Quantitation of the microcystin hepatotoxins in water at environmentally relevant concentrations with the protein phosphatase bioassay

Early physiological and biochemical responses of rice seedlings to low concentration of microcystin-LR

Microcystin-leucine and arginine (microcystin-LR) is a cyanotoxin produced by cyanobacteria like Microcystis aeruginosa, and it's considered a threat to water quality, agriculture, and human health. Rice (Oryza sativa) is a plant of great importance in human food consumption and economy, with extensive use around the world. It is therefore important to assess the possible effects of using water contaminated with microcystin-LR to irrigate rice crops, in order to ensure a safe, high quality product to consumers. In this study, 12 and 20-day-old plants were exposed during 2 or 7 days to a M. aeruginosa extract containing environmentally relevant microcystin-LR concentrations, 0.26-78 μg/L. Fresh and dry weight of roots and leaves, chlorophyll fluorescence, glutathione S-transferase and glutathione peroxidase activities, and protein identification by mass spectrometry through two-dimensional gel electrophoresis from root and leaf tissues, were evaluated in order to gauge the plant's physiological condition and biochemical response after toxin exposure. Results obtained from plant biomass, chlorophyll fluorescence, and enzyme activity assays showed no significant differences between control and treatment groups. However, proteomics data indicates that plants respond to M. aeruginosa extract containing environmentally relevant microcystin-LR concentrations by changing their metabolism, responding differently to different toxin concentrations. Biological processes most affected were related to protein folding and stress response, protein biosynthesis, cell signalling and gene expression regulation, and energy and carbohydrate metabolism which may denote a toxic effect induced by M. aeruginosa extract and microcystin-LR. The implications of the metabolic alterations in plant physiology and growth require further elucidation.

Monitoring approaches for a toxic cyanobacterial bloom

Cyanobacterial blooms, dominated by Microcystis sp. and associated microcystin variants, have been implicated in illnesses of humans and animals. Little is known regarding the formation of blooms and the presence of cyanotoxin variants in water bodies. Furthermore, the role played by ecological parameters, in regulating Microcystis blooms is complicate and diverse. Local authorities responsible for water management are often faced with the challenging task of dealing with cyanobacterial blooms. Therefore, the development of suitable monitoring approaches to characterize cyanobacterial blooms is an important goal. Currently, various biological, biochemical and physicochemical methods/approaches are being used to monitor cyanobacterial blooms and detect microcystins in freshwater bodies. Because these methods can vary as to the information they provide, no single approach seemed to be sufficient to accurately monitor blooms. For example, immunosensors are more suited for monitoring the presence of toxins in clear water bodies while molecular methods are more suited to detect potentially toxic strains. Thus, monitoring approaches should be tailored for specific water bodies using methods based on economic feasibility, speed, sensitivity and field applicability. This review critically evaluates monitoring approaches that are applicable to cyanobacterial blooms, especially those that focus on the presence of Microcystis, in freshwater bodies. Further, they were characterized and ranked according to their cost, speed, sensitivity and selectivity. Suggested improvements were offered as well as future research endeavors to accommodate anticipated environmental changes.

Cyanotoxins: characteristics, production and degradation routes in drinking water treatment with reference to the situation in Serbia

Cyanobacteria are members of phytoplankton of the surface freshwaters. The accelerated eutrophication of freshwaters, especially reservoirs for drinking water, by human activity has increased the occurrence and intensity of cyanobacterial blooms. They are of concern due to their ability to produce taste and odors compounds, a wide range of toxins, which have a hepatotoxic, neurotoxic, cytotoxic and dermatotoxic behavior, being dangerous to animal and human health. Therefore, the removal of cyanobacteria, without cell lysis, and releasing of intracellular metabolites, would significantly reduce the concentration of these metabolites in the finished drinking water, as a specific aim of the water treatment processes. This review summarizes the existing data on characteristics of the cyanotoxins, their productions in environment and effective treatment processes to remove these toxins from drinking water.

Cyanobacterial bioactive molecules — an overview of their toxic properties

Allelopathic interactions involving cyanobacteria are being increasingly explored for the pharmaceutical and environmental significance of the bioactive molecules. Among the toxic compounds produced by cyanobacteria, the biosynthetic pathways, regulatory mechanisms, and genes involved are well understood, in relation to biotoxins, whereas the cytotoxins are less investigated. A range of laboratory methods have been developed to detect and identify biotoxins in water as well as the causal organisms; these methods vary greatly in their degree of sophistication and the information they provide. Direct molecular probes are also available to detect and (or) differentiate toxic and nontoxic species from environmental samples. This review collates the information available on the diverse types of toxic bioactive molecules produced by cyanobacteria and provides pointers for effective exploitation of these biologically and industrially significant prokaryotes.

Decrease in toxicity of microcystins LA and LR in drinking water by ozonation

Unchlorinated treated waters from two Australian reservoirs were spiked with microcystin-LA and -LR extracted from a toxic scum of Microcystis aeruginosa. The two waters had considerably different water quality and therefore ozone demands. The spiked sample waters were ozonated using the batch method of ozonation at a range of doses and the samples were analysed for toxins using high-performance liquid chromatography (HPLC). The toxin content of the samples was also determined using a protein phosphatase type 2A inhibition assay (PP2A) and toxicity via the standard mouse bioassay. The HPLC results correlated well with the PP2A results and toxicity tests for both waters. A loss of both toxins and toxicity was observed with increasing ozone dose, resulting in a complete loss of toxicity for both waters once an ozone residual had been achieved. At this ozone residual no toxin was detected using HPLC. The results indicate that microcystins are not transformed into toxic by-products.

Methods for determining microcystins (peptide hepatotoxins) and microcystin-producing cyanobacteria

Episodes of cyanobacterial toxic blooms and fatalities to animals and humans due to cyanobacterial toxins (CBT) are known worldwide. The hepatotoxins and neurotoxins (cyanotoxins) produced by bloom-forming cyanobacteria have been the cause of human and animal health hazards and even death. Prevailing concentration of cell bound endotoxin, exotoxin and the toxin variants depend on developmental stages of the bloom and the cyanobacterial (CB) species involved. Toxic and non-toxic strains do not show any predictable morphological difference. The current instrumental, immunological and molecular methods applied for determining microcystins (peptide hepatotoxins) and microcystin-producing cyanobacteria are reviewed.

A colorimetric and fluorometric microplate assay for the detection of microcystin-LR in drinking water without preconcentration

Protein phosphatase inhibition assays currently used for the detection of cyanobacterial peptide hepatotoxins in drinking water require an enrichment step using C18 cartridges to achieve lower the detection limit. This paper describes a colorimetric and fluorometric protein phosphatase inhibition method for the direct detection of microcystin-LR (MCYST-LR) in drinking water without complex clean-up steps and preconcentration procedures. In this assay three different substrates, p-nitrophenyl phosphate (p-NPP) and two fluorogenic compounds, 4-methylumbelliferyl phosphate (MUP) and 6,8-difluoro-4-methylumbelliferyl phosphate DiFMUP), were tested. The detection limits of the assay are 0.25 and 0.1 microg/l using colorimetric and fluorometric methods, respectively. These levels are well below the provisional guideline value for MCYST-LR of 1 microg/l of drinking water. The detection limit of the fluorometric method is comparable to that of the classical ELISA test. Although both the latter tests allow the detection of MCYST-LR in drinking water directly without pretreatment, the protein phosphatase inhibition assay remain less expensive and therefore more attractive for use in the routine assessment of drinking water contamination by microcystins.

Cloud-point extraction and preconcentration of cyanobacterial toxins (microcystins) from natural waters using a cationic surfactant

A new cloud-point extraction and preconcentration method using a cationic surfactant, Aliquat-336 (tricaprylylmethylammonium chloride), has been developed for the determination of cyanobacterial toxins, microcystins, in natural waters. Sodium sulfate was used to induce phase separation at 25 degrees C. The phase behavior of Aliquat-336 with respect to concentration of Na2SO4 was studied. The cloud-point system revealed a very high phase volume ratio compared to other established systems of nonionic, anionic, and cationic surfactants. At pH 6-7, it showed an outstanding selectivity in analyte extraction for anionic species. Only MC-LR and MC-YR, which are known to be predominantly anionic, were extracted (with averaged recoveries of 113.9 +/- 9% and 87.1 +/- 7%, respectively). MC-RR, which is likely to be amphoteric at the above pH range, was not detectable in the extract. Coupled to HPLC/UV separation and detection, the cloud-point extraction method (with 2.5 mM Aliquat-336 and 75 mM Na2SO4 at 25 degrees C) offered detection limits of 150 +/- 7 and 470 +/- 72 pg/mL for MC-LR and MC-YR, respectively, in 25 mL of deionized water. Repeatability of the method was 7.6% for MC-LR and 7.3% for MC-YR. The cloud-point extraction process can be completed within 10-15 min with no cleanup steps required. Applicability of the new method to the determination of microcystins in real samples was demonstrated using natural surface waters collected from a local river and a local duck pond spiked with realistic concentrations of microcystins. Effects of salinity and organic matter (TOC) content in the water sample on the extraction efficiency were also studied.

Using an enzyme linked immunosorbent assay (ELISA) and a protein phosphatase inhibition assay (PPIA) for the detection of microcystins and nodularins

Cyanotoxins produced by cyanobacteria (blue-green algae) include potent neurotoxins and hepatotoxins. The hepatotoxins include cyclic peptide microcystins and nodularins plus the alkaloid cylindrospermopsins. Among the cyanotoxins the microcystins have proven to be the most widespread, and are most often implicated in animal and human poisonings. This paper presents a practical guide to two widely used methods for detecting and quantifying microcystins and nodularins in environmental samples-the enzyme linked immunosorbant assay (ELISA) and the protein phosphatase inhibition assay (PPIA).

Determination of microcystins in blue-green algae, fish and water using liquid chromatography with ultraviolet detection after sample clean-up employing immunoaffinity chromatography

Anti-microcystin LR immunnoaffinity cartridges were evaluated for their ability to selectively remove microcystins from extracts of blue-green algae, fish and water samples for subsequent analysis by liquid chromatography with UV absorbance detection at 238 nm. Blue-green algae and fish samples were extracted with 75% methanol in water. A portion of the extract was diluted and passed through an immunoaffinity cartridge. Water samples were applied directly to the cartridge. The cartridge was rinsed with water and 25% methanol in water. The microcystins were eluted with 80% methanol in water containing 4% acetic acid. It was found that the cartridges were effective in isolating the microcystins from blue-green algae, fish and water samples, resulting in extracts that were clean enough to enable direct LC-UV detection down to approximately 0.03 microg/g in the blue-green algae and fish samples, and as low as 0.02 ng/ml for water samples. The cartridges were found to have a capacity of approximately 200 ng each for a mixture of microcystins RR, YR, LR and LA, or as much as 525-800 ng for individual compounds. Recoveries trough the complete analytical procedure ranged from 64 to 115% (all values) with an overall average of approximately 80% at spiking levels of 0.5-4.0 microg/g for the microcystins in blue-green algae. The average recoveries (n=8) from spiked (0.1-0.5 microg/g) fish samples were 73% for RR, 79% for YR, 81% for LR and 77% for LA, while from the spiked (2.0-0.04 ng/g) tap and river water samples (n=6), recoveries were 78% for RR, 86% for YR, 94% for LR and 89% for LA.

Seasonal variation and indirect monitoring of microcystin concentrations in Daechung reservoir, Korea

Physicochemical and biological water quality, including the microcystin concentration, was investigated from spring to autumn 1999 in the Daechung Reservoir, Korea. The dominant genus in the cyanobacterial blooming season was Microcystis. The microcystin concentration in particulate form increased dramatically from August up to a level of 200 ng liter(-1) in early October and thereafter tended to decrease. The microcystin concentration in dissolved form was about 28% of that of the particulate form. The microcystins detected using a protein phosphatase (PP) inhibition assay were highly correlated with those microcystins detected by a high-performance liquid chromatograph (r = 0.973; P < 0.01). Therefore, the effectiveness of a PP inhibition assay for microcystin detection in a high number of water samples was confirmed as easy, quick, and convenient. The microcystin concentration was highly correlated with the phytoplankton number (r = 0.650; P < 0.01) and chlorophyll-a concentration (r = 0.591; P < 0.01). When the microcystin concentration exceeded about 100 ng liter(-1), the ratio of particulate to dissolved total nitrogen (TN) or total phosphorus (TP) converged at a value of 0.6. Furthermore, the microcystin concentration was lower than 50 ng liter(-1) at a particulate N/P ratio below 8, whereas the microcystin concentration varied quite substantially from 50 to 240 ng liter(-1) at a particulate N/P ratio of >8. Therefore, it seems that the microcystin concentration in water can be estimated and indirectly monitored by analyzing the following: the phytoplankton number and chlorophyll-a concentration, the ratio of the particulate and the dissolved forms of N and P, and the particulate N/P ratio when the dominant genus is toxigenic Microcystis.

Colorimetric immuno-protein phosphatase inhibition assay for specific detection of microcystins and nodularins of cyanobacteria

A novel immunoassay was developed for specific detection of cyanobacterial cyclic peptide hepatotoxins which inhibit protein phosphatases. Immunoassay methods currently used for microcystin and nodularin detection and analysis do not provide information on the toxicity of microcystin and/or nodularin variants. Furthermore, protein phosphatase inhibition-based assays for these toxins are not specific and respond to other environmental protein phosphatase inhibitors, such as okadaic acid, calyculin A, and tautomycin. We addressed the problem of specificity in the analysis of protein phosphatase inhibitors by combining immunoassay-based detection of the toxins with a colorimetric protein phosphatase inhibition system in a single assay, designated the colorimetric immuno-protein phosphatase inhibition assay (CIPPIA). Polyclonal antibodies against microcystin-LR were used in conjunction with protein phosphatase inhibition, which enabled seven purified microcystin variants (microcystin-LR, -D-Asp3-RR, -LA, -LF, -LY, -LW, and -YR) and nodularin to be distinguished from okadaic acid, calyculin A, and tautomycin. A range of microcystin- and nodularin-containing laboratory strains and environmental samples of cyanobacteria were assayed by CIPPIA, and the results showed good correlation (R2 = 0.94, P < 0.00001) with the results of high-performance liquid chromatography with diode array detection for toxin analysis. The CIPPIA procedure combines ease of use and detection of low concentrations with toxicity assessment and specificity for analysis of microcystins and nodularins.

Use of protein phosphatase inhibition assay to detect microcystins in Donghu Lake and a fish pond in China

Seasonal variations in the level of total microcystins in water samples collected from Donghu Lake and a fish pond in Wuhan, China, were studied between March 1995 and February 1996 using a protein phosphatase inhibition assay involving a radioactive 32P-labelled substrate. The assay is highly reliable and repeatable, and is probably the most sensitive assay for microcystin detection to date. Results of the survey indicated the presence of microcystins in the water samples, and the concentration of microcystins appeared to be related to the degree of eutrophication and water temperature. There is also a correlative relationship between the quantity of microcystins and the abundance of microcystin-producing cyanobacteria (Anabaena and Oscillatoria) in the water bodies over a year cycle. In the present study, the positive detection of microcystins in water bodies having no signs of algal bloom warns of considerable potential threat of these waters to public health.

Toxicology and risk assessment of freshwater cyanobacterial (blue-green algal) toxins in water

The occurrence of cyanobacterial toxins affects aquatic organisms, terrestrial animals (both wild and domestic), and humans. Detrimental effects have been documented in the scientific literature during the past 50 years. Possible guideline values of some cyanobacterial toxins (microcystins, cylindrospermopsin, and anatoxin-a) are estimated, and they show that children and infants are more susceptible to cyanobacterial toxins than adults. Therefore, particular attention should be paid when cyanobacterial blooms occur, even at relatively low cell counts, to protect children and infants from possible risks. Based on these guideline values and the occurrence of the toxins, it can be concluded that chronic and subchronic exposure to cyanobacterial toxins does occur in some populations, particularly in developing countries where high proportions of the population consume untreated surface water directly, such as pond, ditch, river, or reservoir water. Because wildlife and domestic animals consume a large amount of untreated water daily, they are at higher risk than humans from cyanobacterial toxins. Calculated guideline values in Section X show that a relatively high risk posed by the toxins to these animals is likely to occur, even at low cell densities.

Detection of microcystins using electrospray ionization high-field asymmetric waveform ion mobility mass spectrometry/mass spectrometry

A combination of electrospray ionization, high-field asymmetric waveform ion mobility spectrometry, and mass spectrometry (ESI-FAIMS/MS) was used to analyze standard solutions of microcystins-LR, -RR, and -YR. The ability of FAIMS to separate ions in the gas phase reduced the amount of background in the mass spectrum without compromising the absolute signal for these microcystins. This reduction in background resulted in a ten-fold improvement in the signal-to-background ratio over conventional ESI-MS. Detection limits, using direct infusion, were determined to be 4, 2, and 1 nM for microcystins-LR, -RR, and -YR, respectively.

On-line trace enrichment for the simultaneous determination of microcystins in aqueous samples using high-performance liquid chromatography with diode-array detection

The need for a rapid, sensitive and reliable analytical method for cyanobacterial toxins, microcystins, has been emphasized by the awareness of toxic cyanobacteria as a human-health risk through drinking water. A new high-performance liquid chromatographic method with column switching was developed for the determination of microcystin-LR, -RR and -YR from water samples without pre-purification. The filtered water sample was passed through a Zorbax CN precolumn at a flow-rate of 3 ml/min for on-line trace enrichment. After valve switching, concentrated analytes were eluted in back-flush mode and separated on a Luna C18 column with a gradient of acetonitrile -20 mM phosphate buffer (pH 2.5). The method showed excellent precision, accuracy and speed with detection limits of 0.02 microgram/ml from 100 ml of surface water. The total analysis time per sample was about 90 min. This method improves reliability, sensitivity and sample throughput, and shortens the analysis time compared to analysis methods using off-line solid-phase extraction.

Formation of 3-amino-2,6,8-trimethyl-10-phenyldeca-4E,6E-dienoic acid from microcystin LR by the treatment with various proteases, and its detection in mouce liver

Microcystin LR metabolism in mammals was examined. The degradation of microcystin LR by the enzymes, pepsin, trypsin and chymotrypsin, from porcine or human gastrointestinal tract was examined. Microcystin LR was digested by these proteases, but 3-amino-2, 6, 8-trimethyl-10-phenyldeca-4E, 6E-dienoic acid (DmADDA) was not formed from microcystin LR treated with pepsin and chymotrypsin. The formation of DmADDA was detected from microcystin LR treated with trypsin. Furthermore, DmADDA was detected from the male ddY mice liver orally administered microcystin LR.

Microcystin LR degradation by Pseudomonas aeruginosa alkaline protease

We isolated several species of bacteria from the surface water of a Japanese lake. Of the isolated bacteria, Pseudomonas aeruginosa was the only species which could degrade microcystin LR in vitro. Microcystin LR decreased to 4.5% of the spiked microcystin LR quantity in the P. aeruginosa culture, and was metabolized to (2S, 3S, 8S)-3-amino-2, 6, 8-trimethyl-10-phenyldeca-4E, 6E-dienoic acid (DmADDA). It was possible that P. aeruginosa hydrolysed nucleophilically the peptide bond of microcystin LR. We examined if pyochelin, pyocyanin and alkaline protease produced by P. aeruginosa affected the reduction of microcystin LR. In the result, DmADDA was produced from microcystin LR in the presence of 100 microM H2O2 and 100 microM each of pyochelin, pyocyanin or both. However, the production of DmADDA was slight (2 to 12 mole%). After treatment with P. aeruginosa alkaline protease, DmADDA was produced 75 mole% from microcystin LR. Therefore, we concluded that microcystin was degraded mainly by the action of P. aeruginosa alkaline protease.

Mass spectrometric screening method for microcystins in cyanobacteria

A screening method for microcystins in cyanobacteria, which consists of the formation of 3-methoxy-2-methyl-4-phenylbutyric acid as an oxidation product of microcystins by ozonolysis, and detection of 3-methoxy-2-methyl-4-phenylbutyric acid by thermospray-liquid chromatography/mass spectrometry or electron ionization-gas chromatography/mass spectrometry using selected ion monitoring, was developed. The ozonolysis made it possible to significantly reduce the formation times of 3-methoxy-2-methyl-4-phenylbutyric acid because the previously required extraction, clean-up and other procedures could be entirely eliminated. The resulting intact 2-methyl-4-phenylbutyric acid was directly analyzed by thermospray-liquid or electron ionization-gas chromatography/mass spectrometry, and the procedures from ozonolysis to analysis of microcystins at the pmole levels were performed within only 30 min. The calibration curves obtained by thermospray-liquid or electron ionization-gas chromatography/mass spectrometry analysis showed a linear relationship from 14 to 830 pmole and from 2.5 to 100 pmole of microcystin-LR, respectively. The method was applied to the detection and determination of the total amount of microcystins in bloom and cultured samples, showing that it provided a means of not only screening for microcystins but of their accurate quantitation.

Mass spectral analyses of microcystins from toxic cyanobacteria using on-line chromatographic and electrophoretic separations

The application of capillary electrophoresis and of reversed-phase liquid chromatography coupled to electrospray mass spectrometry is presented for the analysis of microcystins isolated from toxic strains of Microcystis aeruginosa. The separation performance of these two techniques is compared in terms of both sensitivity and of resolution of closely related microcystins. Quantitation of microcystin-LR present at low micrograms/ml concentrations in cell extracts is demonstrated using both techniques. A marked advantage of capillary electrophoresis over liquid chromatography was its ability to resolve different desmethyl microcystin-LR analogues. Identification of these positional isomers was facilitated using capillary electrophoresis combined with tandem mass spectrometry (MS-MS). Rationalization of fragment ions observed in MS-MS spectra of microcystins was made possible through comparison with 15N labelled microcystins obtained from stable isotope feeding experiments. The potential of tandem mass spectrometry in providing selective detection of microcystins in cell extracts, and in structural characterization of novel microcystins, was also investigated.

Use of a colorimetric protein phosphatase inhibition assay and enzyme linked immunosorbent assay for the study of microcystins and nodularins

Microcystins and nodularins are cyclic peptide hepatotoxins and tumor promoters produced by several genera of cyanobacteria. Using a rabbit anti-microcystin-LR polyclonal antibody preparation, the cross-reactivity with 18 microcystin and nodularin variants was tested. A hydrophobic amino acid, 3-amino-9-methoxy-10-phenyl-2,6,8-trimethyl-deca-4(E),6(E)-dienoic acid (Adda), which has the (E) form at the C-6 double bond in both microcystin and nodularin, was found essential for these toxins to express antibody specificity. Modification of -COOH in glutamic acid of microcystin and nodularin did not alter their antigenicity. Antibody cross-reactivity of these toxins was compared with their ability to inhibit protein phosphatase type 1 (PP1). Detection of PP1 inhibition was done by measuring the inhibition effect of the toxins on p-nitrophenol phosphate activity toward PP1. PP1 was obtained as recombinant PP1 expressed in E. coli. The inhibition effect of five microcystins and two nodularins on recombinant PP1 activity toward p-nitrophenol phospate was measured in a microwell plate reader. The concentration of microcystin-LR causing 50% inhibition of recombinant PP1 activity (IC50) was about 0.3 nM, while that of two modified microcystins had a significantly higher IC50. Microcystin-LR and nodularin with the (z) form of Adda at the C-6 double bond or having the monoester of glutamic acid did not inhibit PP1. These three toxins were also nontoxic in the mouse bioassay. These results show the importance of Adda and glutamic acid in toxicity of these cyclic peptides and that PP1 inhibition is related to the toxins' mechanism of action.

Identification and characterization of hydrophobic microcystins in Canadian freshwater cyanobacteria

Hepatotoxic microcystins produced by cyanobacteria in freshwater lakes represent a significant health hazard to humans and agricultural livestock. Liquid chromatography (LC)-linked protein phosphatase (PPase) bioassay analysis of blooms of Microcystis aeruginosa produced in a Canadian drinking water lake identified several PPase inhibitors with significantly greater hydrophobicity than microcystin-LR, based on their retention time on C18 reverse phase LC columns. Seven PPase inhibitors were purified to homogeneity by bioassay-guided fractionation involving Sephadex LH-20 chromatography and two-step reverse phase at pH 6.5 and 2.0. One of the PPase inhibitors, isolated in a final yield of 1.5 micrograms/g lyophilized cyanobacteria, was identified as microcystin-LL by amino acid analysis and mass spectrometry. A further PPase inhibitor (20 ng/g cyanobacteria) was identified as microcystin-LL but with D-Ala replaced by an unknown amino acid. Four PPase inhibitors (< 20 ng/g cyanobacteria) were characterized by amino acid analysis and identified as microcystin-LV, -LM, -LF and -LZ (where Z represents an unknown hydrophobic amino acid). A further microcystin was also identified (< 10 ng/g cyanobacteria) in which arginine was apparently absent. The biological activity of the seven microcystins as inhibitors of the catalytic subunit of protein phosphatase-1 (PP-1c) was compared with microcystin-LR and motuporin (a hydrophobic analogue of nodularin). All of the compounds inhibited PP-1c with IC50 values of 0.06-0.4 nM, consistent with their identification as microcystins. These findings further demonstrate the applicability of a sensitive PPase bioassay for the identification of variant microcystins in the natural environment.