremendous attention. This work presents the results of the allelopathic inhibitory effects of eucalyptus (Eucalyptus grandis × E.urophylla 'GLGU9') extracts of roots (ERE), stems (ESE), and leaves (ELE) on culture solutions of M. aeruginosa and its eco-physiological mechanism. The inhibitory effects of the extracts on the growth of M. aeruginosa varied greatly with ELE exhibiting the highest level of potency. Modes of action by which ELE inhibited M. aeruginosa growth were established. They involved reduction in photosynthesis, disruption of the cell membrane integrity, and inhibition of esterase activities of the cyanobacterial cells. However, ELE did not exhibit any gradients of toxicity towards zebrafish nor Washington grass plant. Species abundance and diversity in the systems remained likewise unaffected by ELE. The synergistic interaction between ELE and single-component allelochemicals (e.g., gallic acid and berberine) was ascribed to the increase in efficacy of allelochemicals in the various systems. The results of this study provide an underlying, novel, and attractive approach for controlling the growth of M. aeruginosa in aquatic environments.

Evaluating putative ecological drivers of microcystin spatiotemporal dynamics using metabarcoding and environmental data

Microcystin is a cyanobacterial hepatotoxin of global concern. Understanding the environmental factors that cause high concentrations of microcystin is crucial to the development of lake management strategies that minimize harmful exposures. While the literature is replete with studies linking cyanobacterial production of microcystin to changes in various nutrients, abiotic stressors, grazers, and competitors, no single biotic or abiotic factor has been shown to be reliably predictive of microcystin concentrations in complex ecosystems. We performed random forest regression analyses with 16S and 18S rRNA gene sequencing data and environmental data to determine which putative ecological drivers best explained spatiotemporal variation in total microcystin and several individual congeners in a eutrophic freshwater reservoir. Model performance was best for predicting concentrations of the congener MC-LR, with ca. 88% of spatiotemporal variance explained. Most of the variance was associated with changes in the relative abundance of the cyanobacterial genus Microcystis. Follow-up RF regression analyses revealed that factors that were the most important in predicting MC-LR were also the most important in predicting Microcystis population dynamics. We discuss how these results relate to prevailing ecological hypotheses regarding the function of microcystin.

Isolation, molecular identification, and characterization of a unique toxic cyanobacterium Microcystis sp. found in Hunan Province, China

Global proliferation of cyanobacterial blooms associated with climate change and eutrophication constitutes a serious environmental threat. In Hunan Province a freshwater pond located in Changsha City was found to contain high concentrations of cyanobacteria, however, the characteristics of these cyanobacteria at present are not known. This study thus aimed to isolate, identify the most common bloom-forming cyanobacteria in this region and determine the toxigenic characteristics of the predominant cyanobacteria. The cyanobacteria were isolated by serial dilution and identified using polymerase chain reaction (PCR). The cyanotoxins generated by the cyanobacterium were detected using high-performance liquid chromatography with an ultra-high resolution LTQ Orbitrap Velos Pro ETD mass spectrometry equipped with electrospray ionization interface (HPLC-ESI-MS). One  species of cyanobacterium was isolated and identified as Microcystis sp. YFM1 according to the sequence of the 16S ribosome deoxyribonucleic acid (16S rDNA). It was found that this cyanobacterium contained microcystin synthetase B gene (mcyB) and produced three types of cyanotoxins including microcystin-LR, RR and YR. Our findings indicate that the Microcystis sp. YFM1 isolated from the freshwater pond in Hunan Province exhibits unique characteristics distinguishable from other known cyanobacteria.

Detection of microcystin-producing cyanobacteria in water samples using loop-mediated isothermal amplification targeting mcyB gene

Microcystin toxin-producing cyanobacteria are known to have harmful effects on humans and animals. We have developed a loop-mediated isothermal amplification (LAMP)-based detection method by targeting the microcystin synthetase B gene (mcyB), the gene responsible for the production of microcystin. The sensitivity of the method was found to be 1 fg per reaction, and it was 1000-fold higher than the conventional PCR. The LAMP method was able to amplify the target gene with a minimum amount of dNTP (0.4 mM), which further reduces the cost of reaction. The improved LAMP assay could detect the presence of the toxin-producing cyanobacteria in water samples within 2 h of time, which demonstrates the rapidness of the method. Freshwater samples were screened using the developed LAMP, and seven water samples collected from lakes and a bird sanctuary tested positive for mcyB gene harboring cyanobacteria, and negative in all other drinking waters. Hence, the developed LAMP could be a possible alternative to the existing molecular methods for screening for microcystin in environmental samples with greater sensitivity.

An overview of the accumulation of microcystins in aquatic ecosystems

Cyanotoxins produced by toxic cyanobacteria pose a major, worldwide environmental threat to freshwater ecosystems. Microcystins (MCs) are considered to be the most hazardous groups. Indeed, some of the largest aquatic ecosystems on the earth are being contaminated with MCs. Questions have arisen regarding their transfer and bioaccumulation in natural environment. This review summarizes the present state of knowledge regarding toxic cyanobacteria and MCs, with a specific focus on their distribution in different components of aquatic ecosystems. Their accumulation in water columns, aquatic animals, plants, and sediments is summarized. MCs have been contaminating all areas of the aquatic ecosystems. Of these, the water column was the most contaminated with MCs and served as an intermediate transmission substation. Via this route, MCs could enter to other stations such as sediment, animals, aquatic and terrestrial plants. Therefore, the use of water contaminated with MCs may induce food chain contaminations with considerable health risks.