Microcystins (MCs) are potent hepatotoxins that comprise a family of more than 90 different variants. MCs can bind to matrices, after which they cannot be completely extracted by the solvent. Currently, the MMPB (2-methyl-3-methoxy-4-phenylbutyric acid) method is the only effective way of determining total MCs without solvent extraction. In this study, a sensitive method to determine the total MCs in water and sediment was developed based on the MMPB method. Specifically, MCs were oxidized to MMPB with an improved oxidation reagent (20 mg mL−1 NaIO4, 4000 mg L−1 KMnO4, pH ∼ 9) and a stable MMPB yield of about 35% was obtained for both water and sediment samples. The minimum volume of the oxidation reagent could be determined by the organic content in the matrix. After being concentrated, the MMPB was derivatized with 1,2-benzo-3,4-dihydrocarbazole-9-ethyl-p-toluenesulfonate (BDETS) and detected by liquid chromatography using a fluorescence detector. Quantification limits of MCs in water and sediment samples were 125 ng L−1 and 100 ng g−1 respectively. The method was successfully applied to Lake Dianchi and the soluble MC concentrations determined by this method were correlated well with those obtained by the enzyme-linked immunosorbent assay (ELISA).

Colony formation plays an important role in the life history of Microcystis. However, analyzing the colony size distribution with a microscope is time consuming, and colony formation also hinders the direct monitoring of cell density. In this study, a quantitative protocol for rapidly analyzing the cell density and colony size distribution of pelagic and benthic Microcystis was developed. Microcystis colonies were disintegrated by alkaline hydrolysis with 0.01–0.05 mol L−1 sodium hydroxide at 85 °C for 6–8 min and automatically measured by the Flow Camera And Microscope (FlowCAM). Benthic Microcystis colonies were isolated from different lake sediments using 40 % Percoll solution. Alkaline hydrolysis was validated as a rapid and universally effective method to disintegrate different morphospecies of Microcystis colonies. The FlowCAM exhibited excellent accuracy, reproducibility, and efficiency in determining the cell density and size distribution of Microcystis. The developed protocol was successfully applied to field samples from Lake Caohai (China) and can accurately monitor the population dynamics and colony size distribution of bloom-forming Microcystis.

The optimization of analytical procedures for the quantification of free and total microcystins (MCs) in natural sediments was systematically examined based on solvent extraction and Lemieux oxidation. In this optimized analytical procedure, a sequential solvent extraction using 50% (v/v) methanol and EDTA-sodium pyrophosphate was selected as the optimal extraction solvent for free MCs analysis, after which the purified extracts and sediment residuals were applied to the optimized Lemieux oxidation for determination of total MCs in lake sediments. The optimized procedures were shown to be efficient and reliable for the routine analysis of both free and total MCs in lake sediment samples, as indicated by the minimal adverse impact of sediment organic matter on the recovery of free MCs and yield of MMPB (2-methyl-3-methoxy-4-phenylbutyric acid). Finally, the developed procedures were applied to field sediment samples collected from Lake Dianchi during a bloom season and seven of thirty samples showed positive results.Graphical abstractHighlights► An optimized solvent extraction for analysis of free MCs in sediments was proposed. ► Best recovery of free MCs in sediments was found via sequential solvent extraction. ► We further optimized the Lemieux oxidation for analysis of total MCs in sediments. ► The yield of MMPB decreased as the content of sediment organic matter increased. ► Seven of thirty sediment samples showed positive results of MCs in Lake Dianchi.

The sorption of microcystins (MCs) to fifteen lake sediments and four clay minerals was studied as a function of sediment/clay properties, temperature, and pH through well−controlled batch sorption experiments. All sorption data for both sediments and clays are well described by a nonlinear Freundlich model (nf varies between 0.49 and 1.03). The sorption process for MCs exhibited different adsorptive mechanisms in different lake sediments mainly dependent on the sediment organic matter (OM). For sediments with lower OM (i.e., less than 8%), the sorption of MCs decreases with increasing OM and is dominated by the competition for adsorption sites between MCs and OM. In contrast, MC sorption to organic-rich (i.e., more than 8%) sediments increases with increasing OM and is dominated by the interaction between MCs and adsorbed OM. The sorption thermodynamics of MCs onto sediments showed that MC sorption is a spontaneous physisorption process with two different mechanisms. One mechanism is an exothermic process for sediment with lower OM, and the other is an endothermic process for sediment with higher OM. Furthermore, the sorption of MCs onto sediments is pH dependent (sorption decreased with increasing pH). These results provide valuable informations for a better understanding of the natural abiotic attenuation mechanisms for MCs in aquatic ecosystems.

Microcystins (MCs) comprise a family of more than 80 related cyclic hepatotoxic heptapeptides. Oxidation of MCs causes cleavage of the chemically unique C20 β-amino acid (2S, 3S, 8S, 9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda) amino to form 2-methyl-3-methoxy-4-phenylbutanoic acid (MMPB), which has been exploited to enable analysis of the entire family. In the present study, the reaction conditions (e.g. concentration of the reactants, temperature and pH) used in the production of MMPB by oxidation of cyanobacterial samples with permanganate-periodate were optimized through a series of well-controlled batch experiments. The oxidation product (MMPB) was then directly analyzed by high-performance liquid chromatography with diode array detection. The results of this study provided insight into the influence of reaction conditions on the yield of MMPB. Specifically, the optimal conditions, including a high dose of permanganate (≥50 mM) in saturated periodate solution at ambient temperature under alkaline conditions (pH ∼9) over 1–4 h were proposed, as indicated by a MMPB yield of greater than 85%. The technique developed here was applied to determine the total concentration of MCs in cyanobacterial bloom samples, and indicated that the MMPB technique was a highly sensitive and accurate method of quantifying total MCs. Additionally, these results will aid in development of a highly effective analytical method for detection of MMPB as an oxidation product for evaluation of total MCs in a wide range of environmental sample matrices, including natural waters, soils (sediments) and animal tissues.